Using arc welding, gas welding and other welding process, it is very difficult to weld the aluminum alloys. Friction stir welding, on the other hand, can be used to join most Al alloys and better surface finishing is achieved. Although the work piece does heat up during friction stir weld, the temperature does not reach the melting point.

1. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
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TENSILE BEHAVIOUR OF ALUMINIUM PLATES (5083) WELDED BY
FRICTION STIR WELDING
Er. Jaitinder Mittal (AP), Er. Kumar Gaurav (Assot. Prof.)
(ME Deptt., GTBKIET, Chhapinwali, Malout/PTU Jalandhar, India)
ABSTRACT
Using arc welding, gas welding and other welding process, it is very difficult to weld the
aluminum alloys. Friction stir welding, on the other hand, can be used to join most Al alloys and
better surface finishing is achieved. Although the work piece does heat up during friction stir weld,
the temperature does not reach the melting point.
In this research work, various welding parameters, like rotational speed, welding speed and
pin diameter was considered for experimentation to weld Al alloy 5083 and its effect on tensile
strength. Mathematical models were developed from the data generated using the two level full
factorial technique. Significance of the coefficients and adequacy of the developed models has been
checked using student‘t’ test and ‘F’ test respectively. Developed model have been found to be
adequate up to 95% of level of significance. The influence of welding parameters has been presented
in graphical form for better understanding. The combined effects of welding parameters on the
mechanical properties are also presented in graphical form. Tensile strength decreases with increase
in rotational speed, tensile strength increases with increase in welding speed, tensile strength
decreases with increase in pin diameter and at low rotational speed of tool and high welding speed,
there is maximum tensile strength.
Key words:- Friction Stir Welding, Tensile Properties, Mathematical Models, FSW Parameters.
1. INTRODUCTION
1.1 Welding
Welding can be defined as the joining of two components by a coalescence of the surfaces in
contact with each other. This coalescence can be achieved by melting the two parts together – fusion
welding or by bringing the two parts together under pressure, perhaps with the application of heat, to
form a metallic bond across the interface. This is known as solid phase joining (Mathers, 2002).
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Welding is the most economical and efficient way to join metals permanently. Welding ranks high
among industrial processes and involves more sciences and variables than those involved in any
other industrial process. In many cases welding is the most cost effective and structurally sound
joining technique. Welding can be performed almost anywhere outdoors, indoors, under sea or in
space (Rossi, 1954).
1.2 Friction stir welding
Friction stir welding is an innovative solid state welding process invented in December, 1991
by Wayne Thomas at The Welding Institute (TWI), Cambridge, United Kingdom. It has been found
as one of the most significant welding process invention from the last two decades. It can be
considered as a hot working process in which a large amount of a deformation is imparted to the
work piece through the rotating pin and the shoulder (Pflluger, 1996). No melting occurs in this
process and the developed welds have a fine grained, hot worked condition with no entrapped oxides
or gas porosity. No shielding gas or flux is used. The joining does not involve any use of filler metal.
1.2.1 Friction Stir Welding Equipment
The major equipments used in FSW are:
• Tool
• Conventional Vertical Milling Machine
• Fixture.
1.2.2 Principle of working (www.google.co.in, friction stir welding process)
Friction stir welding is a variant of friction welding that produces a weld between two work
pieces by heating and plastic displacement caused by a rapidly rotating tool that traverse the weld
joint. Heating is done by both frictional rubbing between the tool and the work pieces and by visco-
plastic dissipation of the deforming material at high strain rates. Friction stir welding uses a non
consumable, rotating welding tool to create heat locally. A common tool design is the shape of a rod
with concave area with a pin, coaxial with the axis of rotation. The work pieces are rigidly clamped
and are supported by a backing plate, or anvil, that bears the load form the tool and constrains
deformation of the material at the backside of the joint.
2. DESIGN OF EXPERIMENTS
2.1 General Quantitative Approach
This approach is based on the principles of statistics. It has been popular for the research in
the field of medicines and agriculture for a long time. However, this approach has been used recently
in the field of welding to predict the effect of input parameters on output parameter or response. This
approach has a number of advantages over the classical approach. The most important advantage is
that several parameters can be studied simultaneously at different levels on the response and in
addition to the process optimization: the interactions between two or more parameters could also be
evaluated. In general the statistical method of designing experiments helps in minimizing the time
and the cost of experimentation. In this technique experiments are conducted as per the design
matrix. Based on the experimental data generated regression equations are derived using the method
of least squares. The statistical significance of coefficients of the regression equations are tested
using students ‘t’ test. The coefficient having higher the value of ‘t’, more significant it becomes.
After dropping insignificant coefficients, adequacy of the final model is determined by applying ‘F’
test. In this manner adequate model within the given confidence level can be developed. These
developed models can be used for prediction the effects of control variables on the required
responses.

