Effect of solid-state phase transformation on residual stress of selective laser melting Ti6Al4V

Abstract

Residual stress in selective laser melting (SLM) is one of the key challenges in terms of precision control, success rate and the performance of deposited components. Ti6Al4V belongs to α+β titanium alloy, the residual stress of selective laser melted (SLMed) Ti6Al4V component maybe affected by solid-state phase transformation result from complex thermal history of SLM. In the present study, effect of solid-state phase transformation on residual stress of SLM Ti6Al4V was investigated. A coupled modeling method of thermo-metallurgical-mechanical considering solid-state phase transition is provided and validated by microstructure observation and residual stress measurements. Then the solid-state phase transformation strain was neglected in the validated model, the computed residual stress is used to investigate the effect of solid-state phase transformation strain on residual stress by comparing with experimental measurements. In addition, the influence of the difference in yield strength and thermal expansion coefficient between the original phase and the transition phase on the residual stress is discussed. It has been found that the residual stress of SLMed Ti6Al4V is related to the direction, the longitudinal residual stress is about twice transverse, and they are all tensile stresses. The microstructure and residual stress predicted by the simulation are in good agreement with the experimental measurements. The microstructure of SLM Ti6Al4V is mainly composed of martensite α', and the average error between the predicted longitudinal stress and measurement is 2.1%. The solid-state phase transformation has a stress relaxation effect during the SLM Ti6Al4V, predicted longitudinal and transverse residual stress exceeds the experimental measurement by up to 80.7% and 53.9%, when neglecting the solid-state phase transformation strain. The influence of the solid-state phase transformation of SLM Ti6Al4V on the residual stress is mainly determined by the volume change between the solid-state phases.