Microstructure, mechanical properties and failure mechanisms of resistance spot welding joints between ultra high strength steel 22MnB5 and galvanized steel HSLA350

Resistance spot welding of 22MnB5/HSLA350 and its weldability are investigated. From base material to the nugget, the microstructure of 22MnB5 side can be expressed as M/tempered M+ granular carbides/F+M/fine M/lath M, and on HSLA350 side, F+M/F+flocculent P+tempered M/F+flocculent P+M/F+M/lath M are observed. The heat-affected zone (HAZ) of 22MnB5 exhibits obvious softening region and strengthening region caused by the tempered M and quite fine M, respectively. The peak load of welded joints undergoes two stages of a dramatic increase and a subsequent decrease with increases in the welding current. The maximum value (13.82 kN) of peak load is achieved at a welding current of 8 kA. The failure modes involving interfacial failure (IF), pullout failure from the galvanized steel (PFG), pullout failure from boron steel and tearing of the galvanized steel (PFB-TG) and pullout failure from the boron steel (PFB) are discussed in detail.