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Table 2.1 Welding parameters used and their limits
Parameters Units Symbols Designation Upper limit Lower limit
Tool RPM Rpm R X1 1100 750
Welding speed mm/min S X2 150 90
Pin diameter Mm D X3 8 6
2.2 Development of design matrix
Table 2.2 Design Matrix
S.No. R
1
S
2
D
3
1 + + +
2 - + +
3 + - +
4 - - +
5 + + -
6 - + -
7 + - -
8 - - -
2.3 Experimental procedure
All the friction stir welds were performed with a vertical milling machine that was set up for
performing such welds. The initial joint configuration was obtained by securing the base plates (100
x 50 x 6 mm) of Al-5083 alloy on to a specially designed fixture. Two friction stir welding tools with
pin diameters (D) 8mm and 6mm, 5.8 mm long (h) and shoulder diameter (25mm) were used. The
three welding parameters were considered i.e. Tool rotation speed, welding speed and tool pin
diameter.
Table 2.3 Coefficients of model
S.No. Coefficient of Regression Due to
1 b0 Combined effect of all parameters
2 b1 Tool RPM (R)
3 b2 Welding speed (S)
4 b3 Pin diameter (D)
5 b12 Interaction of R & S
6 b13 Interaction of R & D
7 b23 Interaction of S & D

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Table 2.4 Design Matrix for calculating the value of coefficients
b0 b1
R
b2
S
b3
D
b12
RS
b13
RD
b23
SD
+1 +1 +1 +1 +1 +1 +1
+1 -1 +1 +1 -1 -1 +1
+1 +1 -1 +1 -1 +1 -1
+1 -1 -1 +1 +1 -1 -1
+1 +1 +1 -1 +1 -1 -1
+1 -1 +1 -1 -1 +1 -1
+1 +1 -1 -1 -1 -1 +1
+1 -1 -1 -1 +1 +1 +1
2.4 Developed model
Developed models could be obtained by putting the values of the regression coefficients
obtained from equation (2) in the selected model,
Y = b0 + b1R + b2S + b3D + b12RS + b13RD + b23SD ----------------(1)
3. EXPERIMENTAL WORK
The friction stir welding process was performed on a vertical milling machine. The specially
designed fixture was clamped on bed of vertical milling machine. The tool was mounted on the
vertical spindle. Then two prepared aluminum pieces were clamped into the fixture. Then the
rotating tool was made to embed into the butt joint. Then after some time, when there was sufficient
heating was achieved due to friction between tool and plates, the bed was given automatic feed,
along the joint direction. Thus the welding was achieved.
Thus total 8 experiments were performed. The welding parameters were selected as per
design matrix. The trials were repeatedly three times for determining the adequacy of mathematical
models. Thus total 24 trials were performed.
After that the pieces were cut into the samples of required dimensions for performing the
tensile tests, impact tests, and micro hardness tests.
3.1 Mathematical model for tensile strength
3.1.2 Observation Table for Tensile Strength
The observed values for tensile strength for the specimens are tabulated as:
Table 3.1 Observation table for tensile strength
Sr. No. T1 (Set 1)
N/mm2
T2 (Set 2)
N/mm2
T3 (Set 3)
N/mm2
1 277 278 281
2 283 282 283
3 278 280 279
4 275 276 278
5 280 283 280
6 288 287 284
7 283 280.5 280
8 283.5 284 280

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3.1.3 Variance of Optimization for Tensile Strength
Variance of optimization for tensile strength is calculated by taking the values of tensile
strength from first two sets. Table 3.2 shows the Variance of optimization for tensile strength.
Table 3.2 Variance of optimization for tensile strength
Tensile Strength N/mm2
S2
y = 2 ΣN
i=1 ∆T2
/N
T1 T2 Tm ∆T ∆T2
277 278 277.5 0.5 0.25
283 282 282.5 -0.5 0.25
278 280 279 1 1
275 276 275.5 0.5 0.25
280 283 281.5 1.5 2.25
288 287 287.5 -0.5 0.25
283 280.5 281.75 -1.25 1.5625
283.5 284 283.75 0.25 0.0625
Σ∆T2
=5.875 S2
y =1.46875
Coefficients of regression for model for tensile strength were calculated (Table 3.3) as
Table 3.3 Coefficients of Regression for Tensile Strength Mathematical Model
Coefficient Factor Value
b0 Combined effects of all factors 281.125
b1 Tool rotation speed (R) -1.1875
b2 Welding speed (S) 1.125
b3 Pin diameter (D) -2.5
b12 Interaction of R & S -1.5625
b13 Interaction of R & D 0.8125
b23 Interaction of S & D 0.25
The model for tensile strength can be obtained by putting the values of coefficients in Equation (1):
T= 281.125-1.1875R+1.125S-2.5D-1.5625RS+0.8125RD+0.25SD
‘t’ values for coefficients of regression for tensile strength were calculated as:
These values were compared with the‘t’ value taken from standard Table 5 [7]. The value
of’t’ is 2.306. Hence the coefficients of regression having‘t’ values less than 2.306 were treated as
insignificant and were dropped from final model.

