Mechanical Properties and Failure Behavior of Resistance Spot Welded Medium-Mn Steel under Static and Quasi-Static Shear-Tension Loading

Medium-Mn (M-Mn) steels have received considerable attention in the last few years as they possess excellent mechanical properties and in situ strain hardening capability due to the transformation-induced plasticity (TRIP) effect. Despite intensive studies being conducted on the uniaxial tensile properties of M-Mn steels, no research has been undertaken on the mechanical properties of resistance spot welded M-Mn steels under quasi-static loading conditions, which are crucial for designing vehicles capable of withstanding high impact loads. The present study has examined the influence of loading speed between 1 and 100 mm/min on mechanical properties and failure behavior of M-Mn steel spot welds during shear-tension loading. The results showed that the strength and ductility of the spot welds enhanced with increasing loading speed. Strain analysis utilizing digital image correlation (DIC) and finite element analysis (FEA) showed that the higher loading speed resulted in a significant increase in plastic strain localization at the fusion zone (FZ) and heat-affected zone (HAZ). Consequently, the higher strain caused the occurrence of strain hardening in the weld areas, which in turn increased the hardness and strength of the weldment. This lastly led to an improvement in the mechanical properties of welded joints with higher loading speeds.