Study On Laser Powder Bed Fusion Additive Manufacturing ZK60 Magnesium Alloy For Vascular Stent And Its Corrosion Resistance Mechanism

Mg and Mg alloy have advantages such as biodegradation,biocompatibility,specific strength and Young’s modulus close to human bone,and induction of bone growth,and are one of the research hotspots of degradable biomedical materials.Laser powder bed fusion（LPBF）technology provides a new approach for the manufacturing of Mg alloy biodegradable human implants.At present,the manufacturing of Mg alloy by LPBF has problems such as difficult formation control,poor corrosion resistance and mechanical properties.Therefore,this paper focuses on the urgent needs of degradable medical metal materials,aiming at the problems of crack defects,low mechanical properties and poor corrosion resistance of LPBF-fabricated Mg alloy,carried out study on LPBF-fabricated ZK60 Mg alloy for vascular stent and its corrosion resistance mechanism.The causes of defects such as spheroidization and cracks in LPBF ZK60 Mg alloy were analyzed,and the microstructure and corrosion behavior were studied.Rare earth element was used to improve the mechanical properties and corrosion resistance of LPBF ZK60 Mg alloy,and the corrosion mechanism was revealed.The influence mechanism of Y on the corrosion resistance of LPBF ZK60 Mg alloys was revealed.Firstly,the forming quality of LPBF ZK60 Mg alloy was studied,and the influence of process parameters on surface morphology and density and the formation mechanism of spheroidization during the forming process were analyzed.It was concluded that spheroidization would be caused in the low energy density（E_v<200J/mm³）and high energy density（E_v>400 J/mm³）,which were formed by the large unmelted particles and pores on the surface caused by the poor wettability,the insufficient liquid phase,and the aggregation of small particles on the surface.The LPBF fabricated ZK60 Mg alloy was suitable for the low-power and slow speed process,and the density of the sample shows a trend of increasing rapidly at first and then decreasing slowly with the increase of energy density.When the laser energy density is 291.6 J/mm³,the highest density is 99.6%,and the surface roughness is17.6μm。Secondly,the microstructure and mechanical properties of LPBF ZK60 Mg alloy were analyzed,the mechanism of crack formation during the forming process was clarified.The microstructures are composed of equiaxed crystal,columnar crystal and heat affected zone（HAZ）.The average grain size of equiaxed grain and HAZ is 8μm and 10μm,respectively。The thermal cracks are caused by the eutectic phase Mg₇Zn₃with low melting point at the grain boundary and the segregation of element Zn at molten pool boundary.At high laser energy density,the thermal cracks are solidification crack and liquefaction crack.At the optimal laser energy density,the thermal cracks are liquefaction crack.The maximum microhardness of LPBF ZK60Mg alloy is 90 HV,the tensile strength is 246 MPa,and the elongation is 15.5%,which is superior to the same series of cast Mg alloys.Subsequently,the corrosion degradation behavior of LPBF ZK60 Mg alloy was studied by electrochemical test,hydrogen evolution,weight loss test and observation of corrosion products,and its corrosion mechanism was analyzed.The corrosion mechanism of LPBF ZK60 Mg alloy is local corrosion caused by uneven grain size at the melt pool boundary,crevice corrosion caused by microcracks,and galvanic corrosion caused by potential difference between the precipitated phase in the grain and the matrix.The tensile strength of the sample formed at a preheating temperature of 180°C was significantly increased to 291 MPa,with an elongation of 14.7%.The presence of a large amount ofβ’₁ precipitate deteriorates the corrosion resistance of the alloy.Finally,the corrosion resistance of LPBF ZK60 Mg alloy was improved by rare earth element and its corrosion resistance mechanism was analyzed.The hot cracks were eliminated by adding rare earth element Y.After adding Y,the average grain size changes from 7.2μm reduced to 2.0μm.The precipitation phase formed at the grain boundary strengthens the grain boundary and improves the strength and plasticity of the sample.By controlling the content of Y,the type,quantity and distribution of precipitates can be changed,and the corrosion resistance can be improved.It was found that ZK60 Mg alloy with 1 wt%Y showed excellent corrosion resistance.The addition of Y reduces the potential difference between Mg₃Zn₆Y phase andα-Mg matrix,weakening galvanic corrosion.Simultaneously continuous grain boundaries and fine grains are conducive to the formation of a dense Y₂O₃ film with continuous distribution,which hinders the diffusion of corrosive media.The samples with tensile strength of 310 MPa,yield strength of 214 MPa,elongation of 24.4%and corrosion rate of 0.62 mm/y were obtained by adding 1 wt%Y,which synergistically improved the strength and corrosion resistance of LPBF ZK60 Mg alloy.