Multi Scale Simulation And Mechanical Property Research Of Additive Manufacturing

Understand and master the relationship of " process—structure—property " in additive manufacturing is the key to realize shape control and property control.However,it is difficult to accurately build the relationship of " process—structure—property " due to the strong physical and chemical changes and complex physical and metallurgical processes in additive manufacturing.Metal additive manufacturing involves heat transfer,mass transfer and phase transformation,which is a multi-scale,multi physical field and multidisciplinary problem.At present,the experimental research can not directly reflect the process mechanism of process physical field solidification structure,and trial and error method dominates the experiment and process design,resulting in high experimental cost and Low efficiency.In this paper,a multi-scale simulation model of additive manufacturing process was built to study the relationship between "process structure performance".The macro scale heat transfer,micro grain evolution and growth behavior and plastic deformation mechanism were studied by numerical simulation.The microstructure evolution mechanism and the cause of anisotropic mechanical properties were analyzed,and the microstructure and mechanical properties of materials were predicted.The main contents and achievements were as follows.Firstly,a three-dimensional finite element model for the simulation of the mesoscopic temperature field of IN718 material in selective laser melting(SLM)was constructed.The influence of laser power and scanning speed on the size of the molten pool was analyzed,and the influence of laser power and scanning speed on the temperature field and solidification mechanism of the molten pool was studied,The changes of temperature gradient and solidification rate at the solid-liquid interface of molten pool under different process parameters were calculated.The transformation conditions of columnar crystal to equiaxed crystal during solidification were analyzed.The microstructure at different depth of molten pool was predicted,which provided theoretical basis for microstructure control.Based on the simulation of temperature field,the coupled model of temperature field and micro grain evolution of IN718 material in laser selective melting was established by using CAFE(cellular automation finite element)model.The cellular automata algorithm based on extended Moore neighborhood was proposed in this paper to improve the computational efficiency and simulate the microstructure evolution process of multi-layer melting pool solidification process.The simulation results show that the heterogeneous nucleation on the upper surface of the molten pool forms equiaxed grains,while the grains at the fusion line at the bottom of the molten pool grow towards the center of the molten pool and form columnar crystals by epitaxial growth.Finally,after multilayer deposition,columnar crystal structure gradually inclines from the deposition direction to the scanning direction and run through several deposition layers.Finally,a research idea was proposed to study the relationship of SLM technology between microstructure and macro mechanical properties of IN718 materials based on microstructure evolution simulation and crystal plastic finite element method.According to the simulation results of the crystal plastic finite element model,the material manufactured by laser additive manufacture has higher strength and stiffness in transverse direction than longitudinal direction.According to the results of microstructure simulation,the transverse and longitudinal schmidt factors were 0.3183 and 0.4714,respectively,and the average grain size is 30 ?m and 90 ?m,respectively.This is the cause of strength anisotropy of SLM.According to the three-dimensional orientation distribution of Young’s modulus of IN718 single crystal and the microstructure simulation results,the average young’s modulus in z-axis direction is 139.5919 GPa,while that in xoy plane is 155.8748 GPa.The transverse stiffness of the material is higher than that in longitudinal direction.