http://www.iaeme.com/IJMET/index.asp 564 [email protected]
International Journal of Mechanical Engineering and Technology (IJMET) Volume 8, Issue 6, June 2017, pp. 564–574, Article ID: IJMET_08_06_059
Available online at http://www.iaeme.com/IJMET/issues.asp?JType=IJMET&VType=8&IType=6
ISSN Print: 0976-6340 and ISSN Online: 0976-6359
© IAEME Publication Scopus Indexed
EFFECT OF WELDING PARAMETERS ON
MECHANICAL PROPERTIES OF FRICTION
STIR WELDED JOINTS OF AA6082 AND AA6061
ALUMINIUM ALLOYS
P. Radha Krishna Prasad and P. Ravinder Reddy
Department of Mechanical Engineering,
CBIT - Chaitanya Bharathi Institute of Technology, Hyderabad, India
K. Eshwar Prasad
JNTU - Jawaharlal NehruL Technological University Hyderabad, India
ABSTRACT
The need of the hour in aerospace and military structures is the joints between
dissimilar aluminium alloys. Friction stir welding (FSW) is a novel solid state joining
technology especially developed for joining low melting temperature alloys like
aluminium and magnesium. In this present investigation, AA6082 Aluminium alloy is
friction stir welded with AA6061 Aluminium alloy for different combination of tool
rotation speed, tool feed and tool tilt angle. A cylindrical tool with a square frustum
probe is employed for friction stir welding. For each of the three factors rotational
speed, tool feed and tool tilt angle two levels are selected. Eight experiments are
designed on full factorial concept and FSW carried out for 8 runs. The tensile
properties and microhardness were measured by UTM and Vickers hardness tester
respectively. Using analysis of variance (ANOVA) optimum parameters are obtained
and presented.
Key words: Friction stir welding, Dissimilar alloys, AA6082, AA6061, ANOVA.
Cite this Article: P. Radha Krishna Prasad, P. Ravinder Reddy and K. Eshwar Prasad.
Effect of Welding Parameters on Mechanical Properties of Friction Stir Welded Joints
of AA6082 and AA6061 Aluminium Alloys. International Journal of Mechanical
Engineering and Technology, 8(6), 2017, pp. 564–574.
http://www.iaeme.com/IJMET/issues.asp?JType=IJMET&VType=8&IType=6
1. INTRODUCTION
Recent trend in the automotive world has been transition from conventional materials to
light materials like Aluminium[1]. Due to the demands for a lower environmental impact
through improved fuel efficiency, weight reduction and load capacity Aluminium is being
used more widely in the auto industry, aerospace and marine structures [2] because of its light
weight. Modern structural concepts demand reductions in both the weight as well as the cost
of production. Fabrication of Aluminium alloys by riveting results in stress concentration and
P. Radha Krishna Prasad, P. Ravinder Reddy and K. Eshwar Prasad
http://www.iaeme.com/IJMET/index.asp 565 [email protected]
increase of the weight of the structure. Therefore, welding processes have proven more
attractive and there is an urgent need to study their potential [3].
Conventional fusion welding methods for joining similar Aluminium materials and
dissimilar Aluminium materials producing joints with defects like porosity, hot cracking and
distortion. Friction stir welding has been found as a best alternative to produce the joints free
from the defects which are generally induced by fusion welding processes since FSW does
not involve melting and recasting.
Friction stir welding is a solid state joining process considered to be the significant
development for the past two decades which was invented and patented by The Welding
Institute (TWI), United Kingdom in the year 1991[4]. FSW involves in employing non-
consumable rotating tool with a shoulder terminating in a profiled pin. The tool rotates and
traverses along the butting surfaces of two plates are shown in Fig. 1. The heat for welding is
produced due to the relative motion between the tool and the two interfaces being joined. The
localized heating softens the material in the vicinity of the pin and combination of tool
rotation and translation results in movement of material from the front of the pin to the rear of
the pin where it is forged into a joint [5].
