Abstract:
Ti-6Al-4V alloys are extensively used in biomedical, aerospace and petro-chemical applications
mainly due to their high specific strength, excellent corrosion resistance and high fatigue strength. In
contrast to conventional techniques, additive manufacturing techniques make possible production of
parts with complex geometries in one step by minimizing the usage of raw materials. However, post
heat treatment processes need to be applied to reduce residual stresses created and to get uniform
controllable microstructure so as to obtain desired mechanical properties. Conventional annealing heat
treatments are effective in eliminating the residual stress and increasing the ductility. But, they usually
degrade the strength and result in coarsening the microstructure. In this study, thermochemical
process, called thermo-hydrogen process (THP), was applied to Ti-6Al-4V alloy parts produced by
selective laser melting (SLM) and electron beam melting (EBM) with the aim of increasing ductility
without degrading the alloy strength. Additively manufactured parts initially hydrogenated at 650oC
for 1 h under H2+Ar gas mixture and then, dehyrogenated at 700oC for 18 h under high vacuum. As a
result of alloying with hydrogen and dealloying turned martensitic microstructure in SLM parts into
fine + lamellar phase mixture. On the other hand, microstructural refinement in EBM parts were
limited since as-manufactured parts were already composed of fine lamellar microstructure. THP
prevented grain growth and grain boundary α-phase formation in SLM parts. Ductility of the alloys
increased by 110 %, while the strength decreased slightly only by 10 %. However, the effect of the
THP on mechanical properties of EBM samples couldn’t be observed due to their high surface
roughness which induced notch effect.