Study On The Microstructure And Properties Of CoCrMo Alloys Fabricated By Selective Laser Melting

Selective laser melting(SLM)technology is one of the most widely used additive manufacturing technologies,which optimizes the time-consuming and complex process of traditional manufacturing,reducing the cost of product manufacturing and raw material cost.It also features personalized customization and fast response prototyping.Among many biomedical materials,CoCrMo alloy exhibits outstanding corrosion resistance,wear resistance and mechanical properties,especially excellent biocompatibility.However,the researches on the microstructure and properties of SLM-formed CoCrMo alloy are not deep and the mechanism of heat treatment on its microstructure and properties is still unclear.In this paper,CoCrMo alloy samples were prepared by different SLM processes,anddetermined the relative density and hardness to determine the optimal SLM process.Then,thesamples formed by optimal SLM process were subjected to solution and aging treatment.The microstructure and phase composition of CoCrMo alloy samples were analyzed by OM,SEM,TEM,XRD,etc.,and the room tempreture tensile,compression,impact properties and hardness were measured to determine the preferred heat treatment process.Finally,different processeselectropolishing and solution treatment were used to improve the surface roughness and hardness of the CoCrMo alloy crowns formed by optimal SLM process.There are mainly conclusions as follows:(1)Within a certain range,the relative density and hardness of CoCrMo alloy samplesformed by SLM increase with increasing surface energy density.When the laser input surfaceenergy density is J=6 J/mm2(corresponding to the optimal process parameters:laser power P=450 W,scanning speed V=1500 mmls and scanning space S=0.05 mm),relative density and hardness reached the maximum of 98.9%and 374 HV1,respectively.(2)The molten pool of CoCrMo alloy samples formed by optimal SLM process is mainly composed of celler and columnar crystals with crystals boundaries of rich in Cr and Mo bright compounds.The microstructure consists of mass y phases and small amount lamellar athermal?-martensite phases.The room temperature tensile strength,yield strength and elongation are 1070 MPa,758 MPa and 9.0%,respectively.Tensile fractures exhibit more cleavage platforms and tearing ribs,indicating typical quasi-cleavage fracture.(3)CoCrMo alloy samples formed by optimal SLM process after solution at 1200 ?,the microstructure consists of y phase and,phase.After solutionl h,the tensile strength,yield strength,elongation and hardness at room temperature are 985 MPa,639 MPa,18.0%and 334 HV1,respectively,possessing the best comprehensive mechanical properties.The tensile fracture is mainly composed of cleavage platforms and tearing ribs,and existing a few dimples,demonstrating cleavage fracture dominating.(4)CoCrMo alloy samples formed by optimal SLM process after aging at 900 ?,the microstructure consists of ? phase,? phase and the ? particle phase rich in Cr and Mo.After aging 4 h,tensile strength,yield strength and elongation at room tempreture are the highest,reaching 1020 MPa,677 MPa and 23.0%,respectively.After aging 12 h,hardness up to 350HV1.The tensile fracture presents cleavage platforms,numerous dimples and ligaments,indicating a mixed mode of typical cleavage fracture and ductile fracture.(5)CoCrMo alloy crowns formed by optimal SLM process after being electropolished,the roughness Ra of the 1#,2#,3#and 4#crowns decreased to 1.14 ?m,1.25 ?m,3.65 ?m and 3.97 ?m,with reduction by 75%.78%,45%,and 38%,respectively.The 1#and 2#crowns after being electropolished,the surface showed no obvious rags and burrs,which became smooth,flat and with metallic luster.The 1#crown after being electropolished,the difference betweenthe bulging and depression extremes was reduced from 79 ?m to 50 ?m.(6)CoCrMo alloy crowns formed by optimal SLM process after solution at 1200 ? X 10 h,the hardness decreased from 395 HV1 to 345 HV1,with decreasing by 12.7%,meeting the hardness standard of international clinical implant crowns.The solution treatment promoted the???,martensite transformation and the ?1??2+? solid phase transformation.As the solution time increasing,the number of massive ? phases increased and coarsened,resulting in a decrease in hardness.