Study Of Flow Law,microstructure And Mechanical Properties Of 316L Stainless Steel By Selective Laser Melting

Selective laser melting(SLM)is one of the important forming methods of additive manufacturing.Comparing to traditional forming methods which cannot be directly processed,small batch production and personalized products,additive manufacturing has obvious advantages,and received extensive attention and research.However,the forming process is very complicated involving multiple physics coupling,and the optimal process parameters are difficult to determine.In addition,the lack of understanding of coupling mechanism between different physics results in difficulty in controlling the forming quality and affecting its potential.In this paper,single-layer multi track experiments to study forming process parameters.The finite element method is used to simulate the flow field and temperature field,and the microstructure and macroscopic mechanical of sample after forming are also studied.The main work and results are as follows:To 316L stainless steel powder,the characteristics of the melt track(morphology of melt track and geometry of molten pool)in the single layer multi track experimental results of 84 sets of process parameters were observed,counted and analyzed.When the input energy density is below 70 J/mm3,discontinuous balled or irregularly track can be easy formed;if the energy density is too large,keyhole phenomenon will occur.By constructing a BP neural network to predicte the size of melt pool.The simulation results show that the prediction accuracy of the neural network is higher than that of the linear fitting method.The neural prediction model can guide the selection of experimental process parameters,thereby avoiding blind selection of appropriate process parameters through empirical and trial-and-error methods.Laser and materials interaction area is small,and process is intense.The process consists of multiple interacting physical processes on multiple scales,experimental methods are difficult to study.Therefore,using the finite element method to study the law of molten pool flow is an effective method.By analyzing the physical phenomena in laser melting process,simplify the model.The latent heat of material phase transition and the Marangoni effect are considered in the model.In addition,the modle also considers the surface active element sulfur,and studies the influence of different sulfur cantent on the flow field and temperature field of the melten pool.Grains epitaxial growth makes grain trend to be columnar structure.When the molten pool is shallow and wide,the preferred orientation of grain growth is(001),if the molten pool is deep and narrow,the preferred orientation of grain growth is(110).Therefor,the morphology of the microstructure can be adjusted by process parameters.316L stainless steel sample obtained via selective laser melting with special cellular and columnar substructure.The nona-scale fine structure,the sub-micron-scale substructure,the micro-scale grain and molten pool,and formed part together constitute a unique multi-scale structure of the selective laser melting.In this research,the formation mechanism of cellular substructure is explained by the theory of solidification.The large temperature gradient and fast solidification rate at the solidification front of the molten pool are the main reason for the formation of the cellular substructure.When the energy input density is 100 J/mm3,samples have high yield and tensile strength formed by different process parameter.The samples also have different mechanical properties performance due to the orientation relationship of microstructure.When the laser power is 75 W and laser scanning speed is 415 mm/s,high strength and elongation can be achieved(yield strength:645 MPa,total elongation:53%).Compared with the rolled-316L stainless steel,the high dislocation density(1.14×1015 m-2)is take responsibility for high strength of SLM-31 6L stainless steel.The special substructure,large number of interface,and high proportion of small angle grain boundaries are also the reasons for the high yield strength.Although the SLM-formed specimens can not be full dense,these pores can act as crack sources and have negative effect on mechanical properties of the material.But twins occur at the initial stage of deformation,and accompany the entire plasic deformation process,which makes SLM-316L stainless steel has a good elongation.Through the research 316L stainless steel process parameter of selective laser melting,flow law of molten pool,microstructure characteristics and macroscopic mechanical properties.Learn how the process affects the microstructure and how the microstructure determines macro performance.The research ideas and methods of this paper can be applied to other powder materials.