EFFECT OF WELDING PARAMETERS ON RESISTANCE
WELDING OF TI ALLOY AND STAINLESS STEEL
Abstract:
Titanium and its alloys have gained acceptance in a wide array of industries due to their
superior and multifarious properties like high strength to weight ration, low density, good
corrosion resistance and brilliant mechanical properties. However, certain cost restraints and
usage implications have highlighted the need of welding of titanium and its alloys with
stainless steel and other light and cheap materials with comparable properties. In the present
research one such welding technique viz Spot Welding will be used for welding Titanium
Grade 5 alloy and Stainless Steel and with varying parameters to find the optimum process
window. The galvanic coupling effect will also be studied alongwith all the mechanical
properties and microstructure analysis of the resulting welds. The results will be used to draw
sound conclusions regarding the effect of welding parameters on the resistance welding of the
said materials.
1. Introduction:
Titanium and its alloys have been the centre of attention for various industries for decades
now, partly because of their low specific gravity, low density, high strength rivaling almost
that of steel and extremely lightweight nature [1]. Titanium and its alloys are also highly
corrosion resistant in air, water and a wide variety of corrosive environment [2]. However
like everything, this material also tends to have its flaws, the most prominent of which is its
cost. Commercially pure titanium is almost 10 times more costly than stainless steel which
greatly affects its practicality [3]. Due to which there arises a dire need to mate titanium with
other relatively inexpensive materials that have mildly similar properties to that of titanium
and its alloys. The most worthy contender is this case is steel and its different types which
give better prospects of having critical parts made of titanium and the rest of the assembly
made up of steel as in aircrafts, boilers and dissolver vessels [4]. Initially, the mechanical
properties of direct joints of titanium with steels are not found to be appreciably high. It is
however possible to use the interlayer technique in welding methods like resistance welding,
friction stir and energy beam welding [5,6]. The second problem faced during such welding
mechanism is the spontaneous pickup of oxygen by titanium from the atmosphere, which
makes the joint highly susceptible to corrosion attack by the formation of a galvanic couple
[7].
In this proposal, spot welding technique is selected and it is aimed to study the effect of
different welding parameters on the strength of welded joints between Titanium alloy and
stainless steel and it is aimed to look for further ways for the inhibition of corrosion
2. Literature Review:
Titanium and its alloys are among the best metals for industrial applications due to their
excellent corrosion resistance and high strength-to-weight ratio. But joining of titanium to
other metals sometimes is needed due to service requirements. Welding of dissimilar metals
is more challenging because of the differences in physical and chemical properties of the base
metals, such as poor wettability and different thermal expansion [8].
Boyer R.R. dictates that composite components of titanium alloy and steel can fully exert the
advantages of these two materials simultaneously. Partial replacement of steel components by
titanium and its alloy would become an important approach to reduce mass of spacecrafts [9].
It is indispensable to weld titanium alloy and steel. Resistance spot welding is a welding
process wherein coalescence is produced by the heat obtained from resistance to the flow of
electric current through the work parts held together under pressure by electrodes. The size
and shape of individually formed welds are limited primarily by the size and contour of the
electrodes [10]. The particular steps controlled are squeeze time, welding time, hold time and
off time. The process is used extensively for joining low and mild carbon steel sheet metal
components for automobiles, cabinets, furniture and similar products. Stainless steel,
aluminum and copper alloys are also spot welded commercially [11]. There are three stages
in making spot weld as shown in Figure 1, first the electrodes are brought together against
the metal and pressure applied before the current is turned on. Next the current is turned on
momentarily. This is followed by the third, or hold time in which the current is turned off but
the pressure continued. The hold time forges the metal while it is cooling [12].
Figure 1: The 3 Stages of Spot Welding
It is a high speed process, wherein the actual time of welding is a small fraction of second
and it is one of the cleanest and most efficient welding process that has been widely used in
sheet metal fabrication. The high speed of process, the case of operation and its adaptability
for automation in the production of sheet metal assemblies are its major advantages.
Limitations of RSW are equipment cost and power requirements, difficulty of disassembly
for maintenance or repair of RSW joints, and the nature of the design needed for the process
[13]. However since accurate method for selection of welding variables i.e. welding current
welding time and electrode force, thickness of sheet, electrode type, electrode tip diameter,
gap in the electrodes, shape of electrode tip, electrode material etc. are lacking, we have to
adopt a trial and error approach whenever we come across a new-type of metal. Field
experience has shown that in resistance spot welding the basic welding variables are
practically linear functions of metal thickness. Austenitic Stainless Steel 316 is an extremely
important commercial alloy due to its excellent corrosion resistance, high strength, good
ductility and toughness. D.S. Sahota et al state that an increase in weld current, weld time and
electrode force results in an increase in weld nugget diameter and width. An increase in weld
current, weld time and electrode force results in an increase in electrode indentation [14].
