On the hot deformation behavior of Ti-6Al-4V made by additive manufacturing

Abstract

Innovative process chains that use additive manufacturing (AM) to produce pre-forms for hot forging have been proposed recently, but almost no scientific knowledge on the hot deformation behavior of AM material exists. The combination of AM and metal forming may allow for producing pre-forms by AM which can be forged to the final shape in a single forming operation, thus saving tooling costs compared to conventional multi-stage forging processes. In this work, Ti-6Al-4 V samples made from powder using Directed Energy Deposition (DED) and Selective Laser Melting (SLM) are investigated under typical hot working conditions. An increasing cooling rate in AM promotes the formation of martensite. When heated to forming temperature, the martensite decomposes and yields intermediate microstructures with improved hot working properties compared to conventional wrought material. Differences in the flow stress and activation energy for hot working are analyzed for conventional and as-built Ti-6Al-4 V. AM material exhibits both lower flow stresses and a faster globularization rate, which is explained by non-equilibrium phase fractions and the spatial arrangement of the β-phase. A model is proposed to predict flow stresses and microstructure evolution in Ti-6Al-4 V specimens with a conventional and AM microstructure, which takes the spatial arrangement into account. The findings can be used to design new process chains that allow for single-stage net-shape forging of Ti-6Al-4 V parts at reduced forming forces and with improved mechanical properties.