Study On Microstructure And Mechanical Properties Of 316L Stainless Steel By Laser Additive Manufacturing

Conventional 316 L stainless steel have been limited in many complex conditions in modern industry due to their low strength,long preparation cycle,and inability to directly prepare complex parts.The additive manufacturing technology just makes up for the shortcomings of traditional stainless steel materials due to its short preparation cycle,extremely high degree of freedom of preparation and good structural properties of the molded parts.At present,the research on additive manufacturing 316 L stainless steel mainly focuses on the influence of material powder characteristics and laser processing parameters on properties,but the integration of microstructure evolution,plastic deformation mechanism and strengthening mechanism is still less,which greatly limits 316 L stainless steel,as a structural component in the field of engineering.In view of the above research background,this paper studied the microstructure and mechanical properties of 316 L stainless steel produced by additive manufacturing,and compared the mechanical properties of 316 L steel prepared by traditional methods,studied the forming mechanism,strengthening and plastic deformation mechanism,and further exploration were carried out by using multi-scale simulation method.Firstly,the material composition,phase structure and microstructure characterization of the materials were studied by experimental methods.The micromorphology and material defects of each scale were observed and analyzed.The results show that the elements in the forming material are evenly distributed and consist of a single austenite phase.The phase structure is affected by the cooling rate and element composition.The shape and boundary of the material molten pool are affected by the process parameters and scanning methods.The grain of the material exhibits completely different morphology and orientation texture in the stacking plane and the scanning plane.The dislocation cells are nested inside the grain with a certain deviation from the grain growth direction.Then the hardness,tensile and fatigue processing properties of the molded specimens were tested by experiments.The microstructure of the specimens after tensile fracture was characterized.The work hardening,strengthening and plastic deformation mechanisms of the materials were studied in combination with the strengthening theory.The theoretical formula of the yield strength and critical uniform strain of the nested structure is derived.The results show that the hardness of316 L stainless steel is 288 HV,and the hardness distribution in the stacking direction is slightly different from the scanning direction.The yield strength of the sample scanning direction is 568.2 MPa,and the elongation after fracture is 43.1%.The hardness and yield strength of the 316 L stainless steel produced by the additive manufacturing are exceed over average 170% and 150% comparing to the conventional 316 L stainless steel,respectively,and the ductility is not reduced.In the structure,there are phenomena such as precipitation of solute atoms at the dislocation cell boundary,deformation twin formation,grain coarsening.After low-cycle fatigue processing,the tensile strength of the sample is improved but the plasticity is weakened.Finally,using the crystal plastic finite element simulation method and the molecular dynamics simulation method,the tensile simulation of the 316 L stainless steel was carried out on the mesoscopic and microscopic aspects respectively,The deformation and strengthening mechanism of the material were further explored.The results show that the stress and strain of each dislocation cell are not uniform during the stretching process,which produces a strain gradient strengthening effect,which increases the material strength.The material accords with the law of yield strength and ductility.However,when the dislocation cell size is lower than 80 nm,the yield strength no longer increases with the size of the dislocation cell.The nested structure of 316 L stainless steel is easy to hinder the movement of dislocations,so that the dislocations accumulate at the boundary,and crack initiation or plastic deformation occurs.