Zhou Y. et al state that due to the established interesting features of brazing, which is its
ability to join dissimilar metals, and spot welding techniques, which are its ability for
automation and forming a spot weld, there is an effort to combine conventional spot welding
and brazing principle methods whereby metal bonding is achieved using resistive heating of
the filler metal [15].
Miyazawa Y et al came with a new approach for the spot welding, which refers to brazing
using a spot welding machine, will require a better understanding of the issues associated
with resistance spot welding of dissimilar metals with the use of a filler metal. This is
because resistance weldability of sheet metals is determined by resistivity of the metal
components between the copper electrodes, as well as other physical properties such as
melting point, latent heat of fusion, and specific heat [16].
The welding of titanium, titanium alloys and austenitic stainless steel involves forming
bimetallic welded joints using special technological measures greatly reducing the intensity
of the interaction of the welded materials. These measures include, for example, the
application of the superplasticity effect in the production of bimetallic tools, introduction of
interlayers between the welded materials (Patent 2215627, Russian Federation) or welding
technology [17].
The titanium–12Cr18Ni10Ti-steel welded joints were produced by S.F. Gnyusova et al using
an interlayer in the form of a copper sheet with a thickness of 1mm. Welding of the titanium
alloy to the austenitic stainless steels with the copper interlayer is accompanied by extensive
dissolution of the steel in the liquid metal copper pool with the formation of a matrix
composite material in which the hardening particles are presented by plate-shaped
precipitates of the a-phase (martensite). An intermediate layer based on the intermetallic
phases with the thickness of 100– 150mm with higher microhardness (4500MPa) forms on
the side of the titanium alloy. The presence of this hard interlayer results in brittle fracture of
the specimens with the welded joint in quasi-static tensile loading [18].
3) Copper electrodes are often used in RSW; however, electrode degradation during welding
is inevitable. E. Gauthier et al. pointed out that in the case of Zn-coated steel, continuous
alloying occurs between them Molten zinc and the electrode surface. This accelerates
electrode erosion [19]. But elemental diffusion from the electrode tip to the welding zone was
ignored in every previous research. According to Feng Chen et al, elemental diffusion causes
a significant change in the chemical composition of the welding zone during MRSW. This is
referred to as a size effect.
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
Increasing the ramping time decreases the peak temperature and thus reduces copper
diffusion into the welding zone as shown by Figure 2. The holding time affect the
amount of copper that sticks to and solidifies on the electrode tip surface.
The effects of welding time and current on copper diffusion are similar. Increasing
either parameter increases the amount of copper that diffuses into the WN. However,
the copper concentration decreases when the current is above a critical value because
of expulsion.
Figure 2: EDS Analysis of HAZ after Welding
Grachino J.W. et al concluded that increasing welding time and electrode force increased
tensile-shearing strength of the resistance spot welded specimens. The welds carried out
under argon gas atmosphere yielded better tensile-shearing strength than those affected in
open air [20]. The weld nugget gave the highest hardness values, and this was followed by
HAZ and weld metal. Microstructural examination showed that deformation during the
welding process was in the form of twinning rather than shearing in the welding zone. It was
also observed that high pressure and welding time increased the twinning [12].
Yimin Tu used 1.0 mm thick commercially pure titanium (Ti) sheet and stainless steel
SUS304 (SUS304) sheets as the base materials; 1.0 mm thick aluminum alloy A5052
(A5052) sheet, used as the insert [21].
Sun D.Q. et al varied the current value and weld time to see the weld growth while the
electrode tips and force remained unchanged. The investigation was carried out to
characterize the spot weld growth for 2 mm base metals by varying the welding current and
weld time, in other word
• Hardness of welded zones is greater than the hardness of the un-welded zone and also the
heat affected zones.
• The welding current and weld time increments have resulted diameters increment at the
welded zones.
• Tensile forces have proportional relationship with current and weld time increments until
the expulsion limit occurs.
• The common three failure modes as shown in Figure 3 were seen detected as interfacial
fracture (IF); the medium weld produces tear form one side (PF); and a good weld produces
button pullout or tear from both sides (TF).
• Chromium to nickel ratio has been increased due to welding process and therefore the
ferritic properties became rich at the welded zones [22].
Figure 3: Modes of Fracture
2.1. Problem Statement:
Although there have been various researches related to the welding of Titanium and its alloys
with dissimilar metals, but very few have been concerning titanium grade 5 alloy and
stainless steel and almost none of them have established an optimum range of welding
parameters and studied their effects on the integrity of the welds. Almost every study made in
the past has focused solely on successful welding of titanium to dissimilar counterparts and
the testing of mechanical properties of the welds. This approach has left a big gap where
there has been practically no corrosion testing of the welds in different temperature and their
effect on the integrity of the welds. In the proposed study, several different ranges of welding
parameters will be tested and the high possibility of corrosion will also be tested and their
resulting effect on the welds will be measured.
2.2. Objectives:
The major objective of the study is to study the effect of welding parameters on the resistance
welding of titanium alloy and stainless steel by performing the welding under different
conditions and obtaining high degree of strength and corrosion resistance from the joint.
