Study On The Forming Process,Microstructure And Property For Biological Cobalt-base Alloy By Direct Metal Deposition

In recent years,with the improvement of people's living standard as well as the increase of life expectancy,the demand for biomedical materials is increasing rapidly,The medical cobalt base alloy,has been widely used in the field about medical implant material,because of its excellent mechanical properties,such as excellent wear resistance,corrosion resistance,young's modulus not change with strength,low coefficient of thermal expansion,high thermal conductivity,good biocompatibility,Cobalt base alloy has been regarded as the most typical biomedical metallic materials with wide application scope currently.Laser direct metal deposition technology combining with CAD technology,computer control technology,laser fused technology,increase material manufacturing technology and other advanced technology,it has many incomparable advantages,such as simple process and short processing cycle,so it is changing the traditional living and productive styles fastly.In order to meet individuals'requirements,most biomedical materials need to realize personalized design and production.However for traditional methods such as forging or casting,it is easy to bring about troubles like high cost and long productive cycle when fabricating production with complex needs of customization.Therefor DMD technique with highly flexible characteristics has be concerned by the medical fleld.This method has solved difficult problems with the traditional forming technology in complex parts.such as high performance and high precision,high flexibility and rapid response.DMD technology make it possible to realize no modeling and digital in design and production of personalized restoration,besides it significantly improve the efficiency and quality,it provides an important way for the production of biomedical materials with high performance.In this research,the effect of different process parameters on microstructure and metallographic properties of cobalt base alloy during DMD process has been studied mainly,The structure,properties and preparation mechanisms of the samples were studied by using means of metallographic microscope,scanning electron microscopy,transmission electron microscopy,XRD,hardness,and electrochemical analysis methods.The results show as flowing:The formability of thin wall is good,and the thin wall has no porosity,crack and other defects,the density of thin wall can reach above 99%,microstructure morphology of grains are columnar dendrite and equiaxedwhich doesn't vary with the variations of laser power,scanning speed and feeding amount.When power is 700 W,scanning speed is 540 mm/min,the spindle speed is 750 rap/min,the dendrite with same grow direction can be gained,and microstructure is relatively uniform and small.hardness value of samples fabricated by appropriate process parameters above is about 380 HV,the average tensile strength,yield strength,compressive strength are 860.27 MPa,835.91 MPa and 1800 MPa,the average elongation is 19.97%.Compared with the traditional casting Co-Cr alloy,Co-Cr alloy prepared by laser direct deposition forming has high strength,plasticityand corrosion resistance,so it can meet the requirements of medical materials,and is suitable for clinical application.Through the way of increasing preheating device on the substrate,which can reduce temperature gradient during process of solidification,problem of crack is solved in process of fabricating block sample,The influence of different scanning methods on forming quality and microstructure are studied as well,as a result,the samples,which has good quality,free crack,tiny microstructure and excellent performance,are deposited by a way of interlayer orthogonal parallel reciprocating scanning mode.The way of deposition can affects the forming quality and microsturcture,including vertical and horizontal deposition way,the growth direction of dendrite with vertical sample is same with stretching direction,so it is ductile fracture with high strength and good toughness,however,microstructure of horizontal sample is disorder and there exist microcracks in the sample,so it is brittle fracture with low intensity and plasticity.After solid solution and aging treatment,grains of samples grow up evenly,hardness of samples decrease to about 330 HV.For mechanics properties,strength declinesand toughness improves greatly as a whole,what's more mechanics performance of specimen with a vertical deposited orientationis still better than specimen with horizontal orientation after heat treatment.