Evolution mechanism of intermetallic compounds and the mechanical properties of dissimilar friction stir welded QP980 steel and 6061 aluminum alloy

Intermetallic compounds (IMCs) play an important role in the welding quality of steel and aluminum (Al) alloys by friction stir welding (FSW). However, the evolution mechanism of IMCs and their effects on the mechanical properties of FSW joints are still unclear and need to be further elucidated. In this work, QP980 steel and 6061 Al alloy are successfully welded by FSW. The microstructure and mechanical properties of joints are characterized. The results show that an IMC layer composed of Al3Fe, Al5Fe2, Al2Fe, and Al6Mn phases is generated at the weld interface. The joint efficiency is about 70.1%. The formation sequence of IMCs in the same binary system is affected by the effective enthalpy of formation, while it is related to the element concentration in different binary systems. Furthermore, the formation sequence is Al3Fe, Al5Fe2, Al2Fe, and Al6Mn phases. The interfacial bonding strength is enhanced by the low lattice mismatch between IMCs and matrix. The interfacial bonding strength of IMCs with Fe and Al matrices in the joint is in ascending order: Al2Fe and Fe, Al2Fe and Al, Al5Fe2 and Fe, Al5Fe2 and Al, Al6Mn and Fe, Al3Fe and Fe, Al6Mn and Al, as well as Al3Fe and Al. It is noted that the bonding strength between the Al2Fe and Fe is lowest, indicating that Al2Fe phase is responsible for the joint fracture during tensile test.