*3.2. Selection of Tool Material to Fabricate Dissimilar Al*/*Cu Lap Joints*

In an attempt to opt an appropriate tool material for successful welding of Al/Cu joints, two dissimilar lap joints of 2219 Al-Cu and Cu-1060 Al were fabricated employing two types of tool materials namely HSCo Steel and WC Carbide. The pin geometry in both cases was square because, as found above, this geometry offered better results in comparison to other considered ones. The complete set of conditions has been listed in Table 6.


**Table 6.** Fabrication of 2219 Al-Cu (1.65 mm) lap joint using WC and HSCo square-pin tool (TD: 22 s).

Lap shear tests were carried out in order to assess the mechanical performance of lap joints. The joints and their strength results are shown in Table 7. As observable, both 2219 Al-Cu and Cu-1060 Al dissimilar joints exhibit fairly good strength when welding was done utilizing the WC tool. Furthermore, the WC tool offers greater joint strength than the HSCo-Steel tool. For example, the joint strength of the 2219 Al-Cu joint is doubled when the tool material is altered from HSCo-steel to WC. The strength gain in the case of the Cu-1060 joint is even better (about 29 times). Figure 9 presents the temperature profiles recorded during welding. The peak temperature obtained with the WC tool is 250 ◦C and that obtained with HSCo-steel tool is 150 ◦C. This difference in the temperature is due to a fact that the WC material has higher conductivity (110 W/mK) than HSCo-steel (60 W/mK). As a result, the workpiece experiences greater heat input that in turn helps in proper mixing and defect free welding when the WC tool is employed. This can be witnessed from the SEM images shown in Figure 10 wherein joints produced with the WC tool are sound while those fabricated with the HSCo-steel tool suffer from defects like pores and voids. These shreds of evidence support why the WC tool produces sound joints with high strength.

**Figure 8.** Dissimilar 2219 Al-Cu lap joints fabricated employing different tool materials: (**a**–**d**) WC square-pin tool at scale, (**e**) HSCo square-pin tool.

**Table 7.** Comparison of joint strength between dissimilar lap joints fabricated with WC and HSCo-steel tools.

**Table 7.** *Cont*.

**Figure 9.** Effect of tool material on temperature profiles during FSW of 2219 Al-Cu joints with square-pin tools.

**Figure 10.** Scanning electron microscopy of 2219Al-Cu lap joint at F 30.0 mm/min using (**a**): WC tool, (**b**) HSCo square- tool.
