Influence of dwell time on microstructure evolution and mechanical properties of dissimilar friction stir spot welded aluminum–copper metals

Abstract

1060 aluminum–T2 copper dissimilar lap joints were produced by friction stir spot welding (FSSW) with various dwell time. All the joints possess a Cu hook extruded upward from the lower Cu plate into the upper Al plate with intermetallic compounds (IMCs) developed on its interface. Increasing the dwell time produced an increase in the heat input during welding and promoted IMC growth. At short dwell time, the interface was characterized by the interruptedly distributed CuAl2 layer partially mingled with CuAl phase. However, continuous CuAl2–CuAl–Al4Cu9 laminated layer developed at the interface at longer dwell time. IMC formation sequence for CuAl2, CuAl and Al4Cu9 were determined by thermodynamic principles. Microhardness was quite different in different zones. Hardness values in the stir zone (SZ) were much higher due to the dispersively distributed IMC particles and the refined grains. Joints with better tensile properties have a higher penetration depth of the hook into the upper Al plate as well as a continuous IMC layer at the hook interface. Results of fracture path analysis indicated that all the fracture initiated at the CuAl2–CuAl or CuAl2–Al interface and then extended along the IMC layer at the hook interface. The dispersed IMC particles in the SZ provided an alternative path for crack extension when strong metallurgical bonding was achieved at the hook interface. Keywords: Friction stir spot welding, Aluminum/copper dissimilar metals, Microstructure evolution, Mechanical properties, Fracture path.