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Application of Taguchi method to optimizefriction stir welding parameters for

polypropylene composite lap joints

ARTICLE in ARCHIVES DES SCIENCES JOURNAL · JULY 2012

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Archives Des Sciences Vol 65, No. 7;Jul 2012

59 ISSN 1661-464X

Application of Taguchi method to optimize friction stir welding

parameters for polypropylene composite lap joints

Hedi Ahmadi

Department of Mechanical Engineering, Shahid Rajaee Teacher Training University, Tehran, Iran

Tel: +982122970052 E-mail: [email protected]

Nasrollah Bani Mostafa Arab (Corresponding author)

Department of Mechanical Engineering, Shahid Rajaee Teacher Training University

PO box 16788-15811, Tehran, Iran


Faramarz Ashenai GhasemiDepartment of Mechanical Engineering, Shahid Rajaee Teacher Training University, Tehran, Iran


Abstract

In this article, friction stir welding has been used for lap joining polypropylene composite plates having 20

wt% glass fiber. The effects of important process parameters such as tool rotational speed, welding speed,

tilt angle and tool pin geometry on tensile shear strength were investigated using the Minitab software and

the Taguchi method of design of experiments. A L16 orthogonal array with four factors at four levels was

employed to evaluate effects of rotational speed ( 630, 800, 1000 and 1250 rev/min), welding speed (12, 16,

20 and 25 mm/min), tilt angle (0, 1, 1.5 and 2 degree) and tool pin geometry (threaded cylindrical tool,

threaded cylindrical-conical tool, simple cylindrical-conical tool and threaded conical tool) on tensile shearstrength of the lap joints. The results indicated that the tensile shear strength was maximum when rotational

speed, welding speed, tilt angle and tool pin geometry were 1000 rev/min, 20 mm/min and 1 degree

respectively with threaded cylindrical-conical tool. Analysis of variance was performed to calculate the

percentage of contribution of each factor on tensile shear strength. It was found that, the rotational speed,

welding speed, tool pin geometry and tilt angle were significant factors respectively.

Keywords: Polypropylene composite, Friction stir welding, Tensile shear strength, Taguchi method,

Analysis of variance

1. Introduction

Friction Stir Welding (FSW) is a non-fusion welding process which is a derivative of conventional friction

welding giving good quality butt and lap joints [1]. The FSW process has proved to be ideal for creatinghigh quality welds in a number of materials including those which are extremely difficult to weld by

conventional fusion welding [2]. High joining speed, autogenous welding, improved metallurgical

properties, and reduced need for human skill are amongst the most important advantages of FSW compared

to conventional fusion welding methods [3-5]. Welding defects such as porosity and hot cracking are not an

issue in FSW and joints with low residual stresses, improved dimensional stability, good mechanical

properties and high surface finish are produced which require no post weld cleaning [6, 7].

In FSW process, the tool rotation speed, welding speed, tilt angle and tool pin geometry influence the weld

properties. A number of research works have been reported to study the effects of the above parameters on




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material flow, microstructure formation and mechanical properties of FSW joints for metals and metal

matrix composites [4, 8-10]. De filippis et al. [11] studied the effects of different shoulder geometries on

the mechanical and microstructural properties of friction stir welded 6082 T2 aluminium alloy in the

thickness of 1.5 mm. The three studied tools differed from shoulders with scroll and fillet, cavity and fillet,

and only fillet. The results showed that, for thin sheets, the best joint has been produced by a shoulder with

cavity and fillet. Kumar et al. [12] studied the effect of tool geometry on microstructural development andmechanical properties of friction stir welded precipitation hardenable Al-Zn-Mg alloy in the thickness of

4.4 mm and concluded that joints welded with frustum-shaped rounded-end pin profile had better

mechanical properties compared to cylindrical flat-end pin profile. Zhao et al. [13] studied the effect of pin

geometry on the weldability and mechanical properties of friction stir welded 2014 aluminium plates and

found out that the shape of the pin had a significant effect on the joint structure and the mechanical

properties. Buffa et al. [14] reported that in FSW of lap joints, tool geometry had significant effect on

mechanical properties. They used three tools with different pin geometry to weld aluminum alloy

