Microstructure and mechanical properties of Kovar/Cu dissimilar alloy joints produced by friction stir spot welding

Developing the Kovar alloy/Copper composite is an effect way to enhance the thermal conductivity of Kovar alloys. In this study, Kovar alloy/Copper composites are successfully prepared via friction stir spot welding (FSSW). Microstructure, diffusion behavior and recrystallization behavior at the welding interface and their effect on mechanical properties of the welding joint are investigated and discussed. During FSSW, severe plastic deformation and high temperature promote the elements’ diffusion at the interface. Using a FSSW parameter of 1000 rpm-90 s, a 6.9 μm diffusion layer was found at the interface, showing a typical metallurgical bonding. Meanwhile, mechanical interlocking was observed at the interface because of the hard wall effect on Kovar alloy. In addition, dynamic recrystallization occurred on Cu side, resulting in complex microstructure in the stir zone. After FSSW, the nano-hardness increased gradually from Cu side to Kovar alloy. At the interface, the nano-hardness are 2.26 GPa, 2.44 GPa and 2.54 Gpa for S-42, S-90 and S-120, respectively. S-90 has the highest tensile-shear stress (2.17 KN) due to the combining effect from metallurgical bonding and mechanical interlocking.