Improving Interfacial Microstructures and Tensile Properties of Active–Passive Filling Friction Stir Repaired ZL210 Aluminum Alloys by Optimizing Shoulder Morphologies

To eliminate the casting defects in the ZL210 aluminum alloys, the active–passive filling friction stir repairing (A-PFFSR) was used. Shoulder morphologies on the surfaces of the pinless tools were designed and optimized to improve the mechanical properties of the repairing joints. The detailed microstructures, interface behaviors, microhardness, and mechanical properties were investigated. The casting defect was first drilled into a keyhole shape by a threaded pin. The keyhole defect was successfully filled by the materials surrounding the keyhole and an extra filler via the A-PFFSR using pinless tools with grooves on the shoulder surface. Increasing the number of groove was beneficial in improving frictional heat and eliminating interfacial defects. The extremely higher heat input easily resulted in severe joint softening. The filling zone consisted of the finer and more uniform grains, leading to the higher microhardness values. The maximum tensile strength of the optimized joint was quasi equivalent to that of base metal.