Corrosion behavior of Mg-11Gd-2Zn-0.4Zr alloy before and after heat treatment: Additive manufacturing versus casting

Additive manufacturing of Mg alloys via laser powder bed fusion (LPBF) has garnered increasing interest due to its capacity to produce superior mechanical properties compared to conventional as-cast alloys. However, the corrosion resistance of as-built Mg alloys relative to their as-cast counterparts remains poorly understood. In this study, a high-strength Mg-11Gd-2Zn-0.4Zr (wt.%, GZ112K) alloy is fabricated using both LPBF and semi-continuous casting techniques. The corrosion behavior of both as-built and as-cast alloys is systematically evaluated before and after solution and aging (T4 and T6) heat treatment processes. Electrochemical and immersion tests reveal the following corrosion resistance ranking: As-built < LPBF-T6 < LPBF-T4 < F-T4 < F-T6 < As-cast. The as-built alloy, despite having finer grains and (Mg,Zn)₃Gd eutectic secondary phase that enhance tensile properties, exhibits poorer corrosion resistance primarily due to the network distribution of (Mg,Zn)₃Gd along grain boundaries, which lacks a corrosion barrier effect while increasing the cathode-to-anode ratio. During T4 treatment, the (Mg,Zn)₃Gd phase transforms into blocky X phase along grain boundaries and lamellar long-period stacking ordered (LPSO) structures within the grains, reducing Volta potential differences, thereby improving corrosion resistance. However, the formation of β′ aging precipitates in the LPBF-T6 alloy increases the micro-galvanic corrosion tendency, reducing corrosion resistance. This study underscores the critical role of T4 and T6 heat treatments in optimizing the corrosion resistance of additively manufactured Mg alloys.