Effect of Al Addition on Microstructure, Mechanical, and Corrosion Properties of Hot Extruded Mg-2.0Zn-0.4Mn Alloy

Microstructure, mechanical and corrosion properties of hot-extruded Mg-2.0Zn-0.4Mn with varying Al addition were investigated in this study. The results demonstrate that adding Al to Mg-2.0Zn-0.4Mn promotes dynamic recrystallization significantly during hot extrusion and improves the tensile mechanical properties and corrosion resistance of the hot-extruded alloy. The dynamic recrystallized grain sizes decrease while the fraction of dynamic recrystallization area increases with Al addition. This is owing to the promoted recrystallization nucleation rates attributed to the grain refinement or increased amount of grain boundaries in solutionized alloy and particle stimulated nucleation by undissolved second-phase particles. The quantity and size of second phase particles of extruded alloys also increase with Al addition, which, in turn, contributes to the refinement of recrystallized grains by grain boundary pinning effect. As a result, tensile properties of the extruded alloy have improved by alloying with Al, owing to the coupling effects of more solute atoms, reduced recrystallized grain size and presence of fine second-phase particles. The improved corrosion resistance of extruded alloys by Al addition is mainly attributed to Al solute atoms and relatively uniform extruded microstructure that facilitate the formation of a more compact passive film. However, galvanic corrosion induced by second-phase particles and the Mg matrix significantly mitigates this effect. Consequently, optimized Al addition to this extruded Mg-2.0Zn-0.4Mn alloy is determined to be 4 wt.%.