Effect of Friction Stir Welding Processing Parameters on Abrasion Wear Resistance of AA5083 Welded Joints

In this study, materials of interest are AA5083 friction stir welded joints. The aim of this study is to investigate the effect of processing parameters, welding speed and rotational speed, on abrasive wear performance of the stir zone in the welded joints. The wear resistances of the studied joints are tested by dry sand rubber wheel abrasion test (ASTM G65-00).

Table 2.1: Chemical composition of Al alloy

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Table 2.2: Mechanical properties of Al alloy used in the experiment

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Experimental

Materials

The chemical nominal composition of 5083A alloy base material used in this investigation is presented in table 2.1. The welds were produced in 6 mm thick plates with mechanical properties shown in table 2.2.

Welding Equipment

A conventional vertical milling machine shown in figure 2.1 was transformed into a friction stir welding machine by designing a fixture that makes the milling machine capable of performing friction stir welding process. The fixture used in the process was designed to be firmly clamped the specimen during the process as shown in figure 2.2

The tool used for FSW process is a high carbon steel with square pin profile which was recommended by several workers [3, 6,10and 11] as it is the most effective tool used in the process. Figure 2.3 shows schematic diagram for the tool.

Figure 2.1: Conventional vertical milling machine transformed to FSW machine

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Figure 2.2 : The fixture used to transform the milling machine into FSW machine

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Figure 2.3: Schematic diagram for FSW tool used in the experiment. All dimensions are in mm

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Welding Procedure

The plates of 6 mm thickness were cut into the required size (300 mm ×150 mm) by manual cutting machine. Square butt joint configuration (300mm × 300 mm) was prepared to fabricate FSW joints. The initial joint configuration was obtained by securing the plates in position using mechanical clamps.

Friction stir welding is done by holding the plates to be welded securely in the fixture designed so that the plates stay in place and do not move or even fly away due to the welding forces as shown in figure 2.2.

The rotational motion of the spindle is started and the tool is introduced to be engaged and in contact with the surface of the plates and the probe is penetrated to a 5.5mm depth in between the faying surfaces of the plates to be welded.

The tool is given some time as it rotates in contact with the surfaces to raise the temperature of the work piece and to soften the material by the frictional heat produced, this time is called as dwell time, and after the dwell time the tool fed to have a feeding motion along the partition between the two work pieces in order to achieve the weld between these two pieces. The tool is withdrawn after the weld is completed.

As it has mentioned earlier many parameters are introduced in FSW process, in the present work the parameters (factors) were considered, i.e. the welding speed, and the rotational speed with five levels for each factor and three replicates for each one. So that the process is carried out 75 times² .Factors levels are being selected by trial and error to fix the working range for these levels in order to get defect free welds, table 2.3 shown the working range for the two factors, welding speed and rotational speed.

After fabricated the joints, Abrasion wear test was carried out based on ASTM G65-00 standard using the Dry Sand/Rubber Wheel Apparatus (see figure 2.4). The 25mm wide and 75mm   in length abrasive wear resistance test specimens were cut from the middle area of the welded joints. The mass of all specimens were to the nearest 0.0001g as required by ASTM G65-00. The applied pressing force the test coupon against the wheel was TL = 130 [N] (test load – TL).

After the abrasive wear resistance test, the test Specimen was weighed at weight sensitivity 0, 0001 [g]. Mass loss of the Specimens were determined.¹²

Table 2.3: working parameters for Al alloy joints

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Figure 2.4: Schematic diagram of test apparatus (Dry Sand/ Rubber wheel abrasion wear)

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Results and Discussion

Abrasive wear test carried out for the specimens prepared from the stir-zone of the welded materials at different welding and rotational speed. The mass of specimen weighed before and after the test.

Effect of Welding Speed on Abrasive Wear Resistance

Figure 3.1 reveals the effect of welding speed on wear resistance. It is shown that abrasion wear resistance increased as the welding speed increased. At lower welding speed 22 mm/min the abrasion wear resistance was lower. When the welding speed was increased from 22 mm/min, the abrasion wear resistance was increased and reached a maximum at 101mm/min. This was found due to the hardening of the material caused by the lower heat input associated with faster welding speed.

Figure 3.1: Effect of welding speed on average loss of weight due to wear

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Effect of Rotational Speed on Abrasive Wear Resistance

Figure 3.2 shows the effect of rotational speed on abrasion wear resistance of the joints. At lower rotational speed 400rpm, the abrasion wear resistance of the joints was higher. When the rotational speed was increased from 400 rpm, abrasion wear resistance was decreased and reaches a minimum at 1200 rpm this could be attributed to softening of material caused by higher heat input.

Figure 3.2: Effect of rotation speed on the average loss of weight due to wear

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Analysis of Variance of Abrasive Wear Test Result

Figures 3.3 and 3.4 shows the main effect plot of welding speed and rotational speed respectively and indicates that the wear resistance increased with increasing welding speed and decreasing rotational speed. Also it can be concluded from table 3.1 that those two factors are the main effecting factors on wear resistance since they explain 99.64% variability in the model It’s clear from the table that welding speed has the largest sum of square 0.364128 so it can be inferred that welding speed has more significant effect on wear resistance followed by the rotational speed which is less significant. 

Figure 3.5 shows the interaction effect plot of rotational speed and welding speed on wear resistance, it can be concluded from the existence of parallels between the lines that the interaction effect of welding speed and rotational speed is not significant which support the result from table 3.1.

Figure 3.3: Main effect plot of welding speed on wear resistance

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Figure 3.4: Main effect plot of rotation speed on wear resistance

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Table 3.1: ANOVA of wear resistance

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Figure 3.5: Interaction effect plot of rotational speed and welding speed on wear resistance

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Conclusion

Abrasion wear resistance of the aluminum welded joints increased with decreasing rotational and it increased with increasing welding speed, because of hardening happened in metal due to grain size reduction associate with lower heat input per unit length.

Effectiveness of FSW process depends mainly on two factors temperature and thermal exposure which are controlled by two competitive processes: heat generation and heat dissipation. The heat generation is mainly determined by the tools rotational speed. Increasing rotational speed result in increased frictional heat, the heat dissipation is mainly controlled by welding speed. Increasing welding speed increases heat dissipation rate because the heated FSW zone is quickly cooled, after the welding tool passes it, during the welding process resulting in reduced temperature and duration of thermal exposure.

ANOVA indicates that welding speed is the dominant factor in the experiment, and interaction effect of rotational speed and welding speed is not significant.

The optimal processing parameter’s in the working range to achieve higher abrasion wear resistance is 400 rpm, 101 mm/min.

Acknowledgements

The authors are grateful to the Departments of mechanical engineering, Al-Balqa’ Applied University for extending the facilities of Workshop and Material Testing Laboratory to carry out this investigation.

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