Research On The Forming Process And Microstructure Of Additive Manufacturing 316L Stainless Steel

Additive manufacturing has been applied in many fields due to its advantages of high manufacturing freedom,high utilization rate of raw materials,and short product development cycle.Therefore,there is also a corresponding demand for the molding quality and performance of additive manufacturing materials.However,due to the complexity of the additive manufacturing process,the current research on the control of the forming process of additive manufacturing materials and the deep basic relationship between the microstructure and properties of materials is not clear enough.hinder.This paper mainly focuses on the forming process,material structure and material properties of additively manufactured 316L stainless steel,from macroscopic forming defects,to material molten pool characteristics,and then to the substructure inside the molten pool.The characteristics of 316L stainless steel are characterized.And the relationship between microstructure and material properties is discussed.So as to provide a reference for the performance control of 316L stainless steel material for additive manufacturing.Different additive manufacturing molding equipment often has certain differences.Therefore,this study firstly explores the molding process of the equipment used in the experiment,eliminates the process parameters that are prone to cracking and warping defects and lower density,and obtains better experimental equipment.Process parameter range.Explore the process parameters within the optimized process range to characterize and count the size and structure of the molten pool of the material,and combine the static tensile properties of the material to explore the correlation between the molten pool structure and the material properties.In the molten pool structure based on the exploration of the relationship with the performance,the performance of the substructure inside the molten pool was further tested and analyzed,and the relationship between the microstructure and the performance of the microdomain was discussed.The main conclusions of this paper are as follows:(1)Under different molding processes,the experimental samples are printed and molded.And the defect and density of the molded samples under each parameter are analyzed.The main selection and optimization process of process parameters is as follows.The thickness of the powder layer is determined to be 40μm according to the size characteristics of the powder material and the printing efficiency and other factors.The selection of the scanning distance is determined according to the width of the single-channel molten pool.Within the laser power of 150～450W and the laser scanning rate of 500～1300 mm/s,the single-channel width of the melt channel ranges from 67.43μm to 145.32μm.The sample is prone to warpage and deformation at low laser power and high scanning rate.Even if it cannot be formed,the sample will have defects such as cracking under high laser power and low scanning rate;the density changes of the samples under different energy densities are compared,and the results show that when the energy density is lower than 70 J/mm~3,the sample The density change of the sample increases rapidly with the increase of the energy density;in the range of 70～170 J/mm~3 energy density,the density change of the sample is also proportional to the energy density,but the change is relatively small,and the overall steady state.When the energy density is greater than 170 J/mm~3,the density of the sample appears a jump stage.(2)In the process of selective laser melting of 316L stainless steel in the process of increasing the bulk energy density from 92.59 J/mm~3 to162.04 J/mm~3,the tensile strength of the sample first increased and then decreased,and the tensile strength of the sample was 145.83 J/mm~3.When the energy density was 145.83 J/mm~3,the strength reaches a peak value of498.48 MPa.The morphology and size of the molten pool are related to the energy density.The approximate area of the molten pool increases first and then decreases with the increase of the energy density.Statistics show that the approximate area of the molten pool and the tensile strength change trend positive correlation.Selective laser melting of 316L stainless steel will damage along the molten pool boundary during the stretching process.The larger the approximate area of the molten pool,the smaller the proportion of the molten pool boundary,and the relatively high tensile strength of the sample.Controlling the energy density of printing,and then regulating the molten pool morphology characteristics such as the size of the molten pool of the selective laser melting of 316L stainless steel,can further control the microstructure of the selective laser melting of 316L stainless steel and improve the material properties.(3)Characterization and observation of the internal substructure of the316L stainless steel molten pool by selective laser melting.The structure shows that there are smaller substructures inside the molten pool,and the morphological characteristics of the substructures are not completely consistent,which can be divided into approximately equiaxed cellular crystals,elongated cellular structures and elongated columnar crystals;nanoindentation experiments were carried out on approximately equiaxed cellular crystals of different sizes,and the results showed that the size of the cell size was significantly correlated with the nanoindentation results.As the cell size increased from 0.4949μm to 0.6503μm,the hardness of nanoindentation decreased by 0.24 GPa.Nanoindentation tests were carried out on equiaxed,elongated and columnar crystals respectively.The increase of the surface angle,that is,during the transition from the elongated column shape to the equiaxed cellular shape,the indentation hardness value shows an upward trend.When the cell diameter is similar,the hardness of the equiaxed cellular structure is higher than that of the columnar structure by 13%.Further characterization analysis of the cellular structure shows that there are a large number of dislocations at the cellular boundary.It is precisely because of the existence of dense dislocations at the cellular boundary that the cellular structure shows better performance than the columnar structure.