金属增材制造件的电化学抛光仿真及实验研究
HOSSAIN MD JABER
日期: 2022 年 9 月
The Simulations and Experiments of Electropolishing on
Additive Manufactured Metal Parts
Candidate Name:
HOSSAIN MD JABER
School or Department:
School of Mechanical Engineering
Faculty Mentor:
Prof. Li-Chaojiang
Chair, Thesis Committee:
Professor Zhang Zhijing
Degree Applied:
Master of Science in Engineering
Major:
Mechanical Engineering
Degree by:
Beijing Institute of Technology
Dissertation Date:
2022 September 12
金属增材制造件的电化学抛光仿真及实验研究
作 者 姓 名:
贾贝尔
指 导 教 师:
李朝将 教授
答辩委员会主席:
张之敬教授
申 请 学 位:
工学硕士
学 科 专 业:
机械工程
学位授予单位:
北京理工大学
论文答辩日期:
2022 年 9 月 12 日
研究成果声明
本人郑重声明：所提交的学位论文是我本人在指导教师的指导下进行
的研究工作获得的研究成果。尽我所知，文中除特别标注和致谢的地方
外，学位论文中不包含其他人已经发表或撰写过的研究成果，也不包含
为获得北京理工大学或其它教育机构的学位或证书所使用过的材料。与
我一同工作的合作者对此研究工作所做的任何贡献均已在学位论文中作
了明确的说明并表示了谢意。
特此申明。
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日期：2022.09.12
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日期：2022.09.12
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日期：2022.09.12
Abstract
Additive manufacturing (AM) technology (3D printing technology) is becoming a
research hotspot in engineering, manufacturing, materials, optics, and other disciplines. The
material-structure integrated net shape of complex structural metal components could be
realized based on AM technology, which is a promising tool for aerospace, automobile, and
medical. However, due to the technical characteristics of layer-by-layer manufacturing and
the stacking of AM technology, the part's surface has its particular surface morphology. In
the selective laser melting (SLM) process with high forming accuracy, the step effect caused
by spheroidization, powder adhesion, and interlayer bonding during the processing will lead
to the surface roughness Ra of SLM forming parts ranging from 6 to 50 μm. It is not easy to
meet the needs of higher precision use, so it is necessary to cooperate with subsequent surface
polishing measures to enhance the surface nature of the workpiece.
Electropolishing (EP), otherwise called anodic polishing and electrochemical polishing,
is a completing process that eliminates materials from metals or alloys in light of the anodic
disintegration process and polishes the surface of the workpiece by removing ions. Regarding
the mechanism of electrolytic polishing, there is currently no unified theory that everyone
can widely accept. The concept of mass transport limitation of anode disintegration is
generally considered to be the main reason for removing the anode surface material to achieve
the polishing effect. Since electropolishing is a complex, multi-factor-influenced process, and
there is no uniform parameter setting for different workpiece materials. Therefore, this paper
carried out experimental research on the influence of different electrolytes, temperature,
electrolyte concentration, polishing time, polishing voltage, and other factors on polishing
quality and used COMSOL Multiphysics to analyze the relevant mechanisms.
NaCl, H2SO4, H3PO4 and mixed different working solutions were used for comparative
experiments. The influence of pH value and working fluid concentration on surface
roughness were analyzed. Orthogonal experiments of polishing voltage, polishing time,
temperature, and electrical conductivity were planned to compare polishing. The surface of
the additively manufactured parts before and after is explored to explore the optimal
electrolytic polishing process of the SLM 316L manufactured parts. The average surface
roughness Ra was obtained at 0.67 μm from 6.375 μm. The electrochemical corrosion module
VII
of COMSOL Multiphysics was utilized used to simulate the relevant mechanism and
influence laws. Combined with the spheroidization, powder bonding, and steps of the surface
of the additively manufactured parts, different uneven distribution surfaces of the workpiece
were constructed, and the secondary current of the corrosion module was utilized. The
physical field of distribution was utilized to analyze the material removal process and laws
of different spacing, conductivity, voltage, and other influencing parameters. From the
simulation, the surface was dominated by steps and powder bonding. The primary and
secondary parameters are different, but the polishing time is the main influencing factor. The
distance between electrode and workpiece has the weakest influence factor. The electrolytic
polishing technology studied in this paper is helpful to the surface finish of laser additive
manufacturing metal parts. The corrosion resistance of additive parts ought to be improved
partially, which can expand the application field of additive manufacturing metal parts.
Keywords: Additive manufacturing, Electropolishing, Orthogonal test, Surface
roughness, Software simulation.
VIII
摘要
增材制造技术（也称 3D 打印技术）正成为工程、制造、材料、光学等学科的研
究热点，基于增材制造技术可实现复杂结构金属构件的材料－结构一体化净成形，
为航空航天、汽车、医疗高性能构件的设计与制造提供了新的工艺技术途径。然而，
由于增材制造技术的分层制造、逐层叠加的技术特点，零件表面有其特殊的表面形
貌。成型精度较高的激光选区熔化工艺（Selective laser melting,SLM），其加工过程
中球化和粉末粘附以及层间结合造成的阶梯效应，会导致 SLM 成型零件表面粗糙度
Ra 在 6~50 μm 之间，难以满足较高精度的使用需求，因此需要配合后续的表面抛光
措施来提升工件的表面质量。
电解抛光 (Electropolishing，EP)，又称电化学抛光、阳极抛光，是一种基于阳极
溶解过程从金属或合金中去除材料的精加工工艺，通过去除离子对工件表面进行抛
光。关于电解抛光的机理，目前人们并没有一个统一的能被大家广泛接受的理论。
阳极溶解的质量输运限制理论通常被认为是造成阳极表面物质被去除，达到抛光效
果的主要原因。由于电解抛光是一个复杂的、多因素影响的过程，对于各种工件材
料没有一个统一的参数设置。因此，本文开展了不同电解液、电解液的温度、电解
液浓度、抛光时间、抛光电压等因素对抛光质量的影响实验研究，并利用 Comsol 软
件进行相关机理和影响规律的分析。
采用 NaCl、H2SO4、H3PO4 及 混合的 不同工作液进行对比实验，分析 PH 值及工
作液浓度对表面粗糙度的影响规定，规划了抛光电压、抛光时间、温度、电导率的
正交实验，对比抛光前后的增材制造件表面，探寻 SLM 316L 制造件的电解抛光较优
工艺，实现平均表面粗糙度 Ra 为 6.375 - 0.67 μm 抛光效果。采用 Comsol 软件的电化
学腐蚀模块进行相关机理和影响规律仿真研究，结合增材制造件表面的球化、粉末
粘结和台阶等特点，构建工件不同的凹凸分布表面，利用腐蚀模块的二次电流分布
的物理场，进行不同间距、电导率、电压等影响参数的材料去除过程和规律的分析。
仿真发现表面以台阶为主和粉末粘结为主的表面，其影响参数的主次不同，但抛光
时间都是主要的影响因素，工件与电极之间距离的影响因素最弱。本文所进行的电
解抛光技术有助于激光增材制造件金的表面精整，对应增材件的耐腐蚀也要一定的
提升，可以扩大增材制造金属件的应用领域。
关键词: 增材制造，电解抛光，正交试验，表面粗糙度，软件仿真。
IX
Acknowledgment
I want to express my sincere gratitude to the Beijing Institute of Technology and School
of Mechanical Engineering for granting me the opportunity to pursue my master’s study and
research under the supervision of their faculty.
My sincere appreciation also goes to Professor Li-Chaojiang for his countless effort and
guidance throughout my studies and research. He was always available for discussions and
readily shared his thoughts on encountered challenges throughout my research. As my
graduate studies supervisor, he was always available to see my progress. He also made
himself available, offering guidance and suggestions throughout my studies.
I am also thankful to Liu-Sheng-Gui and Qu-Rui, who were always there to help me at
any stage of my studies where I found any difficulty with my experiment. I would like to
acknowledge the assistance of all of my friends from mechanical micro-manufacturing
students in room 208. Thanks to them for giving me lots of funny and enjoyable moments.
I would also like to acknowledge the services of our international student office in the
persons of Ms. Niu-Yanting and Ms. Zheng-Ye for everything they did to make our stay and
studies comfortable.
Finally, I would like to thank my family and friends for their prayers, love,
encouragement, trust, and support throughout my life. They kept pushing me to use advanced
education to pursue my career dreams.
X
Table of Content
Abstract ...................................................................................................................... VII
摘要 ............................................................................................................................... IX
Acknowledgment .......................................................................................................... X
Table of Content .......................................................................................................... XI
List of Figures ........................................................................................................... XVI
List of Tables............................................................................................................. XIX
Nomenclature ..............................................................................................................XX
Introduction................................................................................................. 1
1.1
Research Background and Significance ....................................................... 1
1.2
Research Overview ......................................................................................... 2
1.2.1 Additive manufacturing technology ............................................................. 2
1.2.1.1 Residual or internal stresses ................................................................... 4
1.2.1.2 The porosity of additive manufactured parts .......................................... 5
1.2.1.3 Surface morphology ............................................................................... 7
1.2.1.4 Development of additive manufacturing ................................................ 8
1.2.2 Metal surface treatment ................................................................................ 8
1.2.2.1 Chemical mechanical polishing .............................................................. 9
1.2.2.2 Laser polishing ..................................................................................... 10
1.2.2.3 Abrasive flow machining...................................................................... 11
1.2.3 Electropolishing .......................................................................................... 12
1.2.3.1 Overview of electropolishing on AM metal parts ................................ 14
1.2.3.2 Present research status of electropolishing ........................................... 15
1.2.3.3 Advantage of electropolishing .............................................................. 17
1.3
Thesis Structure ............................................................................................ 18
XI
1.3.1 Research purpose ........................................................................................ 18
1.3.2 Research innovation .................................................................................... 18
1.3.3 The main research content of this paper ..................................................... 18
Characterization of the SLM Printed SS316L and Mechanism of the
ECP
2.1
20
Characterization of the SLM Printed SS316L ........................................... 20
2.1.1 SLM processes of printing SS316L samples .............................................. 20
2.1.2 Mechanical properties ................................................................................. 21
2.1.3 Surface roughness ....................................................................................... 23
2.1.4 Surface morphology .................................................................................... 24
2.2
Electropolishing ............................................................................................ 26
2.2.1 The basic theory of electro-chemical polishing .......................................... 26
2.2.2 Mechanism of electropolishing................................................................... 27
2.2.3 Relationship between anode potential and anode current density .............. 30
2.2.4 Cathode, electrolyte, and diffusion layer layers ......................................... 31
2.2.5 Effect of electropolishing on stainless steel ................................................ 33
2.3
Influence of Various Parameters in Electropolishing ............................... 34
2.3.1 Surfaces cleaning ........................................................................................ 34
2.3.2 Polishing gap............................................................................................... 35
2.3.3 Polishing voltage......................................................................................... 35
2.3.4 Influence of the electrolyte composition .................................................... 36
2.3.5 Electrolyte temperature ............................................................................... 38
2.4
Orthogonal Experimental Design ............................................................... 38
2.4.1 Orthogonal test ............................................................................................ 38
2.4.2 Orthogonal array ......................................................................................... 39
XII
2.4.3 Minimizing the experiment ......................................................................... 39
2.5
Summary ....................................................................................................... 40
Simulation Analysis of Electrochemical Polishing ................................. 41
3.1
Multiphysics and Mesh ................................................................................ 41
3.1.1 Multiphysics................................................................................................ 41
3.1.2 Geometry .................................................................................................... 42
3.1.3 Working condition ...................................................................................... 45
3.1.4 Boundary condition..................................................................................... 46
3.1.5 Simulation Research and Analysis of Step Effect ...................................... 47
3.1.6 Meshing ...................................................................................................... 48
3.2
Results Analysis ............................................................................................ 49
3.2.1 Electric potential distribution...................................................................... 49
3.2.1.1 Polishing effect under using various temperature; ............................... 50
3.2.1.2 Polishing effect after changing polishing gap ...................................... 52
3.2.2 Electrolyte current density .......................................................................... 55
3.3
Influence of Initial Surface Roughness on Polishing Results ................... 56
3.4
Simulation Result Analysis in Orthogonal ................................................. 57
3.4.1 Polishing study of the first simulation model ............................................. 58
3.4.1.1 Effect of electrolyte potential: .............................................................. 60
3.4.1.2 Effect of polishing time: ....................................................................... 60
3.4.1.3 Effect of polishing conductivity ........................................................... 61
3.4.1.4 Effect of temperature: ........................................................................... 61
3.4.2 Polishing study of the second simulation model ........................................ 62
3.4.2.1 Effect of electrolyte potential: .............................................................. 63
3.4.2.2 Effect of polishing time: ....................................................................... 64
XIII
3.4.2.3 Effect of polishing gap: ........................................................................ 65
3.4.2.4 Effect of conductivity: .......................................................................... 65
3.5
Summary ....................................................................................................... 66
Experiments and Analysis of Electropolishing Effect on Additive
Manufactured 316L Stainless Steel. .......................................................................... 67
4.1
Electropolishing Procedure ......................................................................... 67
4.1.1 Material and workpiece preparation ........................................................... 67
4.1.2 Post-treatment of electropolishing .............................................................. 69
4.1.3 Electrolyte preparation ................................................................................ 69
4.1.4 Experimental setup ..................................................................................... 70
4.1.5 Profilometer ................................................................................................ 71
4.1.6 Surface analyses .......................................................................................... 73
4.2
Experimental Parameters of Electrochemical Polishing .......................... 74
4.2.1 Polishing voltage......................................................................................... 74
4.2.2 Conductivity of electrolyte ......................................................................... 74
4.2.3 Polishing gap............................................................................................... 75
4.2.4 Polishing time ............................................................................................. 77
4.2.5 Polishing temperature ................................................................................. 77
4.3
Results Analysis ............................................................................................ 78
4.3.1 Factorial design analysis ............................................................................. 78
4.3.2 NaCl based electrolyte ................................................................................ 83
4.3.2.1 LSV curve analysis ............................................................................... 84
4.3.2.2 Polishing graph analysis ....................................................................... 86
4.3.2.3 Roughness analysis ............................................................................... 88
4.3.2.4 Microscopic analysis ............................................................................ 89
XIV
4.3.3 H2SO4 based electrolyte .............................................................................. 89
4.3.3.1 LSV curve analysis ............................................................................... 89
4.3.3.2 Polishing graph analysis ....................................................................... 91
4.3.3.3 Roughness analysis ............................................................................... 93
4.3.3.4 Microscopic analysis ............................................................................ 94
4.3.4 H3PO4 based electrolyte .............................................................................. 94
4.3.4.1 LSV curve analysis ............................................................................... 94
4.3.4.2 Polishing graph analysis ....................................................................... 97
4.3.4.3 Roughness analysis ............................................................................... 98
4.3.4.4 Microscopic analysis ............................................................................ 99
4.3.5 Mixed electrolyte (H2SO4 +H3PO4) .......................................................... 100
4.3.5.1 LSV curve analysis ............................................................................. 100
4.3.5.2 Polishing graph analysis ..................................................................... 102
4.3.5.3 Roughness analysis ............................................................................. 103
4.3.5.4 Microscopic analysis .......................................................................... 104
4.4
Summary ..................................................................................................... 106
Conclusions and Future Works ............................................................. 107
5.1
Conclusions ................................................................................................. 107
5.2
The Future Prospect of Research .............................................................. 108
References .................................................................................................................. 110
Achievements ............................................................................................................. 119
XV
List of Figures
Figure 1-1 Complex geometry of additive manufactured metal feature[3] ......................................... 1
Figure 1-2 Metal additive manufacturing process chain[5] ................................................................. 3
Figure 1-3 a) Residual stress of additive manufacturing parts b) AM parts failure cause of residual
stresse [10]............................................................................................................................................. 5
Figure 1-4 Porosity on metal additive manufactured surface (cause and result [11] ........................... 6
Figure 1-5 SEM image of additive manufactured 316L stainless steel at different scant rate [10] ...... 7
Figure 1-6 Chemical mechanical polishing mechanism [19] .............................................................. 10
Figure 1-7 Mechanisms of laser polishing[21] ................................................................................... 11
Figure 1-8 Mechanism of Abrasive Flow machining process[22] ...................................................... 12
Figure 1-9 Schematic diagram of levelling process in electropolishing[29] ...................................... 13
Figure 2-1 SLM printing process of stainless-steel parts[43] ............................................................ 20
Figure 2-2 Surface morphology images of SLM-processed 316L stainless steel samples at scan speed
of a) 400 mm/s, b)500 mm/s, c) 600 mm/s, d) 700 mm/s, e) .............................................................. 25
Figure 2-5 Working principal of Electropolishing[51] ....................................................................... 26
Figure 2-6 Mechanism of electropolishing[58] ................................................................................... 28
Figure 2-7 Relationship between voltage and current density in electropolishing[62] ..................... 30
Figure 2-8 Electrolyte properties of electropolishing[67] .................................................................. 32
Figure 3-1 Electropolishing simulation model-1 .............................................................................. 43
Figure 3-2 Electropolishing simulation model-2 .............................................................................. 44
Figure 3-3 Polishing gap of the electropolishing simulation model ................................................. 44
Figure 3-4 Boundary condition of simulation ................................................................................... 46
Figure 3-5 Average surface roughness profile .................................................................................. 47
Figure 3-6 Mesh analysis of simulation mode-1 ............................................................................... 48
Figure 3-7 Mesh analysis of simulation mode-2 ............................................................................... 49
Figure 3-8 Polishing simulation result under optimal parameters ................................................... 50
Figure 3-9 Polishing result after the 1200s, a) Temperature 25 ℃ b) Temperature 70 ℃ .............. 50
Figure 3-10 Polishing result after the 2400s, a) Temperature 25℃ b) Temperature 70℃ .............. 51
Figure 3-11 Polishing result after 3600s, a) Temperature 25℃ b) Temperature 70℃.................... 51
Figure 3-12 Polishing result after 6000s, a) Temperature 25℃ b) Temperature 70℃.................... 52
XVI
Figure 3-13 Polishing result after 7200s, a) Temperature 25℃ b) Temperature 70℃.................... 52
Figure 3-14 Polishing result in the initial condition ......................................................................... 53
Figure 3-15 Polishing result after the 1200s, a) Polishing gap 10 mm b) Polishing gap 15 mm ..... 53
Figure 3-16 Polishing result after 3600s, a) Polishing gap 10 mm b) Polishing gap 15 mm ........... 53
Figure 3-17 Polishing result after 6000s, a) Polishing gap 10 mm b) Polishing gap 15 mm ........... 54
Figure 3-18 Polishing result after 6000s, a) Polishing gap 10 mm b) Polishing gap 15 mm ........... 54
Figure 3-19 Average surface roughness changes with time.............................................................. 55
Figure 3-20 Current density of both simulation model ..................................................................... 56
Figure 3-21 Simulation results of various initial surface roughness ................................................ 57
Figure 3-22 Relationship between polishing voltage (V) and average surface roughness (Ra) ....... 60
Figure 3-23 Relationship between average surface roughness (Ra) and polishing time (s) ............. 60
Figure 3-24 Relationship between average surface roughness (Ra) and polishing conductivity (S/m)
........................................................................................................................................................... 61
Figure 3-25 Relationship between average surface roughness (Ra) and polishing Temperature ( ℃)
........................................................................................................................................................... 62
Figure 3-26 Relationship between Ra and polishing Voltage (V) ..................................................... 64
Figure 3-27 Relationship between Ra and polishing time (s) ........................................................... 64
Figure 3-28 Relationship between Ra and polishing gap (mm) ........................................................ 65
Figure 3-29 Relationship between Ra and polishing conductivity (S/m) .......................................... 66
Figure 4-1 Experiment workpiece design............................................. Error! Bookmark not defined.
