Prediction Of Stress And Deformation Of Martensitic Stainless Steel In Selective Laser Melting

The rapid heating and thermal cycle of the metal in selective laser melting(SLM)generates a large temperature gradient,which leads to a large residual stress in SLMed parts,resulting in deformation and even cracks of the parts and affect the performance of the parts.The global residual stress of the SLMed parts can be predicted fast and economically by the finite element(FE)method.However,for materials with phase transformation,there is usually a large error between the measured values and the predicted results by the traditional FE model based on the ideal elastic-plastic hypothesis,because the solid-state phase transformation effect in the SLM process can significantly affect the stress state of the SLMed part.Therefore,based on 6511 martensitic stainless steel,this study established a set of multi-scale thermo-mechanical coupling finite element model considering solid-state phase transformation,so as to describe the solid-state phase transformation behavior and studied the evolution of temperature field,phase transformation field and residual stress field in the SLM process.First,based on the process optimization test,the optimal process parameters of 6511 martensitic stainless steel in the SLM process,namely laser power and scanning speed,were determined through the characterization of relative density,microstructure and mechanical properties of SLMed samples,which were taken as the boundary condition input of the multi-scale finite element simulation model.Then,Gleeble 3500 C thermal simulation machine was used for free thermal expansion test to determine the phase transformation type of 6511 steel during SLM thermal cycle,and the phase transformation temperature,volumetric effect strain and thermal expansion coefficient of the material were also determined.MTS E45.105 universal testing machine was used for high temperature tensile test,and obtained the yield strength,Young's modulus and flow stress of the material with different phase under different temperatures.The true stress-true strain curves at different temperatures were used to describe the elastic and plastic behavior of materials during the SLM process.Finally,a multi-scale thermo-mechanical finite element model considering solid-state phase transformation is established by finite element software MSC.Marc and its pre-and post-processor Mentat,including micro-scale laser scanning model,the meso-scale whole layer model and macro-scale parts model.Not only the dependence of mechanical properties on phase volume fraction and temperature,but also the volumetric change and phase transformation induced plasticity(TRIP)are taken into account.The residual stress and deformation of the parts can be predicted rapidly and effectively by multi-scale modeling method which increases the heat source dimension and the layer thickness.The thermo-mechanical coupling analysis of SLM process of 6511 martensitic stainless steel shaped cantilever was carried out to analyze the effect of solid-state phase transformation on the evolution residual stress in additive manufacturing.It was found that martensitic transformation leads to the decrease of the tensile residual stress and increase the compressive residual stress of the SLMed parts,and the occurrence of compressive stress on the cantilever beam reduces the bending deformation of the cantilever in the build direction after removing the support.