Figure 1 FSW process
Selection of process parameters plays a pivotal role in producing the sound joints. The
influence of process parameters like tool rotational speed, tool feed and tool design on FSW
have been studied previously [6]. The data reported on optimization of process parameters
tool rotational speed, tool feed and tilt angle using square frustum probe tool on FSW of
dissimilar Aluminium alloys is scanty. Thus, in this investigation parametric optimization on
FSW of AA6082 and AA6061 dissimilar aluminium alloys using a tool of square frustum pin
has been studied.
1.1. Taguchi
Taguchi method is a disciplined technique for implementing improvements in products,
processes, materials, equipment and facilities. These improvements are aimed at improving
the desired characteristics and simultaneously reducing the defects by studying the key
variables controlling the process and optimizing the process or design to achieve the best
results.
1.2. Analysis Of Variance (ANOVA)
ANOVA was developed by Sir Ronald Fisher in the year 1930 as a way to interpret the results
from agricultural experiments. It is an objective decision-making tool for detecting any
differences in average performance of the groups of items tested. The decisions are arrived
Effect of Welding Parameters on Mechanical Properties of Friction Stir Welded Joints of AA6082
and AA6061 Aluminium Alloys
http://www.iaeme.com/IJMET/index.asp 566 [email protected]
taking variation into account rather than using pure judgement. The purpose of ANOVA is to
determine the significant factor statistically. This gives a clear picture as to what extent the
process parameter influences the desired response and the level of significance of the factor
considered. ANOVA is a mathematical technique which breaks total variation down into
accountable sources. Some of the components in ANOVA are discussed.
1.2.1. Sum of Squares
The magnitude of each error value can be squared to provide a measurement of total variation
present. This is known as “Sum of Squares”. The basic ANOVA is that the total sum of
squares is equal to the sum of sum of squares due to known components as shown in Eq. 1.
��� = ��� + ��� (1)
Where,
SST - Total sum of squares.
SSm - sum of squares due to mean.
SSe - sum of squares due to error.
1.2.2. Variance Due To Error
Error variance, usually termed just variance, is equal to the sum of squares of error divided by
the degree of freedom of error. Error variance is a measure of variation due to all the
uncontrolled parameters, including measurement of error involved in a particular experiment.
1.2.3. Test of Comparison
The F-test is simply a ratio of sample variances as shown in Eq. 2.
� = �� � / ���
� (2)
When this ratio becomes large enough, the two sample variances are accepted as being
unequal at some confidence level. To determine whether an F ratio of two sample Variances
are statistically large enough, three pieces of information are considered. These are the
confidence level, degree of freedom associated with the sample variance in the numerator and
degree of freedom associated with sample variance in the denominator. F-test values are
found from F-test table.
1.2.4. Percent Contribution
The portion of the total variance observed in an experiment attributed to each significant
factor and/or interaction is reflected in the percent contribution. The percent contribution is a
function of sum of squares of significant factor. The percent contribution indicates the relative
power of factor and/or interactions to reduce variation.
2. EXPERIMENTAL PROCEDURE
2.1. Welding
Rolled plates of AA6061-T6 and AA6082-T6 Aluminum alloys of 4mm thickness were used
in this investigation. Composition and mechanical properties of alloys AA6082-T6 and
AA6061-T6 are presented in tables 1 and table 2 respectively. Plates of equal size
160mm×80mm×4mm were cut and cleaned. The welding was carried out in butt joint
configuration of 160mm×160mm×4mm by placing AA6082-T6 on advancing side and
AA6061-T6 on retreating side. A milling machine modified for friction stir welding was used
to carry out the welding. A tool made of H13 tool steel having square frustum profile pin
shown in Fig. 2 was used for welding. Experiment set up for welding was illustrated in Fig.3.
