Research On Numerical Simulation And Micro-mechanisms During Selective Laser Melting(SLM) Process Of Cemented Carbides

WC-Co cemented carbides have many excellent properties,such as high hardness,high strength,good corrosion resistance and so on.It has been widely used in machining,minning,oil drilling and other fields.The traditional process of cemented carbide mainly includes powder mixing,pressing and sintering.The pressing and sintering process leads to long process period and high fabrication cost.Therefore,it is urgent to develop faster,cheaper and near net shape technologies.Selective laser melting(SLM)additive manufacturing technology has changed the traditional manufacturing mode of material reduction.It has the advantages of high material utilization rate,short research and development cycle and low cost.It has solved many forming problems of complex structural parts which were difficult to manufacture in the past.However,for SLM process,there are significant technical challenges in the preparation of WC-Co cemented carbides.Up until now.there is still a lack of systematic research on the SLM process optimization.SLM liquid phase sintering pool characteristics and sintering mechanisms.WC grain growth mechanisms,microstructure evolution mechanisms and print properties at home and abroad.In this study,based on WC-20Co and WC-32Co system,the relationship modeling between SLM process parameters and carbide density was constructed through response surface methodology,and SLM process parameters was therefore optimized;the morphology,growth model and coarsening mechanism of WC grains were studied by WC particle size analysis;by numerical simulation of heat transfer and liquid flow within SLM liquid-phase sintering pool.the physical characteristics of liquid-phase sintering pool were evaluated to analyze the densification mechanism and sintering mechanism.The study shows that:(1)The chessboard scanning strategy is more conducive to improving the density of SLM processed WC-Co cemented carbides than stripe scanning and spiral scanning.Through Response Surface Methodology,the relationship modeling between relative density.p and SLM processing parameters,including laser power P.W,scanning speed V.mm/s,and hatch spacing S,mm,has been successfully constructed for WC-20Co and WC-32Co.(2)During SLM process,under the action of gradient surface tension,liquid Co flows from the center to the edge of the liquid phase sintering pool.At the same time,the shear force at the liquid-solid interface causes WC-Co at the edge of the liquid phase sintering pool to flow along the solid-liquid interface and rise to the upper surface after the fluid meets at the center of the bottom of the liquid phase sintering pool.Finally,a special fluid vortex is formed in the liquid phase sintering pool.This process belongs to Marangoni convection,which leads to the deformation of the molten pool and the formation of a wide and shallow molten pool with a convex surface.(3)The microstrucuture of SLM processed carbides is composed of WC,Co,and carbon deficient phase,whose distribution is inhomogeneous.The ultrafine grains coexist with ultracoarse grains and ultracoarse grains show incomplete growth.(4)The WC grain morphology and grain growth mechanisms under the instant high temperature liquid phase sintering during SLM process are quite different from those under traditional liquid phase sintering.The laminate step growth and mosaic agglomeration are the main mechanisms for WC grain growth.(5)Densification mechanisms for WC-Co cemented carbide include melting and solidification of Co phase in liquid phase sintering pool and the overlap of adjacent liquid-phase sintering pool in both horizontal and vertical directions.This research focuses on the numerical simulation and micro mechanisms of SLM process of WC-Co cemented carbides,which provides basic theoretical and engineering support for additive manufacturing of WC-Co cemented carbide,and provides theoretical support for design and forming of cemented carbide with complex shapes or multifunctions.