Wasit Journal for Science & Medicine
2014 7(4): (49-61)
The effect of friction stir welding and tungsten inert gas welding
process on mechanical properties of aluminum alloy 6061- t6 joints
Bushra Ebraheem Malik
Institute of Technology /Baghdad
‫ ) على الخواص الميكانيكية لوصالت من سبيكة‬TIG ( ‫تاثير اللحام االحتكاكي واللحام ب‬
6061-T6 – ‫االلمنيوم‬
‫تشري اتراهُى يهك‬
‫ تغذاد‬- ‫يعهذ انركُىنىجُا‬
‫المستخلص‬
‫ ذى نحايهى تطرَقرٍ نحاو يخرهفح‬6061 T6 ‫ذضًٍ انثحث يقارَح انخىاص انًُكاَثكثح نىطالخ نحاو ذُاكثُح يٍ سثُكح انًُُىو‬
‫يهى هًا نحاو انقىش انكهرتائٍ تاسرخذاو قطة انرُكسرٍ تىجىد غاز خايم وسهك نحاو‬10005005 ‫نهحظىل عهً وطهرٍُ تاتعاد‬
800ٍ‫ وطرَقح نحاو انخهظ االحركاكٍ تاسرخذاو ياكُح ذفرَس يثريجح راخ قهى نحاو دائرٌ َذورتسرعح ه‬ER4043 (AlSi5)
. ‫ دقُقح‬/‫يهى‬20 ‫دورج تانذقُقح و سرعح نحاو‬
‫ ذى ذحضُر عُُاخ‬X-radiography ‫تعذ عًهُح انهحاو وانراكذ يٍ خهىانىطالخ يٍ انعُىب يٍ خالل فحظها تىاسطح جهاز‬
ٍ‫ ثى اذثعرها عًهُاخ ذحضُر ي‬ASTM17500 ‫اخرثار انشذ نجًُع انعُُاخ انًهحىيح وغُر انًهحىيح وفق انًىاطفح انقُاسُح‬
‫ذُعُى وطقم الجراء فحض انثُُح انًجهرَح تاسرخذاو انًجهر انضىئٍ رو كايُرا نهرعرف عهً انثُُح انًجهرَح نًُاطق انهحاو‬
. ‫واجرٌ اخرثار طالدج عُاَُح ويكروَح نثُاٌ ذأثُر انهحاو عهً انخىاص انًُكاَُكُح‬
‫أظهرخ انُرائج ذذَُا فٍ انخىاص انًُكاَُكُح نهحاو انقىش انكهرتائٍ ونحاو انخهظ االحركاكٍ يقارَح تانًعذٌ االساش تًُُا‬
‫اعطد َرائج نحاو انخهظ االحركاكٍ قُى افضم نهخىاص انًُكاَُكُح عٍ نحاو انقىش انكهرتائٍ تسثة انرغُراخ فٍ انثُُح‬
. ‫انًجهرَح خالل عًهُح انهحاو‬
Abstract
This work involves a comparison between mechanical properties of welded joints using two
different welding processes : tungsten inert gas (TIG) process and an solid state welding process
known as friction stir welding (FSW) process of 6061 T6 aluminum alloys.
Arc welding processes by tungsten inert gas (TIG) have been carried out on Rolled sheet of 5
mm thickness to obtain many welding joints with dimension of (100 *50* 5)mm using ER4030
(Al Si5 ) as a filler metal and argon as shielding gas. While friction stir welding process carried
out using CNC milling machine with a tool of rotational speed (800 rpm) and welding speed of
(20mm/min) to obtain the same butt joint dimension. The welded pieces were tested by X-ray
radiography and faulty pieces were excluded. Tensile test was implemented for specimens which
prepared in the dimensions according to ASTM 17500 by using testing machine smart series
with preload value 100 N.
All specimens were subjected to Vickers hardness test and microstructure were examination
carried out to show the effect of weld method on the joint microstructure.
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2014 7(4): (49-61)
Results showed a general decay of mechanical properties of TIG and FSW joints comparing with
base alloy while FSW weld joint gives better mechanical properties than TIG weld joint that is
due to the microstructure change during the welding process.
Key words: Mechanical properties, TIG weld, friction stir welding, aluminium alloys 6061-T6,
hardness test
Introduction
The welding of aluminum and its alloys has always represented great challenge for designers and
technologists. As a matter of fact, lots of difficulties are associated to this kind of joint process,
mainly related to the presence of a tenacious oxide layer, high thermal conductivity, high
coefficient of thermal expansion, solidification shrinkage and, above all, high solubility of
hydrogen, and other gases, in molten state. Further problems can arise when attention is focused
on heat-treatable alloys, since heat, provided by welding process, is responsible of the decay of
mechanical properties, due to phase transformations and softening induced in alloy. As a
consequence of all above-mentioned problems, also into a leader industry such as the aeronautic
one, which makes wide usage of aluminum alloys, and especially of the alloy6061-T6 are mostly
used as extruded products, as well as for construction and automotive application which investigated
in present paper (1).
