Numerical Invetigation On Multiphysics Field Problems During Selective Laser Melting Of Ti-6Al-4V Alloy By The Finite Element-Phase Field Method

As the fine horse in titanium alloys,Ti-6Al-4V alloy is widely used in aerospace,biomedical and other industrial fields because of its low density,good biocompatibility,excellent comprehensive mechanical properties and good corrosion resistance.Traditionally,vacuum induction remelting and thermomechanical processing are adopted to tune the microstructure and thus the mechanical properties of titanium alloys.However,traditional processing technology often faces problems such as high cost,low material yield,and long manufacturing cycle.In order to overcome the inherent shortcomings of traditional processing technology and meet the needs of tailor-made and personalized products,additive manufacturing technology has become a new processing route in recent years.Additive manufacturing technology manufactures parts based on the additive principle of three-dimensional design and layer-by-layer printing,which is breaking through the shackles of traditional material removal manufacturing processes.As an important member of the additive manufacturing technology family,the selective laser melting(SLM)process involves multiple subjects such as materials,machinery,heat transfer,powder and laser forming.Based on the finite element-phase field method,the temperature field distribution,the pore evolution and the microstructure evolution of Ti-6Al-4V alloy during SLM process were investigated in this thesis.A moving Gaussian heat model was proposed as the heat input of the temperature filed.The latent heat of phase change during SLM process was calculated by the apparent heat capacity method.Based on the principle of heat balance,the solid heat transfer equation was derived by the weighted residual method and the Gauss theorem.A flow filed model for the SLM temperature field and pore evolution was proposed.Based on the phase field method,a microstructure evolution model was established.A finite element model for calculating the temperature evolution during SLM process of Ti-6Al-4V alloy was proposed.By comparing the numerical accuracy of three different mesh generation strategies,the numerical simulation accuracy of the powder layer refinement mesh generation strategy was found to be higher than the others.The basic characteristics of the temperature field,the influence of process parameters on the temperature field distribution and the molten pool size were investigated in depth.It was found that under the same laser powder(100 W),as the scanning rate increased from 500 mm/s to 1200 mm/s,the heating rate of the powder layer gradually increased,while the depth of molten pool decreased from45μm to 18μm,and the half width of molten pool decreased from 65μm to 39μm,which were in good agreement with the experimental results.It was also revealed that the temperature gradient of the SLM transient molten pool along the z-axis was proportional to the laser power and inversely proportional to the scanning rate.Coupling the SLM flow field equations and the gas-liquid two-phase interface dynamic phase field equation,a finite element-phase field model for the pore evolution during SLM solidification of Ti-6Al-4V alloy was established.It was found that porosity was very sensitive to changes of process parameters,but there was no specific linear relationship with the laser power and the scanning rate.At the same time,the quantitative relation between porosity and energy density was revealed.The critical energy density of Ti-6Al-4V alloy for full densification was in the range of 45 J/mm~3～75 J/mm~3,which was in good agreement with the experimental results.Two-dimensional models of the transient molten pool along the xoz plane and along the yoz plane of the Ti-6Al-4V alloy during SLM process were established.Considering the influence of the scanning rate on the microstructure evolution,the phase field model of the microstructure evolution along the xoz plane was revised.It was found that,when the laser power was 50 W and the scanning rate was 500 mm/s,the columnar crystal structure in the yoz plane was finely arranged and there were almost no secondary dendrites.In this case,the primary dendrite arm spacing(PDAS)was about 1μm.However,under the same process conditions,the phenomenon of primary dendrite coarsening along with secondary dendritic growth and coarsening appearred during the microstructure evolution in the xoz plane,which was due to the low temperature gradient and the relatively low cooling rate of the molten pool.In this cross section,the PDAS was about 2.5μm.