Solid-State Mg Heat Sink Direct Cooling Bonding with Refill Friction Stir Spot Bonding for Power Inverters

In this study, we spotlight the solid-state joining of Cu and Mg alloys for power applications without spatter and heat affected zone (HAZ). Refill friction stir spot welding (RFSSW) was applied to weld assemble Mg alloy <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(\text{AZ}31\mathrm{B})$</tex> and oxygen-free Cu (1100) sheets. By positioning the Cu busbar on the upper side and the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text{AZ}31\mathrm{B}$</tex> alloy on the lower side, and applying rotation and pressure to the Cu plate with the RFSSW tool, plastic flow was successfully realized at the Cu/Mg alloy interface. Consequently, as evidence of plastic flow at welding interfaces, it was revealed that the lap joint was firmly joined by a hook having a width and height of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$700 \mu\mathrm{m}$</tex>, and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$600 \mu\mathrm{m}$</tex> size at the interface of Cu and Mg alloys, and the mechanism by which the MgCu<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> phase was formed and grown as a secondary phase based on the hook during the solidification and cooling process was elucidated. This plastic flow-based hook fastening was substantially effective in bonding Mg alloys with poor weldability without defects. In this study, we investigated the defect of the joint and the microstructure of the RFSSWed joint.