Medium Manganese steel (MMS) is a promising third-generation high-strength steel having the potential to improve the fuel efficiency and crashworthiness of modern vehicles. However, to form larger components, these need to be resistance spot welded in similar and dissimilar configurations, which is a challenging task owing to its high carbon equivalent and the tendency for elemental segregation. In the present work, the welding current range for similar and dissimilar combinations of MMS and high-strength low alloy (HSLA) steel sheets of ~1.5 mm thickness is determined. It is shown that welding simulation using SORPAS can accurately predict the onset of nugget formation and expulsion. An interesting observation of the shifting of the onset of welding as well as expulsion towards lower welding current is noted for the MMS-MMS combination compared to MMS-HSLA and HSLA-HSLA configurations, which is explained based on the online measurement of the dynamic contact resistance (DCR). MMS showed remarkably higher contact resistance due to high solute content and higher density of grain boundaries owing to finer microstructure. In terms of tensile-shear load-bearing capacity, MMS-MMS configuration showed ~1.5 times higher peak load compared to the MMS-HSLA configuration despite an occurrence of interfacial fracture in the former case compared to the pull-out mode of failure in the latter case. Such an improved load-bearing capacity of the MMS-MMS joint is explained based on the higher hardness of the fusion zone coupled with transformation-induced plasticity (TRIP effect) arising from the presence of reverted austenite film in its nugget. The industrial usefulness of these results and the future direction of research are also proposed.

Keywords: Medium Manganese steel; Third generation AHSS; Resistance spot welding; Transformation-induced plasticity; Martensite