Influence of carbon content on the primary solidification mode of high strength steels in resistance spot welding conditions

The primary solidification mode of resistance spot welds made of advanced high strength steels has been investigated as a function of carbon content. Electron backscatter diffraction and dendrite morphology analysis showed for the first time a transition from primary solidification into δ-ferrite (C ≤ 0.22 wt%) to primary solidification into austenite (C ≥ 0.29 wt%) with increasing carbon content. The description of this transition by using the so-called "interface response function" model is not fully satisfactory and strongly depends on input model parameters. Epitaxial growth from the heat-affected zone or from the slowly solidified zone could influence the solidification path.