| Selective Laser Melting(SLM)technology,as the most important branch of metal AM,has been widely used in aviation,automotive,medical and other industries in recent years,attracting extensive attention from the scientific community.However,due to the coupling relationship of transient multi-physical fields and the combination of various process parameters involved in SLM forming process,the forming performance of the specimen can not meet the requirements of precision machining.In the forming process,the input of laser energy density directly affects the mechanical properties and microstructure of the specimen,and only relying on the traditional test method will cost a lot of cost and low efficiency.the method of numerical simulation can effectively help us to study the transient change of molten pool in the forming process and understand the formation mechanism of defects in the forming process,which is of great significance to the selection and optimization of process parameters.Therefore,this paper takes 316 L stainless steel as the research object,establishes a multi-physical field mesoscopic scale thermal-fluid-solid coupling model,simulates the molten pool temperature field and flow field during SLM forming process,and analyzes the mechanism of parts defects.The reliability of simulation was verified by experiments,and the properties and microstructure of SLM forming parts under different process parameters were systematically studied.The research content of this paper is mainly carried out from the following aspects:(1)Combined with the literature related to selective laser melting process at home and abroad,the related research status of SLM technology and numerical simulation are summarized.(2)The discrete element theory and SLM hydrodynamics theory are introduced,and the physical phenomena of selective laser melting process are analyzed,including surface tension,steam recoil pressure,Marangoni convection caused by surface tension gradient and latent heat of phase transformation,etc.,and a mesoscopic numerical model of SLM forming process is established.(3)The evolution rules of temperature field,flow field and morphology of molten pool during selective laser melting at different laser scanning speeds were simulated by hydrodynamics software.the effects of surface tension gradient and steam recoil pressure during laser scanning on the molten pool size,temperature field and flow field are analyzed,and the changing rules of molten pool morphology at different scanning speeds are obtained.the formation mechanism of surface defects of formed parts is revealed.A better processing parameter of SLM is predicted.(4)Taking 316 L stainless steel as the research object,the specimens with different process parameters were formed by SLM,the molten pool sizes of experiments and numerical simulation under the same parameters were compared,and the surface micromorphology and spheroidizing defects of the formed parts at different scanning speeds were observed.it is concluded that with the decrease of scanning speed,the surface spheroidization phenomenon decreases at first and then increases,from large size spheroidization to small size spheroidization.The experimental results are in good agreement with the numerical simulation results.(5)According to the SLM forming specimens with different process parameters,the effects of different laser linear energy density on the density and microhardness of the formed parts are analyzed.When the laser linear energy density is about 240 J m-1,the density and hardness of the formed parts are better.At the same time,a BP neural network is established to predict the density and hardness of forming specimens,and its prediction accuracy is better.Using this prediction model can avoid blindly selecting appropriate process parameters to increase the test cost.The microstructure and elements of the molded parts under different linear energy density were analyzed by SEM and EDS,and the effect of laser energy density input on crystal morphology and element segregation was revealed.In this paper,through the combination of numerical simulation and experimental study of 316 L stainless steel formed by SLM,the evolution law,macroscopic mechanical properties and microstructure of molten pool in the forming process are studied,and the forming mechanism of selective laser melting process is obtained.the related research results provide theoretical guidance and application support for the selection and optimization of process parameters of SLM.Figure [43] Table [7] Reference [85]... |
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