Track-Scale Simulations of Selective Laser Melting to Investigate Development and Mitigation of Thermal Stresses

Abstract

This work investigates the evolution of thermal stresses, dependence of residual stress on important process parameters, critical locations of high stresses and ways to minimize them by adopting the appropriate set of the parameters for selective laser melting of Ti6Al4V at track-scale. A fully coupled 3D thermo-mechanical model is developed. Elasto-plastic material and material properties considered are dependent on temperature. In the coupled modelling approach, the validation of thermal model carried out with the available experimental data. Detailed analysis of temperature field and resulting thermal stresses is then presented. During heating, compressive stress zone is observed in the neighbourhood of the melt pool and balancing tensile stress zone below it. As laser traverses forward, tensile stresses are generated in the solidified melt pool region and a balancing compressive stress zone underneath it is observed. It is quantified that with increase in laser heat source power and interaction time, the magnitude of residual stress increases, but pre-heating the substrate reduces the residual stress. The magnitude of residual stress is also decreased by adopting the alternate-scan strategy over the uni-directional scan-strategy. Apart from delineating the detailed quantitative analysis of residual stress, this work helps understand the evolution of thermal stress in the SLM process at the fundamental level, i.e. at track-scale which is the basic building block. This understanding is crucial to control the residual stresses at part scale. Volume reduction during the conversion of powder material to bulk liquid has been received less attention in the previous thermal models. Thus, its incorporation in the present thermal model makes the modelling approach more realistic, as the predictions of the thermal model act as important inputs to the coupled mechanical model that calculates stresses.