The Effect of Thermohydrogen Process on the Mechanical Properties of the Ti-6Al-4V Alloy Produced by Selective Laser Melting

Selective laser melting (SLM) is a powder bed fusion type additive manufacturing (AM) process that allows fabrication of complex and detailed components from computer aided design (CAD) model in one single step without the need of costly tools.Ti-6Al-4V alloy is widely used in the applications where high specific strength is required particularly in aero engine discs, frames and compressors.Production of the Ti-6Al-4V alloy with a conventional methodis inadequate in manufacturing complex shaped parts used especially in the aero engine industry; therefore, SLM processes arebeing developed in order to enable rapid manufacturing of the Ti-6Al-4V alloy forintricate components. However, in SLM technologydue to large thermal inputs during production ductility of Ti-6Al-4V parts are inferior to conventionally produced ones without post heat treatment.Therefore, post processing of SLM processed parts is highly required to obtain desired mechanical properties which meet the specifications of the aerospace industry. In this study, as an alternative to typical heat treatments, a thermohydrogen process (THP) based on hydrogenation and dehydrogenation steps were applied to SLM processed Ti-6Al-4V alloy to refine the microstructure and to improve its mechanical properties. The starting non-equilibrium Į′ martensitic phase of SLM processedTi-6Al-4V alloy was transformed to į (TiH2) and ȕ phases after the hydrogen treatment at 650°C for 1h. When the hydrogenated alloy was vacuum treated at 700°C for 18h, discontinuous and fine Į/ȕ duplex microstructure formed as a result of this dehydrogenation step. 2-step THP post-process heat treatmentwas found to increase the alloy’s ductility without degrading its strength. The effect of the novel 2-step THP was also compared with that of the 4-step THP which has been applied to cast and wrought Ti-6Al-4V parts conventionally. It was determined that the novel 2-stepTHP ismore beneficial for SLM fabricated Ti-6Al-4V parts in terms increasing their ductility while optimizing their strength in contrast to conventional 4-step THP