Microstructural effects of HIP nitriding on additively manufactured titanium

Sammanfattning

This thesis investigates the feasibility and microstructural effects of combining Hot IsostaticPressing (HIP) and nitriding into a single process for Powder Bed Fusion – Laser Beam (PBF-LB)titanium components, primarily for orthopaedic implant applications. While titanium alloys offersuperior biocompatibility and closer mechanical matching to human bone compared tostandard Cobalt-Chromium alloys, their inferior wear resistance limits their use in articulatinghigh-load applications. A hybrid surface treatment, HIP nitriding, is explored to streamline postprocessing by simultaneously densifying and stress relieving the additively manufactured partsand generating a wear-resistant, bioinert titanium nitride surface layer. The study aims to achievea target nitride layer thickness of 10 μm by systematically varying process time, temperature, andlocal atmospheric conditions.Microstructural characterization, including Scanning Electron Microscopy (SEM), EnergyDispersive X-ray Spectroscopy (EDS), X-ray Diffraction (XRD) and microhardness testing, wasperformed on the as printed and treated samples. The results demonstrated that the HIP nitridingprocesses investigated successfully created a compound nitride layer on the surface of thesamples consisting of TiN and Ti2N. Increasing the treatment hold time and sub β-transustemperatures successfully increased the compound layer thickness and achieved a maximumof approximately 6 μm after a 12-hour hold at 950°C. Shielding samples within an alumina box tostabilize the local nitrogen atmosphere minimised surface contamination and consistentlyyielded thicker nitride layers than unshielded samples. The thermal treatment converted themetastable, highly stressed as-printed martensitic structure into a stable α+β morphology,lowering bulk core hardness while significantly increasing surface and subsurface hardness dueto the creation of the compound layer through interstitial nitrogen diffusion. This study has showthat HIP nitriding shows promise as an effective single step method to densify, stress relieve andsurface harden additively manufactured titanium.