Defect, microstructure and mechanical properties of Mg–Gd binary alloy additively manufactured by selective laser melting

Abstract

The high-performance rare earth magnesium alloys such as Mg–Gd based alloys are becoming the research focus of Mg industry. In this paper, a series of Mg-xGd (x = 3, 6, 10, 15, wt. %) binary alloys were additively manufactured by using the selective laser melting (SLM) technology. The influence of Gd content on the defects, microstructures, and mechanical properties of the SLM alloys were studied. The result shows that the Mg-3Gd, Mg-6Gd, and Mg-10Gd SLM samples are well-built with high density. In contrast, macro-cracks growing perpendicular to build direction occasionally occur on the Mg-15Gd samples. The phase composition of all the SLM samples includes α-Mg matrix and Gd2O3 oxidation inclusion, and the Mg-6Gd, Mg-10Gd, and Mg-15Gd samples also contains eutectic β-Mg5Gd. With the increase of Gd content, the morphology of the eutectic β-Mg5Gd turns from lamellar to network-like, whilst the grain morphology of the α-Mg matrix changes from coarse columnar to fine equiaxed and the texture intensity decreases. The increase of Gd content enhances the effects of fine grain strengthening and secondary phase strengthening, so that the microhardness and tensile strength of the SLM samples improve accordingly. The average microhardness and ultimate tensile strength of the Mg-15Gd SLM sample are as high as 106.9 HV and 386 MPa, respectively. On the other hand, the elongation of the SLM alloys shows a “Λ-shaped” variation trend with the increase of Gd content, reaching the maximum value when the Gd content is 10 wt %, due to the combined influence of the changes in grain size, texture intensity, and the morphology of eutectic β-Mg5Gd. Despite the SLM Mg-15Gd alloy has the highest microhardness and tensile strength, its practicality is restricted by the occasional formation of macro-cracks.