Resistance spot welding of dissimilar maraging steel and 4340 steel: correlation between microstructural analysis and mechanical performance

Abstract There are very similar applications between maraging steel and 4340 steel ranging from aerospace, automobile and other industries where spot welding is necessary. This paper focused on the resistance spot welding of dissimilar steels with a holistic characterisation of weld joints according to macroscopic and microscopic analyses, electron dispersive spectroscopy (EDS), as well as electron beam diffraction spectroscopy (EBSD). The joint configuration has been found to be achievable without macroscopic defects leading to a tensile-shear failure load of 4.3 kN, as well as the displacement at failure reaching 1.23 mm. In the texture analysis of lath martensite phase, ℇ-fibre < 110 >||TD in the weld zone with the high intensity of 16.5 MRD for the rotated goss texture {110} < 110 > appears with strong directional texture and directional properties. A high intensity of partial γ-fibre < 111 >||ND for reverted austenite has been identified, while the intensity could reach 10 MRD in the weld zone due to recrystallisation. EDS mapping suggests small Ti and Al oxide inclusions in the weld zone due to their affinity with oxygen thus protecting oxygen to reach grain boundary, thus enhancing mechanical properties. On the other hand, nano-hardness analysis and application of Tabor relation across all zones in the weld joint indicates that hardness and yield strength of heat-affected zone (HAZ) on the side of 4340 steel could reach 7.23 and 1.68 GPa respectively. The same is due to microstructure at HAZ on the side of 4340 steel reveals harder needle-like martensite phase. Nonetheless, the presence of Mn-Mo intermetallic with the span length of 300 µm causes crack initiation at HAZ on the side of 4340 steel, as evidenced by associated results obtained from EDS mapping analysis of the weld and fractographic analysis. Finally, the crack tends to propagate further through the defects in the weld zone taking place due to molten metal expulsion. The novelty lies in achieving a sound dissimilar joint between maraging steel 300 and 4340 steel with a plug ratio of 0.75.