The microstructure and mechanical properties of friction welded joints of Al–Cu alloy matrix composite and Al–Si–Cu–Mg alloy castings

Microstructures, Vickers hardness, tensile properties and rotated beam fatigue properties of friction welded aluminum-borate whisker reinforced Al–Cu alloy matrix composites (MMC) joints with Al–Si–Cu–Mg alloy (C355) castings were examined. The mixed layer of MMC and C355 is formed near the bonding interface by the shear stress during the rotated friction period. This layer has different composition from both Al–Cu alloy and Al–Si–Cu–Mg alloy, and the mixed layer has lower strength but also has a role to relax thermal stress during water-quenching after solution treatment. Softening occurs near the bonding interface and the joints have lower tensile properties than those of C355 alloy castings in as-friction welded joints. After T6 heat treatment, hardness of the joint is recovered and the joint has higher tensile and fatigue properties than those of C355–T6 alloy castings. It is concluded that a combination of the friction welding and post-weld T6 heat treatment is a reliable method to assure good performance for MMC/C355 joints.