Resistance spot welding behaviour of novel medium manganese (M-Mn) steels - Role of welding parameters on weld microstructure and mechanical properties

Abstract

Medium Mn (M-Mn) steels are classified as third-generation advanced high-strength steels (3G-AHSS) and have demonstrated outstanding mechanical properties. As a result, they are being considered as potential materials for use in the automotive industry. Spot weldability of materials is an essential requirement for the body in white (BIW) structures in automotive vehicles. Resistance spot welding (RSW) M-Mn steel can be challenging because it can lead to the formation of a fully martensitic microstructure. This work established the weld growth curve on M-Mn steel in the 4 to 8 kA current range. Based on this, single pulse (6 kA) and double pulse current (first pulse of 6 kA followed by the second pulse of 4 to 7 kA) were employed to investigate the microstructure, elemental segregation, cross-tensile strength (CTS), and fracture behaviour of the M-Mn steel welds. These welds, in all conditions, exhibited a martensitic microstructure with retained austenite content of <10 %. In all welding conditions, martensite softens along the heat-affected zone (HAZ) boundary, yielding tempered martensite and carbides. A study of chemical homogeneity using electron probe microanalysis (EPMA) demonstrated that Mn and Si segregate along the weld nugget (WN) edge during single (6 kA) and double (6–6 kA and 6–7 kA) pulsing. Mn and Si segregation is less prominent in double pulse welds at lower pulse currents (<6–6 kA). The elemental segregation at higher pulse currents is higher due to the more pronounced thermal gradient within the weld nugget, which promotes elemental segregation at the interface. Based on the cross-tension test results, the double pulse welds (6–7 kA) had the maximum CTS (1.26 kN) and energy absorption (2 J). The failure mode in this condition is a predominantly partial interfacial failure (PIF) with a plug ratio of about 50 %. This is attributed to the larger nugget diameter and refined martensitic structure. Complete interfacial failure (IF) leading to interdendritic fracture and a plug ratio of 0 % was observed when two equal pulse currents (6–6 kA) were applied. This can be attributed to remelting of WN and a high degree of Mn and Si segregation. From the above studies, we can conclude that M-Mn steel is weldable; however, further strategies are necessary to enhance the CTS of M-Mn steel welds.