Simultaneous improvement in strength and ductility of friction stir welded Mg/Al joints by reducing micro-intermixing

Thread of pin has been proved beneficial to increasing shear action of tool and material intermixing, which however induces abundant formation of intermetallic compounds (IMCs) in Mg/Al dissimilar friction stir welding (FSW) leading to deteriorating the performance of joints. In this work, Mg alloy and Al alloy were well joined by threaded pin and unthreaded pin under the same welding parameters. For threaded pin joints, cracks occur in the banded structure (BS) region and bottom interface, then the main crack propagates along the Mg/Al interface, resulting in low strength and elongation. For unthreaded pin joints, necking and fracture occurred at the softened Al alloy to the retreating side (SAA-RS), and the fracture mode changed from brittle fracture (Mode I) to ductile fracture (Mode III). The tensile strength and strain of unthreaded pin joints are 201 MPa and 7.14 % respectively, significantly higher than those of threaded pin joints (i.e. 140 MPa and 1.15 %), and is the highest combination of stress and strain reported up until now. An unthreaded pin could reduce the local micro-intermixing between Mg and Al to suppress the formation of IMCs, which reduced the hardness in the BS region, preventing the formation and propagation of cracks.