AA2198-T4 and came to the conclusion that cylindrical-conical tool in comparison to conical tool and

cylindrical tool results in joints with better quality. Watanabe et al. [15] studied the weldability of FSW

AZ31 magnesium alloy/SS400 steel, and reported that the rotation speed and the position of the pin axis

had a significant effect on the strength and the microstructure of the joint. Cao and Jahazi [16] studied the

effect of welding speed on microstructures and mechanical properties of friction stir welded AZ31B-H24magnesium alloy and concluded that as the welding speed increased, the grain size in the stir zone

decreased while the yield strength increased. Higher welding speeds produced slightly higher hardness in

the stir zone and the tensile strength increased first with increasing welding speed but remained constant

later.

Polypropylene (PP) and PP composites have already been joined by some of the welding or bonding

techniques [17-19]. Arici et al. [20] studied the effects of two parameters of tool penetration depth and

dwell time on the tensile shear strength of lap joints of friction stir spot welds for PP sheets and found out

that both parameters affected the joint strength of the welds. Scialpy et al. [21] used titanium tool for FSW

of PP extruded sheets and compared it with that of common welding methods such as hot gas welding and

extrusion. They concluded that mechanical properties of the welds and the welding speed of FSW is higher

than the other two methods. Bilici [22] studied the effects of friction stir spot welding parameters (dwell

time, tool plunge depth and tool rotational speed) on PP sheet weld strength with the help of the Taguchimethod. It was found that, all the parameter were effective on joint strength of PP friction stir spot welds

with the dwell time being the dominant parameter and the tool rotational speed the least important one.

Arab et al. [23] investigated the effects of FSW process parameters (tool pin geometry, tool rotational speed,

work linear speed and tool tilt angle) on weld appearance and tensile strength of butt joints in PP

composites with 30% glass fiber (GF) and concluded that the tool pin geometry had a significant influence

on weld appearance and the effects of rotational speed and tilt angle on weld appearance and tensile

strength were more than that of work linear speed.

The survey of the previous works shows that FSW has been mostly used for welding of metals and plastics.

Therefore, it deserves to investigate the ability of this process for welding of PP composites reinforced with

GF. Although research on friction stir butt welding of PP composites has been carried out [23], but lap

welding of this composite by FSW has not been reported yet. This study is therefore intended to explain the

effects of FSW parameters on lap shear strength of PP composite welds with 20 wt% GF.Basically, classical experimental design methods are too complex and not easy to use, especially when the

number of experiments increases. To solve this problem, the Taguchi method uses a special design of

orthogonal arrays to study the entire parameter space with only a small number of experiments [24, 25].

The Taguchi design method has been found to be a simple and robust technique for optimizing the welding

parameters [26]. Considering the above facts, the Minitab software [27] and Taguchi L16 orthogonal arrays

(OAs) method are employed to analyze the effect of each FSW process parameter (i.e. rotational speed,

welding speed, tilt angle and tool pin geometry) on tensile shear strength of lap joints in PP composites

with 20 wt% GF.




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2. Experimental procedures

2.1 FSW process parameters

The FSW process parameters that may influence the quality of FSW joint are tool rotational speed, welding

speed, tool tilt angle and tool pin geometry [28]. In the present investigation, four levels of these process

parameters were considered after conducting trial runs. The FSW process parameters and their levels aregiven in Table 1.

Table 1. FSW process parameters and their levels.

Symbol Welding parameter Unit Level 1 Level 2 Level 3 Level 4

N Rotational speed rev/min 630 800 1000 1250

S Welding speed mm/min 12 16 20 25

θ Tilt angle degree 0 1 1.5 2

T Tool pin geometry - 1 2 3 4

2.2 Materials and equipment

Plates of PP composite with 20 wt% GF and 101 mm×50 mm×4mm size were used as the parent material to

be lap welded in this investigation. A vertical milling machine and a clamping fixture as shown in Figure 1

were used to weld the plates together.

Figure 1. Clamping fixture used in experiments.

The rotating tools were made of high speed steel. In order to study the tool geometry effect on weld

strength, four different friction stir tools with different pin geometry were used for experiments. Details of

these tools are shown in Figure 2 and Table 2.