Gas metal arc welding is a process that melts and joins metals by heating them with an arc
established between a continuously fed filler wire electrode and the metals. Shielding of the arc
and the molten weld pool is often obtained using inert gases such as argon and helium. This is
the most widely used arc welding process for aluminum alloy. With TIG welding using
alternating current (AC) polarity and high heat generation end is continuously changing. An
electric arc is formed between inconsumable tungsten electrode and the work piece. The arc
provides the thermal energy to melt the work piece as well as the filler material. For alloys due to
their elevated thermal conductivity, the weld penetration remains very shallow (2, 3).
These problems can be eliminated by Friction stir welding (FSW) process. Frictions stir welding
(FSW) is a solid state joining technique using a tool with a probe attached to its tip rotated while
being pushed against the butt sections of the pieces of metal to be welded. The frictional heat
generated by this process softens the metal to produce a plastic flow that effectively stirs the
metal from the sections on both sides and melting them together to create a weld.
A detailed observation of the material microstructure in the joint section distinguishes different
areas: the parent material in which no material deformation has occurred, the heat affected zone
(HAZ) in which material has undergone a thermal cycle which has modified the microstructure
and/or the mechanical properties, a thermo-mechanically affected zone (TMAZ) in which the
material has been plastically deformed by the tool, and the heat flux has also exerted some
influence on the material. At the core of the weld, the nugget appears to be characterized by fine,
equiaxed recrystallized grains characterized by a nominal dimension of a few µm (4).
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FSW is depend on the welding parameters such as pin rotation speed, traverse speed and stirrer
geometry. In order to increase the welding efficiency mechanical properties of joints must be
maximized and the defects must be minimizing (5).
Many studies concerned this subject as Ratnesh K. Shuklabeen (6) who studied microstructure,
micro hardness distribution, tensile properties and fracture surface morphology of weld butt
joints of 6061 T6 aluminum alloy. Two different welding processes have been considered: a
conventional tungsten inert gas (TIG) process and solid state welding process known as friction
stir welding (FSW) process. Micro hardness distribution results showed a general decay of
mechanical properties of TIG joints, mainly due to high temperature experienced by the material.
Instead, in FSW joint, lower temperatures are involved in the process due to severe plastic
deformation induced by the tool motion and lower decay of mechanical properties. In the nugget
zone a slight recovery of hardness is observed due to recrystallization of very fine grain
structure.
A. Squillace (5) studied microstructure and corrosion resistance of weld butt joints of AA 2024T3 for two different welding processes, a conventional tungsten inert gas (TIG) process and
friction stir welding (FSW), polarization curve and EIS tests allow to point out that in both kind
of joints parent alloy show evident pitting tendency, while weld bead and HAZ show a passive
behavior, even though, in case of FSW joint, such differences are less evident.
M. Ericsson (7) studied the influence of welding lower speed on fatigue strength of friction stir
(FS) welds and conventional arc-welding methods: MIG-pulse and TIG and compares the fatigue
results of FS with results for conventional arc-welding methods: MIG-pulse and TIG, the results
showed that welding speed has no major influence on the mechanical and fatigue properties of
the FS welds. At a significantly lower welding speed, however, the fatigue performance was
improved possibly due to the increased amount of heat supplied to the weld per unit length. And
TIG welds had better fatigue performance than the MIG pulse welds.
S. Rajakumar (3) attempted to develop empirical relationships to predict grain size and tensile
strength of friction stir welded AA 6061-T6 aluminum alloy joints. The empirical relationships
are developed by response surface methodology (RSM) incorporating FSW tool and process
parameters. A linear regression relationship was also established between grain size and tensile
strength of the weld nugget of FSW joints.
Aendraa Azhar Abdul Aziz (8) studied the effect of filler alloy types on mechanical properties
and microstructure of welded AA6061 aluminum alloy using gas metal arc welding Different
filler metal (ER5356, ER4043) gave different microstructure and mechanical properties of
welded AA6061, results showed major alloying element in both filler such as Si and Mg play
important role in determining the microstructure and mechanical properties, where the yield
strength of base metal were 330 MPa and while the yield strength of ER5356 joints and ER4043
joints were 200 MPa and 235 MPa, respectively.
