Fundamental Study Of Selective Laser Melting Of Biodegradable Mg-RE Alloy Components

The study of Mg-RE alloys is one of the hotspots in the field of Mg alloy research.The high-strength Mg-RE alloys represented by Mg-Gd alloys have the advantages of excellent mechanical properties and biodegradability,showing great potential in the field of medicine.Selective laser melting（SLM）is a rapidly developing laser additive manufacturing technology in recent years,which is highly consistent with the needs of intelligent manufacturing and personalized medicine.It will bring great opportunities to apply SLM to Mg-RE alloy components’fabrication.In this study,the densification behavior,microstructure and mechanical properties of Mg-x Gd（x=3,6,10,15）binary alloys fabricated by SLM from pure Mg/Mg-Gd mixed powders in different proportion are systematically studied,and the alloys’prospects in medical application are explored.The effects of scanning velocity,hatching space and laser energy density per volume on densification of Mg-Gd binary alloys are studied by single-factor SLM forming process experiments.For the SLMed Mg-3Gd,Mg-6Gd and Mg-10Gd binary alloys,the optimal process parameters are determined and block samples with high relative density are obtained.However,there are some macro horizontal cracks in the Mg-15Gd binary alloy samples.The microstructure features of the alloys are analysed by XRD,SEM,and EBSD.Low energy input reduces the interaction time of the mixed powder in the molten pool and results in the decreasing of the composition uniformity.By proper energy input,the alloys’microstructure shows typical dendrite morphology withβ-Mg₅Gd phase distributed inα-Mg matrix.With the increasing of Gd content,the content ofβ-Mg₅Gd phase increases,and its shape gradually changes from isolated"dot like"to continuous"network".Besides,the content of Gd has a significant effect on the grain morphology of the SLMed materials.The SLMed Mg-3Gd binary alloy is composed of coarse columnar crystals with obvious preferential orientation and twins driven by residual stress.With the Gd content increasing,the columnar crystals are gradually refined,and the proportion of fine equiaxed crystals is increased.The twins disappear,and the texture intensity is gradually reduced.Mechanical property tests show that the microhardnesses of the SLMed Mg-3Gd,Mg-6Gd and Mg-10Gd binary alloys built with optimal process parameters are better than those of the heat-treated Mg-Gd extrusion alloys,and the tensile strengths are almost the same.But the elongations of the SLMed Mg-Gd binary alloys are generally lower.The relatively high microhardnesses and tensile strengths are the results of fine grain strengthening by rapidly solidified micro molten pools and secondary phase strengthening.The low elongations are mainly related to the hard brittle secondary phase and pore defects.According to the study,SLMed Mg-3Gd,Mg-6Gd and Mg-10Gd binary alloys may have a potential prospect in the field of biodegradable materials for the high relative densities,fine grains and good mechanical performances.