Figure 4-2 Experimental setups in the lab ........................................................................................ 71
Figure 4-3 Profilometers for surface roughness measurement ......................................................... 71
Figure 4-4 Surface roughness profile................................................................................................ 72
Figure 4-5 Surface roughness after using different electrolytes ....................................................... 73
Figure 4-6 Surface morphology characterization equipment ........................................................... 74
Figure 4-7 Diagram of polishing gap at high and low anodes ......................................................... 76
Figure 4-8 The relationship between electropolishing Voltage (V) and average surface roughness Ra
........................................................................................................................................................... 81
Figure 4-9 The relationship between polishing time (s) and average surface roughness Ra ........... 82
Figure 4-10 The relationship between average surface roughness Ra and electrolyte Conductivity
(S/m) .................................................................................................................................................. 82
XVII
Figure 4-11 The relationship between average surface roughness Ra and electropolishing
temperature ℃ .................................................................................................................................. 83
Figure 4-12 LSV curve in NaCl-based electrolyte at a different stirrer speed ................................. 84
Figure 4-13 LSV curves of NaCl-based electrolytes at different electrolyte temperatures ............... 85
Figure 4-14 LCV curve in a different kind of NaCl concentration ................................................... 86
Figure 4-15 Polishing graph of NaCl under different kinds of concentration .................................. 87
Figure 4-16 Polishing effect in NaCl-based electrolyte after using different types of voltage ......... 88
Figure 4-17 Surface roughness analysis before and after polishing in NaCl-based electrolyte....... 88
Figure 4-18 Microscopic picture analysis of NaCl-based electrolyte before and after polishing ... 89
Figure 4-19 LSV curve in H2SO4-based electrolyte at different stirrer speeds ................................. 90
Figure 4-20 LSV curve in H2SO4-based electrolyte at wile temperature is different ........................ 91
Figure 4-21 Electropolishing curve in H2SO4 electrolyte with different potential temperature ....... 92
Figure 4-22 Electropolishing range in H2SO4 electrolyte with the different potential range ........... 93
Figure 4-23 Surface roughness before and after polishing with H2SO4-based electrolyte ............... 93
Figure 4-24 Microscopic picture analysis of H2SO4-based electrolyte before and after polishing . 94
Figure 4-25 LSV cure of H3PO4-based electrolyte in different levels of stirrer speed ...................... 95
Figure 4-26 LSV cure of H3PO4-based electrolyte in different levels of temperature ...................... 96
Figure 4-27 Polishing range of H3PO4-based electrolyte in the different potential range ............... 97
Figure 4-28 Polishing range of H3PO4-based electrolytes in the different temperature range ........ 98
Figure 4-29 Surface roughness analysis before and after polishing in H3PO4-based electrolyte .... 99
Figure 4-30 Microscopic picture analysis of H3PO4-based electrolyte before and after polishing .. 99
Figure 4-31 LSV curve in H2SO4 and H3PO4-based electrolyte at different levels of stirrer speed 100
Figure 4-32 LSV curve in H2SO4 and H3PO4-based electrolyte at different levels of temperature 101
Figure 4-33 The polishing range of mixed acid-based electrolyte at different voltages ................. 102
Figure 4-34 Polishing range of H2SO4 and H3PO4-based electrolytes at different temperature .... 103
Figure 4-35 Analysis of surface roughness before and after polishing with mixed electrolyte
concentration ................................................................................................................................... 104
Figure 4-36 Microscopic analysis of before and after polishing at mixed acid-based electrolyte . 104
XVIII
List of Tables
Table 1 Common metals, as of now utilized in AM as ingredient materials ..................................... 22
Table 2 AISI SS (stainless steel) Composition ................................................................................... 33
Table 3 Factor and levels of orthogonal experiment design ............................................................. 57
Table 4 Orthogonal test arrangement of electropolishing simulation-1 ........................................... 58
Table 5 Range calculation of electropolishing simulation 1 ............................................................. 59
Table 6 Orthogonal test arrangement of electropolishing simulation 2 ........................................... 62
Table 7 Range calculation of electropolishing simulation 2 ............................................................. 63
Table 8 Factors and levels of Taguchi method in the electropolishing............................................. 78
Table 9 Experimental data and polishing result ............................................................................... 79
Table 10 Experimental Polishing range calculation ......................................................................... 80
XIX
Nomenclature
Abbreviations
AM
Additive Manufacturing
PBF
Powder Bed Fusion
EBM
Electron Beam Melting
SLM
Selective Laser melting
CAD
Computer Aided Design
EP
Electro-Polishing
ECP
Electro-Chemical Polishing
ECM
Electro-Chemical Machining
EDM
Electrical Discharge Machining
CNC
Computer Numerical Control
CAM
Computer Aided Manufacturing
RP
Rapid Prototyping
DDM
Direct Digital Manufacturing
DED
Direct Energy Deposition
IEG
Inter-Electrode Gape
DOE
Design Of Experiment
LCD
Liquid-crystal Display
LSV
Linear sweep Voltammetry
DFX
Drawing Exchange Format
LENS
Laser Engineering Net Shaping
DC
Direct Current
XX
北京理工大学硕士学位论文
Introduction
1.1 Research Background and Significance
Additive manufacturing technology has become a new strategic direction in
manufacturing technology because of its unique idea of forced forming and traditional
removal processing. Integrating information, new material, and manufacturing technology is
a hot technology in the manufacturing industry and is valued by major manufacturing
countries worldwide. Laser powdering technology can directly manufacture complex
components whose mechanical properties are the same or even better than castings, which
has extraordinary application potential in automobile, die, and aerospace fields.
Figure 1-1 Complex geometry of additive manufactured metal feature[3]
Additive manufacturing, represented by selective laser melting (SLM) technology,
converts three-dimensional models into a series of two-dimensional models and directly
produces solid parts by melting metal powder with high-energy laser beams through "layered
manufacturing, layer by layer." However, the surface roughness of metal additive
manufactured parts is generally between 10 to 50μm, while the surface roughness of
1
北京理工大学硕士学位论文
mechanical finishing parts can be less than 2.5 μm. The "Balling Effect" and "Powder
Adhesion" are the main factors leading to the poor surface roughness of metal parts in
additive manufacturing.
Therefore, the subsequent finishing of metal additive manufacturing parts is
indispensable. This paper will focus on surface quality improvement of the additive
manufactured metal parts and explore the finishing processing method to improve the surface
roughness. The traditional finishing technology is grinding, honing, grinding, and polishing.
In the late 1970s, a number of new finishing technologies appeared, such as electrochemicalmechanical composite polishing, electrochemical polishing, ultrasonic polishing and so on.
The study and application in the field of electrochemical polishing, while carried out earlier,
but compared with foreign there, is still insufficient, have very big improvement space,
especially in the electrolyte type, optimization of polishing process parameters and control
these a few respects polishing processes. Therefore, the electrochemical polishing and
simulation research of additive manufacturing parts have essential scientific and engineering
significance.
1.2 Research Overview
1.2.1 Additive manufacturing technology
Additive Manufacturing (AM) is the industrial name for a specific 3D printing process.
Based on digital 3D design data, this process creates 3D items by stacking materials layer by
layer. The object material of additive manufacturing may be plastic, metal, concrete, or other
materials.[1] It was also a revolutionary industrial production method that created lighter,
more robust components and systems. AM envelops various innovations, including
subgroups such as 3D printing, layered manufacturing, rapid prototyping (RP), direct digital
manufacturing (DDM), and additive manufacturing.[2] AM happened as a trademark
consequence of both the creation solicitations of Computer-Aided Manufacturing (CAM) and
the rising capacity of Computer-Aided Design (CAD), made possible by extending handling
power.[1] Like any new development, it is sensible that Additive Manufacturing advancement
was used in the beginning phases to override existing collecting strategies would get the most
benefit at this point.[3,4]
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Figure 1-2 Metal additive manufacturing process chain[5]
AM offers a huge opportunity to manufacture creative items. Specifically, additive
manufacturing permits the assembling of mathematical shapes that incorporate complex
highlights that are impractical with conventional manufacturing techniques and has the
chance to manufacture momentary creation runs without critical monetary expense inflictions.
Surface roughness can be an exceptionally pertinent plan characteristic for these situations.
For additive manufacturing, the surface roughness relies upon the particular attributes of the
AM cycle utilized, the related interaction boundaries, and the direction of parts inside the
volume that can be built.[6]
Over the historical backdrop of metals manufacturing, powder bed fusion has been a
moderately new strategy in additive manufacturing. Also, powder bed fusion (PBF) is a
widely used 3D manufacturing process. PBF has four sources of energy: laser smelting,
electron beam smelting, agent energy smelting, and thermal smelting. This energy can melt
plastic or metal powder particles, solidifying and fusing together to form objects.[75] The two
overwhelming PBF 3D manufacturing processes used in the industry are Selective Laser
Melting (SLM) and Electron Beam Melting (EBM).[3]
In all powder bed fusion processes, a couple of stages are reiterated in a cycle until a
segment is finished. The PBF process uses two chambers, a build chamber and a powder
chamber. Also, as well as PBF process uses a coating roller. The coating roller moves and
diffuses the powder material through the fabrication chamber to make these objects,
depositing a thin layer of powder. Some PBF processes use a scraper, scraper, or levelling
roller after the roller is applied to ensure a uniform surface thickness of the material.[7] After
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the coating is scanned and fused, the construction platform is lowered gradually, while the
powder chamber is similarly raised, and repeated this process until the desired object is
complete.[2]
SLM is a new additive manufacturing technology developed in the late 1980s and 1990s.
In the SLM process, successive layers of powder are specifically liquefied to frame the item
through the collaboration of the laser beams to form the product. When irradiated, the powder
material is heated and melts, and forms a liquid pool if applied with enough energy. The pool
then solidifies and cools rapidly, and the solidified material begins to form products. After
checking the cross-section of the layers, the structural stage reduces the sum equivalent to the
thickness of the layers and saves another layer of powder. This interaction is re-hashed until
the project is complete.[8] Selective laser melting uses powerful lasers, and electron beam
melting uses electron beams. Selective laser melting is also inseparable from direct melting
laser melting and laser powder bed melting, both of which describe the basic process of
melting metal powder using a laser heat source.[9]
1.2.1.1 Residual or internal stresses
Residual stress is one of the leading sources of the component or system failure in metal
additive manufacturing, and it is easy to cause crack propagation and structural deformation.
The laser scans the powder layer by layer: Due to solidification, melting, and heating,
different material areas undergo different cooling and heating. Many factors cause residual
stress of parts, but it is mainly controlled by design. Residual stress results from the
temperature gradient from the surface to the centre of an AM part during cooling. It can have
a particularly severe effect on parts with a large amount of material because the material
inside the mass cools more slowly than the material outside, creating stress in part. Residual
stress affects crack propagation behaviour, fatigue strength, corrosion resistance, and fracture
toughness.
Macroscopic stress is related to sample size, and intergranular stress is related to grain
size. These are two types of residual stresses. If the cast plate is not preheated before
deposition, the liquid metal will attempt to contract during cementing. Still, the liquid metal
is squeezed by the cold cast plate, creating an in-plane elastic pressure. Reducing the layer
thickness can reduce the most significant longitudinal residual stress and the residual stress
through the thickness while increasing the torsion of the workpiece. In addition, the residual
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stress can be reduced or mitigated by preheating the network, checking on smaller islands,
shortening the deposition length, and speeding up the deposition rate. Preheating the substrate
is the best process to reduce the infinite residual stress during development. Changing the
inspection speed was seen as the least attractive because it modifies the calculation of the
liquid pool, which could cause troublesome changes to the different nature of the prearranged part.[10]
Figure 1-3 a) Residual stress of additive manufacturing parts b) AM parts failure cause of
residual stresse [10]
1.2.1.2 The porosity of additive manufactured parts
The porosity of metal is a manufacturing defect that can make machines vulnerable to
various potential problems. While porosity may cause structural defects, it does not
necessarily require replacing or rebuilding metal components.[10] Lasers are used to melt a
layer of metal powder for some metal additive manufacturing processes, making the powder
gather and join a formerly melted layer. Frequently, this cooperation brings about
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exceptionally thick parts, generally 98% or denser. The discontinuity can prompt gas pockets
inside the casing of the metal powder, changes in the size of the powder atoms, or delayed
consequences of the additive manufacturing process itself.[11]
There are many causes behind surface porosity. In unambiguous selective laser melting
cycles working at high power densities, powder melting can happen in keyhole mode. In the
keyhole mode, the power thickness of the laser hub is adequate to cause plasma improvement
and scattering. The vanishing of the metal permits the laser to travel farther than anticipated.
In the event that the plan of keyholes is not controlled, they can become shaky and break
down, leaving openings brimming with steam. The second reason for porosity is the catching
of gas inside the powder particles during powder treatment. Also, cautious gases caught in
fluid pools might be delivered. At long last, the blend vanishes due to the upper laser's
passage. Porosity is still up in the air as the contrast between the expected absolute thickness
of the material and the decided thickness. This procedure is essential and non-devastating;
however, it does not give pore transport, size, or numerical information that can be impacted
by the surface break and pores that permit water to enter. [10]
Figure 1-4 Porosity on metal additive manufactured surface (cause and result [11]
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1.2.1.3 Surface morphology
Surface roughness can play an essential role in manufacturing and exploration related to
additional manufacturing, rather than just a device to check the consistency of well-defined
surface prerequisites. Surface estimation can be used to understand the actual uniqueness that
occurs during AM manufacturing by examining the interactive delivery of surface highlights
and by examining the mysterious and interwoven causality between the organizations
involved in the actual uniqueness.
The factors influencing the surface nature of additive-made parts incorporate
combination type, size, powder shape, and morphology. There are additionally two principal
components for surface roughness. One of the primary instruments for surface roughness is
the "stepping stool" impact because of the layer-by-layer estimate of slanting endlessly
surfaces. This impact is significantly more articulated on unsupported shades since no melted
powder supports the surface rather than solid material.[10] The second mechanism that
produces surface roughness is spheroidization. The schizogenesis phenomenon will lead to a
discontinuous scanning trajectory and hinder the uniform deposition of fresh powder,
resulting in porosity. Pellet formation depends on the properties of the powder material but
can be controlled by laser processing conditions.
Figure 1-5 SEM image of additive manufactured 316L stainless steel at different scant rate [10]
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1.2.1.4 Development of additive manufacturing
Metal additive manufacturing goes back essentially to Baker's invention in 1920, which
utilized electric curves and metal electrodes to shape wall structures and enhance objects.[12]
3D printing technology can create complex cooling diverts in infusion molds, setting aside a
ton of cash and significantly shortening the production cycle. Significant cost savings are
achieved by using AM technology wisely, which is increasingly slow and costly than
traditional manufacturing procedures. In 1999, Solidica(Fabrisonic) invented a new
technology for laminate additive manufacturing based on ultrasonic energy. Sciaky started
an EAM technology based on electron beams and wires for printing large parts in 2009.[13]
Metal additive manufacturing begins with DTM. Additive manufacturing may reduce
strategic impressions, costs, and energy associated with additional component bundling,
transportation, and capacity. The impact on strategic inventory networks is not thoroughly
studied at present.[14]
Polyjet Materials Printing, Laser Engineering Net Shaping (LENS), aerosol injection,
ultrasonic additive manufacturing, continuous liquid interface production, selective laser
melting (SLM), and other new technologies have been commercialized in industrial
applications.[15] At the same time, the raw materials were also improved, developed, and
modified to adapt to the use of additive manufacturing. Numerous materials, including metals,
composites, ceramics, and organic materials, are currently accessible for additive
manufacturing.[16] As a result, the technology is no longer considered a prototype but has
been introduced as a manufacturing technique capable of producing anything from miniature
bulls to components sufficiently enormous to be considered infrastructure. Additive
manufacturing alloys have a perplexing thermal history, rapid solidification, directional
heating, and repeated melting. Typically, additively manufactured alloys also undergo
repeated solid phase transitions. These factors introduce a complexity not commonly seen in
traditional crafts. [14,17]
1.2.2 Metal surface treatment
Metal completing is the last move toward the manufacturing system and is utilized to
give stylish and natural security. It can likewise be utilized to lessen surface roughness past
machining limit with respect to parts that should be fitted or fixed. It additionally incorporates
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metal cleaning, descaling, deburring, and different strategies. Metal-finishing machines use
various materials and processes to clean, etch, and plate metal and nonmetal surfaces to
produce workpieces with desired surface characteristics. Surface fruition control is
fundamental since it influences natural reaction and mechanical properties like powerless
strength and strength development execution. There are many surface roughness treatments
referenced as before. Mechanical surface roughness treatment incorporates machining,
sandblasting, vibration grinding, and micromachining. Since one of the most appealing
components of additive manufacturing is the capacity to make inward estimations, this is
basic to working with propels in interior surface roughness. The non-mechanical procedures
considered incorporating chemical polishing, abrasive flow machining, laser polishing, and
electrochemical polishing.
1.2.2.1 Chemical mechanical polishing
Chemical mechanical polishing is often called CMP polishing. This is a process in which
a slurry containing abrasive particles is suspended in an active chemical reagent to polish the
surface of the crystal dome. A chemical-mechanical polishing (CMP) is usually related to a
polishing process that removes surface materials through a chemical reaction. CMP is the
standard manufacturing process used to manufacture integrated circuits and memory disks in
the semiconductor industry. CMP uses three main components, polisher, pad, and grout.[18]
Currently, there are four most delegated and broadly involved commercial CMP devices in
the industry:(a) turning polishers with wafer transporters responding along the platen width;
(b) a rotary polishing machine with a carrier with oscillating motion; (c) Track type polishing
machine with track rotating platen; (d) A linear polishing machine with a linear moving belt
as a polishing pad.[19]
Chemical corrosion softens the material, while mechanical wear removes it, flattening
the topographic features and making the surface flat. Only chemical corrosion is isotropic
and will not flatten the surface morphology, while mechanical wear will flatten the surface
but produce surface defects. This polishing method is unsuitable for precision surface
finishing because it cannot remove relatively large concave and convex surfaces. It is more
suitable for obtaining a smooth surface by removing subtle defects left after pre-polishing.
Industrially, chemical polishing is used for aluminium and its alloys, copper and its alloys,
and stainless steel. Chemical polishing liquid mainly uses sulfuric acid - phosphoric acid,
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nitric acid - sulfuric acid, nitric acid, and other phosphoric acid-based solutions.[20] For
chemical polishing to be effective, the metal surface under study must have a uniform
structure in which the dissolution rate is uniform. Otherwise, we cannot expect good results.
Figure 1-6 Chemical mechanical polishing mechanism [19]
1.2.2.2 Laser polishing
Laser radiation polishing is mainly used for remelting the workpiece with thin surface
and smoothing the surface roughness by surface strain. The progress of laser polishing
technology lies in its dynamic guidance, which is unique to conventional crushing polishing
technology. Laser polishing can decrease the roughness of a few extraordinary materials,
such as hot-worked steels in the mold and decoration industry or titanium composites in
clinical design. The improvement of the presence of the planar surface is accomplished by
creating a selective laser polishing with a double sparkle impact. Laser polishing increases
the chance of selectively treating small areas compared to conventional polishing
processes.[21] With the rapid development of laser additive manufacturing technology, laser
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polishing is becoming increasingly important. Laser polishing is an attractive alternative to
mechanical polishing. Because it is likely to be roboticized entirely without device power, it
is considered for use in applications where mathematical elasticity is crucial. There are three
unique laser lighting polishing systems: massive area removal, nearby removal, and naturally
visible or infinitesimal remelting. Compared with ablation strategies, the study of laser
polishing mainly focuses on the remelting process: profound mechanization degree, short
treatment time, no grinding or composite waste pollution, customer quantifiable surface
roughness, no change in workpiece state, and micro-roughness.
Figure 1-7 Mechanisms of laser polishing[21]
1.2.2.3 Abrasive flow machining
Abrasive flow machining (AFM) is a non-traditional method to remove recast layers,
chamfer, deburr, polish, and generate residual compressive stress.[22] AFM method is the best
alternative for machining parts requiring manual finishing or complex shapes and other
complicated machining methods. In the AFM process, a semi-solid medium, consisting of
transporter as a polymer base, which contains the grinding powder in the desired proportion,
is used to extricate the surface at a given pressure, which is to be machined. It becomes a
versatile device when the medium has restrictions because of an irregular surface. The
particular deformable gadget of the medium empowers it to go through channels of any shape.
Surfaces machined by AFM must have restricted media flow channels in which the media
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moves like a flexible grindstone, grinding the material and providing a good surface finish.
A fixture is usually expected to confine or direct and concentrate the medium to the ideal
position on the workpiece.[22]
Figure 1-8 Mechanism of Abrasive Flow machining process[22]
1.2.3 Electropolishing
Surface polishing is a customary idea that can be separated into surface coatings, surface
medicine, or a combination of the two. Surface finishes include electrochemical polishing,
mechanical polishing, ultrasonic cleaning, material stretching and degreasing, and lowpressure plasma etching.[23] During electropolishing, the real-life mechanisms make it
incredibly reasonable for the EP of additive manufacturing parts. The most generally
perceived method for dealing with a surface from sandpaper-like to a mirror-like culmination
is to include genuine direct contact as the surface advances from cruel to smooth. The
procedures executed change, regularly beginning with machining, progressing to sanding or
significant polishing, and a while later fine polishing.[24] Similarly, the impediment of
machining, polishing, sanding, beading, or sandblasting and polishing is that these techniques
require actual direct contact, which is not generally imaginable in AM parts, complex
channels, designs so forth. There are many polishing methods for metal surface treatment.
The ECP, or electropolishing (EP), is a promising strategy for polishing conductive materials
without including mechanical action. ECP is one of the main applications of electro-chemical
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machining (ECM) and is based on the same principles as ECM.[25] Fortunately,
electropolishing works similarly to existing methods, first reducing large contours and then
reducing more minor features without direct contact.
Electropolishing (EP) is a kind of anodic treatment method widely used in metal or alloy
surface treatment in the industry, which can obtain good stress-free surface and surface
brightness. In 1930, Jacquet was the first to propose electrochemical polishing as an industrial
surface treatment process generally attributed to work.[26] Electropolishing really eliminates
high tops on a surface through an electro-chemical activity. Electro-chemical polishing
eliminates the top layer of metal alongside toxins in the base metal. This eliminates
debasements and surface material and establishes an oxygen-rich climate that considers the
arrangement of a fantastic chrome oxide layer and delivers the material aloof.[27]
Figure 1-9 Schematic diagram of levelling process in electropolishing[29]
In ECP, the positive workpiece is submerged in the electrolyte and goes about as an
anode. Furthermore, the negative terminal of the power supply operates as a cathode. When
the workpiece is immersed in the electrolyte, metal ions are removed from the anode as a
current flow through the circuit. Since most compound arrangements are perilous and harmful,
care should be taken while dealing with electrolytes. The mounting material should not
respond with the electrolyte utilized and ought not to be covered to keep away from pointless
secondary effects. Due to the slow rate of electropolishing, deep scratches or notches from
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past material treatments may not be eliminated entirely because the material has not been
electrically polished for enough time.[29]
The electrolytic polishing system is driven by the difference in potential between anode
and cathode. It is assumed that the second stage of the workpiece is anodic. Moreover, the
disintegration of the metal takes place in the second stage rather than in the primary stage; If
the subsequent stage is cathodic, the material surrounding the subsequent stage will be
cleaned, possibly allowing the cathode to break away and leave pits on the surface.
