*3.4. Role of Heat Sink in 2219 Al-Cu Lap Joints*

In order to avoid melting of the Al plate in the Cu-Al stacking configuration found earlier, a number of trials were made with having the Cu plate as a sink under the Al plate. Two plates of Cu with thickness of 1.65 mm and 3 mm were utilized. The welding speed was varied over a range as listed in Table 10. The cracks and flakes of Cu were observed in many instances although the quality of the weld joint was significantly improved in terms of bubbles formation and cracking in comparison to the scenario(s) when no heat sink was used. The sound joint was achieved when thicker heat sink (3 mm Cu plate) was placed and the joining was performed at the rotational speed of 1500 rpm and welding speed of 30 mm/min as indicated in Table 10.

**Table 10.** Experiments to fabricate dissimilar Cu-2219 Al lap joint with WC square-tool using heat sink (TD: 20 s).


Table 11 compares the effect of stacking sequence on the joint strength. It can be noticed that the Al-Cu sequence offers around 4 times greater strength than the Cu-Al stacking. This happens due to a fact that the peaks temperature in Al-Cu stacking is 220 ◦C and that in Cu-Al stacking is 160 ◦C (Figure 13), which promotes material mixing and defect-free joining. Further, the cooling rate in the former case is higher that promotes the joint strength [27,28]. Moreover, Akbari et al. [14] have reported that weaker compounds are formed when joining is done with the Cu-Al stacking sequence. These, besides defects, impair the interfacial strength.



(**c**)

**Figure 12.** Redesign of dissimilar Cu-2219 Al lap joints fabricated by WC square-tool and using heat sink: (**a**,**b**) Cu 1 and (**c**) Cu 2.

**Figure 13.** Temperature distribution curves of 2219 Al-Cu and Cu-2219 Al fabricated by WC at S 1500 rpm and F 30.0 mm/min.