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Figure2. Different tools used in the investigation.

Table 2. Different tool pin geometry of four friction stir tools.

Number

Tool

Description of the pin Length of the pin

(mm)

Diameter of the pin

(mm)

Diameter of the shoulder

(mm)

1 Threaded cylindrical 7.7 4 12

2 Threaded cylindrical-conical 7.7 4 12

3 Simple cylindrical-conical 7.7 4 12

4 Threaded conical 7.7 4 12

2.3 Design of experiments

In order to examine process parameters effects on tensile shear strength of the joints, the statistical

techniques of Taguchi method and analysis of variance (ANOVA) were selected. Taguchi method utilizes

OAs from experimental design theory to study a large number of variables with a small number of

experiments. OAs are subsets of the full factorial experiment which are balanced, i.e., each variable setting

occurs the same number of times and none of two experiments are the same (or even mirror images). Using

OAs significantly reduces the number of experimental configurations to be studied. Taguchi has simplified

their use by providing tabulated sets of standard OAs and corresponding linear graphs to fit specific

projects [29].

Three stages of Taguchi approach to design the experiments are as follows:

1.

Planning a matrix experiment to determine the effect of the control factors,

2.

Conducting the matrix experiment,

3.

Analyzing and verifying the results [29].

In this work, four factors at four levels were selected based on the literature and qualitative experiments.

The matrix experiment was designed according to the Taguchi parameter design methodology, L16 OA

shown in Table 3 to investigate the effect of four controllable factors (rotational speed, welding speed, tilt

angle and tool pin geometry) on the tensile shear strength. Each row of the OA represents a run, which is a

specific set of factors. Lap welding of the plates was performed randomly according to Table 3. Each run

was replicated twice to minimize the noise factor.




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2.4 Tensile shear strength test specimen preparation

The tensile shear strength test specimens with the dimensions given in Figure 3 were prepared after welding

according to ASTM D5868 standard [30] from the middle of the welded plates to eliminate the start and

end effects of the welding process. Tensile shear strength tests were conducted using a SANTAM Universal

Testing Machine-STM-150 keeping the cross-head speed at 2 mm/min during the loading conditions.Figure 4 shows the tensile test setup.

Figure3. Overlap shear test specimen[30].

Figure 4. Specimen and fixture during tensile shear strength testing.

3. Results and discussion

3.1 Signal to noise ratio

Taguchi method uses a statistical measure of performance called signal-to-noise (S/N) ratio to analyze the

results. In its simplest form, the S/N ratio is the ratio of the mean response (signal) to the standard deviation

(noise). S/N parameter is gained by minimizing the loss function and defined in three different conditions:

lower-the-better, larger-the-better, and nominal-the-better. Applying this method, it can be guaranteed that

the effect of noise factors (which are not controllable or are unidentified to be controlled) such as stirring

specimen




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machine and environmental condition will be the minimum in comparison to the main factors, and it means

that the final result shows the least sensitivity to noise factors. For the friction stir weld strength, the

larger-the-better quality characteristic is taken [25]. The S/N ratio for the larger-the-better quality is

expressed as [24]:

(1)

Where, n is the number of experiments and yi is the response at each experiment.

In this work, 16 means and 16 signal to noise (S/N) ratios were calculated and the estimated tensile shear

strength, means and S/N ratio are given in Table 3.

Table 3. Orthogonal array for L16 with response (raw data and Mean values and S/N ratio).

Experiment

number

Input parameter Response Mean

value

(MPa)

S/N

ratioRotational

speed

Welding

speed

Tilt

angle

Tool pin

geometry Trial 1 Trial 2 N S θ T (MPa) (MPa)