Electrolytic polishing is a complex technology, but it has been widely used. [28,30]
1.2.3.1 Overview of electropolishing on AM metal parts
Electrolytic polishing was first mentioned in 1907 by Buetel, who noticed that the outer
layer of gold had a silky sheen in an acidic solution.[31] In 1910, Spitalsky patented a cyanide
whitening process for a variety of metals, such as silver, gold, and stainless steel. One of his
electrolysis methods was silver nitrate potassium cyanide, which is basically the electrolytic
polishing of silver in the industry today. Electrolysis progressed slowly over the next decade
until the late 1920s, when the steel industry discovered the role of sulfuric and phosphoric
acid, which also brightened steel surfaces. As per Metz, Jacquet found in 1929, "an
exceptionally fine anode for electrolysis of copper wire in a solution of phosphate and
perchloric acid in an o natural solvent could be polished."
In the 1970s, with the upgrading of products, the reduction of production volume, and
the improvement of product quality, the demand for processing accuracy is getting higher
and higher.[32] Processing, passive electrolyte, power frequency pulse current (pulse current),
vibration feeding, and other technological measures, especially pulse current electrolytic
polishing equipment in the Soviet Union, the United States, Germany, Britain, Japan and so
on have been widely studied. During this period, in-depth research on the cathode design of
electrolytic polishing equipment has also been carried out.
In the 1980s, competition became increasingly fierce due to the development of other
technologies, such as the development of extensive CNC cutting machine tools, the largescale development of EDM machine technology, and a large number of die processing turned
out to be by CNC cutting and EDM. The number of electrolytic polishing equipment was
gradually decreasing, and the application range of electrolytic polishing equipment was
constantly tested and challenged. During this period, the processing of electrolytic sleeves in
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China reached or partially exceeded the world's advanced level. In the mid-1980s, computer
technology began penetrating electrolytic polishing equipment.[33] Therefore, the outer shell
of electrolytic polishing equipment has become a booming application field of electrolytic
polishing equipment in Britain, the United States, Russia, and other countries.
In China, electrolytic polishing technology is relatively mature and has been widely used
in automobile manufacturing, filtration machinery, the petrochemical industry, medical and
health care, and architectural decoration.
Since the mid-1980s, Nanjing University of Aeronautics and Astronautics has also
researched electrolytic polishing equipment by development method.[33] Xi'an Kunlun
Machinery Factory and the former Beijing Institute of Technology have jointly conducted
research on the microcomputer control of the barrel inlet and outlet size. However, it has not
been used for production but laid a good foundation for future research. The composite
research of electrolysis at the Dalian University of Technology and North-western
Polytechnical University was also carried out during this period. Many units have also carried
out CAD/CAM research on tool electrodes.[33] The electrolytic polishing equipment method
can significantly improve processing efficiency and quality, and the depth of the processing
hole is significantly increased. The various types of holes, grooves, and lugs on alloy bushing
are examples of successful application on production lines, giving full play to the tributes and
benefits of electrolytic polishing equipment. There are numerous electropolishing companies
in China. Dongguan Senyuan Metal Polishing Factory is China's most professional stainless
steel electropolishing and chemical polishing factory. It specializes in stainless steel
electropolishing, copper polishing, and aluminum polishing.[34]
From 1985 to 2021, there were 539 pieces of literature related to electropolishing, mainly
in the fields of metallurgy and metal technology, chemical industry, machinery, instrument
industry, etc., including 183 journal papers, 19 conference papers, and 337 patent kinds of
literature.[35] The related literature on electrolytic polishing was contributed by 1145 authors,
including Zhang Yongjun, Li Chaoyang, Xing Pifeng, etc.
1.2.3.2 Present research status of electropolishing
As of late, with the development of the semiconductor industry, aerospace, medical
equipment technology, and the improvement of surface processing technology, domestic and
foreign scholars gradually began to conduct in-depth research on precision stainless and the
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surface polishing process.
[36,37]
Several thematic studies have been undertaken on the
different issues that have arisen. Currently, the methods of processing the inner surface of the
pipeline can be divided into manual polishing, chemical polishing, mechanical honing and
polishing, magnetorheological finishing grinding, abrasive flow polishing, and ultrasonic
vibration polishing, electro-chemical
polishing, and their
composite processing
technology.[38]
Electrochemical polishing is otherwise called electrolytic polishing. Electrolytic
polishing is a method to lessen the surface roughness and improve the surface brightness by
using the principle of electro-chemical anodic corrosion to remove the residual microinhomogeneity of the surface during cutting. In the process of electrolytic polishing, insoluble
metal material as the cathode and the workpiece to be polished is used as the anode. Both
electrodes are immersed in the electrolyte simultaneously, and the anode surface is selectively
dissolved under the activity of an electric current, thus developing the surface finish of the
workpiece.[39] For the principle of electro-chemical polishing, researchers from all over the
world have many disputes. At present, the mainstream theory of electro-chemical polishing
is the mucosal theory.[40] According to the theory: in the process of polishing with phosphoric
acid electrolyte, the metal cation falls off from the surface of the polished workpiece,
combines with the phosphate in the electrolyte, and the phosphate film formed will be
adsorbed on the workpiece surface. The highest point of the protrusion film is thinner than
the depression film due to its more robust ability to diffuse into the electrolyte. Nonetheless,
this kind of film has a more significant impedance. The current density of the convex part of
the workpiece surface is higher because the dissolution rate of the convex part is higher than
that of the concave part. The greater the difference in film thickness between the depressions
and the projections, the greater the difference in current density. With the progress of
polishing, the high points of the workpiece surface to be polished are gradually eliminated,
the thickness of the mucous membrane changes and the rough surface is progressively
smooth.[36]
In 2000, Zhejiang Zhide Iron and Steel Co., LTD used the electro-chemical polishing
method to polish the inner surface of a 316L stainless steel tube with a total length of 40mm
and inner diameter of 9mm and achieved a good polishing effect.[41] In the electrolyte
environment, the impact of current density, polishing time, electrolyte temperature,
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machining gap, tool electrode, surface roughness, and polishing quality of the inner pipe
surface was analyzed by experiments, and the process parameters were optimized. The
original 2.0μm is reduced to 0.05μm, and a mirror effect is achieved. However, limited to the
application background of this type of pipeline, this study did not conduct in-depth research
and analysis on small-diameter stainless steel pipes with smaller diameters. Simultaneously,
in this review, the aspect ratio of precision polished stainless-steel tubes is about when the
diameter of smaller stainless-steel tubes is more significant than ten or more. The electrolyte
concentration is very high during the polishing process. The stray corrosion caused by the
action of the chemical, air bubbles, and polishing sediments will be more apparent. The actual
effect of using this method to polish the inner surface of sizeable length-diameter ratio pipes
is not apparent at present.
1.2.3.3 Advantage of electropolishing
The advantage of electropolishing is achieved through the process's ability to process
materials that are often difficult to process using mechanical polishing processes. One of its
main advantages is the flexibility of tool selection; relatively soft tools can be used to polish
hard materials. In an exemplary electropolishing process, tool wear is zero. However, even
in practical applications, tool wear is negligible, saving many installations and tool
replacement costs. The electrolyte used to complete the circuit can be filtered and reused,
saving the cost of the entire polishing process. Polished parts are burr-free and can be
automated to achieve repeatable results. The main benefits of electropolishing of metal
surfaces can be summarized below:
1) The electropolishing improves surface physical appearance
2) Enhance Mechanical properties.
3) Reduced dirt adhesion.
4) Better corrosion protection
5) Ability to remove sharp edges (burrs, corners).
6) Removes cold-worked metal oxides.
7) Removal of H2 (hydrogen) from alloy.
8) Easier cleaning and maintenance.
9) Can process a large number of parts simultaneously
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1.3
Thesis Structure
1.3.1 Research purpose
This project aims to study the essential parameters that affect the electropolishing of additive
manufactured metal parts. Electrochemical experiments and electropolishing simulation
studied the reaction mechanism. The effects of polishing time, polishing temperature,
electrolyte conductivity, and polishing voltage on surface finish were studied.
The Main Research purpose includes
1. Understanding the primary working process of electropolishing.
2. To investigate the essential parameters affecting the electropolishing of stainless steel.
3. To reduce the micro surface roughness of 3D printed parts.
4. The main objective is to reduce dirt and improve the surface's cleanability.
5. To achieve high performance of surface roughness.
6. Investigate the electropolishing affecting factor in electropolishing simulation.
7. The proposed design and simulation process should meet the successful polish rate
with better electrolyte potential at the end of the simulation.
8. Understanding the polishing effect under different electrolyte concentrations.
1.3.2 Research innovation
The main Innovative points of this thesis are summarized as follows:
1) An environmentally friendly electrolyte polishing method for additive manufactured
316L stainless steel parts based on NaCl was proposed and compared with other electrolytic
liquid systems such as sulfuric acid and phosphoric acid.
2) The best surface roughness (Ra 0.67 μm) has been obtained in this study. The use of
pulse current electrochemical polishing additive manufacturing metal parts reduces the
surface roughness of the parts and improves their corrosion resistance. The polishing factors
such as polishing time, polishing voltage, electrolyte conductivity, and the polishing gap in
additive manufacturing of 316L stainless steel were analyzed and verified by orthogonal
experiment.
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3) Combined with the spheroidization, powder bonding, and steps of the surface of the
additively manufactured parts, different uneven distribution surfaces of the workpiece were
constructed, and the secondary current of the corrosion module was utilized for simulation.
1.3.3 The main research content of this paper
This paper mainly studies the experimental and simulation parameters of electrochemical
polishing on the surface of additive manufacturing parts, explores the influence of the
experimental parameters on the polishing effect, and summarizes the polishing result.
The main content includes:
1. The standard metal additive manufacturing technology and the surface morphology
of additive manufacturing parts are introduced. The principle and characteristics of additive
manufacturing parts surface polishing technology are summarized.
2. This paper introduces the principle of electrochemical polishing, including the main
reaction, the polarization of electrodes, and the mechanism of electrochemical polishing. It
introduces the application and advantages of electrochemical polishing.
3. This paper analyzed the experimental process of electrolytic polishing, including the
influence of electrolyte concentration, polishing voltage, polishing temperature, and
polishing time. The results of surface roughness and surface morphology were analyzed.
4. AutoCAD drew the surface features of the additive parts, and the electrochemical
polishing process was simulated by COMSOL software. The simulation results were
analyzed, and the influence of machining parameters on the polishing effect was explored.
The influence curve was drawn, the law was analyzed, and the conclusion was discussed.
5. The conclusion of this paper is summarized and the future research direction
prospects.
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Characterization of the SLM Printed
SS316L and Mechanism of the ECP
2.1 Characterization of the SLM Printed SS316L
2.1.1 SLM processes of printing SS316L samples
Selective laser melting (SLM) is a kind of powder laser-based compounding process,
most of which is carried out on structural plates. Import the project CAD file into a 3D printer
and build parts as needed using legal interaction boundaries. This cycle takes place in a
vapour atmosphere where the substance of the gas varies depending on the powder substance.
A laser beam melts and joins different metal powders in a printing system. When a laser beam
hits a thin material layer, it selectively joins or welds the particles together. After a full print
cycle, the printer adds another fuel layer on top of the previous one. The item is then removed
by accurately measuring the thickness of a single layer. Someone must remove unused
powder from the item when the printing process has finished.[42]
Figure 2-1 SLM printing process of stainless-steel parts[43]
A large number of metal powders have been verified in SLM processes, including cobaltchromium, copper, aluminium, superalloys, titanium, tool steel, chromium, and stainless steel.
While the majority of the unused powder can be reused for additional AM processes, the
development volume of the filling SLM studio is badly arranged and wasteful, particularly
when enormous parts should be created.[43] SLM-treated stainless steels show more grounded
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consumption obstruction and extensively higher biocompatibility than customary stainless
steels. Because of its astounding mechanical, consumption and organic properties, SLMhandled stainless steels have been effectively utilized in atomic, aviation and clinical inserts.
Even though SLM-handled stainless steel offers better execution, many moves should
be tended to before a far-reaching modern application. The anisotropy of properties is one of
the principal issues in SLM machining materials and parts, which has been drawn broad
consideration. Many examinations have zeroed in on the impact of building direction on the
microstructure and tensile properties of stainless-steel tests processed by SLM. The
technology promises to revolutionize the future essence of industrial manufacturing and
client producer association. For modern engineers, the use of additive manufacturing and its
combination with computers and information technology has made design a more
straightforward task. Due to this effortlessness, more individuals can now contribute to
design and manufacture, given the abilities of modern software packages in design and design
development and the possibilities for file storage, transfer, and sharing through the Internet
and cloud-based frameworks. Additive manufacturing is a globally recognized component
manufacturing technology that enables on-demand components production, energy
utilization, and reducing costs and carbon emissions. [44]
2.1.2 Mechanical properties
Mechanical properties depend on the microstructure characteristics of parts. It is possible
to find certain similarities when comparing the properties of materials processed by different
manufacturing routes. However, it is well known that specific properties can be improved
depending on the manufacturing process. In the SLM process, construction direction
significantly affects the mechanical properties of parts. It is essential to consider all possible
construction directions during the printing process. In addition, the combination of structural
orientation, layer thickness, material type, surface finish, and post-treatment shows an
essential aspect of the mechanical properties of polymer sprayed parts. [37]
Powder quality essentially affects the capacity to create parts that reliably meet rigid
determinations. The powder layer thickness ought to be essentially as high as conceivable to
create thick parts with high examining pace and high efficiency along these lines. The density
of the powder layer is mainly dependent on the size, shape, or more precise size distribution
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of the particles. [17] Particle shape affects porosity, as more significant deviations from the
spherical shape result in lower density and thus more porosity. In addition, sphericity also
provides better flow performance in the coating. Particle size determines its fusibility and
eventual pore size. Although fine powder granulation usually has better density and surface
quality than coarse powder, the grain size distribution must conform to the background of the
selected layer thickness. [46]
Table 1 Common metals, as of now utilized in AM as ingredient materials
Type of metals
Alternative name
Stainless steel 304
Stainless steel 410
Stainless steel 316L
Stainless steel
Stainless steel 440
Stainless steel 15-5 PH/PH1
Stainless steel 17-4 PH/GP1
Stainless steel CX
AISI 420
Maraging steel 300
Maraging steel 18Ni300
20MnCr5/EN10084
Tool steel
H13
AISI D2
AISI A2
AISI S7
AlSi10Mg
Aluminium alloys
AlSi7Mg
AlSi7mg0.6/SAF AMS 4289
AlSi12
AA 2139 (AlCu, Mg)
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There are a variety of metals that can be utilized as ingredient materials for additive
manufacturing. The quantity of these metal materials is increasing. It is important to note that
even directly machined additive manufacturing parts sometimes require post-treatment heat
treatment, machining, or surface treatment to improve their characteristics and achieve
specified performance. Type 316L SS is austenitic and has been used for quite a while in the
chemical and petrochemical industry and seaward designs. The significant erosion
obstruction of stainless steel is credited to the game plan of a stable passivated oxide layer;
however, stainless steel is powerless against limited consumption by chloride particles.[47]
Specifically, the static stacking capabilities of SLM parts, including elasticity, hardness,
and tensile profiling, are determined and distributed. The SLM machine vendor also
evaluated the above implementation. Given the data sheets available in the laboratory, the
distributed results, and the centralized progress, it is well assumed that the mechanical
properties of SLM parts are the same as those of mass materials, except for the flexibility,
which is lower for parts produced using SLM. In any case, the mechanical properties of SLM
parts and different parts depend on the material structure, as well as the microstructure
obtained and the presence or absence of deformation in the final product, as determined by
process parameters and manufacturing strategy. In terms of mechanical properties, the unique
stacking limit (fatigue), high-temperature properties, and the relationship between
mechanical properties and microstructure still need to be considered.[48]
2.1.3 Surface roughness
The surface roughness of parts has a high demand on components' performance and longterm performance. Metal additive manufacturing processes by themselves typically do not
meet surface roughness necessities requiring slow and expensive post-processing such as
polishing or machining. In order to select the ideal manufacturing process, it is necessary to
grasp the surface roughness capabilities of metal additive manufacturing, as well as the posttreatment methods and their related time and cost.
The surface roughness of parts machined by SLM is typically somewhere in the range
of 8 and 20 μm, which is higher than that of parts machined by different techniques like
milling processing (~1 μm). There are two fundamental purposes behind the unpleasant
surface in the SLM cycle. One is the Marangoni force during the time spent vanishing and
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powder melting. Gas development weakens softened streams, and profoundly sporadic and
temperamental liquefy pools increment surface roughness and porosity. Thicker powder
layers make more powder material be liquefied by the laser beam, bringing about additional
gas development. Thusly, a little covering thickness can decrease the surface roughness
somewhat. Be that as it may, finishing a part comprising of slight layers is tedious. The
second reason for surface roughness is ill-advised powder melting and palletization (the
arrangement of metal drops during laser melting, as opposed to the ideal uniform
dissemination of liquid metal across the liquid surface). When the laser power is low, the sent
energy is insufficient to liquefy the powder particles totally, and the strong powder particles
stick to the surface of the part.[49]
Surface roughness is a proportion of topological changes on the surface of a section.
Engineering prerequisites for most parts incorporate surface roughness particulars.
Roughness affects component aesthetics and mechanical properties such as crack initiation,
fatigue life, wear resistance, fit, bearing, seal, and fluid dynamics. Because metal AM
machining produces a relatively rough surface, a secondary post-treatment is required. This
requirement has a significant impact on total production time and cost.[50]
2.1.4 Surface morphology
The surface morphology and surface unpleasantness of 316L stainless steel treated by
SLM are displayed in Figures 2-2. This diagram shows the effectiveness of laser remelting
in increasing density. A thick surface can be noticed for SS 400 and SS 500 examples.
Constantly checked tracks can be obviously seen from the surface, with no air openings or
holes between tracks showing the acoustic limits between adjoining tracks. Also, customary
fluid fronts were seen from the surface, uncovering the dependability of the liquid pool during
SLM handling. While the checking speed is expanded to 600 mm/s, a few voids and openings
can undoubtedly be tracked down between contiguous tracks. Contrasted and the examples
with lower filtering speed (SS 400 and SS 500), SS 600 additionally had more balls appended
to the surface. With speed up, the number of holes and pores increases, showing a poor
interlace limit. Also, the filtering direction becomes broken, demonstrating that the liquid
pool is unsound during SLM. The surface unpleasantness of stainless steel handled by SLM
is represented. The surface harshness increments with speeding up, which is steady with SEM
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perception.[41] The relative density, microhardness, and corrosion resistance of 316L stainless
steel treated by SLM decrease gradually with the increase of laser scanning speed. The 316L
sample with the highest relative density was selected for SLM treatment to study the
difference in its microstructure and properties along the direction of the building.
Figure 2-2 Surface morphology images of SLM-processed 316L stainless steel samples at scan
speed of a) 400 mm/s, b)500 mm/s, c) 600 mm/s, d) 700 mm/s, e)
800 mm/s and f) surface roughens[41]
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2.2 Electropolishing
2.2.1 The basic theory of electro-chemical polishing
Electropolishing is an electrochemical surface treatment process that eliminates a flimsy
material layer from metal parts. Commonly stainless steel or comparative composites are
utilized as electrolytic polishing workpieces. The interaction leaves a smooth, glossy, super
clean completion. Electropolishing is otherwise called anodic polishing, electrochemical
polishing, or electrolytic polishing.[51] Electrolytic polishing is particularly helpful for
delicate or complex geometric parts. During this interaction, the workpiece is associated with
the power supply's positive (anode) end at a constant pressure of 0V to 20V.[52]
Figure 2-3 Working principal of Electropolishing[51]
A metal part or workpiece acts as a positive pole in electrolytic polishing. The workpiece
is connected to the positive axis of the DC power supply. The cathode is connected to the
negative axis of the DC power supply. The anode and cathode are immersed in a temperaturecontrolled electrolyte. The electrolyte is usually sodium chloride, phosphoric acid, sulfuric
acid, and mixtures of different acid concentrations—the current travel from the anode to the
cathode through the electrolyte. Metal ions on the part's surface dissolve into an electrolyte
due to the current flow carried to a tightly controlled micron-thick surface.[52]
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The electrolytically polished samples were anodized in an electrolytic cell. The material
is eliminated from the anode surface to significantly eliminate anomalies, after which the
polished surface becomes shiny and smooth. This conduct should be a specific instance of
broader anode conduct and relies upon working circumstances. Metal anodes may be etched,
partially passivated, polished, or completely passivated. The electrical properties
accompanying these processes are easy to note and should be considered a prerequisite for
all speculations about electrolytic polishing processes.[53]
This experiment uses periodic intermittent current rather than DC. The voltage of pulse
current is generally a square wave. The polishing voltage is the external force of
electrochemical polishing. It is the main factor in accelerating the electrons on the electrolyte
and the metal surface to move in a fixed direction. Generally speaking, the polishing voltage
is positively correlated with the polishing current, and the ratio of the polishing current to the
polishing area is the current density.[54]
However, the electrochemical reaction on the workpiece surface will be too weak to
remove enough surface material when the current density is too low.[37] The conductivity of
the polishing electrolyte is also an irreplaceable part of the electropolishing process. Good
performance of the electrolyte can make the surface roughness of the polished far lower than
other electrolytes. The electrode potential occurs at the connection point between the
workpiece and electrolyte due to the exchange of charged species at the point of interaction,
specific adsorption/orientation of polar molecules, and specific adsorption/orientation of ions
(including solvents) at the interface. The anode potential is the potential through the cathode
component. There is a negative terminal potential at the cathode in the experiment and a
positive electrode potential at the anode. The potential contrast of a cell is equal to the
potential difference between two electrodes.[56]
2.2.2 Mechanism of electropolishing
Many scientists have analyzed the mechanism of electrochemical polishing. Jacquet first
described the process in the 1930s. According to Jacquet's article, the most important factor
affecting surface smoothness during electrolytic polishing is the presence of a deep solid layer
on the anode. The coating is shaped on the surface of the anode due to the polarization of the
processed material.[57] Figure 2-4 shows a schematic of uniform anodic dissolution of metals
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in an electrolyte containing few acceptor species. This electropolishing mechanism model
was arranged by Matlosz et al.[58] The oxidized metal cations are adsorbed on the anode
surface to form an adsorption layer, which is then solvated by the recipient species. The
adsorption layer increases the overpotential of metal dissolution and prevents the solvation
of metal ions and receptors. According to this model, receptors (such as water or water-related
species) are key factors in initiating an effective electropolishing process. Diffusion of the
acceptor species controls the dissolution of metal on the anode surface. A limiting current
can be observed when the concentration of acceptor species on the electrode surface is zero,
similar to metal deposition at the cathode.[59]
Figure 2-4 Mechanism of electropolishing[58]
During the electrolytic polishing process, metal particles are fed into the electrolyte to
remain at the cathode. The surface finish is described in terms of smoothness and brightness.