P. Radha Krishna Prasad, P. Ravinder Reddy and K. Eshwar Prasad
http://www.iaeme.com/IJMET/index.asp 567 [email protected]
In this investigation three factors and each of them at two levels are taken and illustrated in
Table 3. L8 standard orthogonal array matrix presented in Table 4 has been selected to
conduct the welding trails. The orthogonal array after assigning the values of factors
presented in Table 5. Welding was performed according to the matrix given in Table5. For
each run three specimens were welded.
Table 1 Composition of Parent Materials
S. No Material Si Fe Cu Mn Mg Cr Zn Ti Al
1 AA6061 0.6 0.23 0.25 0.12 1.04 0.12 0.017 0.007 REM
2 AA6082 1.07 0.3 0.046 0.58 0.96 0.021 0.014 0.006 REM
Table 2 Mechanical properties of Parent Materials
S. No Parameter AA6061-T6 AA6082-T6
1 Tensile strength (MPa) 292 332
2 0.2% Proof strength(MPa) 202 314
3 Elongation (percent) 24 13.5
4 Hardness-HV5 102 113
Figure 2 FSW tool
Figure 3 FSW set-up
Effect of Welding Parameters on Mechanical Properties of Friction Stir Welded Joints of AA6082
and AA6061 Aluminium Alloys
http://www.iaeme.com/IJMET/index.asp 568 [email protected]
Table 3 Factors and their levels
Table 4 L8 Orthogonal Array
RUN ROTATION SPEED
A
WELD SPEED
B
TOOL TILT ANGLE
C
1 1 1 1
2 2 1 1
3 1 2 1
4 1 1 2
5 2 2 1
6 2 1 2
7 1 2 2
8 2 2 2
Table 5 L8 Orthogonal array after assignment of factors
S.No ROTATION SPEED A
(rpm)
WELD SPEED B
�������
TOOL TILT ANGLE C
( 0 )
1 900 80 1
2 1400 80 1
3 900 100 1
4 900 80 2
5 1400 100 1
6 1400 80 2
7 900 100 2
8 1400 100 2
2.2. Tensile Test
Tensile specimens were prepared as per ASTM E 8 standard shown in Fig4. The tensile tests
conducted using 25KN, Universal tensile testing machine. A sample of tensile test specimen
before and after the test are shown in Fig5. and Fig.6 respectively. The mechanical properties
ultimate tensile strength(UTS), 0.2% yield strength and percentage elongation obtained from
experimentation are presented in Table 6.
(All dimensions are in mm)
Figure 4 Tensile specimen
FACTOR LEVEL 1 LEVEL 2
Speed (rpm) 900 1400
Feed(mm/min) 80 100
Tilt angle (degree) 1 2
P. Radha Krishna Prasad, P. Ravinder Reddy and K. Eshwar Prasad
http://www.iaeme.com/IJMET/index.asp 569 [email protected]
Figure 5 Tensile specimen before test
Figure 6 Tensile specimen after test
2.3. Microhardness Test
The microhardness tests were performed using Shimadzu, Japan, model HMV-G on a cross
section perpendicular to the weld line at mid thickness across the weld zone applying 200gf
load for a dwell time of 10sec.
2.4. Microstructure
Microstructural examination conducted using the Metascope metallurgical microscope. For
the microstructure, the samples were cut in the direction perpendicular to the welding
direction. These samples were then polished successively on the Sic papers of grit 220 to 600.
Then samples were polished on a disc polishing machine to obtain mirror finish. The samples
were then etched using a solution of Keller’s reagent. Keller’s reagent is a mixture of
Hydrogen fluoride (HF) of 1ml, Hydrochloric acid (HCL) of 1.5 ml, Nitric acid(HNO3) of
2.5ml and Distilled water (H2O) of 95ml. Samples were prepared as per ASTM E407.
