Research On The Solidification Microstructuer Evolution Of TC4 Alloy Fabricated By Selective Laser Melting

In the process of selective laser melting,the metal powders melted and cooled rapidly under the action of the laser heat with high energy density.And the temperature gradient and cooling rate are very high during this process.The temperature distribution has great influence on the quality and microstructure of forming parts.The microstructure of the material determines the mechanical properties.However,its so difficult to measure the actual temperature during the forming process of SLM.In this paper,the temperature field distribution and microstructure evolution of TC4 alloy fabricated by SLM were studied from the melting and solidification of micro-molten pool.And the influence of process parameters on temperature field and microstructure was considered.In this article,the temperature field of TC4 alloy formed by SLM was simulated using ANSYS finite element software.Considered that the thermophysical parameters of TC4 alloy varied with temperature nonlinear,and the particle size and morphology effects on the powder density and thermal conductivity.Using APDL language to realize the dynamic loading of heat source.The temperature field distribution of the process is obtained.With the loading of laser heat source,the heat accumulates and the temperature increases.The instantaneous temperature of the center of the spot is the highest.And the isotherm at the front of the spot center is more dense than the back end,because of the heat conduction is different.The temperature gradient of the molten pool is up to 107K/m,and the cooling rate is up to 106K/s.The thermal cycling curves were obtained at several nodes in the laser-induced region.It was found that the nodes experienced similar thermal cycling.The thermal cycling were similar under different process parameters,but the temperature peak of the thermal cycling curve was affected.During the process of temperature field simulation,it was found that the micro-melting pool was formed during the TC4 alloy powders loaded by the laser heat.The width and depth of the molten pool are only tens of microns,and the temperature distribution will affect the micro-bath shape and size.The process parameters affect the size of the molten pool.The width and depth of molten pool increase with the increasing of laser power or reducing scanning speed.The scanning distance has no significant effect on the bath size compared with the laser power and scanning speed.The TC4 alloy powders for SLM forming are approximately spherical,and the microstructure is fine acicular martensite which is less than 10?m.The single-channel with single layer of TC4 alloy is formed by SLM and the structure is consist of a large number of acicular martensite.The length of martensite is about from 30?m to 40?m,which is thicker than the powders'.The shape of the molten pool is very sensitive to the process parameters.With the laser power increasing or the scanning speed decreasing,the depth and the width of the molten pool increases,the angle between the single-channel pool and the upper surface of substrate is also increases gradually.The microstructure of TC4 alloy formed by SLM was observed and analyzed.It was found that the upper surface structure of the block samples was vertical chessboard pattern,and the longitudinal section was a columnar crystal,which is epitaxial growth along the z-axis.There were a large number of acicular martensite in the columnar crystal.The main phase is ?' phase at room temperature.And the process parameters also affect the microstructure.