Smoothing occurs at detectable levels, and the height depends on the thickness of the anodic
film formed during the EP process. Lighting occurs at microscopic levels and is determined
by shaping a thin oxide layer on the anode. The workpiece is immersed in an acidic electrolyte
solution. The direct current flows through the rectifier to the anode shown in Figure-1. The
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anode polarizes during this process, and the metal particles decompose from the anode.
Hydrogen also forms at the cathode, while oxygen may advance from the anode, depending
on the voltage. Faraday's electrolysis law can explain the discharge of electropolishing
materials, as displayed in equation (1).
𝑊𝑙𝑜𝑠𝑠 =
ItM
nF
(1)
Where Wloss is the complete material loss, n is the valence condition of the metal particle,
F is Faraday's constant, M is the anode sub-atomic weight, I is the interaction current, and t
is the polishing time. M, n, and F are constants.
The two factors that influence the complete material misfortune during the
electropolishing system are; the polishing current (I) and the electropolishing time (t). The
rate of electropolishing is essentially reliant upon the inclination for the metal particles to
break down from the anode into the medium. In this way, the higher the current thickness,
the simpler the metal particles break up from the anode. Oxygen precipitation at the anode in
the electrolytic polishing process is displayed in Formula-3. The arrangement interaction of
the oxidation layer on the anode surface is displayed in Equation 3. Passivation of the anode
assumes a part in decreasing the pace of erosion and forestalling further oxidation of mass
materials. Then, at that point, the electrochemical polishing impact on containing NaCl
electrolyte was additionally contemplated, and earthy-colored items were found close to the
anode surface. [60,61]
Whenever Na+ particles slam into the negative shaft, the battery conveys a massive
potential to compel the particles to snatch electrons and structure metallic sodium.
ItM
𝑊𝑙𝑜𝑠𝑠 =
cathode:
nF
(2)
Cl-particles crashing into the positive cathode are oxidized to Cl2 gas, which air pockets
off at this anode.
Anode:
2 Cl-
Cl2 + 2 e-
(3)
The net impact of an electric flow going through the liquid salt in this holder is to separate
sodium chloride into its components, metallic sodium and chlorine gas.
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Electrolysis
Cathode (-): Na+ + e-
Na
(4)
process of NaCl:
Cathode (-): Na+ + e-
Na
(5)
2.2.3 Relationship between anode potential and anode current density
The basic observation of anodized dissolved polished metal is obtained by plotting the
current thickness against the voltage applied at both ends of the cell. A regular current-voltage
bend, 1d = f(Vt), is displayed in Figure 2-4. The current thickness-voltage relationship of the
potential cell changes enormously for various metals and electrolytes. The base curve can be
utilized to give fundamental data that can be applied to explicit circumstances. The polishing
operation requires an appropriate harmony among voltage and current and can be etched,
polished, pitted, or gas precipitated.[62] At the point when the current thickness is shallow,
etching happens, prompting vague expulsion of the metal. During electropolishing, assuming
the current thickness is too high, valleys and metal surface pinnacles break up quicker,
bringing about pitting and gas precipitation. Dominating the joined impact of current and
voltage for top-notch electrolytic polishing is vital.[63]
Figure 2-5 Relationship between voltage and current density in electropolishing[62]
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There are three fundamental regions on the curve shown in Figure 2-5, which are; etched
temperamental disintegration (B-C), (C-D) represents the stable polishing platform, and (DE) represents the pitting dissolution platform. The metal surface retains its original
appearance up to point B and changes in voltage and current. In the (B-C) region,
simultaneous decreases in current density are observed. The surface of the metal was etched.
The current thickness stays steady, and the voltage expansions are in the (C-D) range. The
polishing impact happens around here. At point D, the principal gaseous oxygen bubble
appears on the workpiece. In (D-E), oxygen precipitation joins metal disintegration at higher
voltages. When oxygen bubbles are caught on a metal surface, pitting occurs on the surface
during that period. The increment of film thickness on the current limiting platform
diminishes the disintegration site, increases the local-current concentration, and causes
pitting corrosion. Although the ideal voltage level varies with the combination of metal and
electrolyte, current-limiting platforms are common electropolishing characteristics.[63]
2.2.4 Cathode, electrolyte, and diffusion layer layers
In electrolytic polishing, the negative shaft is connected to the cathode, and the anode is
associated with the positive shaft of the DC power supply. With the help of the cathode
terminal, the current flows back to the power supply. Depending on the device and how it
works, the polarity of the cathode can be positive or negative concerning the anode. While
negatively charged anions create some distance from it, positively charged ions generally
push toward the cathode. The cathode work as the negative pole, and the anode serve as the
positive pole in the electropolishing device. An electrolyte is a substance that creates a
conductive arrangement when broken down in a polar dissolvable, like water. The broken-up
electrolyte is isolated into cations and anions and disseminated uniformly in the dissolvable.
During the electrolytic polishing process, cations in the arrangement are drawn to terminals
with additional electrons, while anions are drawn to anodes with fewer electrons. An electric
current is shaped when anions and cations move in inverse bearings in an answer. Various
metals might require multiple kinds of electrolytes. The structure and properties of
electrolytes are vital to the nature of electrolytic polishing. [64,65]
Notwithstanding metal salts, electrolytic polishing electrolytes frequently contain an
assortment of added substances to meet various purposes. A few reagents are utilized to
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expand the conductivity of electrolytes (supporting electrolytes). Others might be utilized to
develop shower solidness (stabilizers) further, initiate surfaces (surfactants or wetting
specialists), further develop evening out or metal dispersion (evening out specialists), or
upgrade the chemical, physical or specialized properties of surfaces. These properties
combine barriers to the use of mechanical strength, mass or reflectivity, hardness, abrasion
resistance, adaptability, internal tension, or weldability. The properties of electrolytes are
normally portrayed by electrolytic conductivity, in contrast to metals. Electrolytic
conductance can be improved by using inorganic and natural salts, acids, and bases. The
conductivity of the electrolyte is a combination of the ease of individual particles,
temperature, and synthesis of the electrolyte. [66]
Figure 2-6 Electrolyte properties of electropolishing[67]
As the electroplating system advances, emphatically charged particles (metal or
hydrogen) structure on the anode surface, move to the cathode, and are stored on the cathode
surface as adversely charged particles push toward the anode and release. A particle move
process involves arranging a meager electrolyte district (dispersion layer) close to the cathode
with a particle focus slope.[64] The concentration of metal particles on the cathode surface
attenuates nonlinearly in the cathode dispersion layer.
At the anode surface, the metal breaks down into an electrolyte with a higher particle
fixation than the general arrangement. Convection-instigated electrolyte disturbance
(agitation) brings about a decrease in the dissemination layer. The dispersion of particles
through the layers controls the exchange and statement paces of materials.
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𝐽 = D(
Fick's law:
𝜕
)
∂F 𝑥=0
(6)
J is particle motion in the x-direction; C - ion focus as a component of cathode surface
distance X; Diffusion coefficient of D - ion in the electrolyte. The dispersion layer causes
focus polarization of the electrochemical interaction.
2.2.5 Effect of electropolishing on stainless steel
Among the known surface pretreatments of stainless steel, electrochemical polishing is
considered on the grounds that it enjoys the accompanying benefits:1) It can without a doubt
be completed on complex formed examples, for example, endovascular frameworks, with
smooth, defectless, and contamination-free surfaces; 2) It can eliminate non-metallic
considerations and anomalies related with the commencement of the erosion interaction,
mainly confined consumption. It is valuable to decide the justifications for why numerous
gear makers fundamentally participate in the electrolytic polishing of treated steel.
Electrolytic polishing is the favored innovation in regions where mechanical polishing is
challenging to reach and has low work costs. Bacterial defilement levels are definitely
diminished, making keeping a high degree of tidiness in these polished regions more
straightforward.
[61]
Subsequently, electropolished treated steel gives a superior surface
completion and is more specific to clean. This trademark makes tempered steel broadly
utilized in food handling, drug, clinical and different ventures.[68]
Table 2 AISI SS (stainless steel) Composition
Composition
Weight %
Iron
65
Nickel
12
Chromium
17
Manganese
2
Molybdenum
2.5
Sulphur
0.03
Silicon
1
Carbon
0.03
Phosphorus
0.045
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While managing composites, for example, stainless steel, an extra element is especially
dissolving at least one part. All stainless-steel grades are iron-based amalgams containing
massive measures of chromium and nickel. The evacuation pace of various elements is
unique. Iron particles separate quicker than chromium and nickel particles. The
electropolishing of SS will make the surface wealthy in chromium, which assists in
decreasing the erosion pace of the amalgam. At the point when chromium on the outer layer
of stainless steel interacts with oxygen, a slim detached oxide layer is framed.[17] Electrolytic
polishing upgrades the passivation oxide layer and streamlines consumption obstruction.
Along these lines, stainless steels innate strength and intrinsic erosion obstruction make it a
suitable material for most equipment producers.[69]
2.3 Influence of Various Parameters in Electropolishing
2.3.1 Surfaces cleaning
Surface cleaning is an essential aspect of electropolishing. The chemical and physical
properties of the metal surface combine the properties of the electrodeposited metal. The
presence of external contaminants (such as grease, oil, dirt, corrosion products, etc.) can
negatively affect the adhesion, continuity, and durability of electroplated materials because
cleaning surfaces by physical, chemical, or mechanical means increases the surface energy
of the substrate.[70] The term "cleaning" has different meanings for different communities,
but it means removing unwanted or unwanted materials from surfaces of interest. Surface
cleaning can be accomplished through the steps discussed below. Any scientific field is the
object of constant research and development, and surface pre-treatment has seen the
application of the most advanced technology. Cleaning includes utilizing specific solvents,
like soluble cleaners, water, or acidic cleaners, to eliminate layers of oil from the surface.
First, clean each part with fresh water. This will ensure that any residual material left by
blasting, sanding, or solvents will be adequately removed from the metal surface. We need
to clean them again. Once the parts have been cleaned and dried, it is recommended to clean
the parts for 10 minutes with a quick etching de-rusting solution, rinse them with cold water,
and place them directly into the plating bath without drying them.
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It is essential to place the prepared parts into the plating tank as they are adequately
cleaned. It is also essential to wear a clean pair of gloves when handling each clean part. If
we use bare hands, scratches or traces of dirt and oil will be left on the metal surface, which
will cause consistency problems in the plating process. So, when preparing electrolytically
polished metal parts, be sure to be thorough and clean so that our new plates will come out
exactly as we want them to.[71]
2.3.2 Polishing gap
The polishing gap or interelectrode gap is the distance between the workpiece (anode)
and the tool (cathode). The electrolyte acts as a bridge between the two electrodes, allowing
ions to move during the polishing process. The larger IEG provides a wider polishing area
because the more extensive scatters the active region, while the smaller IEG provides a
narrower scope. The role of IEG is critical to the outcome of the ECP process, so maintaining
and controlling IEG is critical for adaptive applications. A small electrode gap allows current
to be located on the anode surface, in this manner decreasing wanderer expulsion. The ECP
cathode gap is mainly inside the range of ≤10μm. A small electrode gap creates a high
electric field, bringing about enormous current and high material expulsion. This is due to
the electrode gap being too large.[71] The study also shows that pits will be formed at specific
minor points due to the high current density when the gap is narrowed. Therefore, an
appropriate IEG must be selected to electropolish the workpiece.
2.3.3 Polishing voltage
The polishing voltage is the external force of electrochemical polishing, which is the
main factor in accelerating the movement of electrolyte and metal surface electrons to a fixed
direction. Therefore, it is essential to select the appropriate polishing voltage for the metal
surface roughness of the final parts. As a rule, the polishing voltage is positively correlated
with the polishing current, and the proportion of the polishing current to the polishing region
is the current thickness.[72] Controlling the polishing voltage relates to keeping up with the
current density. Since the material expulsion rate is straightforwardly connected with the
current density, when the current density is too enormous, the electrochemical response on
the workpiece surface will be excessively rough, and the workpiece cannot be evened out.
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Be that as it may, when the current density is too little, the electrochemical response on the
outer layer of the workpiece is excessively powerless to eliminate sufficient surface material.
For electrolytic polishing of stainless steel, keeping it below 24V DC is recommended. If the
voltage is below 24 volts, the device can electropolish up to 50 amps.[73]
Anodic dissolution includes just the metallic stage while the environment decreases. The
erosion of metals in the fluid arrangement will experience a few different cathode responses.
for example,
Proton reduction (acid media):
2𝐻 + (𝑎𝑞) + 2𝑒 − → 𝐻2
(7)
Water reduction (basic media):
2𝐻2 𝑂(1) + 2𝑒 − → 𝐻2 + 2𝑂𝐻 − (𝑎𝑞)
(8)
Metal ion reduction:
𝑀3+ (𝑎𝑞) + 𝑒 − → 𝑀3+ (𝑎𝑞)
(9)
𝑀2+ (𝑎𝑞) + 2𝑒 − → 𝑀
(10)
Metal deposition:
Reduction of dissolved oxygen:
𝑂2 + 4𝐻 + (𝑎𝑞) + 4𝑒 − → 2𝐻2 𝑂(1) (𝑎𝑐𝑖𝑑 𝑚𝑒𝑑𝑖𝑎)
(11)
𝑂2 + 2𝐻2 𝑂(1) + 4𝑒 − → 4𝐻 − (𝑎𝑞) (𝑏𝑎𝑠𝑖𝑐 𝑚𝑒𝑑𝑖𝑎)
(12)
Proton decrease is regular in acidic media, as is oxygen decrease since water
arrangements in touch with air hold back much broken-down oxygen.
2.3.4 Influence of the electrolyte composition
The composition of electrolytes used in electrolytic polishing has been the subject of
much research.[74] The primary function of the electrolyte is to provide optimal conditions for
machining. The electrolyte as a conductive medium between the two electrodes creates a
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favourable environment for the dissolution of the workpiece material. The ideal electrolyte
used in the polishing process must be highly conductive, low viscosity, non-corrosive and
non-evaporative. The electrolyte should be able to maintain its performance for a long time
and can be used repeatedly after filtration. In polishing, because of the little gap between the
device and the terminal, the current density is high, warming and vaporizing the
electrolyte.[75] Therefore, the chosen electrolyte should not evaporate but should carry debris
away from the workpiece. The toxicity of electrolytes is a major reason for the widespread
implementation of ECP. However, using citric acid as the electrolyte to achieve ECP is one
of the attempts to overcome this limitation.[76]
The centralization of the electrolyte influences this interaction because the higher the
conductivity of the electrolyte, the higher the current, expanding how much material is
eliminated. Typical arrangements comprise at least one concentrated acid, like sodium
chloride, perchloric, sulfuric, hydrochloric, phosphoric, and acidic. The high sharpness of
these chemicals permits them to consume metal surfaces rapidly and productively.
Phosphoric acid guarantees slow and uniform disintegration under appropriate working
circumstances without consumption or disintegration, while sulfuric corrosive increments
current density by causing beginning disintegration of the workpiece surface. During the
electropolishing system, the grouping of the receptor close to the anode is low. The focus
angle increments at the small pinnacle, so it is disintegrated by the anode first. The
disintegration cycle is somewhat deferred in a small space until metal particles near the small
pinnacle are set free from the metal surface.
During the electrolytic polishing process, the convergence of metal particles in the
electrolyte builds, and its consistency changes. Electrolyte arrangement and fixation, current
density, and it are correlative to blend rate. The conductivity of the shower relies upon the
centralization of its parts and the density of the electrolyte.[51] The expansion in Temperature
will fundamentally influence the conductivity of the arrangement and the abatement of
electrolytic clip voltage. Assuming the arrangement consistency expands, the quantity of
adsorbed particles on the electropolished metal surface might increase, reducing
unpleasantness.[71]
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2.3.5 Electrolyte temperature
Temperature is one of the significant factors influencing surface qualities. Temperature
affects the proficiency of electropolishing with the ionic arrangement. As the Temperature
builds, the plating effectiveness of specific arrangements increments, while others show the
best plating temperature. As the Temperature builds, the electrolytic conductivity likewise
increments because the temperature increases, making the ionization and ionic movement.[77]
Each corrosive base or salt has a trademark bend of its fixation and conductivity. Raising the
Temperature gives the particles more energy, making them move quicker and expanding
conductivity. Electrolyte temperature (normally 170°F - 180°F). The effect of temperature is
significant because the diffusion coefficient of the rate-limiting species in the
electropolishing leach is affected by:
𝑛𝐹𝐷0 𝐶
𝑄𝑎
𝑖𝐿 =
𝑒𝑥𝑝 (− )
𝜕
𝑅𝑇
(13)
In question n is the molar number of the complete charge of the particles, F is Faraday's
consistency, δ is the density of the anode dispersion layer, and C is the immersion
centralization of metal particles in the arrangement.
This equation relates the tank temperature to the current density, or at least the steady
current density, at the stage. This condition shows that an expansion in Temperature prompts
a dramatic increase in current density. Nonetheless, it ought to be noticed that as the
Temperature builds, the consistency of the dispersion layer on the anode surface declines,
making it harder to keep up with the thickness of the anode layer.[71] This will unavoidably
influence the nature of the metal surface completion. In that regard, the temperature area
should be painstakingly chosen to accomplish the ideal electrolytic polishing impact for a
specific framework.
2.4 Orthogonal Experimental Design
2.4.1 Orthogonal test
The orthogonal test is often used to study multi-factor and multi-level tests. When more
than three factors are involved in the experiment, it is very complicated and challenging to
carry out the experiment of all parameter groups or even impossible to carry out. According
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to the orthogonality of the orthogonal test, some delegate parameter groups can be selected
to carry out tests instead of comprehensive tests. These parameter groups are evenly
distributed and neat.[78]
The essential instrument of orthogonal experimental design is the orthogonal table. The
experimenter can inquire about the relating orthogonal table as per the number of variables
and the number of considered levels in the experiment. Select some agent boundary
gatherings to analyze by depending on the orthogonality of the orthogonal table to accomplish
the same consequence of the far-reaching explore different avenues regarding the most unnumber of tests. The orthogonal trial is an effective multifaceted investigation design
technique along these lines.
2.4.2 Orthogonal array
The orthogonal table gives the general arrangement of the orthogonal experiment design
and is the primary tool of the orthogonal experiment. Test using levels orthogonal table. Such
as a standard orthogonal table with marker L 𝑛 𝑚 𝑘 said, including L saying the orthogonal
table, n is the number of tests is needed, m is the factor level, and k is the most arrangeable
factor.
Before the orthogonal test, the test's purpose should be clear, and the test objective should
be determined. The test objective is the typical surface harshness of the polished surface.
Then, it is essential to decide the variables to be researched. In this experiment, the four
factors are distributed as polishing voltage, electrolyte conductivity, polishing gap, and
polishing time. After that, the appropriate level is selected according to the actual situation.
After determining the test factors and factor levels, an appropriate orthogonal table is selected
to list the test scheme. Columns without scheduled test factors become empty columns. Next,
the test is carried out. During the test, each test in the list of test schemes needs to be
completed according to the specified scheme, but the test order can be randomly arranged.
After the completion of the test, the range is calculated to determine the order of factors.[79]
2.4.3 Minimizing the experiment
The following three symbols are usually defined in the orthogonal test analysis: Ki, Ki,
and R. Where Ki addresses the amount of the comparing test results when the level number
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is I in any column; 𝑘𝑖 =
𝐾𝑖
𝑠
, where S addresses the recurrence of the event of each level in
any column;[78]
R represents the range and is determined as follows：
𝑅 = 𝑚𝑎𝑥{𝐾𝑖 } − 𝑚𝑖𝑛{𝐾𝑖 }
(14)
𝑅 = 𝑚𝑎𝑥{𝑘𝑖 } − 𝑚𝑖𝑛{𝑘𝑖 }
After calculation, the greater a factor's R, the more critical it is. If the range of the blank
column is more extensive, there may be the following reasons: 1. An essential factor is
omitted when arranging test factors; 2. There may be non-negligible interactions between
factors.