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Experimental work
Table (1) showed the chemical analysis of aluminum alloy (AA) 6061-T6 by using ARL
Spectrometer. , where 5 mm thick plates were machined to the required dimensions (50*50) mm,
to weld the TIG joint Fig.(1) and FSW joint Fig.(2).
Table (1): Chemical analysis of the used metal 6061- T6 (9)
Elements w%
Si
Fe
Cu
Mn
Mg
Cr
Zn
Al
Measured
value
Slandered
value
0.6
0.4
0.3
0.12
1.0
0.2
0.18
Rem.
0.15-0.4
Max 0.15
0.8-1.2
0.04-0.35
Max 0.25
Rem.
0.4-0.8 Max 0.7
Figure: (1.a) :Dimensions of prepared TIG weld joint
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Figure :( 1.b) Photograph of TIG weld joint
Figure :( 1.c) Photograph of FSW weld joint
5
Figure (2): FSW weld operation (10)
Welding process
Butt joints for TIG welding were prepared using ER4030 (AlSi5) as filler material where its
chemical composition is shown in Table (2) and argon as shielding gas, the other parameters
were welding current190amperes, voltage 18 volts, filler rod diameter 3 mm, welding speed 120
mm/min, gas flow 20 lit/min.
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The plates with same dimensions were friction stir welded by putting two plates together with a
rotating tool assembly at 800 rpm rotational speed and welding speed of 50 mm/min. CNC
milling machine with motor of 11 KW was used for performing the weld. The material for the
tool was tool steel which was hardened to57 HRC. The used tool had shoulder diameter of 14
mm, top pin diameter of 5.5 mm, bottom pin diameter of 3 mm and pin height 3.85 mm. The pin
was left hand threaded and tool was rotated in counter clock wise direction.
Table (2): Chemical composition of the filler metal (Filer wire ER 4043) Al Si5 (11)
Elements
Wt%
Si
Actual
value
5.0
Nominal
value
4.5-6
Fe
Cu
Mn
Mg
Cr
Zn
Sn
Al
0.4
0.1
0.08
0.06
0.25
0.15
0.15
93.44
<0.3
< 0.15
< 0.2
-
< 0.1
-
Rem.
<0.6
Categorization of weld joint
After completing the specimen, they were categorized to groups as shown in Table (3)
Table (3): Ccategorization of specimens
Specimen symbol
state
A
Native metal
B
TIG weld joint
C
FSW weld joint
Microstructure test
Micro structural changes from weld zone to the unaffected base material were examined with
optical microscope. Specimens were prepared for microstructure test including wet grinding
operation using emery paper of SiC with different grits of (220,320,500, and 1000). Polishing
process was done by using diamond paste of size (3μm) with special polishing cloth. They were
cleaned with water and alcohol then dried with hot air dryer.
Etching for the structure by using Keller’s reagent consisted of 95 ml distill water, 2.5 ml
HNO3, 1.5 ml HCl and 1 ml HF then washed after that with distill water then dried by hair. The
friction stir welded joint samples were examined by Nikon ME-600 computerized optical
microscope provided with a NIKON camera, DXM-1200F as shown in Fig. (3)
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Mg2Si
Specimen (A) Native metal
B.M
W.Z
HAZ
Specimen (B) TIG Weld
Nugget
Zone
TMAZ
HAZ
B.M
Specimen (C) FSW Weld
Figure (3): Microstructure of specimens at 400X
Hardness test
1-Macro hardness
Specimens were cut from sheet metal and weld joint in dimension of length (20*20 *5mm) to
implement macro Hardness test by using Vickers hardness method
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The result are shown in table (4) by using the below equation
-------------------------------(1)
Where
(p): is the applied load Kg)
(d) Penetration diameter in mm
2- Micro hardness
The Vickers hardness profile of the weld zone was measured on a cross section perpendicular to
the welding direction using micro hardness tester with 4.5N for 10sec and Fig (3) shows the
results.
140
Microhaness (HV)
120
100
80
60
TIG
40
FSW
20
0
-20
-10
0
10
20
Distance from weld center (mm)
Figure (4): The Vickers hardness profiles of a transverse cross section welds zone
Tensile test
Tensile test was implemented for all specimens, Fig.(5) showed the tensile test specimen
dimensions according to ASTM 17500 by using Testing machine smart series with preload value
(N) 100 and cross head speed (mm/min) or rate. 20. Extension the obtained results are shown in
table (4) and the relationship between stress and elongation are shown in Fig. (6).
