Effect of Mg remelting and mechanical hooks of steel on the mechanical and fatigue responses of resistance element welded AZ31/DP780 joints: Experimental, FEM and thermodynamic calculation studies

This study focuses on the fatigue fracture response and microstructure behavior of a resistance element weld (REW) in assembling AZ31B magnesium alloy and dual phase (DP) 780 plates. With the increase of the welding current the prior austenite grain size (PAGS) increased, which resulted in a decrease in the fraction of an alpha prime martensite at the welding interface and had little influence on the dual phase at the welding interface. It is clear that the formation of a suitable hook accompanied by the phase transformation between S20C elements and DP780 steels at the interface can have a positive effect on the mechanical properties. However, the Mg melting and the formation of a partially melted zone (PMZ) affected the elongation decrease significantly. Furthermore, the insufficient penetrations of the welding interface, the severe melted Mg and the formation of porosities in the PMZ were fatal to the strength and fatigue performance. Morphologies and microstructure characteristics including phases of the REWed joint were quantitatively analyzed by electron back-scatter diffraction (EBSD) and thermodynamic calculation by coupling the phase diagrams (CALPHAD) method, respectively. The experiment results were incorporated into the finite element method (FEM) to numerically elucidate their effects against mechanical responses including fatigue behaviors. The results explained why the mechanical strength, fatigue endurance, and fracture modes are radically different for joint morphologies and microstructure developments.