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Table 3.4‘t’ Values for tensile strength mathematical model
Coefficient Factor Value
|bi|
‘t’ Value
b0 Combined effects of all factors 281.125 656.1007 Significant
b1 Tool rotation speed (R) 1.1875 2.771435 Significant
b2 Welding speed (S) 1.125 2.62557 Significant
b3 Pin diameter (D) 2.5 5.8346 Significant
b12 Interaction of R & S 1.5625 3.646625 Significant
b13 Interaction of R & D 0.8125 1.896245 Insignificant
b23 Interaction of S & D 0.25 0.58346 Insignificant
The table shows that the coefficients b3 and b13 are insignificant, so these coefficients have to
be dropped from the model. Now the final model becomes:
T= 281.125-1.1875R+1.125S-2.5D-1.5625RS
3.1.4 Variance of Adequacy for Tensile Strength
Variance of adequacy for tensile strength is calculated by taking the third set values and
estimated values.
Table 3.5 Variance of adequacy for tensile strength
Tensile Strength N/mm2
S2
ad = 2 ΣN
i=1 ∆T2
/N
Tp T3 ∆T ∆T2
279.75 281 -1.25 1.5625
285.25 283 2.25 5.0625
280.125 279 1.125 1.265625
279.375 278 1.375 1.890625
279.25 280 -0.75 0.5625
284.75 284
0.75
0.5625
280.625 280 0.625 0.390625
279.875 280 -0.125 0.015625
Σ∆T2
=11.3125 S2
ad = 2.828125

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3.1.5 Analysis of Variance for Tensile Strength
‘F’ values thus obtained and denoted as Fm were compared from the standard Table 8.4 [7]
of Ft at (4,8, 0.5). as Fm<Ft, it was found that the model was adequate at 95% level of significance
thus justifying the use of assumed polynomial.
Table 3.6 Analysis of variance of mathematical model
Degree of
Freedom
Variance of
Adequacy
Variance of
Response
‘F’–Ratio
Model (Fm)
‘F’-Ratio
Table
Adequacy of
Model
F N S2
ad S2
y Fm= S2
ad/
S2
y
at 4, 8, 0.5 Whether
Fm<Ft
4 8 2.828125 1.46875 1.925532 3.84 Yes
It is observed that the value of Fm is less than the value of Ft, so the developed model is an
adequate model.
4. RESULT & DISCUSSION
4.1 Results
The final proposed mathematical models for tensile strength is given below:
T= 281.125-1.1875R+1.125S-2.5D-1.5625RS
These mathematical models can be used to predict the effects of the parameters of the tensile
strength. These can be used to predict tensile strength substituting the values of respective factors in
coded form. These models are also useful to enhance the productivity and quality of weld.
4.2 Influence of Tool Rotational Speed on Tensile Strength
It can be concluded that as the rpm increases, the tensile strength decreases significantly. At
high rpm, high heat is evolved, due to which the coarse micro structure is produced, as a result the
tensile strength is decreased. Also a higher rotational speed causes excessive release of stirred
material to the upper surface, which resultantly leaves the voids in the FSW zone. Due to it, there is
decrease in tensile strength of the material.
When a force is applied to a metal, the layers of atoms within the crystal structure move in relation to
adjacent layers of atoms. This process is referred to as slip. Grain boundaries tend to prevent slip.
The smaller the grain size, larger will be the grain boundary area. Decreasing the grain size of the
metal tends to retard slip and thus increases the strength of the metal.
4.3 Influence of Welding Speed on Tensile Strength
It can be concluded that as the welding speed increases, the tensile strength also increases. It
is because by increasing the welding speed, there is slow heat input. The slow heat input give
relatively high cooling rates, which generates fine grained heat affected zones and less brittlement.
This results in increase in tensile strength.
4.4 Influence of pin diameter on Tensile Strength
It can be concluded, that as the pin diameter increases, the tensile strength will decreases. It is
because of, if we increase the pin diameter, welding will affect the more area of the plates.

8. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
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4.5 Interactive Effect of tool rotation speed and Welding Speed on Tensile Strength
The tensile strength decreases from 285 to 279.5 with increase in rotational speed 750 to
1100 rpm and at the maximum welding speed i.e. 150mm/min. And on the other hand at the
minimum welding speed i. e. 90mm/min, tensile strength remains constant.
5. CONCLUSION AND FUTURE SCOPE
• Two Level Factorial Design is found to be effective tool to investigate the interaction effects
of parameters on the required response.
• Proposed models are adequate at 95% confidence level, thus justifying the use of assumed
polynomials.
• Tensile Strength decreases with increase in rotational speed of tool.
• Tensile Strength increases with increase in welding speed.
• Tensile Strength decreases with increase in pin diameter.
• Prediction of the Tensile Strength at any combination of the two parameters welding speed
and Rotation speed can be done.
• At low rotational speed of tool and high welding speed, there is maximum Tensile Strength.
Scope for future work
• Effects of welding parameters on the other mechanical properties like impact strength,
bending strength, fatigue behaviour, compressive strength can also be studied.
• Dissimilar metals can be joined with the help of FSW process and study of welding
parameters can be done.
• Effects of different types of tool materials can be studied on FSW process.
• Effects of different types of tool shapes can be studied on FSW process.
• FSW process can be studied for suitability for welding other materials like copper,
magnesium, titanium, steel etc.
• Effects of vertical downward force can be studied on the defects and grain structure produces
in the weld metal zone.
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