Research On Forming Process And Microstructure Properties Of TC4 By Selective Laser Melting

Selective Laser Melting(SLM)forming is a high-precision and environment-friendly laser additive manufacturing technology using the powder as raw material.It has unique advantages in the field of processing and forming of refractory metals.Relying on the advanced forming process,high-complexity and high-precision structural parts could be customized.However,as a complex metallurgical process,the SLM process spans space and time scales,so inevitably involves more complex thermophysical changes.This physical process usually leads to spheroidization,defects,cracks and even forming failures in the formed parts.Since SLM is an instant melting-solidification process,in order to intuitively study the influence of powder particle size and laser process parameters on the morphology of the molten pool,forming quality,and the formation of defects during the forming process,numerical modeling?is used in this paper at the mesoscopic scale and the microstructure and properties of TC4 titanium alloy formed by SLM were studied.The main content of this paper is as follows:(1)Using the discrete element method DEM to establish a mesoscopic selective laser melting TC4 powder bed model.Firstly,define the initial particle size of the powder to be 15-53 microns,and follow the Gaussian distribution,using metal material parameter simulation software to simulate the temperature-dependent physical properties of TC4,and considering Marangoni convection,surface tension and metal evaporation recoil pressure in the melting-solidification behavior of the metal powder.The influence of the cell behavior,the model can better reflect the real state and process of selective laser melting.(2)Through numerical simulation,several objective factors that have a greater impact on the quality of single-pass molten pool and formed parts in the laser selective melting process are studied: including laser process parameters(laser power,laser scanning distance),powder layer thickness and particle size of the powder.The simulation results show that the influence of laser power on the morphology of the molten pool is mainly on the surface quality and whether it can be fully melted.With the increase of laser power,the surface quality of the molten channel gradually becomes better,and the depth and width of the molten pool are also significantly improved..When the laser power is increased from 100 W to 350 W,the width of the molten pool increases from 62.6 ?m to 116.2 ?m,and the depth of the molten pool increases from 24.9 ?m to 32.4 ?m.At the same time,the spheroidization effect caused by insufficient melting of the powder when the laser power is low causes the formation of pores in the molten pool.When the laser power is high,the mobility of the melt in the molten pool is enhanced,and splashing behavior is prone to occur.As the hatch spacing and powder layer thickness increase,the morphology of the melt channel and the uniformity of the temperature field become worse.(3)According to the numerical simulation of the formation of the surface quality and internal defects of the formed part,the formation mechanism and control methods of the spheroidization phenomenon,the splash phenomenon and the unevenness of the melt channel are analyzed.The spheroidization phenomenon is related to parameters such as laser power,powder coating thickness,and powder original particle size.Whether the spheroidization occurs or not depends on whether the bottom of the powder is melted,and its essence is that the melt tends to move in the direction where the surface energy decreases under the action of surface tension.The splash behavior mainly includes the splash of the melt under the recoil pressure and the splash of the particles under the action of steam.The unevenness of the melt channel is related to the initial particle size parameters of the powder.(4)According to the relevant printing process of numerical simulation of laser selective melting,the laser selective melting of Ti6Al4 V titanium alloy formed parts was carried out,and the relative density,porosity,microhardness and microstructure of test parts were characterized.The results of density and hardness are consistent with the simulation results.In addition,the width of the melt channel was measured,and the average error of the melt pool width of the formed parts in the numerical simulation and laser selective melting experiment in this study was measured to be5.53%,indicating that the mesoscopic numerical simulation method used in this paper is feasible,and The simulation results are reliable.