Molten Pool Structure In Selective Laser Melting(SLM) Additive Manufacturing Process

Selection laser melting(SLM)forming process is a kind of technology developed in recent decades for forming metal parts with fine size and complex structure.Because SLM technology is free from mold limitations,and the forming process is not restricted by the structure of parts,it can directly produce parts close to the final shape,greatly reducing the time and material consumption from the design to the industrialization of complex structural parts.Therefore,it has been widely used in the design and production of medical instruments,aircraft structural parts and military equipment.In this paper,A finite element simulation software—ANSYS,is used to simulate the temperature field and stress field in the selection laser melting forming process of 316 L stainless steel,and the change characteristics of temperature and stress are studied.Firstly,the physical changes involved in SLM processing are analyzed.Based on the theory of finite element method,the three-dimensional transient finite element models of single-channel single-layer SLM forming process,multi-channel single-layer SLM forming process and multi-channel multi-layer SLM forming process are constructed respectively.The model had considered the nonlinear change of material property parameters with temperature,the convection heat transfer between the metal powder bed and the outside environment,the phase transition of the material during processing,and the material property transition from powder to solid.Then,according to the selected process parameters,the SLM molding experiment of 316 L stainless steel was carried out,and the characteristics and molding performance of 316 L stainless steel SLM forming processing were obtained through the detection and analysis of the surface morphology,density and the phase composition of the sample.The anisotropic thermal conductivity method was used to modify the finite element model,after that,the relative errors of the depth and width of the molten pool were reduced to 7.8% and 2.39%,respectively.Then,based on the finite element model,the temperature field distribution,molten pool structure characteristic and the influence law of process parameter change on temperature field distribution and molten pool size is simulated and analyzed.The simulation results show that the molten pool center is not at the center of the heat source during the processing,but is shifted backward to the direction of the heat source by about 0.02 mm.The forming temperature was positively correlated with the laser power and decreased with the increase of scanning speed and scanning spacing.Proper selection of process parameters can guarantee the processing efficiency and get a better size of the molten poor,that is,the depth is approximately twice the thickness of the powder layer.Then the forming process can not only get a good metallurgical bonding effect between different layers,but also avoid excessive re-melting.Finally,the evolution of the stress field in the process of single-layer multi-channel and multi-layer multi-channel with different process parameters was simulated and analyzed.The stress of forming parts mainly comes from the non-uniform deformation caused by the sharp change of temperature in the process of processing.The edge of the forming area is so less affected by laser action that the residual stress is smaller,while the situation in the middle of the forming area is the opposite.The residual stress in the middle region fluctuates with the scanning trace changing.The residual stress in the adjacent two intervals is larger,and the residual stress on both sides of the adjacent area is smaller.Compared with the traditional process,the thermal cycling in SLM process can effectively reduce the residual stress of the molded parts.The forming of the latter layer has the effect of stress-relieving annealing on the forming area of the former layer,and the effect of stress-relieving annealing is greater than 40%.