Evolution of Residual Stress in Ti6Al4V components fabricated using Selective Laser Melting

During the Selective Laser Melting (SLM) process large temperature gradients can form, generating a mismatch in elastic deformation that can lead to high levels of residual stress within the additively manufactured metallic structure. Rapid melt pool solidification causes SLM processed Ti6Al4V to form a martensitic microstructure with a ductility generally lower than a hot working equivalent. Post-process heat treatments can be applied to SLM components to remove in-built residual stress and improve ductility.
This investigation sought to investigate and understand the root cause of residual stress formation and lower ductility in Ti6Al4V components when processed by SLM, with the aim of ultimately being able to reduce the residual stress and enhance ductility by SLM parameter adjustment. The effect of individual SLM parameters on residual stress was studied by using hole drilling method. Microstructural analysis, tensile, and Vickers hardness testing was used to understand the effect of SLM parameters on mechanical properties of Ti6Al4V components. Experimental study was carried out using a commercial Renishaw AM250 SLM machine and a modified Renishaw SLM125 machine. FEA modelling in ABAQUS with user subroutines DFLUX and USDFLD was used to predict the correlation between SLM parameters, cooling rates and temperature gradients.
The experimental investigation included studying the effect of scanning strategy, layer thickness, rescanning, power and exposure variation keeping energy density constant, and bed pre-heating temperature on residual stress and mechanical properties of SLM Ti6Al4V parts.
Finally an I-Beam geometry was created to identify the geometrical dependence of residual stress in SLM Ti6Al4V components. Stress reduction strategies from the study of individual SLM parameters were strategically applied to high stress regions of the I-Beam geometry to devise techniques for stress reduction across the cross section of a complex geometry.