1 630

12

0

1

3.51 3.56 3.535 10.96

2 630

16

1

2

4.47 3.71 4.09 12.23

3 630

20

1.5

3

2.90 3.60 3.25 10.23

4 630

25

2

4

2.61 2.56 2.585 8.24

5 800

12

1

3

3.46 3.39 3.425 10.69

6 800

16

0

4

3.52 3.38 3.45 10.75

7 800

20

2

1

3.63 3.60 3.615 11.16

8 800

25

1.5

2

3.56 3.58 3.57 11.05

9 1000

12

1.5

4

4.47 3.73 4.10 12.25

10 1000

16

2

3

4.62 3.60 4.11 12.27

11 1000

20

0

2

4.01 5.04 4.525 13.11

12 1000

25

1

1

3.63 4.62 4.125 12.30

13 1250

12

2

2

4.40 4.31 4.355 12.77

14 1250 16 1.5 1 4.34 4.30 4.32 12.70

15 1250

20

1

4

4.61 4.76 4.685 13.41

16 1250

25

0

3

3.39 3.33 3.36 10.52

All the tensile shear strength test specimens fractured from the weld zone as shown in Figure 5 indicatingthis zone is the weakest part of the joint.

Figure 5. The tensile shear strength test fractured specimen.




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The analysis of mean for each of the experiments will give the better combination of parameters levels that

ensures a high level of tensile shear strength according to the experimental set of data. The mean response

refers to the average value of performance characteristics for each parameter at different levels. The mean

for one level was calculated as the average of all responses that were obtained with that level. The mean

response of raw data and S/N ratio of tensile shear strength for each parameter at levels 1, 2, 3 and 4 were

calculated which are given in Table 4.

Table 4. Results of experiments (means and S/N ratio).

Process

parameter

Level

Means S/N ratio

Rotational

speed

Welding

speed

Tilt

angle

Tool pin

geometry

Rotational

speed

Welding

speed

Tilt

angle

Tool pin

geometry

N S θ T N S θ T

Averagevalue

L1 3.365 3.85 3.71 3.89 10.53 11.70 11.38 11.79

L2 3.515 3.99 4.08 4.135 10.91 12.01 12.21 12.32L3 4.215 4.01 3.81 3.53 12.49 12.06 11.61 10.95

L4 4.18 3.41 3.66 3.70 12.42 10.65 11.26 11.36

The means and S/N ratio of the various process parameters when they changed from the lower to higher

levels are also given in Table 4. It is clear that a larger S/N ratio corresponds to better quality characteristics.

Therefore, the optimal level of process parameter is the level of highest S/N ratio [31]. The mean effect and

S/N ratio for tensile shear strength calculated by Minitab statistical software indicate that the tensile shear

strength was maximum when rotational speed, welding speed and tilt angle were 1000 rev/min, 20 mm/min

and 1 degree respectively with tool number 2. From the results of Table 4, diagrams were drawn to display

the welding parameters effects on weld strength. These diagrams are shown in Figures 6 to 9.

Figure 6 shows the effect of tool rotational speed on means (tensile shear strength) and S/N ratio of the joint.When the rotational speeds were 630 and 800 rev/min, the heat generated by the friction between the tool

shoulder and the base material may be low. Under this condition, the material transportation from the

advancing side of the tool to the retreating side of the tool may not occur sufficiently which end in

formation of joints with lower strength (Figure 7 a and 7 b). When the rotational speed was 1250 rev/min,

the generated frictional heat is too high which may cause turbulence of the plasticized material under the

tool shoulder which in turn may lead to reduction of joint strength (Figure 7 c) [32]. However with the

rotational speed of 1000 rev/min, the generated heat and turbulence are enough to produce a weld of

highest strength, i.e. 4.21 MPa (Figure 7 d). Better weld surface may be an indication of a better weld.




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Figure 6. Effect of tool rotational speed on means and S/N ratio.

Figure7. Effect of tool rotational speed on weld surface appearance, (a) 630 rev/min, (b) 800 rev/min, (c)

1250 rev/min, (d) 1000 rev/min.

Figure 8 shows the effect of welding speed on means and S/N ratio of the joints. Similarly, when the

welding speeds were 12 and 16 mm/min, high frictional heat and turbulence between the tool and the parent

materials were generated which had a significant role in reduction of weld strength (Figure 9 a and 9 b).

When the welding speed was 25 mm/min, low heat was generated at the joint which led to insufficient

stirring of the material around the tool pin in the joint which may be the reason for low strength of the joint

(Figure 9 c). At the welding speed of 20 mm/min, the highest tensile shear strength of joint (4.01 MPa) was

obtained due to the same phenomena (Figure 9 d) [32].




