Research On Microstructure,Textures And Mechanical Properties Of Co-Cr-Mo Alloy Produced By Selective Laser Melting

Co-Cr-Mo alloy has been widely used in the field of biomedical materials,including hip implants,dental dentures and cardiovascular stents,because of its excellent biocompatibility and mechanical properties.However,Co-Cr-Mo alloy is prone to produce debris during friction and wear,especially at the interface of the two metal joints of the artificial hip joint,which will affect the operation of the artificial limb in human body.Increasing the martensitic phase content of HCP phase in Co-Cr-Mo alloy can effectively improve its hardness and wear resistance.Therefore,it is very important to understand the transformation and regulation mechanism of HCP and FCC structure,which will contribute to the preparation of Co-Cr-Mo alloy with high martensitic content.Selective laser melting(SLM)technology is widely used to produce metal parts with high performance and complex shapes.Therefore,this work mainly focuses on the regulation and transformation mechanism of HCP and FCC structure in Co-Cr-Mo alloy,as well as the effects of the relative contents of HCP and FCC structure on the mechanical properties and the strengthening and toughening mechanism of the alloy.The main contents are as follows:(1)Co-Cr-Mo alloy with main structure of martensite was prepared by SLM technology,where the martensite content reaches ?95 %,significantly higher than that of the casting sample 60 %.The reason for the high content of martensite phase in SLM Co-Cr-Mo alloy lies in the high density dislocation and stacking faults produced during the preparation of SLM.These internal defects are easy to become nucleation sites and promote the transformation of FCC structure to HCP structure.The yield strength of the alloy is 735 MPa,the tensile strength is 1211 MPa and the elongation is 12.1%.The strengthening mechanism is mainly caused by the internal high-density dislocation,martensite and cellular structure impeding dislocation slip.The strengthening model of the alloy with cellular structure and martensite striation structure is established.Good plasticity is derived from the plastic bending deformation of the residual FCC structure between the multistage structure and the martensite striations,which contributes to the stable work hardening rate.(2)The effects of different heat treatment temperatures on the microstructure and mechanical properties of SLM Co-Cr-Mo alloy were studied.When the heat treatment temperature is 873 K,the yield strength is increased to 893 MPa,the tensile strength is 1214 MPa,and the elongation is 13.8%.When the temperature is 1173 K,the yield strength and ductility decrease,which is due to the coarse-grained grains and the precipitation of a large number of discontinuous second phases at the grain boundary,which promotes the intergranular fracture.The yield strength increases to 860 MPa,the tensile strength is 1078 MPa,and the elongation is 2.5%.(3)XRD,EBSD and neutron diffraction techniques were used to study the texture of printed and heat-treated SLM Co-Cr-Mo alloy.It is found for the first time that SLM Co-CrMo alloy has fivefold texture in three-dimensional dimension,which is different from that in two-dimensional dimension.This is due to the special printing strategy(72° annular scanning mode)used in the experiment.Fivefold textures on the polar map are obtained by the rotation of the same group of diffraction spots around the center of the polar map by 72° in turn,that is,a group of(110)textures rotates four times to produce five groups of spots.Five groups of spots correspond to five groups of scanning planes,and each scanning plane presents a weak Gaussian texture.The reason of the isosceles trigonometric distribution of martensite in two dimensional scale is explained.The transformation from FCC phase to HCP phase occurs when the heat treatment temperature increases,and four martensite variants are formed when the phase transformation follows the N-S relationship.