Influence of dissimilar plate thickness on temperature during friction stir welding between AA5083 and AA6061 aluminium alloy grades

This paper investigates the influence of dissimilar plate thickness on the temperature profile of AA5083 and AA6061 aluminium alloy. Plates of dissimilar aluminium alloy grades are welded in a butt joint configuration by using a milling machine and a custom FSW tool made of H13 tool. Datalogger and type K thermocouples embedded in the workpieces at different distances from the weld line are used to collect the temperature data during the experiment. One thermocouple is placed at each side of the advancing side and retreating side at equally distance from the center line except for Joint 1, 2 and 3. The results show the range of temperature measurement during the experiment is between 300°C and 600°C. FSW process between similar plate thickness produces the highest value of peak temperature and better surface appearance than dissimilar plate thickness. The thermocouples located near the weld line contributed the higher temperature since the heat source comes from the rotating tool. Besides, higher temperatures were recorded at 5 mm thickness of AA6061 aluminium alloy plate located on the advancing side than the retreating side.


Introduction
In recent years, joining metals to produce permanent joints has become one of the most popular joining technologies.Friction stir welding (FSW) is a solid-state joining process that is becoming increasingly popular as an alternative to fusion welding because it does not produce fumes and is environmentally friendly.Multi-material components, such as joints between dissimilar aluminum alloy grades, are often used in industrial structural applications because they can reduce cost and weight while improving performance [1].In FSW, the frictional heat generated between the rotating pin shoulder and the workpiece material causes severe plastic deformation [2].However, the tendency of this process to form defects is one of the main challenges of FSW between dissimilar aluminum alloys.These defects can occur due to improper selection of common FSW parameters, such as rotational speed, tool offset, plunge depth, and travel speed.In a previous study by Khan et al., it was found that tunneling defects can occur due to a decrease in heat input.This results in higher flow stress and poor mechanical properties, which can lead to a decrease in tensile strength [3].
In most FSW process, the formation of defects such as flash defects occurred at very high temperature due to an excessive material flow [4].Generally, the defect formation in FSW process is caused by incorrect settings of the parameters such as higher value of travel speed, slow rotational speed, inadequate plunge depth as well as tool pin offset.
Joining materials with dissimilar plate thickness can also be cost-effective for the manufacturing and automotive industries.A study by Sahu et al. on joining materials with dissimilar plate thickness showed that this parameter affects the distribution of heat generation during FSW [5].The concept of plate thickness ratio is used to investigate the dissimilar plate thickness during FSW.FSW between dissimilar plate thicknesses can be challenging due to improper surface contact between the shoulder of the rotating tool and the plates.A common method that researchers use to collect and measure temperature data is to embed thermocouples in the workpiece plates at different locations from the weld line or center line [6], [7].
In this study, the influence of dissimilar plate thickness on temperature during friction stir welding (FSW) butt joint between aluminum alloy grades was investigated.A conventional milling machine was used to perform the experiment.The experiment involved butt joining two dissimilar aluminum alloy plates, AA5083 and AA6061.A datalogger with type K thermocouples embedded in the workpieces was used to measure the temperature during FSW of dissimilar aluminum alloy grades.The effect of different placements of AA6061 and AA5083 aluminum alloy plates on the advancing and retreating sides during FSW on the temperature profile was investigated.

Experimental Procedure
AA5083 and AA6061 aluminum alloy plates with dimensions of 70 mm × 100 mm and thicknesses of 3 mm, 4 mm, and 5 mm were prepared.The plates were measured and cut using a shear cutter machine.The joining process was performed at the 70 mm side with a butt joint configuration using a milling machine.A clamping jig was used to clamp the two plates in a rigid position during FSW.A FSW tool made of H13 tool steel with a shoulder diameter of 20 mm, pin diameter of 5 mm, and pin length of 4.5 mm was fitted in the quill of the conventional milling machine during FSW.FSW tools with long cylindrical shapes are used for better joint performance, especially in the joining process between two dissimilar aluminum alloy plates in a butt joint configuration [8].
Four K-type thermocouples were equally spaced at the same distance from the center line on both sides of the plates for joints 1, 2, and 3.One thermocouple was placed on the advancing side and another one on the retreating side for all joints except for joints 1, 2, and 3. Holes were drilled at 1.5 mm and 4.0 mm from the center line for both specimens of AA5083 and AA6061 aluminum alloy plates to insert the thermocouples into the bottom of the holes, as shown in Figure 1 and Figure 2. The thermocouples embedded in the workpieces were used to measure the temperature readings, while a datalogger recorded the temperature data during the FSW process.
Several common parameters in the FSW process were kept constant for all joints: rotational speed at 1110 rpm, plunge depth at 0.2 mm, tool tilt angle of 3°, travel speed at 60 mm/min, and zero tool offset.The parameter that was studied in this experiment was the plate thickness of 3 mm, 4 mm, and 5 mm.Table 1 shows the 12 experiments that were investigated at different conditions using the specified parameters.3.This is because the higher plate thickness ratio results in lower heat generation rate due to improper surface contact between the shoulder of the rotating tool and the plates [5].FSW between two dissimilar grades with similar thickness (Joint 7) resulted in the highest temperature readings due to the higher heat generation compared to Joints 8 and 9.As shown in Figure 4, Joint 7 has the best surface appearance with the absence of flash defect, followed by Joint 8 with a small amount of flash defect.Joint 9 shows the presence of a high amount of flash defect due to improper surface contact between the rotating tool and the specimens, which leads to the formation of flash defect in this joint.Therefore, the higher maximum temperature obtained during FSW process with similar plate thickness could prevent the formation of defects.

