Accelerated Corrosion Rate of Wire Arc Additive Manufacturing of AZ91D Magnesium Alloy: The Formation of Nano-scaled AlMn Phase

Additive manufacturing (AM) technologies, with their high degree of flexibility, enhance material utilization in the fabrication of large magnesium alloy parts, effectively meeting the demands of complex geometries. However, research on the corrosion resistance of magnesium alloy components produced via AM is currently limited. This study investigates the microstructural and corrosion characteristics of AZ91D magnesium alloy fabricated by wire arc additive manufacturing (WAAM) compared to its cast counterpart. A large-sized AZ91D bulk part was deposited on an AZ31 base plate using a layer-by-layer stacking approach. The results showed that the WAAM AZ91D was featured by obviously refined grains from 228.92 μm of the cast one to 52.92 μm on the travel direction-through thickness (TD-TT) and 50.07 μm on the normal direction-through thickness (ND-TT). The rapid solidification process of WAAM inhibited the formation of β-Mg₁₇Al₁₂ phase while promoting the formation of uniformly distributed network of dislocations, the dispersive precipitation of nano Al₈Mn₅ phase, as well as Zn segregation. WAAM AZ91D demonstrated the occurrence of pitting corrosion and inferior corrosion resistance compared to cast AZ91D, attributed to the micro-galvanic corrosion between the α-Mg matrix and Al₈Mn₅ particles and the increased number of grain boundaries.