After determining the importance of the factors, it is necessary to determine the optimal
scheme. The optimal scheme is to choose the level combined with the best level of each factor
within the test range. Suppose the larger the indicator, the better the level that makes the
indicator more significant should be selected. The index of this topic is the typical surface
roughness; the smaller, the better, so select the level to make the average surface roughness
smaller. In addition, improving efficiency and other situations should be considered to reduce
consumption. The optimal scheme is often not included in the orthogonal test scheme, which
must be verified.
2.5 Summary
This part discusses the basic principle of additive manufacturing and the development of
AM. The properties of additive manufactured metal surface, and metal surface roughness
were analyzed to understand AM parts' characteristics. The basic theory of electrochemical
polishing includes the electropolishing process, primary polishing reaction, electrode
polarization, and electrochemical polishing mechanism. It introduces the application,
influence of electrolyte concentration, polishing voltage, polishing temperature, and
polishing time. Orthogonal design experiment discussed to understand the process and test
arrangement of multi-factor and multi-level tests. The experimenter can inquire about the
relating orthogonal table as per the number of variables and the number of considered levels
in the experiment.
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Simulation Analysis of
Electrochemical Polishing
AutoCAD drew the surface characteristics of additive parts, and COMSOL simulated
the electrochemical polishing process. The COMSOL is a finite component displaying
program for settling numerous incomplete differential conditions, with incredible
applications in liquid flow, acoustics, and robust mechanics.[87] COMSOL can tackle
numerous material science issues used in designing purposes, especially Multiphysics issues.
This incorporates a complete demonstrating climate for portraying all actual peculiarities
utilizing PDEs.[88] COMSOL programming is sufficiently adaptable to oblige different PDEs
in a solitary space model. The process of software library analysis is basically the same as
the fundamental experiment analysis, such as polishing voltage, polishing time, electrolyte
conductivity, polishing gap, material selection, etc. The problem is modelled by using a
secondary current distribution interface, which connects the electrolyte-electrode boundary
nodes on the two electrode surfaces.[89]
Due to the faster dynamics and larger anode area, the initial value of the electrolyte was
set to correspond to zero anode polarization. The problem was solved by a static method, and
the disk's radius was changed by parametric scanning. Triangular meshes were used for
meshing, and a smaller size setting was used to increase the resolution of the contact points
between anode and cathode. The rest of the process adds physics, sets parameters, selects
boundary conditions, and meshes. After this process is complete, we will calculate our
simulation.
3.1 Multiphysics and Mesh
3.1.1 Multiphysics
The product's electrochemical etching module gives three fields of current distribution:
essential current distribution, optional current distribution, and current tertiary
distribution.[89] The following aspects are mainly considered under the corrosion module:
First, the mass transfer equation, namely, the conservation of mass in the electrolyte:
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𝜕𝑐𝑖
+ 𝛻 • 𝑁𝑖 = 𝑅𝑖,𝑡𝑜𝑡
𝜕𝑡
(15)
Type of 𝑐 𝑖 as the concentration of a substance, N 𝑖 as the material flux, 𝑅 𝑖, 𝑡 𝑜 𝑡 source
term for reaction. The reaction source term considers the different chemical reactions in the
electrochemical process. Generally, the reaction source term is zero in the electrochemical
reaction process. The reaction source term is not zero only when there are additional chemical
reactions in the electrolyte region. Chemical reactions occurring in the interfacial region
between the electrode and the electrolyte do not make the reaction source term nonzero.
When the steady-state calculation is performed, the first term in Equations (20) is equal
to 0; Material I total flux 𝐍 𝒊 by Nernst Planck formula is given:
𝑁𝑖 = −𝐷𝑖 𝛻𝑐𝑖 − 𝑧𝑖 𝑢𝑚,𝑖 𝐹𝑐𝑖 𝛻𝜑𝑙 + 𝑐𝑖 𝑢 = 𝐽𝑖 + 𝑐𝑖 𝑢
(16)
There are three mechanisms of mass transport for dilute solutions: diffusion,
electromigration, and convection. The 𝑫 𝒊 as diffusion coefficient, and material related to the
concentration gradient; Charge number z 𝑖, u 𝑚, 𝑖 as the number of electromigration, F for
Faraday constant, c 𝑖 is concentration; 𝒖 is the fluid velocity. The net current in the electrolyte
can be described in terms of the total material flux (A/cm²).
𝑖𝑙 = 𝐹 ∑ 𝑧𝑖 𝑁𝑖
(17)
In addition, according to Kirchhoff's law, there is the charge conservation equation：
𝛻𝑖𝑙 = 𝑄𝑙
(18)
Type of 𝑄 𝑙 said the current source in the electrolyte, usually to 0. However, the
electrochemical reaction problem cannot be solved according to the above four formulas, so
the local electric neutral equation needs to be introduced to consider that no ion leaves the
interface area between the electrode and electrolyte during the reaction, as shown in Equation
23.
∑ 𝑧𝑖 𝑐𝑖 = 0
(19)
3.1.2 Geometry
Under the COMSOL electrochemical corrosion module, there are usually two ways to
model. One is to draw the shape of the electrolyte in the process of processing, take the
polygon as the research area (hereinafter referred to as the domain), set the edge of the
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polygon as the electrode surface, and then define the boundary through the relevant
parameters, as the boundary conditions of simulation calculation, and set the initial value for
calculation. The other is to draw the electrolyte, cathode, and anode simultaneously set the
three as the simulation domain, set the boundary of the intersection of the two domains as the
internal electrode surface, define its parameters, and set the initial value for calculation.[90]
The two models are very similar. The difference lies in the slightly different set of the
electrode surface, and the simulation results are the same.
In this paper, two models have been established by AutoCAD., and the model was saved
in DFX format. Each model has a 10 mm length. In Figure 3-1, six particles are on the anode
surface. Furthermore, Figure 3-2 has seven particles. Take the model with a polishing gap of
10 mm as an example. Figure 3-1 shows the position and melting state of each particle. From
Figure 3-2, The left end and right big particle melt the least completely, and only the bottom
part melts and is bonded to the part's surface. The lower particles in the middle are affected
by the heat of both ends, which melt particles entirely. At the end of the simulation, most of
them are melted, and only the upper part of them is bonded between the two melts; There are
also five small particles on the surface that have melted most of the particles. Their primary
function is to compare the large particles at both ends, mainly their removal in the polishing
process.
Figure 3-1 Electropolishing simulation model-1
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Figure 3-2 Electropolishing simulation model-2
Same polishing gaps have been used in both models. Figures 3-3 show the distance
between anode and cathode. From left to right, the polishing gaps are 5 mm, 10 mm, 12mm,
15 mm, and 20mm, respectively. For other polishing gaps, it is only necessary to increase or
shorten the length of both sides to complete the corresponding model creation.
Figure 3-3 Polishing gap of the electropolishing simulation model
When COMSOL imports external graphic features, it automatically fixes errors in the
graphic surface. Therefore, COMSOL automatically uses the particles as error repair when
drawing the surface of the powder particles and parts according to the regular scale and
importing the graphic features. As a result, the particles will not be shown in the model.
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3.1.3 Working condition
The essential electrolyte property in the simulation is the cell temperature conductivity.
In our simulation, we used two models for analysis. In addition, the electrolyte conductivity
is also temperature-dependent. So, we used different temperatures for the first simulation.
The first model analyzes the polishing result with different temperature conditions, while a
fixed polishing gape was used. Moreover, the second model analyzes the polishing result
under the different polishing gaps. For the first experiment, the electrolyte considered was a
solution of sodium chloride (NaCl). The five-level concentration of the solution was 1 S/m,
3 S/m, 6 S/m, 10 S/m, and 14 S/m, respectively. The five levels of potential were 1.5 V, 2 V,
3 V, 4.5 V, and 6 V. Five level of temperature was 25℃, 40℃, 70℃, 80℃, 55℃, 70℃, and
80℃ used for this simulation. The polishing gap was 10mm. The anode electrode was
stainless steel, and the cathode was Silver (Ag). Almost all the same parameters were used
for the second experiment. Room temperature was used for the second simulation, and
polishing gape was used at 5mm, 10mm, 12mm, 15mm, and 20mm.
A first request approximation was utilized to decide the material evacuation rate.
Equation (20) expresses that the material expulsion rate (U) from the anode corresponds to
how much current (I) and its course compared with the outer layer of the anode (n).
Consequently, the material evacuation rate will be moderately more enormous in areas of
high current concentration. The proportionality consistent (K) represented the property's
electrode material. The steady was determined utilizing Equation (21), where u was the
nuclear mass of the material, C was Faraday's Constant, ρ was the density of the material,
and N was the quantity of valence electron of the material.
𝑈 = 𝐾. 𝐼. 𝑛
𝑢
K=
𝐶𝜌𝑁
(20)
(21)
A voltage drop was set between the cathode's outer layer and the anode's outer layer, and
it was held steady all through the length of the simulation. The simulation yielded two plots:
Displacement in X and relocation in Y (Delta X and Delta Y). The relocation in X and Y
plots portrays how much material was eliminated (mm) by adjusting the distance of the
surface from its underlying state to its last disintegrated state.
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3.1.4 Boundary condition
The breakpoint condition in the simulation model describes this problem. The method of
electrical reaction is described by the ordinary current of electrolyte and potential on the
electrode. The fluid flow is determined by the flow rate of the delta, the power source, and
the voltage divider. Furthermore, the anode (lower limit) and cathode (maximum breakpoint)
limits should satisfy explicit likelihood conditions according to Faraday's and Ohm's criteria:
∅𝑎𝑛𝑜𝑑𝑒 = 𝑉1 − 𝑉2
(22)
∅𝑐𝑎𝑡ℎ𝑜𝑑𝑒 = 0
𝑛𝐽∅𝑠𝑖𝑑𝑒 = 0
Where ϕ anode and ϕcathode are the expected on the workpiece and hardware, separately,
and nJside is the run-of-the-mill current density on the edge limit of the model. The key is to
find an anode limit that fulfills every one of the restricting states of the Laplace equation kept
in equation (22) of the electric likely appropriation ∇ 2N in the electric polishing leeway
region. The cathode (furthest breaking point) and workpiece (lower limit) are addressed as
the separating lines of the model, and the short side is viewed as the open limit.
Figure 3-4 Boundary condition of simulation
The etching module is solved by coupling the deformation geometry with the primary
current distribution, so the deformed electrode surface and the non-deformed boundary need
to be defined. In the simulation, it is considered that the electrolyte flow is incompressible,
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so the surface of the deformed electrode has only the anode boundary, and the non-deformed
boundary is the rest of the surface. When setting the flow field, it is necessary to set the
cathode boundary as the fluid inlet, the anode surface as the fluid outlet, and set the relevant
velocity field and fluid properties. At this point, the physical field model of the powder
bonding surface has been set up.
3.1.5 Simulation Research and Analysis of Step Effect
In order to study the polishing effect, the average surface roughness is taken as the basis
to judge the polishing effect. As shown in Figure 4-20, within the sampling length, the
average arithmetic value of the absolute value of contour offset is Ra, which can be expressed
as：
1 1
𝑅𝑎 = ∫ 𝑦(𝑥)𝑑𝑥
𝐿 0
(23)
Although surface roughness data cannot be obtained directly in COMSOL software, it
can be calculated in a certain way. Before the simulation, the integral function INTOP1 and
the average value function AVEOP1 needed to be defined, and the scope of these two
functions was set as the anode surface. After obtaining the result, the average height is first
calculated by means of the average value function to obtain the position of the X-axis
aveop1(y) in Figure 3-5, where z is the height coordinate of any point in the polishing result.
Then the absolute value of offset is calculated as abs(z-aveop1(z)), which is integrated and
divided by length to obtain the expression of surface roughness: intop1(abs(yaveop1(y))/intop1(1).
Figure 3-5 Average surface roughness profile
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3.1.6 Meshing
The administering equations were addressed mathematically utilizing COMSOL
Multiphysics 5.5a, relying upon the finite-component method. COMSOL Multiphysics is a
successful, intelligent unit intended for demonstrating and tackling a wide range of issues in
science and design. The expanded number of the cross-section components in the space of
interest accomplishes higher precision in that space while keeping a coarser lattice in the
remainder of the simulation math trying to improve computational assets.
In meshing, the density of the anode surface area needs to be redefined. Element size
(finer) was selected for the whole geometry. Free triangular mesh and the maximum mesh
density were used in this simulation to guarantee the accuracy of simulation results. The
anode boundary area unit was set as 0.02, as shown in Figures 3-6, and Figures 3-7.
Figure 3-6 Mesh analysis of simulation mode-1
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Figure 3-7 Mesh analysis of simulation mode-2
3.2 Results Analysis
Two significant electrochemical boundaries should be inspected to investigate pitting
consumption ready to go with the assistance of COMSOL Multiphysics programming. The
first is the electrolyte potential, and the other is electrolyte current density. Electrolyte
potential demonstrates the potential produced because of the particles inside the electrolyte.
Regarding pipelines, we can say that higher upsides of electrolyte potential in any area show
that the specific district of the surface has less erosion obstruction. The current created by the
development of particles inside the electrolyte has alluded to as electrolyte current density,
which is relative to the number of species or particles moved across the anode (conelike pit)
surface.
3.2.1 Electric potential distribution
By expanding the voltage, the circuit will prompt an expansion in the pace of
electroplating the metal as more current will flow effectively all through the circuit. Current
density expanded as well while expanding voltage.
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3.2.1.1 Polishing effect under using various temperature;
The optimal parameters can be obtained by selecting the minimum value of the
corresponding mean value of each factor. Namely, the polishing voltage is 1 V, the polishing
gap is 10 mm, the electrolyte conductivity is 1 S/m, and the polishing time is 600 S. while
different temperature level was simulated. The initial surface condition is shown in Figures
3-8.
Figure 3-8 Polishing simulation result under optimal parameters
Figures 3-9(a) show the simulation result after 1200 s with room temperature. And
Figure 3-9(b)) shows the simulation result under using a 40-degree temperature while the
time is the same as Figure 3-9(a). this figure shows that little amount of bulge on the anode
surface has been removed. It is seen that 70℃-temperature used surface polished better
compared with room temperature.
Figure 3-9 Polishing result after the 1200s, a) Temperature 25 ℃ b) Temperature 70 ℃
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When the polishing time reached the 2400s, as shown in Figures 3-10, the bulge on the
anode surface was almost removed. However, the surface is still uneven.
Figure 3-10 Polishing result after the 2400s, a) Temperature 25℃ b) Temperature 70℃
After 3600 seconds, as shown in Figure 3-11, the bulge on the anode surface has been
totally removed, but the surface is still rough. The reaction continues to make the surface
smooth.
Figure 3-11 Polishing result after 3600s, a) Temperature 25℃ b) Temperature 70℃
However, when it reaches 6000 s, it can be found that the surface is still clearly uneven,
but part of the matrix has been removed, as shown in Figures 3-12
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Figure 3-12 Polishing result after 6000s, a) Temperature 25℃ b) Temperature 70℃
The simulation continued, although as the polishing continued, the surface roughness
became smaller and the substrate's corrosion became more serious. The thickness of the
substrate material removed was about 2 mm, as shown in Fig. 3-13. Although the surface
roughness was 0.293μm when the maximum parameter was used, the corrosion of the
substrate is severe during the polishing process, so it is impossible to use the reformatted
parameter.
Figure 3-13 Polishing result after 7200s, a) Temperature 25℃ b) Temperature 70℃
3.2.1.2 Polishing effect after changing polishing gap
In this simulation, part polishing voltage is 1V, polishing temperature is 25℃, polishing
time is 600 seconds, and polishing conductivity is 1S/m. Two polishing gap (10mm and
15mm) is used to compare polishing results. The influence of polishing gap variation on
electrolytic polishing was studied in this part. The surface's initial condition before polishing
is shown in Figures 3-14. The initial surface roughness is 4.96 μm. At this time, particles are
too high.
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Figure 3-14 Polishing result in the initial condition
Figure 3-15 shows the polishing result after polished for1200s. The bulge on the surface
became smooth, but the bulge was not removed properly. During this time, the surface
roughness of 0.7 μm was polished.
Figure 3-15 Polishing result after the 1200s, a) Polishing gap 10 mm b) Polishing gap 15 mm
After 3600 seconds, as shown in figure 3-16, the bulge on the anode surface has been
removed, but the surface is still uneven.
Figure 3-16 Polishing result after 3600s, a) Polishing gap 10 mm b) Polishing gap 15 mm
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However, when the polishing time reached 6000 seconds, it was found that the anode
surface of the simulation model was still irregular. Nevertheless, the matrix has been removed,
as shown in Figures 3-17.
Figure 3-17 Polishing result after 6000s, a) Polishing gap 10 mm b) Polishing gap 15 mm
The surface becomes smooth after 7200 seconds, as shown in Figures 3-18. The
thickness of the substrate removed is approximately 2mm. When the maximum parameters
were used, the polishing time was 7200 seconds, and the final surface roughness was 0.490
μm. Under surface roughness requirements, the overcasting amount is controlled within a
specific range.
Figure 3-18 Polishing result after 6000s, a) Polishing gap 10 mm b) Polishing gap 15 mm
The average surface roughness variation curve of the whole simulation process is shown
in Figures 3-19. The variation grape showed that the surface roughness increased with time.
The surface reduction rate is high over a duration of 2000 seconds. After that, the roughness
decreases slightly. It is because of the increase in distance and the decrease in particle size.
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Figure 3-19 Average surface roughness changes with time
3.2.2 Electrolyte current density
The electrolyte current distribution in the created model has been exhibited in Figures 320. It can be realized that the electrolyte current density is the summation of all the motion
of particles present in the electrolyte, which can be viewed as one of the significant
electrochemical boundaries for the erosion study to compute the consumption rate. A higher
worth of electrolyte current density at a specific area demonstrates that the dissolution pace
of material at that part is exceptionally high contrasted with different regions. Both anodic
and cathodic cycles happen at an electrode-electrolyte interface. The current density across
the point of interaction is the result of the electronic trades in the two cycles. Significantly,
the anodic response is more prevailing at the pit locale because of negative region proportion
and the staleness of the electrolyte in the pit district, along these lines, bringing about the
corruption or dissolution of material at a lot quicker rate when contrasted with different areas
of the surface.
Since the simulation model has been streamlined partially, confirming its effectiveness
is vital. Simulated the electrochemical polishing interaction of treated steel and got the
current density distribution on the anode surface as displayed in figures 3-20.
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Figure 3-20 Current density of both simulation model
This paper used the AC/DC current module and free deformation module to analyze the
simulation process, but the specific electrolyte concentration was not indicated. Therefore,
the current density value was different from the original text when the corrosion module was
used to verify the current density in this paper. The current density distribution on the anode
surface was intercepted, and it was found that the current density distribution was basically
the same as figures 3-20. In both cases, the current density is more significant at the sharp
point and minimum at the gentle point, so the simulation model in this paper can be
considered adequate, as shown in figures 3-20.
3.3 Influence of Initial Surface Roughness on Polishing Results
With the increase in initial surface roughness, final surface roughness also increased, as
shown in Figures 3-21. The reasons are as follows: when the surface roughness is large, the
material removal amount of polishing processing under the same parameters is not enough to
reduce the surface roughness to a certain extent, and then the surface roughness will be large.
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Figure 3-21 Simulation results of various initial surface roughness
At the point when the surface roughness is minor, in light of the surface being relatively
smooth, the level contrast at the undulating place is tiny, so the material evacuation speed
distinction is little. As the electrochemical response goes on, the passivation film steadily
shapes, which further dials back the material expulsion rate and, in the end, expands the
surface roughness. Consequently, a specific level of pre-polishing is expected before splash
electrochemical polishing to lessen the surface roughness to one particular reach, and
afterward, electrochemical polishing is utilized for finishing.
3.4 Simulation Result Analysis in Orthogonal
According to the explanation in table 3, there are four factors in this topic: polishing
voltage, electrolyte conductivity, polishing gap, and polishing time. Five factors are selected
for the test. If a comprehensive test is to be carried out, 54 = 625 experiments need to be
completed, and the number of experiments is very large. Therefore, orthogonal tests need to
be arranged.
Table 3 Factor and levels of orthogonal experiment design
Polishing
Polishing
Polishing gap
Polishing
Electrolyte
Voltage (V)
Time (T)
(mm)
temperature (℃)
conductivity
(S/m)
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1.5
1200
5
25
1
2
2400
10
40
3
3
3600
12
55
6
4.5
6000
15
70
10
6
7200
20
80
14
According to the experimental parameters of electrochemical polishing described above,
the main parameters are polishing voltage, electrolyte conductivity, polishing time, and
polishing gap. Number according to the number of factors and levels, selecting orthogonal
table L2556, including the last two as a null column. This paper, for the most part, centres
around the simulation of surface polishing and the removal of the step effect.
3.4.1 Polishing study of the first simulation model
According to the factors and levels shown in Table 3, the corresponding orthogonal table
was selected, and the test parameters were arranged since the selected orthogonal table is an
orthogonal table with four factors and five levels. The first simulation's specific test
arrangement and test results are shown in Table 4.