3. RESULTS AND DISCUSSIONS
3.1. Tensile Strength
Tensile strength and percentage elongation obtained for three samples of each run are shown
in table 6. Runs 1,3,5,6 and 7 have been found defect free and produced higher UTS. Among
these runs 3 and 6 gave superior tensile strength over the other runs and got joint strength of
65%. The true stress and true strain diagram for runs 3 and 6 is shown in Fig.7.The UTS
obtained for all the welds depicted in Fig.8.
Effect of Welding Parameters on Mechanical Properties of Friction Stir Welded Joints of AA6082
and AA6061 Aluminium Alloys
http://www.iaeme.com/IJMET/index.asp 570 [email protected]
Table 6 L8 MATRIX with UTS and %elongation
Run Rotation
speed A
(rpm)
Weld
speed
B
�������
Tool
tilt
angle
C
( 0 )
UTS (MPa) %elongation
T1 T2 T3 T1 T2 T3
1 900 80 1 162.637 183.48 172.115 13.161 13.046 13.317
2 1400 80 1 147.422 154.018 150.321 11.261 12.738 12.012
3 900 100 1 168.598 188.397 178.469 14.137 14.502 14.353
4 900 80 2 156.255 149.295 156.188 14.606 13.407 14.217
5 1400 100 1 170.524 175.03 178.288 11.321 11.701 11.476
6 1400 80 2 177.254 163.076 189.671 9.126 9.714 10.681
7 900 100 2 171.234 177.905 175.081 12.345 12.945 13.702
8 1400 100 2 120.123 117.402 115.836 5.456 5.711 5.231
Figure 7 True stress and strain diagram for runs 3 and 6
Figure 8 UTS obtained for the welds
-50
0
50
100
150
200
250
1
83
16
5
24
7
32
9
41
1
49
3
57
5
65
7
73
9
82
1
90
3
98
5
10
67
11
49
12
31
13
13
13
95
14
77
15
59
16
41
17
23
18
05
18
87
19
69
20
51
21
33
Tru
e s
tre
ss(M
Pa
)
True strain (mm)
X3 X3 X6 X6
P. Radha Krishna Prasad, P. Ravinder Reddy and K. Eshwar Prasad
http://www.iaeme.com/IJMET/index.asp 571 [email protected]
3.1.1. ANOVA Result Discussion
ANOVA is carried out to find out the most influencing parameters and their relative
influence. From the ANOVA results table 7 for UTS, it is observed that the most dominant
influencing factor for the strength is the combination of speed and tilt angle with a
contribution of 44.5% with a confidence level of 99%.The next influencing factor on UTS is
combination of speed and feed with a contribution of 13.46% at a confidence level of
99%.The next factor which influences UTS is combination of feed and tilt angle with a
contribution of only 11.78% at a confidence level of 99%.Among the individual effects of
speed, feed and tilt angle only feed has influence on UTS with a contribution of 10.56%,
whereas the other two factors have an insignificant(dismal) effect on UTS. The individual
effects of speed, feed and tilt angle on UTS is negligible when compared with interaction
effects of factors.
Table 7 ANOVA result table for UTS
Source SS DOF V F P%
Speed 24.95532204 1 24.95532204 0.451722856 0.247019541
Feed 1066.78667 1 1066.78667 19.31018645 10.55955734
S XF 1359.903095 1 1359.903095 24.61596405 13.46096189
Tilt angle 146.41666 1 146.41666 2.650326522 1.449301122
SXT 4495.385676 1 4495.385676 81.37215998 44.49744654
FXT 1189.717772 1 1189.717772 21.5353947 11.77638734
SXFXT 935.48858 1 935.48858 16.93352515 9.259906953
SST 10102.56998 23
SSE 883.9162047 16 55.24476279 8.749419271
From the ANOVA results table 8 of %elongation it is evident that the most dominant
factor on %elongation is the combined effect of speed and feed with a contribution of 51.55%
at the confidence level of 99%. The individual influence of feed, speed and other factors of
%elongation has been found as 2nd
and 3rd
ranking factors with a contribution of 15.43% and
12.499% respectively at a confidence level of 99%.
