Research On Gas Flow And Forming Process Of Single Track In Selective Laser Melting

Selective laser melting(SLM)is a kind of additive manufacturing technology,which uses high energy density laser beam to irradiate the metal powder according to the model geometry,and makes the metal powder melt quickly and then solidify.The forming parts have good mechanical properties and are widely used in aerospace,biomedical and other fields.On the one hand,it is difficult to ensure the uniformity of gas flow in SLM forming process,on the other hand,the scale of SLM process is small,so it is difficult to carry out experimental measurement.The method of numerical simulation and experimental verification is used to optimize and adjust the gas flow to ensure that the gas flow can effectively remove the black smoke impurities.Further,the numerical simulation model of the basic physical process of the laser scanning powder bed is established to study the influence of the forming process on the forming quality.This project is supported by the National Natural Science Foundation of China(No.51975073).A large number of black smoke impurities(molten spatter)will be produced during laser scanning on metal powder,which will prevent the laser from irradiating the powder bed surface and depositing in the molten pool and affect the final forming quality.It needs to be timely discharged by the gas flow formed by the circulating inert gas in the forming bin.The energy density,scanning speed and other parameters of laser acting on metal powder determine the evolution of molten pool morphology and the basic morphology of forming single track,but the scale of forming process is small and rapid,which is difficult to be observed directly by experiments.In this paper,the method of numerical simulation and experimental verification is used to optimize and adjust the gas flow to ensure that the gas flow can effectively remove the black smoke impurities.On this basis,the basic physical process numerical simulation model of laser scanning powder bed is further established to study the influence of forming process on the forming quality.The research work of this project is supported by NSFC project(No.: 51975073)and supported by the experimental equipment of Chongqing Xianlin 3D Technology Co.,Ltd.(1)In view of the problem that the uneven gas flow of EP-M250 affects the forming quality,the numerical model of the gas flow in the forming bin is established to reveal the gas flow distribution in the forming area,and the root of the gas flow problem is found.In order to reduce the standard deviation of gas flow velocity distribution and improve the uniformity of gas flow in the forming area,the flow-path of the gas inlet forming the gas flow is improved and optimized,and the forced diversion grid is embedded and the numerical iterative optimization is carried out.The uniformity of gas flow in forming area is improved.(2)After the optimization of the gas flow,several groups of surface morphology samples and mechanical properties tensile samples were processed in the EP-M250 forming area.It is divided into two parts: low flow speed area and normal flow speed area.The residual oxides and spheroidized particles on the surface of the formed samples in the normal flow speed zone are significantly lower than those in the low flow speed zone,and the mechanical properties of the samples in the low flow speed zone are worse than those in the normal flow speed zone.It shows that the gas flow has a great influence on the forming quality of SLM.And the optimized gas flow can better ensure the consistency of forming quality.(3)According to the characteristics of powder spreading,geometric structure of equipment,motion parameters and the basic morphology of 316 L stainless steel powder,the dynamic physical model of powder spreading process is established by using the numerical modeling software EDEM,based on the discrete element method.And the discrete point data model of particle position and size in the local powder bed under certain process parameters is obtained.(4)In ICEM,a local computational grid model of single track scanning is established.The numerical iterative solution is carried out in FLUENT,and the model of powder bed is imported,the Gauss heat source is loaded,and the material thermophysical properties are defined by writing the self defined function;the two-phase flow model and melting / solidification model is used to capture the evolution of the surface morphology of the molten pool and the basic change process of the single track and single track lapping.Finally,the influence of laser power,single track scanning and its lapping quality is studied,which shows the influence of laser power and single track filling distance on single track scanning and lapping quality.