Table 4 Orthogonal test arrangement of electropolishing simulation-1
Polishing
Polishing
Polishing
Polishing
Conductivity
Temperature
Polishing result
Voltage(V)
Time (s)
(S/m)
(℃)
(Ra)
1
1.5
1200
1
25
3.810
2
1.5
2400
3
40
3.241
3
1.5
3600
6
55
2.68
4
1.5
6000
10
70
1.99
5
1.5
7200
14
80
1.52
6
2
1200
3
55
3.46
7
2
2400
6
70
2.798
8
2
3600
10
80
2.23
9
2
6000
14
25
1.492
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10
2
7200
1
40
3.154
11
3
1200
6
80
3
12
3
2400
10
25
2.30
13
3
3600
14
40
1.695
14
3
6000
1
55
3.309
15
3
7200
3
70
2.38
16
4.5
1200
10
40
2.54
17
4.5
2400
14
55
1.76
18
4.5
3600
1
70
3.18
19
4.5
6000
3
80
2.275
20
4.5
7200
6
25
1.52
21
6
1200
14
70
2.135
22
6
2400
1
80
3.255
23
6
3600
3
25
2.455
24
6
6000
6
40
1.452
25
6
7200
10
55
0.743
Table 5 Range calculation of electropolishing simulation 1
Level
Polishing
Polishing
Polishing
Polishing
Voltage (V)
Time (T)
Conductivity
Temperature
(S/m)
(℃)
1
2.648
2.989
3.342
2.315
2
2.627
2.671
2.762
2.416
3
2.537
2.448
2.290
2.390
4
2.255
2.104
1.961
2.497
5
2.008
1.863
1.720
2.456
Mean value
0.640
1.126
1.621
0.181
The importance of each factor can be known from the range of results of the orthogonal
tests. In order to make intuitive analysis more convenient, the relationship curves between
the level of various factors and surface roughness are investigated below:
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3.4.1.1 Effect of electrolyte potential:
As shown in Figure 3-22, the surface roughness of the anode decreases with the
increment of polishing voltage. This is because the voltage increments and the current density
likewise increments, along these lines expanding the material expulsion sum, so the
evacuation pace of the lump is higher.
Figure 3-22 Relationship between polishing voltage (V) and average surface roughness (Ra)
3.4.1.2 Effect of polishing time:
Figures 3-23 show that surface roughness decreased when the polishing time increased.
The longer the polishing time is, the more material is removed, making the anode surface
smoother.
Figure 3-23 Relationship between average surface roughness (Ra) and polishing time (s)
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3.4.1.3 Effect of polishing conductivity
As should be visible from Figure 3-24, with the persistent increment of electrolyte
conductivity, the surface roughness diminishes first and afterward increases. When other
conditions remain unchanged, the electrolyte concentration increases, and the conductivity
of the electrolyte is positively correlated with it. Therefore, when the electrolyte
concentration is too large, the concentration polarization will be generated on the electrode
surface, making the anode surface accumulate a layer of passivation film that is not easy to
oxidize, resulting in the difficulty of continuing to energize and remove the material.
Figure 3-24 Relationship between average surface roughness (Ra) and polishing conductivity
(S/m)
3.4.1.4 Effect of temperature:
From figures 3-25, surface roughness has fluctuated depending on temperature. After
increasing temperature, surface roughness also increased until a specific period. Furthermore,
surface roughness decreases first and then increases with increasing temperature. It is because
polishing conductivity decreases while increasing the temperature, as we know that low
conductivity has less polishing rate.
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Figure 3-25 Relationship between average surface roughness (Ra) and polishing Temperature
(℃)
3.4.2 Polishing study of the second simulation model
For the second simulation model, the specific test arrangements and results are shown in
Table 6.
Table 6 Orthogonal test arrangement of electropolishing simulation 2
Polishing
Voltage(V)
Polishing
Time (s)
Polishing
Conductivity
(S/m)
1
1.5
1200
1
5
4.68
2
1.5
2400
3
10
4.325
3
1.5
3600
6
12
3.76
4
1.5
6000
10
15
3.085
5
1.5
7200
14
20
3.01
6
2
1200
3
12
4.625
7
2
2400
6
15
4.06
8
2
3600
10
20
3.635
9
2
6000
14
5
0.875
10
2
7200
1
10
4.20
11
3
1200
6
20
4.453
62
Polishing gap
(mm)
Polishing result
(Ra)
北京理工大学硕士学位论文
12
3
2400
10
5
2.14
13
3
3600
14
10
2.178
14
3
6000
1
12
4.26
15
3
7200
3
15
3.554
16
4.5
1200
10
10
3.397
17
4.5
2400
14
12
2.534
18
4.5
3600
1
15
4.475
19
4.5
6000
3
20
3.676
20
4.5
7200
6
5
0.91
21
6
1200
14
15
3.263
22
6
2400
1
20
4.646
23
6
3600
3
5
2.415
24
6
6000
6
10
1.101
25
6
7200
10
12
1.112
Table 7 Range calculation of electropolishing simulation 2
Level
Polishing
Polishing
Polishing
Polishing
Voltage (V)
Time (T)
Conductivity
gap
(S/m)
(mm)
1
3.772
4.084
4.452
2.204
2
3.479
3.541
3.719
3.040
3
3.317
3.293
2.857
3.258
4
2.998
2.599
2.674
3.687
5
2.507
2.557
2.372
3.884
Mean value
1.265
1.526
2.080
1.680
The relationship curves between the level of various factors and surface roughness are
studied below:
3.4.2.1 Effect of electrolyte potential:
It is apparent from the voltage feature that the current density will increase respectively
if the voltage increases, as shown in Figure 3-26. The surface roughness of the anode
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decreased while the polishing voltage increased constantly. It has helped to remove bulges
quickly.
Figure 3-26 Relationship between Ra and polishing Voltage (V)
3.4.2.2 Effect of polishing time:
From figure 3-27, the graph shows that a longer polishing time can remove more
materials from the anode surface. The initial surface roughness at the beginning of polishing
was more than 4μm. Surface roughness became less than 2.5 μm.
Figure 3-27 Relationship between Ra and polishing time (s)
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3.4.2.3 Effect of polishing gap:
Figure 3-28 shows that as the polishing gap increases, the surface roughness increases.
This is because the gap increases and the effect of points is weakened, thus reducing the
amount of material removal. The smaller polishing gaps have higher current, so surface
polishing increased due to the decreasing polishing gap.
Figure 3-28 Relationship between Ra and polishing gap (mm)
3.4.2.4 Effect of conductivity:
The amount of surface polish increased as the electrolyte conductivity increased shown
in Figures 3-29. More conductivity improves the current density and brightens the
surface. Higher conductivity can create higher current density; however, in the event that the
current density is excessively high, it is not difficult to deliver nearby overheating, burning,
and pitting; when the current density is lower than as far as possible, the metal surface is
effectively consumed and turns out to be unpleasant, which cannot be cleaned Effect.
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Figure 3-29 Relationship between Ra and polishing conductivity (S/m)
3.5 Summary
In this chapter, two group simulations are carried out to analyze the polishing effect of
electrolytic polishing with different polishing factors. The orthogonal experiment is carried
out to analyze the two groups of simulation. Through the observation of the simulation results,
it is realized that the surface roughness should not be regarded as the only standard to measure
the polishing effect. However, the amount of overturning should be considered. The optimum
parameters can be obtained only by considering the surface roughness and overturning degree.
The initial surface roughness has a significant effect on the polishing effect. Electrolyte
pressure has a certain influence on the polishing effect but does not directly influence the
chemical reaction, so its influence is limited.
Polishing time, electrolyte conductivity, polishing voltage, polishing temperature, and
polishing gap are the main factor of the electropolishing simulation. A high concentration
solution will increase the probability of concentration polarization of the anode, which will
affect the polishing effect. As the polishing voltage increases, the average surface roughness
of the material decreases, and the polishing voltage is positively correlated with the polishing
current. When the polishing current increases under the condition that the polishing area
remains unchanged, the anode current density also increases so that a smoother surface can
be obtained.
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Experiments and Analysis of
Electropolishing Effect on Additive
Manufactured 316L Stainless Steel.
4.1 Electropolishing Procedure
4.1.1 Material and workpiece preparation
There is a specific relationship between the current appropriation on the sample surface
and the electropolishing instrument, which depends on the geometry that affects the
electropolishing mechanism. For instance, geometric aspects can be the anode-cathode
distance, the aftereffect of charge move by potential, the geometry of the initial surface, and
the primary conditions for keeping the surface smooth by electropolishing. The SS-316L
samples are ready for the electropolishing tests. This sample was prepared with additive
manufacturing. The additive manufacturing parts were created by selective laser melting of
~20mm thick layer. The sample size is 10mm × 10mm × 0.5mm shown in Figure 3-1. This
sample work as a workpiece or anode. On the other hand, Silver (Ag) was used for the cathode.
A cathode is a present adjacent to the portion of the work that requires electropolishing.
Although Solidworks is not specifically designed for parts manufactured using SLM, it
can be used to create 3D representations of components. 3D models are converted to STL
files. The first step was to find the most appropriate location to maximize success and
minimize the amount of support. This additionally implies picking the right support type with
support limits. When the support + component STL documents are sliced, they can be stacked
onto the SLM machine and individually allocated the proper machining and "machine"
parameters—not entirely settled during the basic process and parameter improvement stages.
Substrate temperatures and machine parameters for support and component were improved
during process advancement. Metal 3D printing uses SLM-280 (SLM Solutions GmbH,
Germany) with laser power of 200W, the print layer thickness of 20-80um, and the print line
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width of 80-200um. The spherical 316L stainless steel powder was fused by selective laser,
and the average particle size was about ∼ 30 μm.
a)
b)
c)
d)
Figure 4-1 a) Experiment workpiece design b) Input design data of 3D printer c) and d) Output
result of 3D printer.
For the workpiece preparation, we should follow some steps,
✓ Remove grease from the workpiece with washing solution in the ultrasonic bath
for several minutes.
✓ To remove the detergent workpiece should be rinsed with water.
✓ Wash the workpiece in an ultrasonic bath with deionized water for several
minutes.
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✓ Flush entirely with water.
✓ Dry the workpiece was utilizing air.
4.1.2 Post-treatment of electropolishing
The sample is polished to the ideal polishing time, the power is switched off, and the
anode workpiece is eliminated from the electrolytic polishing tank. Run cold water rinsing to
ensure that all surface dust is thoroughly removed. The workpiece is now air-dried, and
surface roughness values are recorded with a profilometer.
In metal finishing systems, rinsing has two main functions :(1) Removing chemical
residues from previous operations by dilution; (2) Obstruction to forestall hauling to the
following activity. Oxidation of the outer layer of a part during electropolishing can influence
the nature of the last completion, particularly for basic applications like semiconductors and
medical and pharmaceutical products. The primary substance separated from the cleaning
specialist tank will obliterate the all-out corrosive substance of the electrolytic polishing tank,
bringing about quality issues. The scaling conditions and expulsion strategies are totally
different. Nonetheless, in the electropolishing system, some consideration should be
practiced to guarantee that the strategy picked is viable with the general design of the
electropolishing groove and the electropolishing line.[81]
4.1.3 Electrolyte preparation
Electropolishing is directed in a bath made out of one or a few corrosive blends with the
expansion of mediators and different added substances. A natural, inorganic combination can
be used as an electropolishing electrolyte. Most understand electrolytes are coordinated to
the polishing of stainless steel. NaCl focus directly affects the conductivity worth of NaCl
arrangement. The higher the NaCl concentration, the higher the conductivity of the
structure.[82] For the electropolishing process of 316L stainless steel, the combination of
sulfuric corrosive, phosphoric corrosive, and a few different testing agents has been broadly
utilized as the electrolyte concentration. The electrolyte containing sulfuric acid can obtain a
smooth surface in 316L stainless steel under electropolishing; However, the solid corrosive
utilized is not harmless to the ecosystem. Its stable destructive properties are likewise hurtful
to operators, effectively damaging laboratory equipment.[83] We used four types of chemicals
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with different concentrations for polishing our workpiece in this experiment such as sodium
chloride (NaCl), Ethanol, Phosphoric acid (H3PO4), and Sulfuric acid (H2SO4). The acid part
of the electrolyte usually comprises phosphoric acid and sulfuric acid mixed in a 1:1 or 2:1
proportion. We used several types of concentration in our experiment, listed below.
1. 1M, 3M, 6M, 10M, and 14M NaCl in pure water.
2. H2SO4 (60wt.%)
3. H2SO4 (60wt.%) H3PO4 (60wt.%). 1:1
4. H3PO4 (60wt.%)
The composition of different bath fluids was calculated every few minutes by
approximating the ratio of phosphoric acid to sulfuric acid in aging fluids. The electrolyte
containing sulfuric acid can obtain a smooth surface in 316L stainless steel under
electropolishing.
4.1.4 Experimental setup
The EP process was performed in a conventional three-electrode cell consisting of the
reference electrode and electrolyte of the silver reference element. The application potential
was provided using a CH 700E instrument shown in Figure 4-2. A hot plate agitator is used
to heat the electrolyte, and a heat sensor controls the temperature of the electrolyte. A
magnetic stirring rod to stir and polish electrolytes were used in this study. The polishing gap
width between the anodes and the counter electrode was set to 10 mm. Keeping a protected
separation is essential because fixed techniques limit the gamble among anode and cathode
connections. The potential was cleared from 0 V to 7 V at a filtering velocity of 20 mV/s,
and the polarization curves of different electrolytes were measured. In order to obtain reliable
electropolishing repeatability, all tests were rehashed on numerous occasions under similar
circumstances.
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Figure 4-2 Experimental setups in the lab
4.1.5 Profilometer
Mitutoyo Surftest SJ-210 contact convenient profiler is displayed in Figure 4-3. The
gadget can rapidly and precisely measure the surface roughness of SS parts. The typical
roughness esteem (Ra) in microns showed by the roughness analyzer. Mitutoyo Surftest SJ210 is lightweight and small, making it exceptionally helpful to utilize. It utilizes a pointer to
connect with the outer layer of the SS workpiece. Mitutoyo Surftest SJ-210 shows surface
roughness waveform on a variety of LCD screens. The end length set by the gadget
determines the distance the pointer should travel to quantify the surface roughness of the
workpiece, which is still up in the air.
Figure 4-3 Profilometers for surface roughness measurement
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Ra does not give a complete portrayal of the unpleasantness intricacy. Consequently, a
sign deterioration procedure, otherwise called recurrence examination, has been applied to
the electropolished tests. The objective of the recurrence examination is to deteriorate a given
sign as several rudimentary signs (Figure 4-4). This disintegration permits exposing a few
rudimentary parts, which Ra alone does not give.
Figure 4-4 Surface roughness profile
Primary systems for electrolytic polishing: "anodic evening out" and "miniature
smoothing." Anodic evening out is because of the distinction in nearby charge dissemination,
and the top on a superficial level specially disintegrates. Micro-smoothness alludes to the
perfection created by hindering the impact of surface imperfections and precious stone
direction on the disintegration interaction.[16] Electrolytic polishing has been accomplished
at different times. It very well may be seen that for electropolished tests with a current density
of 130 mA/cm², the ghastly density diminishes at frequencies beneath 100µm. Nonetheless,
a few signs stay at frequencies higher than 100µm. Interestingly, electropolishing at lower
current densities (50 mA/cm²) debases signal just at extremely low frequencies under a couple
of µm.
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Figure 4-5 Surface roughness after using different electrolytes
4.1.6 Surface analyses
Metal inserts' surface qualities are significant in their usefulness, biocompatibility, and
security.[84] In this review, electropolishing (EP) utilized the 316L-SS surface to investigate
the superficial level bio-compatibility of the material. An MV3000 computerized microscope
described the surface morphology shown in Figures 4-6. The surface geography estimated a
non-contact optical profile. In this review, all surface roughness was estimated inside an area
of 230 mm × 150 mm, and the frequency cut-off esteem was 0.025 mm after slant expulsion
and Gaussian relapse separating. In order to obtain a very reliable surface assessment, each
electropolished surface is measured in five unique areas to obtain very representative surface
geology. The structure of the components was portrayed by field emanation checking electron
microscopy.
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Figure 4-6 Surface morphology characterization equipment
4.2 Experimental Parameters of Electrochemical Polishing
4.2.1 Polishing voltage
Polishing voltage is an integral part of the polishing parameter. Current density increased
with the increase of polishing voltage. The oxidization and etching layers are generated to
increase the potential voltage range. The repeated polishing method is adopted to control the
voltage and polishing time of each polishing within the allowed range to minimize the surface
roughness and achieve a good polishing impact. Through the analysis of the material, five
voltage levels are adopted. The voltage level is 1.5V, 2V, 3V, 4.5V, and 6V. Voltage levels
were used between 1.5V to 6V because of the better polishing result. The current density
level remains zero until the voltage level is 1.5V.and the electrolyte produces a large number
of bubbles when the operating voltage is higher than 6V; also polishing surface becomes
black. Perhaps high voltage contains high current density, which is the cause of surface
combustion, and also high voltage can damage the surface quality.
4.2.2 Conductivity of electrolyte
The electrolyte is also an indispensable part of the electrochemical polishing process.
Electrolytes with good performance can make the surface roughness after polishing far less
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than other electrolytes. Traditional electrochemical polishing usually USES the robust acid
solution because this kind of solution has strong corrosive; during transport, storage, and
processing, all need to consider security issues.[91] in addition, the need to consider the liquid
waste processing after processing and the use of strong acid solution after finishing
processing of products could also be seen as problems such as corrosion, hydrogen
permeability, and the need for additional working procedures. Therefore, this topic uses the
NaCl electrolyte. Through literature research, several widely used electrolytes were
investigated. The price of sodium chloride solution was low; despite there was stray corrosion
on the workpiece surface, it did not affect the final result. The processing of NaCl solution is
moderate, and the polishing effect is good. Although the conductivity of the neutral salt
electrolyte is lower than that of the strong acid solution, it is less corrosive, so the storage
standard can be reduced in the process of transportation and processing, and the polishing
process equipment and operation process can be significantly simplified.[82] The NaCl
contains five levels of concentration. NaCl concentrations are 1M, 3M, 6M, 10M, and 14M,
respectively.
In this review, the electropolishing arrangement likewise contains three corrosive
arrangements. 316L stainless steel was electropolishing with sulfuric acid-free H3PO4
electrolyte. Interestingly, H2SO4 free electrolyte was utilized to concentrate on EP process
attributes to investigate an all the more harmless to the ecosystem and more secure electrolyte
to supplant the generally involved sulfuric corrosive based electrolyte for EP interaction 316L
SS. The impact of electrolyte temperature was considered. Phosphoric acid 60(wt.%, Sulfuric
corrosive 60(wt.%), and mixed acid utilized in this paper. Mixed acid concentration-based
electrolyte arrangement made out of phosphoric corrosive 60(wt.% v/v) and sulfuric
corrosive 60(wt.% v/v). Acidic mixer solutions were utilized to contrast NaCl-based
electrolytes to track down an option in contrast to the usually utilized combination of H2SO4
and H3PO4, considering ecological and security concerns.
4.2.3 Polishing gap
During the time spent on electrochemical polishing, the polishing gap also influences
surface roughness. As per Ohm's regulation, the material expulsion rate on a superficial level
ordinary of the anode workpiece can be approximated by the simultaneous equation：
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𝑣𝑟 =
𝜂𝜇𝜎𝑈
𝑑
(24)
Where: 𝑈 is the polishing voltage, 𝜎 is the electrolyte conductivity, 𝑑 is the polishing gap.
Figure 4-7 Diagram of polishing gap at high and low anodes
As shown in Figure 4-7, according to formula (25), material removal rates at high and
low places can be expressed as：
𝑣ℎ =
𝜂𝜇𝜎𝑈
ℎ
(25)
(26)
𝜂𝜇𝜎𝑈 ′
𝑣𝑙 =
ℎ + ℎ′
Then the ratio of the removal speed of the two materials can be expressed as：
𝑣ℎ ℎ + ℎ′
ℎ′
(27)
=
= 1+
𝑣𝑙
ℎ
ℎ
According to the definition of localization, when the value K of localization is larger,
the polishing effect is better. According to Formula 26, when the polishing gap h is smaller,
the ratio of dissolution speed between high and low parts is more prominent because the
localization is more significant, so the polishing effect is better.[71] Nevertheless, on the other
hand, if the distance between electrodes is too small, there may be a discharge phenomenon,
so it is necessary to find a suitable machining gap. In the way of a superior polishing rate, the
terminal gap between the anode and the cathode was set at 10 mm and kept up with by gap
two openings in the acrylic plate at 10 mm dispersing.
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4.2.4 Polishing time
The control of polishing time is also an essential part of the experiment. Longer polishing
time can destroy the workpiece characteristics. Too short polishing time is insufficient to
eliminate surface material, and the workpiece surface cannot be totally evened out. Polishing
time is excessively lengthy; material will eliminate overly, which can diminish the size of the
workpiece. Furthermore, a lot of the design size deviation will prompt rejecting parts. Hence,
the polishing time should not be entirely settled by the base material's properties, the surface's
first condition, the organization of the electropolishing arrangement, the current density, and
the temperature. This paper uses five grades of polishing time to comprehend the impact of
electropolishing, such as the 1200sec, 2400 sec, 3600 sec, 6000 sec, and 7200 sec.
