Laser Powder Bed Fusion Process Of Rare Earth Magnesium Alloy Implants And Studies Of Degradation Properties

Rare-earth（RE）magnesium alloys have excellent mechanical properties,moderate elastic modulus,and good biocompatibility,showing great application prospects in bone repair field.Laser powder bed fusion（LPBF）,as a typical metal additive manufacturing technology,is particularly suitable for rapid manufacturing of personalized medical implants.However,the presence of tissue defects in as-built part tends to affect mechanical and degradation properties.Therefore,LPBF technology is first used to prepare RE magnesium alloy implants in present study,and improves the parts forming quality through optimizing process strategies.Then,heat treatment is adopted to regulate the microstructure and improve the mechanical properties of as-built part,and finally introduces a long period stacking ordered（LPSO）structure to further improve corrosion resistance.The main conclusions and innovations are as follows:（1）The effect of process parameters on microstructure of RE magnesium alloy prepared by LPBF was investigated.At low energy density,the insufficient liquid phase in molten pool leads to high viscosity and difficult flow,resulting in a large number of underfusion pores;With the energy density increased,a large amount of magnesium vapor drives the formation of keyhole,and some vapor is easily trapped in the solidification layer,which leads to the formation of pores.At a suitable energy density,the keyhole can be closed in time under the surface tension of molten pool,thereby achieving the densification of deposited layer during cooling process.Under the conditions of laser power 60 W and scanning speed 800 mm/s,the density of the as-built parts can reach 99.9%.（2）The effect mechanism of heat treatment process on microstructure and comprehensive properties of as-built parts is revealed.It is found that the residual stress of as-built part can be eliminated after heat treatment.Besides,the recrystallization ofα-Mg grains effectively weakens the texture strength,thereby improving anisotropy.At the same time,the relatively coarse Mg₄₁RE₅ phases are transformed into fine and uniform Mg₂₄Y₅ phase during heat treatment.Therefore,the tensile yield strength and ultimate strength were increased to 250.2±3.5,312±3.7 MPa,and the elongation was maintained at 15.2%.In addition,uniform microstructure reduces the tendency for local corrosion,which promotes the formation of uniform passivation films,and thus improves corrosion resistance.（3）The mechanism by which the long-range ordered stacking（LPSO）structure improves the corrosion resistance of magnesium alloys was identified.During heat treatment,the precipitatedβ-（Mg,Zn）₃Gd phase dissolved inα-Mg,and reduced the energy threshold of stacking faults on basal planes,which finally triggered the formation of long period stacking ordered（LPSO）phase.The LPSO phases owned minor potential difference withα-Mg,thus causing less micro-galvanic corrosion tendency as compared toβ-（Mg,Zn）₃Gd phase.More importantly,they were uniformly distributed within theα-Mg grains and showed different orientations between adjacent grains.As a result,the LPSO-reinforced Mg-Zn-Gd tended to expand laterally during corrosion evolution,and achieved uniform degradation with a considerably reduced degradation rate of 0.34 mm/year.