Numerical Study On Melt Pool Evolution And Microstructure Solidification In Electron Beam Selective Melting

As an important development direction in the field of metal additive manufacturing,electron beam selective melting additive manufacturing has been widely used in aerospace,biomedical,automotive manufacturing,and other fields.This manufacturing process involves the coupling of various non-linear physical mechanisms such as phase change,convection,heat and mass transfer,which requires an in-depth understanding of the melt flow characteristics and heat-mass transport laws during the evolution of the melt pool,and the exploration and prediction of the microstructure formation process of solidified parts to improve the metallurgical quality and mechanical properties of the formed parts.With the rapid development of numerical computing technology and the continuous improvement of computer performance,numerical simulation technology has become an essential scientific and technological means to study the additive manufacturing process.Predicting the forming process in advance through numerical simulation technology is important guidance for optimizing the manufacturing process,reducing R&D costs and lead times,and improving product quality.In this thesis,numerical models are developed for the processes of powder laying,selective melting,and microstructure solidification during electron beam selective melting.Based on the basic principles of the discrete element method,a numerical model is constructed for the simulation of particle motion and collision in the powder laying process.A three-dimensional heat and mass transfer model describing the melt two-phase flow,heat-mass transport,and phase change phenomena during the evolution of the electron-beam-selected melting pool is developed based on the basic principles of the Lattice Boltzmann method.And a lattice Boltzmann-finite volume mixed-format model is proposed for microstructure solidification based on the kinetic theory of crystal growth.The model is validated quantitatively by simulating thermal convective phase changes,droplet deformation,wetting effects,and binary alloy dendrite growth,and comparing the simulation results with the analytical solution and the benchmark solution in the literature.The model developed allows accurate simulation of melt two-phase flow and solid-liquid phase change phenomena.The multi-scale model was applied to numerically simulate the electron beam selective melting process.The effects of surface tension,Marangoni effect,and recoil pressure on the morphology of the melt pool are analyzed based on a single track selective melting process.Surface tension is a crucial contributor to the fusion of molten powders.Marangoni convection makes the melt flow around the pool,accelerating the convective cooling process,and the presence of recoil pressure results in significant depressions on the pool surface and deepens the pool depth.The effect of process parameters such as electron beam power,scanning speed,effective diameter,and acceleration voltage on the melt pool evolution was investigated.Increasing the power,reducing the scanning speed,and decreasing the effective diameter can all increase the melt pool volume.When the energy input is insufficient,the substrate and powder do not melt sufficiently and the balling phenomenon occurs under surface tension.When the electron beam acceleration voltage is suitably increased,the depth of the energy absorption peak increases,as does the depth and volume of the melt pool,which promotes the fusion and integration of the metal powder with the substrate material.Based on the multi-track selective melting process,the morphological differences between the ‘Z’ and ‘S’ type scanning strategies are analyzed.The spacing between tracks should be smaller than the width of the trajectory obtained in single-track selective melting to ensure a reasonable overlap and avoid porosity.The temperature gradient and solidification rate at the solid-liquid interface are calculated based on the simulation results of the melt pool evolution process.The transformation from liquid to solid phase during alloy solidification is further simulated and the effect of temperature gradient and solidification rate is investigated and analyzed.The structure shows a columnar crystal shape along the temperature gradient.With the higher solidification rate and temperature gradient,the crystal growth can get a greater drive and the microstructure grows faster.The multi-scale model developed in this thesis and the related findings demonstrate potential value in understanding the transport and phase transition patterns during electron beam selective melting to optimize the additive manufacturing process and improve the final formed parts’ quality.