Residual stress modelling and experimental analyses of Ti6Al4V ELI additive manufactured by laser engineered net shaping

Abstract

This paper focus on the experimental analyses and modelling of the residual stresses build up during laser additive manufacturing by Laser Engineered Net Shaping. Currently, additive manufactured parts employ heat treatment for the reduction of internal stresses, but then additional advantages are also possible from heat treatment. The experimental analyses focus on stress relieving heat treatment temperatures to reduce the residual stresses during laser processing of LENS Ti6Al4V ELI specimens. LENS parts out of Ti6Al4V ELI will illustrate the mechanical property possibilities resulting from the selected stress relieving heat treatments in this study. The primary aim of heat treatment in this case of Ti6Al4V ELI is the reduction of internal stresses. Due to the mechanical behaviour of Ti6Al4V as built additive manufactured parts, the heat treatment seems to be necessary to increase the mechanical behaviour, such as the fatigue performance and the breaking elongation. Optical Microscope, Scanning Electron Microscope and Vickers hardness test was employed to carry out detailed study of the resulting microstructures and Hardness. The model by COMSOL Multiphysics was employed to predict the residual stresses of as built LENS Ti6Al4V ELI and to better understand the residual stresses amounts in the Ti6Al4V ELI alloy that need to be minimized by heat stress relieving heat treatment methods. The results included the β-phase that formed in the stress relieving heat treatment process that was transformed to martensite α during the cooling process and a fine basket-weave structure emerged. The microhardness of LENS Ti6Al4V ELI alloy gradually decreased with increasing stress relieving heat treatment temperature. The computed model revealed the maximum stress of 1.78x109 MPa, the Model strongly recommended the LENS process parameters suitable to obtain Ti6Al4V ELI samples with minimal residual stresses and a further possible method to alleviate the attained residual stresses in the model to the desired elasticity.