Table 8 ANOVA result table for %elongation
Source SS DOF V F P
Speed 22.61265067 1 22.61265067 84.34167865 12.49913215
Feed 27.915894 1 27.915894 104.1219535 15.43049744
S XF 93.2598375 1 93.2598375 347.8447247 51.54933184
Tilt angle 18.9783735 1 18.9783735 70.78638868 10.49028713
SXT 5.841066667 1 5.841066667 21.78627243 3.228646886
FXT 5.396016667 1 5.396016667 20.12630498 2.982645705
SXFXT 2.620204167 1 2.620204167 9.772955019 1.448316636
SST 180.9137658 23
SSE 4.289722667 16 0.268107667 2.371142211
3.2. Microhardness
The microhardness variation measurement help the interpretation of microstructure and
mechanical properties. Microhardness tests were performed in order to characterize the
hardness in the vicinity of the weld affected area and help determine to what extent the HAZ
has been spread. The microhardness variation tests were conducted on a cross section
perpendicular to the weld line, at mid thickness across the weld zone applying load of 200gf
for a dwell time of 10 seconds. The variation of microhardness for runs 3 and 6 was illustrated
in Fig.9 and Fig.10 respectively.
Effect of Welding Parameters on Mechanical Properties of Friction Stir Welded Joints of AA6082
and AA6061 Aluminium Alloys
http://www.iaeme.com/IJMET/index.asp 572 [email protected]
The microhardness variation reveals that the microhardness values in the nugget region
were close to the parent metal AA6061-T6 and less than parent metal AA6082-T6. The
hardness recorded in the nugget zone higher than that of the TMAZ and HAZ of either side.
The average hardness in the nugget region of run 3 when compared to other runs. Better
hardness is attributed to fine grains which are resulted due to better mix and flow of metal in
weld nugget. The hardness on the advancing side and retreating side depicts that hardness
decreased from the nugget zone towards TMAZ and HAZ. The hardness observed on
advancing side in the NZ, TMAZ and HAZ region are more than the retreating side. The
lowest hardness observed on the HAZ of AA6061 which is the softest region where tensile
fractures occurred.
Figure 9 Microhardness variation for run 3
Figure 10 Microhardness variation for run 6
3.3. Microstructure
Microstructural examination revealed that there are three distinct microstructural zones weld
nugget zone (WNZ) or stir zone(SZ), thermomechanically affected zone(TMAZ) and heat
0
20
40
60
80
100
120
-22 -19 -16 -13 -10 -7 -4 -1 0 1 4 7 10 13 16 19 22
Mic
rohar
dnes
s hv 2
00gf
Distance from center of the weld (mm)
0
20
40
60
80
100
120
-22 -19 -16 -13 -10 -7 -4 -1 0 1 4 7 10 13 16 19 22
Mic
rohar
dnes
s hv 2
00gf
Distance from weld center(mm)
P. Radha Krishna Prasad, P. Ravinder Reddy and K. Eshwar Prasad
http://www.iaeme.com/IJMET/index.asp 573 [email protected]
affect zone(HAZ) were present in the weld. The weld nugget zone (WNZ) the central portion
of the weld (i.e pin influenced region) is characterized by the mixture of the two alloys. The
nugget zone undergoes greater strain and is liable to recrystallization. TMAZ is at the two
sides of the nugget zone which terminates at the tool shoulder. TMAZ is subjected to heat and
severe plastic deformation marked by the highly deformed grains. The region outside TMAZ
which is affected only by the heat but not plastic deformation is HAZ.
