Numerical Simulation On Temperature Field And Stress Field During Selective Laser Melting Of Titanium Alloy

During the process of selective laser melting(SLM),due to the highly located energy input and the movement of laser source,the selected powder beds will undergo circulation of heating and cooling with extremely high rate of temperature variation,which will cause thermal stress within products.As a result,the formability and dimensional accuracy of the parts may be affected,and the products may even be cracked.Therefore,the finite element method was adopted to study the distribution of temperature and thermal stress in SLM process under different process parameters and scanning strategies.In this thesis,the thermal-mechanical indirect coupling finite element model of SLM processing Ti-6Al-4V alloy was established by using the ANSYS parametric design language(APDL),which utilized the Gaussian moving heat source model and thermaldependent material properties.In order to improve the accuracy of simulation,the anisotropic enhanced thermal conductivity was used.After that,the predicted results were compared with experimental measurements at multiple scales including molten pool dimensions,grain growth direction and residual stress.And the simulations were proved to be consistent well with experiments.The finite element model can successfully predict the temperature and thermal stress distribution of SLM process.Based on the established finite element model,the effects of processing parameters on temperature distribution and characteristics of molten pool were analyzed.The investigation showed that during the SLM process,the deposited layers underwent high-frequency thermal circulation in both horizontal direction and vertical direction.Under the effect of complicated thermal circulation,the materials were melted for multiple times.Furthermore,both the molten pool dimensions and liquid lifetime elevated with the laser power increasing and the scanning velocity decreasing.And increasing the layer thickness reduced the peak temperature and numbers of re-melting.In addition,at high scanning spacing,the heat accumulation between the scan tracks was reduced and then the higher maximum temperature was achieved.The effects of process parameters and scanning strategies on the distribution of residual stress and the evolution of thermal stress were further investigated.The influences of process parameters were mainly reflected in the numerical value,and raising laser power and scanning velocity as well as reducing layer thickness and scanning spacing led to higher residual stress.While the scanning strategies can significantly change the distribution of residual stress.During the partition scanning,the residual stress at overlapping area was higher than the stress within the sub-region.And the thermal stress on surface was lower than that within the blocks.What's more,the unique complex thermal circulation in SLM process will induce complicated thermal stress circulation,and cause stress-relief annealing on solids.The stress-relief annealing caused by thermal circulation in vertical direction was characterized by variable temperature and pulsed action.In each circulation,the thermal stress went down rapidly and then rose up slowly,which resulted in the maximum thermal stress between the circulation decreasing gradually.With the increasing of deposition layers,the thermal circulation in vertical direction was weaker and led to the magnitude of the maximum thermal stress reduction within a circulation decreasing.Under the effect of multiple thermal circulation,the maximum thermal stress could drop by more than 40 %.Compared to the thermal circulation in horizontal direction,the stress-relief annealing caused by thermal circulation in vertical direction was more significant.The investigation showed there was significant reduction under the influence of thermal circulation.