Microstructure And Mechanical Properties Evolution Of 316L Stainless Steel Based On Additive Manufacturing Technology

Selective Laser Melting(SLM)is an important additive manufacturing method.It has obvious advantages for the metal parts which cannot be directly shaped by traditional shaping methods,and small batch products or specially customized metal parts.At present,that process has gradually received widespread international attention.However,in the shaping process,due to the short existence of the micro-melt pool and the coupling of multiple physical fields,it is difficult to determine the optimal parameters of the SLM process.In addition,forming quality is difficult to guarantee due to the unstable control SLM forming process,which affects the application of SLM formed parts as important structural parts.This paper uses the secondary development of the module based on flow-3d to predict and analyze the mesoscopic simulation of the molten pool problem in the SLM forming process.Through analysis and research on SLM-316L samples formed by different processes and parameters,the evolution law of the microstructure and macroscopic mechanical properties of the formed samples were studied.In order to study the evolution of the mechanical properties and microstructure of SLM formed 316L specimens,the experiment used simulation method to predict and analyze the distribution of SLM mesoscopic molten pool and residual stress.The experiment also used variable laser power and scanning distance to form SLM-316L specimens,and heat-treated part of the formed specimens.The experiment used Epiphot 300 metallurgical microscope,D8 Advance polycrystalline X-ray diffractometer,JSM-6610LV scanning electron microscope to characterize the SLM-316L sample,and study the change rule of microstructure and the change rule of mechanical properties.This article uses WDW-100E universal testing machine and Lian Er HV-1000 Vickers hardness tester to test the mechanical properties of the formed samples.Mesoscopic simulation can effectively predict the surface geometric characteristics of SLM formed parts.Through the mesoscopic simulation of 316L-SLM formed samples,it can be concluded that the laser power is increased,the temperature of the spot center is increased,and the depth and width of the molten pool are increased.Too low laser power will cause the powder to melt incompletely,and the surface of the molten pool caused by the laser recoil pressure is lower than the surface of the cladding track.Spatter in SLM is not affected by laser power.The smaller the scanning distance,the better the surface quality of the track formed by multiple cladding channels.The microstructure of 316L stainless steel formed by SLM is an austenitic layered molten pool.There are columnar and cellular substructures between the molten pools.There are obvious nanoparticles between the substructures,and there is an obvious phenomenon of element aggregation.As the laser energy density decreases,the scaly structure gradually transforms into a layered structure,and unmelted powder appears.The Vickers hardness distribution range of the unheated SLM molded parts is 240-290HV,and the tensile strength distribution range is 635±7 MPa increases and then decreases with the change of laser energy density.The forming hardness of SLM is higher than that of hot-rolled parts.With the increase of heat treatment temperature,ferrite and M₂₃C₆ appear,the fusion line gradually disappears,and the columnar substructure Transformed into equiaxed cell substructure to form massive crystal grains,and then the substructure disappears,while the Mo,Ni,and Mn elements are distributed along the grain boundary from the dispersed distribution of SLM,and no element aggregation occurs.The molded part after heat treatment The hardness distribution is 192.2HV-279.6HV.The hardness of the molded parts is reduced and is similar to that of the hot-rolled molded parts.The influence of the laser power on the hardness of SLM molding is gradually weakened.The tensile strength distribution range of the sample is 574MPa-684MPa,and the elongation is 20%-40%.With the increase of laser power and heat treatment temperature,the tensile strength and yield strength of SLM molded parts increase first and then decrease The elongation rate decreases first and then increases with the increase of heat treatment temperature.Under the condition of unidirectional tensile stress,the substructure inside the molten pool of the SLM molded part elongates or rotates in the direction of the force.The tensile fractures of SLM formed samples are all plastic fractures.With the increase of heat treatment temperature,the size of fracture dimples gradually increases,and the plasticity of the material increases.Through the study of the evolution of the microstructure and mechanical properties of SLM formed 316L steel,the essence of the influence of process parameters and heat treatment temperature on the microstructure is revealed.The corresponding relationship between the microstructure and the mechanical properties,the research ideas and methods in this article can also provide a reference for further research on the molding of the same or other materials of SLM.