Effect of HIP and gas nitriding on microstructure, hardness and tribological behavior of EBM-fabricated Ti6Al4V alloy

This study investigated the effects of Hot Isostatic Pressing (HIP) and gas nitriding on the microstructure, hardness, and tribological behavior of Ti6Al4V alloy fabricated by Electron Beam Melting (EBM). As-built specimens exhibited a lamellar α + β microstructure governed by the EBM thermal history. HIP treatment caused no significant α-lamellae coarsening; instead, it produced a more homogeneous phase distribution and slight lamellar refinement, indicating diffusion-driven phase redistribution. In contrast, gas nitriding led to the formation of a dense TiN/Ti 2 N surface layer, resulting in a pronounced gradient in both microstructural and mechanical properties between the surface and the bulk. Microhardness measurements revealed that the hardness decreased from approximately 380 HV in the as-built condition to around 320 HV after HIP treatment, corresponding to a reduction of about 15.8%. In contrast, gas nitriding significantly increased the surface hardness to approximately 540 HV, representing an improvement of nearly 42.1%. Tribological results showed that the average coefficient of friction decreased from 0.44 for the as-built samples to 0.38 after HIP treatment, corresponding to a reduction of 13.6%, and further decreased to approximately 0.10 after nitriding, indicating a substantial improvement of about 77.3%. The findings indicate that the response of EBM-fabricated Ti6Al4V to post-processing is governed by diffusion-based kinetics and the initial microstructure. While nitriding markedly enhances surface hardness and frictional performance, the observed surface cracking and localized delamination indicate an increased susceptibility of the hardened nitride layer to brittle damage under sliding conditions.