Effect of different distance between the thermocouple and weld line on the temperature profile
Temperature readings for joints 1, 2, and 3 were obtained at two locations on each side of the plates to study the effect of different placements of the thermocouples from the weld line towards the temperature profile.The results indicate that the different distances of the thermocouples from the weld line affect the temperature readings during welding, as shown in Table 3 and Figure 5.The highest maximum temperature was recorded at 1.5 mm from the weld line, compared to 4.0 mm from the weld line on both sides of the plates.This shows that the maximum temperature increases as the distance between the thermocouples and the weld line decreases.This is because the heat source comes from the rotating tool during welding, and the thermocouples that are closer to the heat source will record higher temperatures [7].

Effect of different placements for dissimilar aluminium alloy plates on the advancing side and the retreating side towards the temperature profile
The results of the experiment between Joints 7, 8, 9, 10, 11, and 12 show that the maximum temperature of AA6061 aluminum alloy plates was higher on the advancing side than on the retreating side, as shown in Table 4 and Figure 6.Joints 7, 8, and 9 were joining processes between dissimilar aluminum alloy grades, where the 5 mm plate thickness of AA6061 aluminum alloy was located on the advancing side, while Joints 10, 11, and 12 were joining processes between dissimilar aluminum alloy grades, where the 5 mm plate thickness of AA6061 aluminum alloy was located on the retreating side.The results clearly show that AA6061 aluminum alloy plates with 5 mm thickness reached a higher peak temperature at Joints 7, 8, and 9 on the advancing side than at Joints 10, 11, and 12 on the retreating side.The advancing side of FSW joints tend to experience a higher temperature because the material on that side is subjected to higher strains, which creates more friction and heat [9].When the thicker plate is located on the advancing side and the thinner plate is located on the retreating side, the higher maximum temperature is recorded during the welding.However, Joints 11 and 12 have a better surface appearance than Joints 8 and 9 for the welding process with dissimilar plate thickness due to a small amount of flash defect formed on the plates, as shown in Figure 7 and Figure 8.Therefore, a better surface appearance on the plates is obtained when the thinner plate is located on the advancing side and the thicker plate is located on the retreating side.

Effect of similar and dissimilar material on the temperature profile during FSW process
Joining two dissimilar plates of AA6061 and AA5083 aluminum alloy resulted in higher maximum temperature than similar plates.The results show that the maximum temperature obtained during FSW process between dissimilar aluminum alloy plates at Joint 7 was the highest compared to the other joints with similar thickness plates of 5 mm, as shown in Table 5 and Figure 9. Joint 7 and 10 had little flash defects formed on the plates during welding compared to Joint 1 and 4.However, it was observed that Joint 1 had better surface appearance with the absence of tunneling defects due to proper heat input and adequate material mixing during FSW process [10].

Conclusion
In this study, an experiment was conducted to investigate the influence of dissimilar plate thickness during FSW process on the temperature profile.It was observed that the lower plate thickness ratio resulted in a higher maximum temperature during welding, as well as better surface appearance of the joint.Thermocouples were inserted at different locations from the weld line.The difference in temperature profiles showed that the thermocouple at the nearest location from the weld line tended to have a higher maximum temperature, since the heat source comes from the rotating tool during welding.It was observed that a higher peak temperature was obtained when the thicker plate was located on the advancing side than the retreating side during FSW process between AA6061 and AA5083 aluminum alloys.In summary, joining two plates with similar thickness produced better surface appearance due to little flash and tunneling defects after welding.The thermocouple located near the weld line gave the highest maximum temperature.However, there is a high probability that the thermocouple will touch the pin of the rotating tool if it is too close to the weld line, which can affect the temperature readings during welding.Finally, FSW process between dissimilar aluminum alloy grades with similar thickness gave the highest maximum temperature as well as better surface appearance during welding.

Figure 1 .
Figure 1.Placements of thermocouples from the centre line for Joint 1, 2 and 3.

Figure 2 .
Figure 2. Placements of thermocouples from the centre line for Joint 4 to 12

Table 1 .
Specimens used in this experiment at different conditions.

. Results and discussions 3
FSW was performed on aluminum alloy plates with dissimilar thicknesses of 3 mm, 4 mm, and 5 mm to investigate the influence of dissimilar plate thickness on the temperature profile.The results indicate that the dissimilar plate thickness of AA6061 and AA5083 aluminum alloy affects the temperature readings during welding, as shown in Table2and illustrated in Figure .1.Effect of dissimilar plate thickness on the temperature profile

Table 4 .
Peak temperatures when AA6061 of 5 mm thickness placed at AS and RS.