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Figure 8. Effect of welding speed on means and S/N ratio.

Figure 9. Effect of welding speed on weld surface appearance, (a) 12 mm/min, (b) 16 mm/min, (c) 25

mm/min, (d) 20 mm/min.

Figure10 shows the effect of tilt angle on means and S/N ratio of the joints. Friction between the tool and

the material causes heat and this heat results in plastic deformation of the material. The material at the joint

is deformed due to the applied downward force. Tool tilt angle affects the flow behavior of the plastically

deformed material at the joint. At high tilt angle of 1.5° and 2°, the material at the joint as shown in Figures

11 a and 11 b leave the joint which results in a low strength weld. When the tilt angle was 0 °, downward




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force might not be generated sufficiently, therefore, the materials were not plasticized enough to be led into

the weld joint (Figure 11 c), and as a result the weld strength decreased [33]. With the tilt angle of 1 ° , the

downward force and hence the plasticized material were enough to produce a weld strength of about 4.08

MPa with better surface appearance as shown in Figure 11 d compared to other welds.

Figure 10. Effect of tilt angle on means and S/N ratio.

Figure 11. Effect of tilt angle on weld surface appearance, (a) 1.5°, (b) 2

°, (c) 0

°, (d) 1

°.

The shoulder diameter, the pin length and diameter are geometric features of the tool which affect the




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amount of heat generated and material flow during welding [34-36]. After contacting the work piece the

shoulder creates friction and causes heat generation which consequently results in weld width. The more

the heat created by the shoulder, the material flow will occur more and easier, however, this heat should not

exceed a certain limit because in that case, partial melting occurs and the material sticking to the tool

surface results in low weld quality. If the length of the pin is short for the thickness of the two plates, the

second piece which is under the first one will not be welded well, therefore, lack of penetration defect willoccur. Although the shoulder has the main role of creating heat, the pin to a lesser degree also plays a role

in creating heat [34-36]. Therefore, shortness of the pin because of decrease in contact surface with the

lower piece results in decrease of the created heat in the lower piece. Compensating the shortness of the pin

through pushing the tool more in the work piece causes pressure increase which forces the materials out of

the molten pool, therefore, flash defect shown Figure 12 shows occurs. As a result, the pin length should be

chosen accurately. To do so, it is better to choose the pin length 0.2 mm to 0.3 mm shorter than the sum of

thickness of the two pieces [37].

Figure 12. Formation of flash defect while welding.

As seen in Figure 13, the resulted tensile shear strength (means) and S/N ratio with tool number 2 is higher

than all other tools. Tool number 2 results in higher tensile shear strength, because its contact surface with

the work piece is more and greater friction created results in more heat [38]. On the other hand, the threadaround the tool pin causes great amount of turbulence on the weld seam and molten material mixes better

and as a result a higher tensile shear strength is produced. The tensile shear strength from tool number 3 is

lower than other tools, because it is the only tool whose pin is simple and this does not cause the materials

to mix well when the tool pin enters the molten material and creates lesser turbulence compared to other

tools. When material turbulence in the weld pool drops, tensile shear strength of the weld decreases. In

addition, it can be observed that tools number 2 and 3 are alike except for their pins. The presence of thread

on tool pin is significantly important, therefore tool number 2 is the best and tool number 3 is the worst one.

Tool number 1 has a lesser contact surface with the work piece in comparison to tool number 2 and this

contact surface decrease results in friction and heat drop and ultimately decreases tensile shear strength of

the weld. Tool number 4 whose pin is threaded conical, creates lesser contact surface with the work piece

compared with tool number 1, which causes tensile shear strength in tool number 4 to decrease in

comparison to that of tool number 1 because lesser friction and heat are generated at the contact area.




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Figure 13. Effect of tool pin geometry on means and S/N ratio.