4.2.5 Polishing temperature
The temperature impacts the electropolishing system at the point when the current
density is steady. As the temperature of the electrolyte builds, the speed of electropolishing
increments. As the temperature builds, the consistency of the solution diminishes, which
fortifies the convection impact and speeds up the dispersion rate. The solution close to the
anode can be quickly restored, which is helpful for the dissolution of the anode.[85]
If the temperature is too high, the conductivity of the treated steel electrolytic polishing
solution can be expanded, and the surface brilliance of the workpiece can be expanded;
however, causing lopsided dispersion of current density pitting is simple. Change the
temperature to the ideal temperature range. This way, ideal electrolytic polishing conditions
may be obtained at high temperatures, when the temperature improves the mass transfer rate
to complete the anodic dissolution expansion.[74]
Whenever the temperature is low, the conductivity of the solution is poor, and polishing
is slow; while the polishing temperature is high, the dispersion of the electropolishing
solution is solid.[85] Requesting a thin film to frame the current density should be expanded
simultaneously. If the polishing temperature is too high, the electrolyte on the outer layer of
the anode is inclined to overheat. The produced gas and fume might release the electrolyte
from the outer layer of the terminal in this way, decreasing the polishing impact, and
accomplishing the reason for polishing by expanding the ongoing density is troublesome. In
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this review, through the investigation of the workpiece, five degrees of polishing temperature
(25°C, 40 °C, 60°C, 70°C, and 80°C) were chosen.
4.3 Results Analysis
4.3.1 Factorial design analysis
The technique of selecting a small number of experiments that provide sufficient
information is called a partial factorial experiment. Although this approach is well known,
there are no general guidelines for it. Taguchi has established a general design guideline for
part analysis because of experiments covering many applications. Taguchi's method uses a
particular set of arrays, called orthogonal arrays, to maximize understanding of all the factors
influencing the results with a minimum number of experiments. The core of the orthogonal
array method is selecting the input factor level for each experiment. Taguchi's approach uses
fewer arrays to assess the impact of process factors. Taguchi is a simple, time-saving
approach that can be used for various technical problems. The larger-is-better quality
characteristic has been chosen for this experiment.
Table 8 Factors and levels of Taguchi method in the electropolishing
Polishing
Polishing Time
Polishing
Electrolyte
Voltage (V)
(T)
temperature (℃)
conductivity (S/m)
1.5
1200
25
1
2
2400
40
3
3
3600
55
6
4.5
6000
70
10
6
7200
80
14
Taguchi technique was acquainted with screening these key factors to find the key factors
influencing the roughness of 316L tempered steel. In light of the writing audit, the
accompanying electrolytic polishing factors were concentrated on in the plan of the
Experimental Analysis (DOE) study: A) Polishing voltage; b) Polishing time; C)
Conductivity of electropolishing; D) Polishing temperature. The fixed levels of these four
variables are listed in the table (8). meanwhile, the L25(54) design matrix with the
experimental data was given in Table 4, where 25 experiments were conducted. This study
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used 10mm electrolyte polishing gap and 450 Rpm stirrer speed for electropolishing. DOE
only analyzed the electropolishing experiment in NaCl-based electrolytes in this study. At
the same time, some other electrolyte concentrations were used in this paper to analyze the
electropolishing results. Such as H2SO4, H3PO4, and mixed concentration. The roughness of
electropolishing materials is measured using the Mitutoyo Surftest SJ-210 profiler. Table 9
contains the determined parameters and average surface roughness after electropolishing.
Table 9 Experimental data and polishing result
Polishing
Polishing
Voltage(V)
Time (s)
Polishing
Polishing
Surface
Total Reduced
Conductivity Temperature Roughness (μm) Roughness(μm)
(S/m)
(℃)
Before
After
1
1.5
1200
1
25
8.088
6.439
1.649
2
1.5
2400
3
40
9.525
5.488
4.037
3
1.5
3600
6
55
7.383
4.698
2.685
4
1.5
6000
10
70
6.997
5.946
1.051
5
1.5
7200
14
80
7.058
6.930
0.128
6
2
1200
3
55
7.369
4.261
3.108
7
2
2400
6
70
8.199
4.131
4.068
8
2
3600
10
80
6.899
4.241
2.658
9
2
6000
14
25
7.403
4.591
2.812
10
2
7200
1
40
7.567
4.327
3.24
11
3
1200
6
80
8.048
4.817
3.231
12
3
2400
10
25
7.247
2.898
4.349
13
3
3600
14
40
7.366
3.387
3.979
14
3
6000
1
55
7.855
3.809
4.046
15
3
7200
3
70
7.565
4.754
2.811
16
4.5
1200
10
40
6.611
3.617
2.994
17
4.5
2400
14
55
6.673
4.341
2.332
18
4.5
3600
1
70
7.197
3.531
3.666
19
4.5
6000
3
80
6.930
4.737
2.193
20
4.5
7200
6
25
8.296
3.593
4.703
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21
6
1200
14
70
7.190
5.874
1.316
22
6
2400
1
80
6.701
4.652
2.049
23
6
3600
3
25
7.230
3.269
3.961
24
6
6000
6
40
8.542
4.237
4.305
25
6
7200
10
55
7.905
4.540
3.365
Optimum surface roughness was acquired at array no. 12 when the parameter is;
Polishing voltage 3V, polishing time 2400s, conductivity 10s /m, and temperature 25℃. The
surface became smoother at this stage. Array number 20 showed the highest reduced surface
roughness when the parameters were polishing voltage 4.5V, polishing time 7200s,
conductivity 6s /m, and temperature 25℃. Moreover, the lowest surface roughness reduction
occurred at array number 5 was the Polishing voltage of 1.5 V, polishing time of 7200 seconds,
the conductivity of 14s /m, and temperature of 80℃. From array number 5, temperature and
conductivity greatly impact surface polishing. At this stage, polishing occurs until 3600
seconds. However, roughness decreased with the increase of time. After the workpiece is
absorbed in the electrolyte for a long time, the workpiece becomes black due to the high
temperature, and the result of the polishing surface becomes uneven.
Table 10 Experimental Polishing range calculation
Level
Polishing
Polishing Time
Polishing
Polishing
Voltage (V)
(s)
Conductivity
Temperature
(S/m)
(℃)
1
5.900
5.002
4.552
4.158
2
4.310
4.302
4.502
4.211
3
3.933
3.825
4.295
4.330
4
3.964
4.664
4.248
4.847
5
4.514
4.829
5.025
5.075
Mean Value
1.967
1.117
0.776
0.917
The primary response data of mean for electropolishing surface roughness is in Table 10.
These parameters show the average surface roughness for each factor—this table assesses the
impact of process factors. The response table also shows which operational factors impact
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more significantly, primarily on responses calculated by assigning a level to model
parameters. The optimal value is usually obtained from the response data set. In the response
table, the lowest mean of the predicted concentration and surface roughness of the
electrochemical polishing rate has the highest importance relative to the specific criteria. It
can be seen from the mean response table that the effects of technological factors have
different effects on the electropolishing rate and metal surface roughness.
Figure 4-8 The relationship between electropolishing Voltage (V) and average surface
roughness Ra
Figure 4-8 shows the relationship between average surface roughness and polishing
voltage in the electropolishing process. Surface roughness reduction occurs slowly until the
voltage level is 1.5V. Surface roughness decreased dramatically with the increasing voltage
in the range of 1.5V to 3V. after that; surface roughness slightly increased with the expanding
voltage in the range of 3V to 4.5V. However, surface roughness increased quickly after
increasing voltage from 4.5V to 6V. Current density also increased due to the increasing
voltage, resulting in many bubbles and pitting corrosion. This pitting corrosion makes the
surface rougher and destroys its surface quality.
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Figure 4-9 The relationship between polishing time (s) and average surface roughness Ra
Figure 4-9 shows the drawing results of the relationship between average surface
roughness and electrolytic polishing time. It can be seen from the curves that the surface
roughness decreases with the increase of polishing time. Electropolishing lasts up to 3600
seconds. During this time, the surface roughness becomes smoother. After that, with the rise
in polishing time, pitting occurs slowly, resulting in uneven surface roughness. Because the
electrode is immersed in the electrolyte for a long time and the workpiece characteristics are
damaged.
Figure 4-10 The relationship between average surface roughness Ra and electrolyte
Conductivity (S/m)
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The relationship between polishing conductivity and average surface roughness showing
in Figures 4-10. the polishing curve showed that surface roughness decreased in the
conductivity range from 1 S/m to 6 S/m. at this level, polishing happened correctly, and
surface roughness became smooth. However, surface roughness increased with the increasing
conductivity because higher conductivity produces higher current density. Furthermore,
higher current density generates numerous bubbles, obstructing the electropolishing of metal
surface roughness and creating surface irregularity due to the electrolyte anodization.
Figure 4-11 The relationship between average surface roughness Ra and electropolishing
temperature ℃
Figure 4-11 shows the relationship between electrolytic polishing temperature and
average surface roughness. The results show that the ambient temperature works better for
NaCl-base electrolytes in electropolishing. Surface roughness was lower when the
temperature was low. After all, surface roughness increases slightly with the increasing
temperature. When the polishing temperature is too high, the outer layer of the electrolyte
anode is inclined to overheat. The gas and smoke generated may release the terminal of the
outer electrolyte layer. In this way, to reduce the impact of polishing, achieving polishing by
expanding the current density of the cause is troublesome.
4.3.2 NaCl based electrolyte
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4.3.2.1 LSV curve analysis
Figure 4-12 shows the LSV curves of NaCl-based electrolytes at different stirring speeds
at room temperature, where NaCl concentration was 1M. The applied potential range was 0V
to 8V. In sodium chloride-based electrolytes, active anodic dissolution begins at a potential
range of 1.5 V. In the range of 1.5V to 5.5± 0.5V, the current density increases sharply with
the increase of potential and decreases with the increase of stirring speed, as shown in Figure
3-8. The current density gradually decreases in the potential range of 5.5±0.5V to 6.5±0.5V,
and the potential range becomes stable after 7±0.5V. When the stirring speed is 400 RPM,
the current density reaches higher than others. During the stirrer speed range of 500rpm,
600rpm and 700rpm, the current density decreased slightly due to the formation of many
bubbles with increased stirring speed. It very well may be seen that the restricting current
density fluctuates with the revolution rate. The increment of the pivot rate brings about the
increment of restricting current density.
Figure 4-12 LSV curve in NaCl-based electrolyte at a different stirrer speed
Figure 4-13 shows the sodium chloride-based electrolyte's linear swipe voltammetry
curve at different temperatures, While the stirrer speed was 450rpm, the Electrolyte
concentration was 6M, and the electrolyte scan rate was 20mV/s. The curve result shows that
with the increasing temperature, current density increased fastly. For the temperature ranges
25°C and 40°C, the potential ranges from 1.5V to 2.8V current density increased. In the EP
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process, this region is characterized as the etching area. A current peak happened at a
potential range of 1.8V. Then decreased for a while due to the new passivated oxide layer
generated. Current density increased dramatically from the voltage level of 2.4V due to the
generated oxygen bubble. Current density decreased again from the potential range of 5V.
For the others temperature levels, the current density curve is different. Current density
growth happened too quickly with the increasing temperature. Since the temperature
expanded, the dissemination cycle of the broke up metal particles into the mass electrolyte
brought about a slenderer thickness of the viscous film layer, which caused the rising current
density
Figure 4-13 LSV curves of NaCl-based electrolytes at different electrolyte temperatures
LSV curves at different NaCl concentrations are shown in Figures 4-14. LSV curves at
different NaCl concentrations are shown in Figures 4-14. The stirring speed was 450 RPM,
and the temperature was room temperature. The five types of concentration are 1M, 3M, 6M,
10M, and 14M, respectively. From the curve, it can be seen that the current density is lower
during concentration was 1M. For 1M concentration, current density increased from the
voltage level of 1.5V. After that, current density increased with the increasing voltage. The
null current density state occurred similarly for the rest of the concentration, and current
density increased from the voltage level of 2.3V. The current density increases with the
increase of electrolyte concentration density. The current density highest pick level occurred
at the voltage level of 7V during 3M concentration. The highest current density pick points
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are the same for 6 M,10M, and 14mol, although voltage levels are different. For 6M
concentration highest current density pick points occurred at the voltage level of 4.2V.
Similarly, for 10mol concentration highest current density pick points occurred at the voltage
level of 5.2V, and for 14M concentration, pick occurred at the voltage level of 5.5V. From
then on, the current density decreases as the voltage increases due to the generated countless
bubbles.
Figure 4-14 LSV curve in a different kind of NaCl concentration
4.3.2.2 Polishing graph analysis
Figure 4-15 shows the electrolytic polishing graph with different concentrations of
sodium chloride-based electrolyte. The applied potential was 3V, and the electropolishing
duration was in the 50s. While the polishing temperature was room temperature and the stirrer
speed was 450 rpm. As can be seen from the figure, the current density is low when the
electrolyte concentration is low, and the current density increases with the increase of the
electrolyte concentration.
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Figure 4-15 Polishing graph of NaCl under different kinds of concentration
Figure 4-16 shows that the current density varies with different types of potential.
Respectively, 1.5V, 2V, 3V, 4.5V, and 6V. Electrolyte concentration was used at 6mol,
stirring speed was 450rpm, and electrolytic polishing time was 50 Seconds. The current
density is too low for the potential 1.5V and 2V, which cannot be used for polishing. Because
it will take lots of time, that could damage workpiece characters. Due to the limited current
density, a better polishing result was obtained from the polishing voltage range of 3V. Surface
quality will damage if the current density is too high, and also electropolishing will not
happen properly if the current density is too low. The best electropolishing result has been
found at potential level 3V in this experiment, shown in the table-9. When the applied
potential is 4.5V, and 6 V, the current density of EP shows a typical current density transition.
When the potential is 4.5V, the active dissolution of the anode increases the current density.
By way of alternative, the potential 6V shows different current density characteristics.
When a constant potential is applied, the current density decreases due to the formation of a
passive oxide film on the anode surface. At this point, the current density reaches a steady
state, and passivated oxide film's formation and dissolution rates reach equilibrium.
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Figure 4-16 Polishing effect in NaCl-based electrolyte after using different types of voltage
4.3.2.3 Roughness analysis
Figure 4-17 shows the surface roughness profile of the 3D-printed metal surface before
and after polishing in sodium chloride-based electrolyte. The numbers inserted in the figure
represent average surface roughness, showing an overall description of average height
variation (Ra), root mean square roughness (Rq), and average peak to valley roughness (Rz).
The average surface roughness (Ra) decreased from 6.184 μm to 2.98 μm after polishing in
NaCl electrolyte for 1200 seconds. Surface polishing achieved 75% of improvement. The
results show that the surface roughness profile became smoother, reducing the distortion after
polishing.
Figure 4-17 Surface roughness analysis before and after polishing in NaCl-based electrolyte
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4.3.2.4 Microscopic analysis
Figure 4-18 shows the microscopic image of a polished surface with sodium Chloridebased electrolyte at the magnetic stirrer speed of 450rpm. Electrolyte concentration was 6M,
and electrolyte temperature was room temperature. The NaCl-based electrolyte shows a
significant difference between before and after polishing the surface and a better
electropolishing effect at the electrolyte room temperature. Electropolished roughness
decreased from 6.184 μm to 2.898 μm. There were many bulges before polishing, and the
metal surface was irregular. However, the bulge reduced significantly after polishing. The
results showed that the sodium chloride electrolyte produced some utilization pits on the
electropolishing surface. It is envisaged that Cl- particles combine with uninvolved oxide
film structures to form porous oxide films that can be indeed eliminated. Then, the anode
surface pursues pitting utilization due to the presence of Cl- particles.
Figure 4-18 Microscopic picture analysis of NaCl-based electrolyte before and after polishing
4.3.3 H2SO4 based electrolyte
4.3.3.1 LSV curve analysis
Figure 4-19 shows the anodic polarization curves at different stirring speeds. In addition,
the temperature was the ambient temperature. Sulfuric acid was used to concentrate the
electrolyte. The passivation zone appeared in the potential range of 0V to 8V. The dynamic
dissolution begins at 1.5 V, and the current density increases significantly with the increase
of applied potential. When the stirring speed is 400rpm, the average current increases in the
range of 1.5V to 2V. This area is described as an attractive electrochemical polishing area.
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The anode surface is emptied conventionally. The peak occurs at the 2V potential, and the
current density decreases as the potential increases from 2V to 2.3V. This directly results
from a new passivated oxide film on the anode surface, which achieves reduced current
density. A limited level of end current density can be seen after the passivation layer, where
the best electrolytic polishing results can be obtained. Due to the formation of a thick salt
film or adsorbent receptor layer on the anode surface, the current density is consistent with
the increase in applied potential. The thickness of the salt film or adsorbent receptor layer
varies with the increase of applied potential to achieve a constant current density.[86]
Figure 4-19 LSV curve in H2SO4-based electrolyte at different stirrer speeds
Then, at that point, with the increment of the applied potential, the current density
increments until above 3V, and this area is named the gas precipitation locale. Because of the
electrochemical response, oxygen bubbles are delivered on the outer layer of the anode and
should be visible on the outer layer of the anode around here. A thick film is framed on the
anode surface as the dissolved metal particles cannot scatter from the anode surface in time.
The concentration of dissolved metal particles in the viscous film was the descent. The
concentration on the anode surface was higher, and the concentration diminished with the
separation from the anode surface increment; as should be visible from Figure 4-19, the
current density increments with speed up, which is basically because of the impact of the
stirring system on the mucosal layer. Stirring electrolyte builds the dissemination cycle of
dissolved metal particles to the general electrolyte, bringing about a thinner viscous film
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thickness.[74] With speed up, electrode gap opposition decreases, expanding current density.
The current density changed a lot after changing the stirring paces, as shown in Figure 3-19.
In this manner, the stirring pace has a restricted effect on the increment of the current density.
Figure 4-20 shows that various temperatures have been utilized at a stirring speed of 400
RPM to comprehend the polishing range. The temperature level was 25°C, 40 °C, 60°C, 70°C,
and 80°C. When the polishing temperature was 40°C to 80°C, the current density was steady
until the voltage was 1.3V. Be that as it may, the current density was steady until 1.5 v for
25 °C temperature. As the potential expanded, overall current density grew in the possible
scope of 1.5V ± 0.5V to 2V ±0.5V. Numerous bubbles were generated due to the increasing
voltage and stirrer speed. The result of current density decreased over a specific period. These
numerous bubbles blocked the electropolishing process and created pittings on the metal
surface.
Figure 4-20 LSV curve in H2SO4-based electrolyte at while temperature is different
4.3.3.2 Polishing graph analysis
Figure 4-21 shows the polishing result of H2SO4-based electrolytes at different
temperatures. The applied electrolyte potential range was 2V, and the stirrer speed was
450rpm. Electrolyte concentration was H2SO4 (60wt.%). Furthermore, the polishing gap was
10mm. Here electropolishing graph was analyzed only for the 50s. Figures 3-21 show the
current density transitions of polishing temperature range of 25°C, 40°C, 55°C, 70°C, and
80°C. The temperature of the electrolyte is directly related to the mass transfer in the
electropolishing process. Sulfuric acid provides SO42− ions, which act as acceptor species in
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electrolytic polishing systems. Subsequently, the lower temperature results in slower
diffusion of ions from the surface, with SO42− ions diffusing slowly to the surface. High
temperatures also increase the solubility of metal ions in the solution, which will also affect
increasing current density. Figure 4-21 shows that the current density increases with
increasing temperature. At all potentials, the current density decreases rapidly with time due
to the growth of the passivated oxide film on the workpiece surface. The material on the
surface of the workpiece is switched over entirely to metal oxide, which then disintegrates
into the electrolyte, prompting the material evacuation process. At this point, the
disintegration and development of the oxide film are in a state of equilibrium, resulting in a
stable current density.
Figure 4-21 Electropolishing curve in H2SO4 electrolyte with different temperature
Figure 4-22 shows the polishing of H2SO4-based electrolytes in different voltage ranges.
The concentration of electrolyte has been H2SO4 (60wt.%). The electrolyte temperature was
room temperature. The stirrer speed was 450rpm. The previous discussion shows that
different voltage levels have different current densities, as shown in Figure 4-22. The current
density is too high until the potential range is 3V. The range of current density decreases with
the increase of potential. Peak current density is generated by the passivation process when
higher voltages are applied. The result shows that the passivation tendency of 316L stainless
steel decreases with the increase of potential after 3V. The current density reaches a stable
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state, and the formation rate of the passivated oxide film is in equilibrium with the dissolution
rate. In this study, good polishing results were obtained in the range of 2V potential.
Figure 4-22 Electropolishing range in H2SO4 electrolyte with the different potential range
4.3.3.3 Roughness analysis
Figure 4-23 shows the surface roughness profile of the 3D-printed metal surface before
and after polishing in Sulfuric acid-based electrolyte. The average surface roughness
decreased from 6.375μm to 0.676μm after polishing the H2SO4-based electrolyte for 3600
seconds. The figure represents that root mean square roughness (Rq) reduced from 7.780 μm
to 0.935 μm, and average peak to valley roughness (Rz) decreased from 34.551 μm to 6.594
μm. Surface polishing achieved 95% of improvement. The results show that the surface
roughness profile became smoother after polishing.
Figure 4-23 Surface roughness before and after polishing with H2SO4-based electrolyte
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4.3.3.4 Microscopic analysis
Figure 4-24 shows the microscopic picture with sulfuric acid-based electrolyte at room
temperature. While the stirrer speed was 450rpm and the potential was 2V. The microscopic
picture shows the polishing result after electropolishing for 3600 seconds. Sulfuric acid has
better polishing results compared with other concentrations. The figure shows that the surface
had a better surface quality and the obtained surface was comparatively smooth and uniform.
The surface has many bulges before polishing, but the bulge is almost reduced after polishing.