Following will be the primary objectives:
i) Establishing a standard process window for the welding of Grade 5 Titanium alloy –
Stainless Steel – several parameters of electrode tip diameter, current, weld cycle and
electrode force will be tested for optimal solution
ii) Studying the effect of those parameters on the integrity of the welds –
microsturucture, mechanical properties as well as weld life will be the key aspects under
observation
iii) Corrosion testing of the weld and working out methods of preventive process in case
of negative results – surface treatment as well pre weld heat treatment will be utilized to
prevent the onset of a galvanic corrosion.
2.3. Significance:
The importance of this research can be assessed by the fact that in different critical
applications there arises a need to join titanium with materials of comparable properties but
having a lower price. Moreover conventional welding techniques do not produce a sound
weld and energy beam welding proves to be too complex and not cost efficient and can also
not be used for plates above a certain thickness. So resistance spot welding has been selected.
Moreover the study of effect of galvanic coupling will tell us how the formation of galvanic
couple affect the corrosion resistance of the two materials when it is exposed to different
environments of varying corrosion resistance.
3. Methodology:
The materials employed in the proposed research will be Ti-6Al-4V Grade 5 alloy and
Stainless Steel. Since the Spot Welding technique will be used from all the resistance welding
methods, the materials will be cut in sheet thickness between 1 – 3 mm. A third sheet of
Copper of maximum thickness 1 mm will be used as an interlayer. The sheets will be stored
in moisture free environment before welding. Finite Element Analysis will be used to predict
the formation of nugget beforehand. Six sample groups of material (Ti6Al4V and Stainless
Steel) will be made where for very single group, a single parameter will be changed in the
welding process and results will be compared to reach an optimum value of that parameter.
Similarly for the next group, another parameter will be changed and results of the weld will
be checked. A spot welder with a Cu-Cr alloy spherical electrode tip will be used. To study
this parameter, different electrode tip diameters will be used i.e. 6, 8, 10 mm. The whole
welding process will be carried out in nitrogen atmosphere as it prevents the brittle
detachment of the nugget from titanium sheet. A different range of the following welding
parameters will be tested in order to find the best mechanical properties in the weld:
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
Welding Current
Electrode Force
% Power
Presence / Absence of Nitrogen or Argon
Ramp Time
Welding Time
Since welding current is the most decisive parameter in weld strength, it will be tested in the
range of 4 to 9 kA. Electrode force varies greatly in cases when similar plates are welded and
when interlayer is used so in order to find the optimum value, a wide range of electrode force
i.e. 2, 4, 6 kN will be tested. Welding time is represented as cycles (1 cycle = 1/60 of a
second). The weld and the hold times will be used as per literature to find the most optimum
value of weld cycle for the given thickness. Ramp time of 1 ms will be used initially.
Furthermore, joints will be made in three different environments i.e. air, N2 and Argon to see
the effect of welding atmosphere on the nugget formation. For structural analysis SEM of the
welds will be carried out. To check the mechanical strength and hardness of the welds, tensile
shear test of the welds will carried out. Radiography of the nugget will also be carried out to
establish the level of porosity of the nugget. Corrosion behavior of the joint will also be
checked. Potentiodynamic polarization test will be carried out in 3.5 % NaCl solution with
SCE as a reference electrode. The galvanic coupling effect will be checked at both high and
low temperatures to replicate different working environments. The corrosion effect will be
studied by welding another pair of plates in which the size of titanium plate will be larger
than that of stainless steel. In that pair of plates, the large sized titanium alloy plate will be
anodized beforehand in ethylene glycol and ammonium biflouride solution to further increase
the passivity of Ti6Al4V. Then the same welding process and characterization of the joint
will follow to study the effect of the corrosion inhibition. In the end, all the results will be
worked upon to find the best welds with perfect mechanical properties, microstructure and
corrosion resistance and the combination of parameters for that weld will be considered
optimal.
4. Time Table
The time table for the research proposed above has been given in the form of a Gantt Chart.
All the research activities have been given in days.
The duration of certain activities might be subject to minor delays depending upon working
conditions and other unforeseen factors but the total time of completion of the research will
remain unchanged.
24-Jun-19 07-Sep-19 21-Nov-19 04-Feb-20 19-Apr-20 03-Jul-20 16-Sep-20 30-Nov-20 13-Feb-21 29-Apr-21
Research Proposal
Sample Cutting & Group Setting
Weld by Varying Tip Dia (G1)
Weld by Varying Current (G3)
Weld by Varying Time Cycles (G5)
Tensile Shear Test of G2
Tensile Shear Test of G4
SEM Analysis of G1
SEM Analysis of G3
SEM Analysis of G5
Radiography of G2
Radiography of G4
Corrosion Testing of G1
Corrosion Testing of G3
Corrosion Testing of G5
Corrosion Tetsing of G6
Thesis Writing
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