Investigation on microstructure and mechanical properties of dissimilar friction stir lap welding of AE44 Mg alloy and DP340 steel

Abstract

The effective connection of magnesium and steel is of great significance to the weight reduction of vehicles. Mg-3.66Al-2.29Ce-1.32La (wt%) magnesium alloy (AE44) and DP340 steel was successfully jointed by friction stir lap welding (FSLW) with significant plunge depth of the tool. The joint shear strength exceeds 396 N/mm, which derived from multi-scale strengthening mechanisms. The microstructure and formation mechanism of the macroscopic Mg and steel hooks embedded into the other's plates were thoroughly analyzed. Multivariate linear regression of geometrical parameters of hooks shows that a small depth of Mg and steel hook, narrow width of steel hook, large width of Mg hook and small aspect ratio of magnesium hook can increase joint strength. Simultaneously, microscopic interlocks like sawtooth formed on the interface makes the Mg/steel interface fully contact and limits the initialization of deformation, resulting in an effective joining. Due to deep plunge depth, considerable element diffusion occurred at the interface and an interlayer composed of Fe₂Al₅ phase was formed. Fe₂Al₅ phase and adjacent steel grains exhibited an orientation relationship of [111]_Fe//[100]_Fe2Al5 and (002)_Fe2Al5//(10$\overline{1}$)_Fe. Furthermore, the semi-coherent interplanar mismatches occurred at Fe₂Al₅/Fe interface.