Simultaneous Improvement in Load-Bearing Capacity and Energy Absorption of Resistance Spot-Welded Medium Mn Steel: An Integrated Simulation and Experimental Approach

Abstract

Third-generation advanced high-strength medium manganese steels (MMS) are a promising candidate for crash-resistant applications in the automotive industry due to their superior strength-ductility synergy. However, their resistance spot welding, which is the predominant joining technique in the automotive sector, is a challenge due to the high carbon equivalent and the destruction of tailored microstructure due to the weld thermal cycle. In this work, an integrated simulation and experimental approach is utilized to thoroughly understand the weldability of a medium Mn steel with ~ 850 MPa tensile strength and a high carbon equivalent of 1.08. The nugget diameter obtained through thermal simulation of spot weld joints after coupling the electrical resistivity and thermodynamic data led to an excellent match with the experimentally observed values. Further, an adequate weld nugget diameter meeting the 4√t criteria was achieved at 6 kA and above welding currents. Spot welds at 7 kA showed a remarkably improved load-bearing capacity (25 pct higher) and energy absorption (80 pct higher) compared to that fabricated at 6 kA welding current despite the occurrence of interfacial failure in both the cases. The improved toughness manifested as a transition in failure mode from interdendritic brittle fracture at 6 kA to quasi-cleavage fracture at 7 kA. The simultaneous improvement of strength and toughness was attributed to (i) weld joint free from solidification cracks despite extensive Mn segregation, (ii) the greater extent of auto-tempering in the fusion zone, and (iii) notable deformation-induced austenite → martensite transformation (TRIP effect) for 7 kA weld joints. The present study provides the pathway for designing strong and tough weld joints of third-generation AHSS to harness their full potential in load-bearing applications.