Microstructure evolution in refill friction stir spot weld of a dissimilar Al–Mg alloy to Zn-coated steel

Abstract

In the present study, dissimilar welds of an Al–Mg–Mn alloy and a Zn-coated high-strength low-alloy steel were welded by refill friction stir spot welding. The maximum shear load recorded was approximately 7.8 kN, obtained from the weld produced with a 1600 rev min⁻¹ tool rotational speed. Microstructural analyses showed the formation of a solid–liquid structure of an Al solid solution in Mg–Al-rich Zn liquid, which gives rise to the formation of Zn-rich Al region and microfissuring in some regions during welding. Exposure of steel surface to Mg–Al-rich Zn liquid led to the formation of Fe₂Al₅ and Fe₄Al₁₃ intermetallics. The presence of defective Zn-rich Al regions and Fe–Al intermetallics at the faying surface affects the weld strength.