Electropolished surface roughness decreased from 6.375 μm to 0.676 μm. It can be seen that
the dark metal surface became clean and shiny after polishing in the sulfuric acid-based
electrolyte.
Figure 4-24 Microscopic picture analysis of H2SO4-based electrolyte before and after
polishing
4.3.4 H3PO4 based electrolyte
4.3.4.1 LSV curve analysis
Figure 4-25 shows the straight range voltammetry bends at various stirring velocities.
The electrolyte concentration is phosphoric corrosive, and the temperature is encompassing
temperature. The LSV potential in the figure goes from 0V to 8V. Dynamic anodic
dissolution starts from the capability of 1.5 V, and the current density increments pointedly
with the increment of applied possibly in the scope of 1.5V to 2V. This range is known as the
passivation area in electrochemical polishing. The anodic surface produced pitting erosion
regularly. The pinnacle worth can see at the applied capability of 2 V, and with the increment
expected in the scope of 2V to 2.5V, the current density diminished. This is a consequence
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of improving a new passivated oxide film on the anode surface, decreasing current density.
A narrow end current density stage region can be seen after the passivation. However, the
current density extended in the extent of potential from 2.5V±0.5V to 3.5V±0.5V. This area
is known as the oxidization area. More oxygen bubbles are generated around here. The
current density changed little at an alternate stirring pace during this period.
Figure 4-25 LSV cure of H3PO4-based electrolyte in different levels of stirrer speed
As displayed in figure 4-26, With the decline of electrolyte temperature, the restricting
current density diminishes, and the width of the stage region increments. The breaking point
current stage district difference is steady with the past consequences of electrolyte
temperature decrease. The viscous film and passivation film shaped on the workpiece surface
during the electropolishing system does not entirely settle the current density. The lower
electrolyte temperature decreases the dissemination pace of dissolved metal particles to the
workpiece surface and the dispersion pace of PO43+ particles to the workpiece surface dials
back. In this manner, as the thickness of the thick film builds, the gap opposition increments,
and the restricting current density diminishes.
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Figure 4-26 LSV cure of H3PO4-based electrolyte in different levels of temperature
Likewise, the low electrolyte temperature decreases the dissolvability of metal particles
in electrochemical cells. It lessens the restricting current density because of the expansion in
the thickness and security of the viscous film, and the width of the restricting current stage
district increments with the abatement of the electrolyte temperature. Figure 3-26 shows the
current density transient graph when the electrolyte temperature is 25℃,40℃,55℃,70℃, and
80℃ individually and the stirrer speed is 400 rpm. For H3PO4-based electrolytes at various
temperatures, the dynamic dissolution layer began at 1.5V potential. The current density of
electrolyte has little contrast when the temperature of the electrolyte is somewhere in the
range of 25℃ and 40. The current density expands at the scope of potential 1.5V to 2V, then,
at that point, current density diminishes in the expected range of 2.5 ±0.2V, and afterward,
current density expands again with the potential territory growing from 2.5±0.2V to 4±0.2V.
With the increment of electric potential, the thickness of the passivation layer increments
correspondingly, the film opposition increments, and the polishing current essentially stays
unaltered. From 2V to 2.5V, oxygen escapes rapidly, and the neighborhood solution warms
rapidly, bringing about the best polishing results. This region is the best for electropolishing
with the fantastic, massive expansion in brilliance. Around here, assuming the potential is
high, the polishing quality is better because of the tremendous nearby temperature, yet the
activity cycle is not difficult to control. At the point when the potential ascents once more,
the film breaks, and the opposition drops. Pits show up during this time, and the consequence
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of polishing impact becomes poor. In the event that the potential is shallow, passivation film
will not be framed, and electrolytic polishing will not happen.
4.3.4.2 Polishing graph analysis
Figure 4-27 shows the polishing result of H3PO4-based electrolyte in different potential
ranges at room temperature, while the stirrer speed was 450rpm. The polishing gap indicates
that the potential range from 1.5V to 4.5V has the same characteristics; the current density
increased with the increasing potential. However, the current density became lower at the
potential range of 6V due to the passivation layer. Peak current density was generated by the
passivation process when higher voltages were applied. 2V potential has been used for further
investigation In this experiment. The potential range of 2V is more stable than the other
potential range. Also, the potential range of 2V provided desired current density range for
better electropolishing.
Figure 4-27 Polishing range of H3PO4-based electrolyte in the different potential range
Figure 4-28 shows the polishing result of H3PO4-based electrolytes at different
temperatures. The potential was 2.5V, the stirrer speed was 450rpm, the polishing time was
50sec, and the electrolyte concentration was H3PO4 (60wt.%). The polishing gap shows that
current density became higher with the increasing temperature. As we know, High
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temperatures increase the solubility of metal ions in the solution, which affects increasing
current density. In the beginning, the current density decreases rapidly with time due to the
growth of the passivated oxide film on the workpiece surface. After that, the current density
becomes stable due to the oxidation layer. High temperature generates high current density,
leading to surface quality and surface pitting destruction.
Figure 4-28 Polishing range of H3PO4-based electrolytes in the different temperature range
4.3.4.3 Roughness analysis
Figure 4-29 shows the surface roughness profile of the 3D-printed metal surface before
and after polishing in Phosphoric acid-based electrolyte. The figure represents that the
average surface roughness decreased from 6.538μm to 1.112μm after polishing the H3PO4based electrolyte for 3600 seconds. Also, root means square roughness (Rq) reduced from
8.007 μm to 1.720 μm, and average peak to valley roughness (Rz) decreased from 35.422 μm
to 14.071 μm. Surface achieved 85% of improvement in this experiment. The results show
that the surface roughness profile became smoother after polishing.
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Figure 4-29 Surface roughness analysis before and after polishing in H3PO4-based electrolyte
4.3.4.4 Microscopic analysis
Figure 4-30 shows the microscopic picture of Phosphoric acid-based electrolytes at room
temperature. While the stirrer speed was 450rpm and the potential was 2.5V. The microscopic
picture shows the polishing result after electropolishing for 3600 seconds. It is found that the
bulges are reduced significantly after electropolishing. Electropolished surface roughness
decreased from 6.538 μm to 1.112 μm. It can be seen that the irregular and dark metal surface
became smooth, clean, and bright after polishing in the phosphoric acid-based electrolyte.
Figure 4-30 Microscopic picture analysis of H3PO4-based electrolyte before and after
polishing
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4.3.5 Mixed electrolyte (H2SO4 +H3PO4)
4.3.5.1 LSV curve analysis
Figure 4-31 shows H2SO4, and H3PO4-based electrolyte polarization curves at various
stirring velocities. The impacts of various stirring velocities were seen at a checking pace of
20 mV/s. The polarization bend was acquired from the possible scope of 0V to 8V. In the
polarization bend, the invalid condition of current density is 0 to 1.4V. Dynamic anodic
dissolution starts at a capability of 1.4V. The current density range expanded consistently
when the applied potential territory expanded from 1.4V to 2.3V, y. This region is
characterized as the drawing region in electrolytic polishing. Then, at that point, there is a
top at the applied expected scope of 2.3V. From that point onward, the current density
diminishes with expansion possible in the range of 2.3V to 2.5 V. This is because of the
development of a new passivated oxide film on the anode surface, bringing about a
diminishing current density. After the passivation region, a tight breaking point current
density stage locale is noticed, where the best electropolishing results can be acquired—the
current density increments with the increment of potential. Then, the current density increases
forcefully in the applied possible scope of 2.5V to 3.3V, the gas passivation locale. Because
of the electrochemical response, oxygen bubbles are delivered on the outer layer of the anode
and should be visible on the outer layer of the anode around here.
Figure 4-31 LSV curve in H2SO4 and H3PO4-based electrolyte at different levels of stirrer
speed
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Linear swipe voltammetry curves using different temperatures at mixed acid
concentrations are shown in Figures 4-32. While the stirrer speed was 450rpm and mixed
acid concentration was H2SO4 (60wt.%) H3PO4 (60wt.%) at a scan rate of 20mV/s. The
polarization curves were measured by sweeping the potential from 0 V to 8V. from figure 432; it can be seen that the range of polishing voltage from 0V to 1.5V is the null state for
current density. Polishing did not occur During this time. Current density increased from the
voltage range of 1.5V. For the temperature level of 25℃, the current density increased from
1.5v to 2.2V. This region is called the etching region in electropolishing. Next, a pick
observes at the potential of 2.2V. After that, the current density decreased when increased the
potential range from 2.2V to 2.5V. That is because of new passivation layer was generated.
The best polishing result was obtained in this region. After that, the current density increased
perilously by increasing the potential range from 2.5V to 3.1V. Due to the numerous oxygen
bubbles generated, current density decreased sharply from the possible range at 3.1V. LSV
curve shows almost the same result for the temperature level 40℃. However, the rest of the
curves for different temperatures show different curve results because higher temperature
produces higher current density and quickly generates an oxidization layer.
Figure 4-32 LSV curve in H2SO4 and H3PO4-based electrolyte at different levels of temperature
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4.3.5.2 Polishing graph analysis
Figure 4-33 shows the polishing result of Mixed acid-based electrolyte at different
voltage levels at room temperature. While the stirrer speed was 450rpm, the polishing time
was 50 sec, and the electrolyte concentration was H2SO4 (60wt.%) H3PO4 (60wt.%). The
polishing result shows that the current density increased with the increasing potential from
1.5V to 3V. Nevertheless, the current density level decreased after increasing the voltage
level due to the numerous bubbles generated. The current density was too low at the potential
range of 1.5V, which could not polish the surface. A great electropolishing result was
obtained for the surface at the potential level of 2V due to a more stable current density. The
current density reached a higher level during the potential of 3V, which occurred pitting on
the surface due to the high current density. For the potential range, 3V,4V, and 5V. Due to
the growth of passivated oxide film on the workpiece surface, the current density decreases
rapidly with time.
Figure 4-33 The polishing range of mixed acid-based electrolyte at different voltages
Figure 4-34 shows the mixed acid concentration electropolishing graph at different
temperatures. The applied electrolyte potential range was 2.5V, the stirrer speed was 450rpm,
the electrode gap was 10mm, and the polishing time was 50sec. The mixed electrolyte
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concentration was H2SO4 (60wt.%) + H3PO4 (60wt.%). Figure 4-34 shows the density
changes in the current polishing temperature range of 25℃, 40℃, 55℃, 70℃, and 80℃.
Current density simultaneously increased with the increasing temperature. The temperature
of the electrolyte is directly related to mass transfer in the electropolishing process. The
current density decreased due to the oxidization layer. The temperature range of 40℃ is more
stable, which can provide desired surface polishing. Due to the dissolution and formation of
the oxide film in equilibrium, the current density is stable, and there is no lasting current
density at different temperatures. Therefore, it is best to avoid these temperature ranges, as
high currents and different current densities may cause surface pitting.
Figure 4-34 Polishing range of H2SO4 and H3PO4-based electrolytes at different temperature
4.3.5.3 Roughness analysis
Figure 4-35 shows the surface roughness profile of the 3D-printed metal surface before
and after polishing the mixed acid-based electrolyte. The mixed electrolyte concentration was
H2SO4 (60wt.%) + H3PO4 (60wt.%). The figure represents that the average surface roughness
decreased from 6.804μm to 0.800μm after polishing the H2SO4 (60wt.%) + H3PO4 (60wt.%)based electrolyte for 3000 seconds. Also, root means square roughness (Rq) reduced from
8.240 μm to 1.117 μm, and average peak to valley roughness (Rz) decreased from 36.694 μm
to 7.141 μm. Surface achieved 90% of improvement in this experiment. The results show that
the surface roughness profile becomes significantly smoother after electropolishing.
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Figure 4-35 Analysis of surface roughness before and after polishing with mixed electrolyte
concentration
4.3.5.4 Microscopic analysis
Figure 4-36 shows a micrograph image of mixed acid-base electrolytes at room
temperature. The stirrer speed was 450rpm, and the potential was 2.5V. The electrolyte
concentration was H2SO4(60wt.%) + H3PO4(60wt.%). The electrolyte volume ratio was 1:2.
When 316L stainless steel is electrolyzed by phosphoric acid and sulfuric acid mixed
electrolyte, H2PO4 - ion or its complex product may be a limited species of the chemical
receptor. It was found that the bulge was reduced obviously after electropolishing. The
surface roughness of electropolishing decreased from 6.538 μm to 1.112 μm. As we can see,
the irregular, dark metal surfaces are polished to a smooth, clean, and shiny finish with a
mixture of acid-base electrolytes.
Figure 4-36 Microscopic analysis of before and after polishing at mixed acid-based electrolyte
4.4 Comparisons between simulation and experiment result
Experimental and simulation both have different processes and different characteristics.
Due to the influence of environment and surface characteristics, the experimental and
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simulation results are not easy to get the same results. However, this paper's experiment and
simulation results are nearly the same. Table 11 shows the comparison between simulation
and experiment results of electropolishing. It was found that the average surface roughness
difference between the simulation result and experiment result was 1.02 μm. At the initial
stage of electropolishing, the current in the convex part of the anode surface is dense and
concentrated, and the current density on the surface is larger than that in the depression. As
shown in the simulation, the reduction effect of the surface spheroidization effect is
significantly greater than that of other positions. After a period of time, the surface roughness
tends to be uniform. The simulation result shows that the average surface roughness reduced
with the increasing electrolyte conductivity. This roughness reduction process similarly
accrued during the experiment. Likewise, the experiment result was achieved as the same as
the simulation result from the polishing time, polishing temperature, and polishing voltage.
Table 11 Comparison between simulation and experimental output result
Polishing
Polishing
Polishing
Polishing
Average Surface
Voltage
Time (s)
Conducti
Temperat
Roughness (μm)
vity (S/m)
ure (℃)
(V)
Simulation
Experiment
Result
Result
Differences
1
1.5
1200
1
25
4.68
6.439
1.759
2
1.5
2400
3
40
4.325
5.488
1.163
3
1.5
3600
6
55
3.76
4.698
0.938
4
1.5
6000
10
70
3.085
4.946
1.861
5
1.5
7200
14
80
3.01
3.93
0.92
6
2
1200
3
55
3.625
4.261
0.636
7
2
2400
6
70
4.06
4.131
0.071
8
2
3600
10
80
3.635
4.241
0.606
9
2
6000
14
25
0.875
2.591
1.716
10
2
7200
1
40
4.2
4.327
0.127
11
3
1200
6
80
4.453
4.817
0.364
12
3
2400
10
25
2.14
2.898
0.758
13
3
3600
14
40
2.178
3.387
1.209
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北京理工大学硕士学位论文
14
3
6000
1
55
3.26
3.809
0.549
15
3
7200
3
70
3.554
4.754
1.2
16
4.5
1200
10
40
3.397
3.617
0.22
17
4.5
2400
14
55
2.534
4.341
1.807
18
4.5
3600
1
70
3.475
3.531
0.056
19
4.5
6000
3
80
3.676
4.737
1.061
20
4.5
7200
6
25
0.91
3.593
2.683
21
6
1200
14
70
3.263
5.874
2.611
22
6
2400
1
80
4.646
4.652
0.006
23
6
3600
3
25
2.415
3.269
0.854
24
6
6000
6
40
3.101
4.237
1.136
25
6
7200
10
55
3.112
4.54
1.428
4.5 Summary
The results show that the electrical conductivity, metal concentration, viscosity, and
specific gravity increase with the plating bath operation, and the composition of the bath
changes. The polishing quality decreases with the increase of dissolved metal ions and the
change of bath properties. The surface finish can be improved by applying a higher voltage
along with the current density platform while keeping other experimental parameters
unchanged. The improvement rate of surface finish decreases with the increase of
electropolishing time. This indicates that a certain depth of material needs to be removed to
achieve a particular surface finish. After all, the polishing temperature has shown a different
characteristic in this study. Surface roughness increased with the increasing temperature.
The sulfuric acid-based electrolyte has a better electrochemical polishing effect than
other electrolytes at room temperature. The Sulfuric acid-based electrolyte obtained a better
surface roughness from 6.375 μm to 0.066 μm. However, the sulfuric acid-free electrolyte is
safer and environmentally friendly. Phosphoric acid has a significant polishing effect. The
phosphoric-based electrolyte also obtained an excellent polished surface roughness from
6.538μm to 1.112 μm.
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Conclusions and Future Works
5.1 Conclusions
The main contributions of this thesis are summarized as follows:
1) For the 3D printed mechanical surface, the factors from primary to secondary are
polishing time, polishing voltage, electrolyte conductivity, and polishing gap. With the
increase in polishing time, the average surface roughness decreases continuously. With the
increase in the polishing gap, the average surface roughness keeps rising. This is because the
tip effect weakens with the electropolishing gap increase, and the bulge's current density
decreases. Therefore, removing enough materials to achieve the desired surface roughness is
impossible. With the increasing polishing conductivity of the electrolyte, the surface
roughness at first decreased and then increased. When the electrolyte conductivity is larger
on the surface of the anode, and the current density is more extensive, that can remove more
material. However, conductivity is often associated with the concentration of electrolytes.
Therefore, a more neutral salt concentration of electrolyte conductivity is also bigger. A high
concentration solution will increase the probability of concentration polarization of the anode,
which will affect the polishing effect. As the polishing voltage increases, the average surface
roughness of the material decreases, and the polishing voltage is positively correlated with
the polishing current. When the polishing current increases under the condition that the
polishing area remains unchanged, the anode current density also increases to obtain a
smoother surface.
2) Brown products were produced by NaCl-based electrolytes near the anode surface,
which diminished the diffusion pace of broken-down metal particles and new electrolytes to
the anode surface, hence lessening the electrolytic polishing impact. It is guessed that the
earthy-colored items primarily contain broken-down metal oxides and metal chloride. The
expansion of ethanol in NaCl-based electrolytes reduces the effect of brown products on the
impact of electrolytic polishing because the solvency of particles in brown products increases.
In contrast, the bonding ability of earth-coloured products on the anode surface decreases.
3) At room temperature, the sulfuric acid electrolyte has a better electrochemical
polishing effect compared with other electrolytes. The potential range of the limit current
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北京理工大学硕士学位论文
density platform region of H2SO4-based electrolyte is comprehensive, and the surface
roughness Ra is low. In this study, the surface roughness of sulphuric acid electrolyte is 6.375
μm to 0.066 μm. Sulphuric acid-free electrolytes, however, are safer and more
environmentally friendly.
4) Phosphoric acid has a considerable polishing impact. With the abatement of the
temperature of the electrolyte, the dissemination pace of broken-up metal ions and PO43 ions in the electrolyte dials back, the solvency of metal ions in the electrolytic cell diminishes,
and the restricting current density diminishes. With the decline of the dispersion paces of
metal ions and PO43 - ions and the diminishing dissolvability of metal ions, the soundness of
the restricting current stage locale is improved, and the width of the restricting current stage
increments. The surface roughness subsequent to polishing with phosphoric acid electrolyte
is 6.538 to 1.112 μm.
5) Through the orthogonal test design, using six factors and five levels of the orthogonal
table, consider four factors. The four factors are polishing voltage, polishing gap, electrolyte
conductivity, and polishing time. The five levels of polishing voltage are 1.5V, 2V, 3V, 4.5V,
and 6V, respectively. The five levels of the polishing gap are: 5mm, 10mm, 12mm, 15mm,
and 20mm; The five levels of electrolyte conductivity are: 1S/m, 3S/m, 6S/m, 10S/m, 14S/m;
The five levels of polishing time are the 1200s, 2400s, 3600s, 6000s, and 7200s, which are
under the influence of different factors.
However, although the influence of each parameter on the polishing effect is
qualitatively analyzed by simulation in this paper, it is necessary to accurately remove the
material of corresponding thickness in the actual machining process, so it is necessary to
carry out a quantitative analysis of each parameter on the basis of this paper. Experimental
results were analyzed based on simulation results. The experiment result reached almost the
same level as the polishing result. In this paper, the simulation analysis of additive
manufactured metal surface was carried out, but other surface morphology characteristics of
additive manufacturing parts need to be studied.
5.2 The Future Prospect of Research
Future research includes:
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1) Extend the current study to other grades of stainless steel (400 series steel, duplex
steel)
2) The influence of electrolytic polishing on the metal surface using different
concentrations of electrolytic.
3) The capacity of electropolishing to lessen surface roughness by changing interaction
factors. Like electrode gap, electrolyte concentration, polishing temperature, voltage,
time, and so on.
4) Investigate the polishing effect on complex 3D printed metal geometry.
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Achievements
1) Academic papers published while studying for a master's degree: Liu, S., Li, C.,
H. M. Jaber, Yang, Y., Zhang, F., & Jin, H. (2022). Influence of pre-adsorbed tio2
particles on the nucleation and growth mechanism of Ni in deep eutectic solvent electrocodeposition.
The
Journal
of
Physical
Chemistry
C,
126(2),
957–964.
https://doi.org/10.1021/acs.jpcc.1c09961 (SCI, IF 4.12; Q2）
2) Paper accepted for publication: Rui Qu, Xin Jin, H. M. JABER, DongYi Zou,
ZhongXin Li, ChaoJiang Li (2022) Reducing Surface Roughness of Selective Laser
Melting of 316 Stainless Steel Component by Electropolishing. IEEE. Paper ID:
MEIECT-201
3) Awarded as a Distinguished international student of BIT 2021.
4) Selected as a Future leader and youth ambassador 2021. Organised by foreign
affairs of Beijing.
5) Nominated member of Northeast Asia Youth Sustainable Development
Workshop 2021. Organised by China Ching Ling foundation and Korea SK
group.
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