Numerical Simulation And Experimental Study On Selective Laser Melting Of Cobalt-chromium Alloy

Selective Laser Melting(SLM),as one of the metal forming technologies with rapid rise and great development potential,has attracted great concern by many researchers because of its ability to process high-precision parts rapidly with arbitrary complex structure.Cobalt-chromium alloy has been widely used in orthopedic materials and various metal structure materials due to its excellent performance in biocompatibility,strength,toughness,wear resistance and corrosion resistance.However,in the process of forming cobalt-chromium alloy with SLM,the huge temperature gradient and extremely high cooling rate are not only conducive to obtaining fine microstructure,but also able to result in the formation of cracks and pores and other defects.In view of the above problems,the effects of various processing parameters on the molten pool behavior and mechanical properties of cobalt-chromium alloy formed by SLM were studied by combining numerical simulation and experimental study.The effects of heat treatment temperature on the surface morphology,micro-hardness,tensile properties and residual stress of the samples were analyzed.The main research contents and conclusions are as follows:(1)The relationship between processing parameters and molten pool behavior during SLM forming of cobalt-chromium alloys was investigated.As the laser power enlarged or the speed of movement slowed down,both the temperature gradient and size of the molten pool increased,and the liquid phase time was prolonged.With the enhancement of the laser power or the scanning speed,the maximum cooling rate elevated.In addition,within the research range,it was found that the average micro-hardness of the weld pool boundary elevated gradually with the enlargement of laser power,while the micro-hardness enhanced firstly and then reduced with the enhancement of scanning speed.Compared with the micro-hardness of the molten pool boundary,the micro-hardness inside the molten pool did not change significantly.(2)The effect of different combinations of laser power and scanning speed on the molten pool behavior and mechanical properties of cobalt-chromium alloy under the same laser volume energy density were studied.The phase change and microstructure were analyzed by XRD and SEM.The results showed that with the simultaneous enhancement of laser power and scanning speed,the heat-affected zone of molten pool broadened firstly and then tended to be stable,the maximum temperature and maximum cooling rate of molten pool increased,the size of molten pool enlarged,the liquid phase time protracted,the ? phase gradually changed from ?(200)to ?(111),while the micro-hardness and tensile properties advanced firstly and then weakened.In addition,it was found that the Marangoni convection-related phenomena were obvious at high power and high scanning speed.(3)The effects of different heat treatment temperature on the microstructure,micro-hardness,tensile properties and residual stress of Co Cr alloy samples were investigated.With the enhancement of heat treatment temperature,the molten pool boundary gradually disappeared,and the micro-hardness,ultimate tensile strength,yield strength and residual stress gradually decreased,while the elongation elevated little by little.In addition,the micro-hardness,ultimate tensile strength and residual stress of the specimens increased slightly compared with the samples before heat treatment only when the heat treatment temperature was 750 ?.In this work,a union of finite element simulation and experiments was implemented to study the effect of processing parameters on mechanical properties and the molten pool behavior of the cobalt-chromium alloy formed by SLM,and explained the limitations of using the laser volume energy density formula for the SLM parameter design.The processing range was optimized and a certain reference for the preparation of high-performance cobalt-chromium alloy was provided.