Intermetallic compounds: Formation mechanism and effects on the mechanical properties of friction stir lap welded dissimilar joints of magnesium and aluminum alloys

Abstract

The formation mechanism of intermetallic compounds (IMCs) during dissimilar friction stir lap welding (FSLW) of Mg and Al alloys, as well as the effects of IMCs on the shearing properties of FSLW joints, are still ambiguous and require clarification. In this study, FSLW dissimilar joints of AZ31 Mg alloys and Al–Zn–Cu–Mg alloys are investigated to clarify the formation mechanism of IMCs and their effect on the joint strength. Microstructures and shearing properties of the joints are characterized. The results showed that new phases of Al12Mg17, Al3Mg2, MgZn2, and Al2Cu are generated at the welding interface, and the original Cu2Mg phase is dissolved into the Al matrix during FSLW. Furthermore, 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 concentrations in different binary systems. The order of IMC formation is as follows: Al12Mg17, Al3Mg2, MgZn2, and Al2Cu. During shearing deformation of the FSLW joint, the crack is initiated at the hook zone in the retreading side and propagated along the welding interface, where IMCs aggregate and the strain concentrates. The small lattice mismatch between the IMCs and the matrix improves their interfacial bonding strength, which increases in the following order: Al12Mg17 and Al, Al12Mg17 and Mg, Al3Mg2 and Al, Al3Mg2 and Mg, MgZn2 and Al, MgZn2 and Mg, Al2Cu and Al, Al2Cu and Mg, Cu2Mg and Mg, and Cu2Mg and Al.