Synergistic optimization of strength-ductility and corrosion resistance of selective laser melted AZ91D magnesium alloy by controlling β-Mg₁₇Al₁₂ phase

Selective Laser Melting (SLM) has great potential for fabricating high performance and complex parts but its application in Mg alloys is rather limited due to the trade-off between corrosion resistance and the strength-ductility properties. In this work, the properties of SLM-fabricated AZ91D alloy have been investigated as a function of heat treatment conditions and the evolution of microstructure has been quantified. It has been found that the size, distribution, and morphology of the β-Mg₁₇Al₁₂ phase are the core factors determining the comprehensive performance of the alloy. At T4 heat treatment, nearly complete dissolution of the β-phase has been achieved, while the formation of a coarse discontinuous β-phase network along grain boundaries has been observed after T5 heat treatment. Dispersed precipitation of fine needle-like β-phase within the matrix are observed after T6 heat treatment, resulting in optimal strength-ductility balance of the alloy. The strengthening mechanism was dominated by dislocation strengthening, and supplemented by Orowan strengthening, reaching an ultimate tensile strength of 308 MPa. Micro-galvanic corrosion between the β-phase and the a-Mg matrix has been observed, whereas the uniform distribution of precipitates improves corrosion resistance. Therefore, the mechanism of tailoring both strength-ductility together with corrosion resistance by controlling β phase size and distribution has been found for SLM-fabricated AZ91D alloy, providing further process optimization strategy for additively manufactured Mg alloys.