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Figure (5): Tensile specimenc dimensions according to ASTM 17500 for weld joint
Table (4) : Mechanical prorerties for all specimens in table 3
Specimen
A
B
C
Ulstermen tensile
MPs
350.45
139.6
181
Yield strength
MPs
295.3
35
40
speceimen A
57
Elongation%
10
8
5.5
Vickers Hardness
Kg/mm2
100
57
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Specimen B
Specimen C
Figure (6): Relationship between stress and extension for all specimens
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Results and Discussion
1- Microstructure test
Fig (3) showed the micro structure weld region of AA 6061T6 .The TIG method which is
represented by symbol (B) give the Lower amount of strengthening precipitates compared to the
base metal specimen symbol (A). The higher strength of the base material is mainly attributed
due to presence of alloying elements such as silicon and magnesium. These two elements
combine and undergo precipitation reaction and form strengthening precipitates Mg2Si as shown
by darken particles in Fig.( 3) Therefore, the strengthening of Mg2Si precipitates is weak in TIG.
The base metal microstructure contains coarse and elongated grains with uniformly distributed
strengthening precipitates (Mg2Si.).
The fusion zone of TIG welded joints contain dendritic structure and this may be due to fasting
welding heating of the base metal and fast cooling of molten metal.
While a comparison between the base metal and stir zone indicating by FSW we can see a fine,
equated grains due to the temperature difference between the tool shoulder side and base size and
the tool centerline and the edge of the weld nugget which causes the grain size variations, this
can be attributed to the mechanical forces operative during welding which causes both
refinement and re –alignment of the matrix grains and should be beneficial with respect to
various mechanical properties (12).
2- Hardness result
Table (4) showed the mechanical properties for all specimens in Table (3). The macro hardness
of the base metal was 100 Hv. and of TIG joint in the weld metal region was 57 Hv. The macro
hardness of FSW joint in the weld region is 70 Hv .This showed that the hardness is reduced in
TIG joint due to higher heat input and low cool rate and the use of lower hardness AlSi5 filler
metal.
The hardness is lower than the base metal due to dissolution of strengthening precipitates during
the weld thermal cycle. However, FSW showed higher macro hardness compared to TIG joint
due to shear stresses induced by tool motion which lead to generation of very fine grain
structure, which allows a partial recovery of hardness
2- Hardness Profiles.
Fig. (4) Indicated the cross-sectional hardness profile from retrieving side metal through center
of the weld to advance metal at a constant rotation speed and welding speed for welded region
The stir zone shows a great change in the hardness distribution from the weld center, nugget,
TMAZ to HAZ and base metal.
Hardness of the stir zone was lower than base metal. The average hardness values in the TMAZ
are slightly lower than in the weld nugget while the HAZ region gives the lowest value of
hardness, this is because HAZ zone affected only by conducting heat and slow cooling which
could cause a coarsening and /or dissolution of precipitates elements However, FS W showed
higher micro hardness compared to TIG joint due to shear stresses induced by tool motion which
lead to generation of very fine grain structure, which allowed a partial recovery of hardness. In
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case of TIG welding, very high arc temperature increases the peak temperature of the molten
weld pool causing a slow cooling rate. This slow cooling rate, in turn, caused relatively wider
dendritic spacing in the fusion zone.
These microstructures generally offered lower resistance to indentation and this may be one of
the reasons for lower hardness and inferior tensile properties compared to FSW joints.
3-Tensile properties
The ultimate tensile strength of base and welded metals are shown in Table 4, Fig. (6). The
tensile properties and elongation of both TIG and FSW joint is far lower than the native metal
These results indicated that softening effect has occurred in aluminum alloy. The tensile
properties and fracture locations of the joints are to a large extent depended on the hardness
distributions of the joint. Hardness degradation region composed of weld zone and HAZ has
occurred in the joints, thus the tensile properties of the joints are lower than the parent material.
The weld zone in specimen (C) is composed of fine equiaxed grains and specimen (B) is
composed of coarse bent recovered mainly due to high temperature experienced by the material.
In FSW joint, lower temperatures are involved in the process due to severe plastic deformation
induced by the tool motion and lower decay of mechanical properties, fine grain structure and
uniform distribution of the alloy precipitates metals throughout the aluminum matrix provides
higher strength and hardness to FSW joint, these results were consistent of the observations
reported by (7) and (13).
Conclousions
1- Grain refinement in weld metal has been achieved due to frictional heating and plastic flow.
2-In FSW operation, hardness of weld zone is low comparing with base alloy because of the
plastic deformation and cooling rate
3-FSW joint exhibited higher strength values (51%of base material) compared to TIG joint
(44.5%).
4-The formation of fine, equiaxed grains and uniformly distributed very fine strengthening
precipitates in the weld region is the reason for superior tensile properties of FSW joints
compared to TIG joints.
Reference
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