In-situ residual stress reduction, martensitic decomposition and mechanical properties enhancement through high temperature powder bed pre-heating of Selective Laser Melted Ti6Al4V

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. The use of high temperature pre-heating during an SLM build can assist in reducing thermal gradients, enable a more controlled cooling with the possibility to control/tailor as-built mechanical properties. In this work a high temperature SLM powder bed capable of pre-heating to 800 degrees C is used during processing of Ti6Al4V feedstock. The effect of powder bed temperature on residual stress formation, microstructure and mechanical properties was investigated. It was found that increasing the bed temperature to 570 degrees C significantly reduced residual stress formation within components and enhanced yield strength and ductility. This pre-heating temperature enabled the decomposition of alpha' martensitic microstructure into an equilibrium alpha +beta microstructure. At 570 degrees C the yield strength and elongation of components was improved by 3.2% and 66.2% respectively.