Effect of external magnetic field on microstructure and mechanical properties in resistance spot welding of microstructurally inhomogeneous high-pressure die casting aluminum alloy

Abstract

High-pressure die casting (HPDC) aluminum alloys are widely used in lightweight automotive body-in-white structures. In addition to large single-piece castings, segmented HPDC components joined by resistance spot welding (RSW) remain common, offering cost and manufacturing flexibility for high-volume production. However, the microstructural inhomogeneity inherent in HPDC aluminum alloys presents substantial challenges to RSW. To address this, this study employs a novel multi-pulse magnetically assisted resistance spot welding (MPMA-RSW) process, which combines multi-pulse (MP) scheduling with an external magnetic field (EMF) to enhance the weld quality of 2.5 mm AlSi7MnMg sheet joints. The results indicate that repeated melting and solidification of a microstructurally inhomogeneous base material (BM) lead to the formation of thick and low-hardness partially melted zones (PMZ) within the nugget. Cracks and low-hardness PMZ influence the fracture path of AlSi7MnMg sheet joints. The EMF creates a three-dimensional composite flow pattern inside the nugget, which interrupts the growth of columnar grain zones (CGZ), transforms directional solidification into nearly simultaneous solidification, thereby suppressing hot crack formation in the nugget, and promotes the development of high-hardness equiaxed grain zones (EGZ) that extend to the edge of the nugget. Ultimately, the MPMA-RSW process increases nugget diameter by 26.6 %, peak lap-shear force by 23.2 %, and peak energy absorption by 58.1 %. These findings validate the weldability of the new material and provide a theoretical basis for process optimization.