HAZ 6061 consists of elongated grains in the direction of rolling with randomly dispersed
particles of FeSiAl, MgSi. TMAZ 6061 consists of elongated grains in the direction of stirring
with coarse and fine particles (FeSiAl, MgSi) aligned in the direction of stirring. WELD
NUGGET consists of fine equiaxed recrystallized grains with randomly distributed particles
in the matrix. TMAZ 6082 consists of elongated grains with coarse and fine FeSiAl, MgSi,
AlMn particles aligned in the direction of stirring. HAZ 6082 consists of elongated grains
with mixture of coarse particles of FeSiAl, MgSi and AlMn aligned in the direction of rolling.
From the microstructure study of weld samples, we can find that finer and equiaxed grains
were present in the run 3 and run 6 welds nugget zone when compared with the runs of other
runs. The fine grain structure in WNZ may be due to the good mix of parent metals into the
weld nugget and the uniform homogeneous material flow. The microhardness readings in the
nugget zone of runs 3 and 6 are also higher. Run 3 tensile fracture occurred in HAZ of
retreating side. The HAZ of retreating side of run 3 weld grain size is smallest when
compared to the HAZ retreating side of other sides characterized by higher UTS. Hence a
good correlation is observed between mechanical tests and microstructure. As a sample
microstructures of weld nugget zones of runs 3 and 6 presented in Fig. 11 and Fig.12
respectively.
Figure 11 Microstructuree of WNZ of run 3
Figure 12 Microstructuree of WNZ of run 6
Effect of Welding Parameters on Mechanical Properties of Friction Stir Welded Joints of AA6082
and AA6061 Aluminium Alloys
http://www.iaeme.com/IJMET/index.asp 574 [email protected]
4. CONCLUSIONS
In the present work the effect of speed, feed and tilt angle on mechanical properties of friction
stir welded AA6082-T6 and AA6061-T6 alloy joints was studied. The following conclusions
are drawn.
1. The optimum parameters for the ultimate strength are with the rotational speed of 900rpm,
feed of 100mm/min and tilt angle of 10. In this condition optimum, tensile strength is
189MPa.
2. The most significant contributing factor for UTS has been found to be the combination of
speed and tilt angle.
3. The most dominant factor for percentage elongation is the combination of speed and feed.
4. The lower speed, higher feed, lower tilt angle or the higher speed, lower feed and higher tilt
angles results in superior joint strength welds and characterized by fine equiaxed grain in the
nugget zone.
REFERENCES
[1] Praveen P, Yarlagadda PKDV, Meeting challenges in welding of Al-alloys through pulse
gas metal arc welding. Journal of material processing technology, 164, 2005, pp.1106-
1112.
[2] Mathers G., The welding of Aluminum and its alloys, Wood head publishing ltd, England,
2002, pp.4-5.
[3] Thomas WM, Nicholas ED, Needham JC, MurchMG, TemplesmithP, Dawes CJ., Friction
Stir Butt Welding, GB Patent No. 9125978.8, 1991.
[4] Dawes CJ, Thomas WM, Friction Stir Welding of Aluminium Alloys, TWI Bulletin,
1995, pp.6: 124-127.
[5] Nandan R, DebRoy T, Bhadeshia HKDH, Recent Advances in Friction Stir Welding
Process, Weldment Structure and Properties, Progress in Material Science, 53, August,
2008, pp. 980-1023.
[6] Mishra RS, b Ma, ZY, Friction stir welding and processing, Materials Science and
Engineering R: Reports, 50(1-2), 2005.
[7] M. Satya Narayana Gupta and K. Shiva Shankar, Evaluation of Electro-Mechanical
Properties of Friction Stir Weld ed Al/Cu Bimetallic Lap Joints. International Journal of
Civil Engineering and Technology, 8(4), 2017, pp. 1967-1976
[8] Rajiv Ranjan Kumar, Ashok Kumar and Shalendra Kumar. Evaluation of Processes
Parameter and Mechanical Properties in Friction Stir Welded Steels. International Journal
of Mechanical Engineering and Technology, 8(2), 2017, pp. 183–193.