3.2 Analysis of variance (ANOVA)

After performing the statistical S/N analysis, ANOVA needs to be employed for determining the relative

importance of various factors. ANOVA demonstrates whether observed variations in the response are due to

alteration of level adjustments or experimental standard errors. ANOVA as a common statistical method is

also used to analyze the results of the experiments on response and also to determine contribution of each

influencing factor. In this study, the purpose of the ANOVA is to investigate the significance of the effect of

the FSW process parameters on the tensile shear strength of the welds. The ANOVA results for tensile shear

strength of means and S/N ratio are given in Table 5 and 6 respectively [32].

Table 5. ANOAV for lap shear strength (Means).

Source DF Seq SS Adj SS Adj MS F P SS' PC

N 3 2.3427 2.3427 0.78089 15.93 0.024 2.1957 47.21

S 3 0.9540 0.9540 0.31799 6.49 0.079 0.807 17.35

θ 3 0.4100 0.4100 0.13665 2.79 0.211 0.263 5.65

T 3 0.7966 0.7966 0.26555 5.42 0.099 0.6496 13.96

Error 3 0.1470 0.1470 0.04901 - - 0.735 15.8

Total 15 4.6503 - - - - - 100

DF:Degree of freedom, Seq SS:Sequential sum of squares, Adj SS:Adjusted sum of square, Adj

MS:Adjusted mean square, F:Fisher ratio, P:Probability that exceeds the 95% confidence level, SS':Pure

sum of squares, PC: Percentage of Contribution.




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Table 6. ANOAV for lap shear strength (S/N ratio).

SourceDF Seq SS Adj SS Adj MS F P SS' PC

N 3 12.824 12.824 4.2747 11.76 0.036 11.734 44.09

S 3 5.722 5.722 1.9072 5.25 0.103 4.632 17.40

θ 3 2.427 2.427 0.8089 2.23 0.264 1.337 5.02

T 3 4.545 4.545 1.5151 4.17 0.136 3.455 12.98

Error 3 1.090 1.090 0.3634 - - 5.45 20.48

Total 15 26.608 - - - - - 100

The percentage of contribution (PC) is the portion of the total variation observed in the experiment

attributed to each significant factor and/or interaction which is reflected. The PC is a function of the sum of

squares for each significant item; it indicates the relative power of a factor to reduce the variation. If the

factor levels are controlled precisely, then the total variation could be reduced by the amount indicated by

the PC [32]. The PC of the rotational speed, welding speed, tilt angle and tool pin geometry for means isshown in Figure 14.

Figure 14. Percentage of contribution of each factor on means (tensile shear strength).

3.3 Confirmation test

Once the optimal level of the welding parameters has been selected, the final step is to predict and verify

the quality characteristic using the optimal level of the welding parameters. The predicted S/N ratio ( )

using the optimal level of the welding parameters can be calculated as:

(2)

Wherem

is the total mean S/N ratio,i

is the mean S/N ratio at the optimum level, and O is the number

of the main design parameters that affects the quality characteristic. Table 7 shows the comparison of the




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predicted and the actual weld lap shear strength using the optimum welding parameters. Good agreement

between the predicted and the actual weld lap shear strength is observed.

Table 7. Results of the confirmation test for weld lap shear strength.

Optimal welding parameters

Prediction Experiment

Parameter levels N3, S3, θ2, T2 N3, S3, θ2, T2

Tensile shear strength (MPa) 5.29 4.83

S/N ratio 14.46 13.67

4. Conclusion

In this paper, the effect of friction stir welding process parameters on lap shear strength of PP composite

plates with 20 wt% GF were evaluated with the help of the Taguchi method. The results indicated that:

1.

Tool 2 (threaded cylindrical-conical tool) was the best tool compared to other tools for producing

higher strength welds.

2.

For maximum weld strength, the optimum values of rotational speed, welding speed and tilt angle

were determined to be 1000 rev/min, 20 mm/min and 1° respectively when using tool 2.

3.

The weld strength was maximum when rotational speed and welding speed were at the

intermediate level of 3 whereas the tilt angle level was 2.

4.

It was found that the rotational speed, welding speed, tool pin geometry and tilt angle had 47.21%,

17.35%, 13.96% and 5.65% contribution on weld strength respectively. Hence, rotational speed is

the most significant parameter whereas tilt angle is the least important one.

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