Microstructure and mechanical properties of Mg-12Gd-3Y-0.4Zr magnesium alloy with layered heterogeneous microstructure fabricated by wire arc additive manufacturing

Interlayer cooling is an essential strategy for Wire arc additive manufacturing (WAAM) magnesium alloys to improve the strength. However, layered heterogeneous microstructure and anisotropy mechanical properties have also been improved by the interlayer cooling, which may affect the microstructure. In this study, an Mg-12Gd-3Y-0.3Zr (GW123K) alloy with a layered heterogeneous microstructure fabricated via WAAM with interlayer cooling strategy was investigated and a heat treatment method was proposed to reduce the anisotropy. The as-deposited WAAM-GW123K alloy primarily consisted of equiaxed α-Mg grains and eutectic phase Mg₂₄(Gd, Y)₅, and fine α-Mg grains were formed at the bottom of the layers while coarse α-Mg grains were formed at the top of the layers, leading to the heterogeneous microstructure. The average tensile strength and elongation of the as-deposited H-direction specimens (274.0 ± 5.3 MPa, 3.9 ± 0.8 %) were notably higher compared to those of the V-direction specimens (242.8 ± 10.9 MPa, 1.7 ± 0.9 %). During solution treatment, fine grains were more susceptible to coarsening than coarse grains due to the lack of eutectic phases at the grain boundaries. Thus a uniform microstructure can be made by proper solution temperature and time. After 480 °C/2 h solution and 200 °C/60 h aging, the average tensile strength of the H-direction specimens reached to 352.5 ± 15.1 MPa with an average elongation 0.7 ± 0.1 %, and the average tensile strength and elongation of the V-direction specimens were 336.7 ± 13.9 MPa and 0.5 ± 0.1 %. Decreased anisotropy was the result of microstructure homogenization.