*3.1. Selection of Tool Geometry to Fabricate Dissimilar Al*/*Cu Lap Joints*

Experiments were performed to select suitable tool geometry to successfully produce Al/Cu lap joints. Based on literature reports [24–26], two shapes of pins namely round tapered and square were utilized (Figure 1). The shoulder diameter in each case was 16 mm, the pin height was 3.2 mm, and the other details were as listed in Table 2. The commercial Cu sheet (1.65 mm thick) was placed on the top of the 1060 Al sheet (4 mm thick) to fabricate dissimilar lap joint. The welding was performed employing the tapered-pin tool made of high-speed cobalt (HSCo) steel (Figure 1a). The attempts were made by employing the rotation speed of 1500 rpm and varying the feed from 23.5 mm/min to 47.5 mm/min as listed in Table 4. As shown in Figure 7a and indicated in Table 4, surface cracks were generated while welding thus rendering all of the attempts to be unsuccessful. Moreover, the HSCo tapered-pin tool was observed to have a shorter life in these tests wherein pin detachment led to complete failure of the tool.

The second experimental plan was launched to select appropriate tool geometry with the square-pin tool. As summarized in Table 5, the square-pin tool contrary to the tapered-pin tool apparently produced successful joints as observable from Figure 7b,c. However, this tool also encountered failure after a few tests as indicated in Figure 7c whereby a pin left inside the material can be noticed. The observation from these primary tests revealed that the square-pin tool produced comparatively successful joints because its squared corners while rotating in a circle generates waves that help in the breaking and mixing of materials [24]. However, the issue regarding the breakage of the tool-pin that probably occurred due to tool softening owing to heating while welding yet needed to be resolved. Therefore, further tests were performed by varying the tool material (WC) as detailed in the coming section. A test experiment was performed through the WC square-pin tool exactly on the same parameters (S 1500 rpm and F 36.5 mm/min) as applied for the HSCo square-pin tool to check the success of the welded joint (Cu-Al) and the strength of WC tool as shown in Figure 7d.

**Figure 7.** 1060Al-Cu lap joints produced; HSCo (**a**) tapered-pin tool, (**b**,**c**) square-pin tool, and (**d**) tungsten carbide (WC) square-pin tool.


**Table 4.** Experimental conditions employed to fabricate Cu 1-1060Al lap joints using HSCo-steel tapered-pin tool (TD: 22 s).

**Table 5.** Experimental conditions employed to fabricate Cu 1-1060Al lap joints using HSCo square-pin tool (TD: 22 s).

