Investigation Of Process Parameters And Heat Treatment On Microstructure And Properties Of Biomedical CoCrWMo Alloy Fabricated By Selective Laser Melting

Cobalt-chromium alloys are widely used in the biomedical field due to their excellent mechanical properties,corrosion resistance,wear resistance and biocompatibility.Selective laser melting?SLM?forming technology can meet the customization requirements of surgical implants,which has further developed the clinical application of cobalt-chromium alloys.In this paper,quantitative characterization of the domestic CoCrWMo alloy powders with high performance was carried out,then the powers were fabricated by selective laser melting and the process parameters were optimized.At the same time,heat treatment was used to further strengthen the performance of the formed parts,and focuses on the effect of heat treatment on the microstructure,mechanical properties,friction and wear properties,and corrosion resistance of selective laser melted CoCrWMo alloys.Analysis of particle size,shape and microstructure of CoCrWMo alloy powders showed that:D₁₀,D₅₀,and D₉₀ of the powders are 17.2?m,27.4?m,and 44.4?m,respectively.The average value of Occhio bluntness of the powders is 87%,the scatterplot shows that the values of circularity and ellipse ratio are densely distributed in the part close to 1,the sphericity is higher,and the powder of non-adhesive satellite powder accounts for 63.4%.Their microstructures are cellular crystals and columnar crystals,and the phase composition is mainly?phase with FCC structure.The above CoCrWMo alloy powders were fabricated by selective laser melting,and the single factor method was used to study the effect of laser energy density on the top surface quality and the relative density of SLM formed specimens.The results indicate that the optimum parameters are:laser power of 280 W,scanning speed of 800 mm/s,layer thickness of 0.03 mm,scanning space of 0.10 mm,and scanning strategy is rotation scanning method?rotation 15°between adjacent layers?.The laser energy density of specimen fabricated by optimal parameters is 117 J/mm³,the relative density is 99.4%,surface roughness Ra is 4.98?m,and the tensile strength is 984 MPa,the yield strength?_0.2 is663 MPa,the elongation is 12.9%,and microhardness is 386 HV,which are all higher than cast alloys.The selective laser melted CoCrWMo alloys were heated by different solution-aging treatment,and the microstructure and properties of the CoCrWMo alloy under different solution-aging treatment conditions were investigated.The results show that:the??FCC?phase is dominant in the microstructure at 1200? solution for 1 h.At this time,the microhardness of alloy decrease,both the strength and the elongation increase after solution treatment.In addition,the phase were transformation from?-FCC phase to?-HCP phase and the fine carbides M₂₃C₆?M=Cr,Mo,W?were precipitated during the aging treatment at600?,700?,750?,800?,900?,respectively.Increasing the aging temperature would promote martensite transformation and precipitation of M₂₃C₆.There is a maximum amount of M₂₃C₆ after aging at 800?,but some M₂₃C₆ re-dissolved and grew up after aging at 900?.The microhardness of the alloys are significantly improved after aging treatment.As the aging temperature increases,the microhardness and strength of the aged alloy tend to increase first and then decrease,and the elongation continues to decrease.As the alloy solution at 1200? for 1 h and aging at 750? for 10 h,the mechanical properties of the alloy reached their optimum combination,the tensile strength is 1076 MPa,the yield strength?_0.2 is 820.8 MPa,and the elongation is 10.5%.The effect of solution-aging treatment on the wear and corrosion resistance of selective laser melted CoCrWMo alloy is mainly related to its microstructure.The main wear mechanism of the alloy is abrasive wear and adhesive wear under dry friction conditions.When the load is increased from 40 N to 80 N,the width of the wear track of the specimens are increased,but the average friction coefficient are decreased,and the change of the average wear rate was affected by the strain-induced martensite transformation and the oxidation of the spalling carbide in the CoCrWMo alloy during the dry wear process.The specimens with high content of??HCP?phase and M₂₃C₆?M=Cr,Mo,W?have lower wear rate,in which the alloy with 1200? solution for 1 h and 800? aging for 10 h has the lowest wear rate under the load of 40 N,60 N,80 N,respectively:4.4×10^-5 mm³/N·m?6.2×10^-5 mm³/N·m?5.0×10^-5 mm³/N·m.The electrochemical corrosion test shows that:in 0.9%Na Cl solution at p H 6.8,the alloy with 1200? solution for 1 h has the minimum₍?8?and the maximum R_ct:1.50?10^-7 A/cm² and 2.23?10⁶?·cm²,respectively.This is because the?martensite transformation and the precipitation of M₂₃C₆ carbides will increase the pitting corrosion sensitivity of the samples,which will lead to the reduction of the corrosion stability of the alloys.Lastly,the acetabular cup implants with high quality were fabricated by SLM using domestic CoCrWMo alloy powders and optimized process parameters,which provides a technical reference for domestic selective laser melted biomedical implants.