Multi-scale precipitates for improved strength and ductility of AZ91D magnesium alloy fabricated using laser penetrating strip directed energy deposition

Abstract

To reduce the cost of additive manufacturing and improve the strength and plasticity of AZ91D magnesium (Mg) alloy, an effective directed energy deposition (DED) technique was proposed using metal strip as raw material and high energy density laser as heat source. Laser penetration of strips for additive purposes, defined as laser-strip DED process. The deposited components showed that equiaxed α-Mg grains, discontinuous β-Mg₁₇Al₁₂, and multiscale Al₈Mn₅ phases were generated. The values for yield strength (YS), ultimate tensile strength (UTS), and elongation (EL) acquired from the build direction (BD) were 141.0 ± 1.2 MPa, 251.0 ± 0.9 MPa, and 14.0 ± 0.3 %, respectively. In contrast, the measurements taken from the traveling direction (TD) resulted in 156.0 ± 1.6 MPa, 257.0 ± 1.1 MPa, and 14.0 ± 0.5 %, respectively. The introduction of laser oscillations facilitated uniform heat distribution, promoting overall grain refinement. The multi-scale Al₈Mn₅ phase was in-situ formed during deposition, and Al-Mn nanophase provided heterogeneous nucleation sites for the refinement of the β-Mg₁₇Al₁₂. The deformation process induces tensile twins, forming a composite phase with the β-Mg₁₇Al₁₂ phase and transfer of dislocations, which relieves the stress concentration and improves the plasticity of the material. During deformation, the multi-scale Al₈Mn₅ phase hinders dislocations within the matrix and induces {10–12} tensile twin by stress concentration. This paper provides ideas for additive manufacturing technology with low cost, and high performance.