Current post-process heat treatments applied to selective laser melting produced Ti-6Al-4V do not achieve the same microstructure and therefore superior tensile behaviour of thermomechanical processed wrought Ti-6Al-4V. Due to the growing demand for selective laser melting produced parts in industry, research and development towards improved mechanical properties is ongoing. This study is aimed at developing post-process annealing strategies to improve tensile behaviour of selective laser melting produced Ti-6Al-4V parts. Optical and electron microscopy was used to study α grain morphology as a function of annealing temperature, hold time and cooling rate. Quasi-static uniaxial tensile tests were used to measure tensile behaviour of different annealed parts. It was found that elongated α’/α grains can be fragmented into equiaxial grains through applying a high temperature annealing strategy. It is shown that bi-modal microstructures achieve a superior tensile ductility to current heat treated selective laser melting produced Ti-6Al-4V samples.

This study contributes to the limited understanding of the initial stages of martensite decomposition and stress relief of SLM-produced Ti6Al4V. Samples are annealed at a temperature range of 427–610°C for hold times ranging from 5 minutes and 30 hours. Stress relief is measured through x-ray diffraction, while phase transformation is observed using microscopy and x-ray diffraction techniques. Tensile tests and hardness measurements are used to obtain mechanical properties. Key findings of this study are that ultra-fine vanadium-rich precipitates form at α’ dislocations and grain boundaries during preliminary phase transformation. While initial formation of vanadium-rich precipitates results in the embrittlement of the material, further growth of these precipitates into measurable β-phase is extremely slow but allows for an improvement of material ductility. The study reveals that phase transformation is incomplete after 30 hours at 610°C. While an increase in annealing temperature accelerates phase transformation and improves ductility, this comes at a cost of material strength.

INFLUENCE OF HEAT TREATMENTS ON THE MICROSTRUCTURE AND TENSILE BEHAVIOUR OF SELECTIVE LASER MELTING-PRODUCED TI-6AL-4V PARTS

This study investigates the feasibility of using Laser Powder Bed Fusion (L-PBF) additive manufacturing (AM) to fabricate porous titanium Porous Transport Layers (PTLs) for Proton Exchange Membrane Water Electrolysis (PEMWE) systems. We explore L-PBF as a potential solution to overcome limitations of traditional PTL manufacturing methods, such as limited control over structural morphology and inefficient material use. Using spheroidized titanium powder, we produced 1 mm thick plates with varying porosities by manipulating laser process parameters. The internal structure and surface morphology of AM-produced PTLs were characterized and compared to conventional press-sintered PTLs. L-PBF successfully produced PTLs with porosities in the recommended 30-50% range, featuring spherical particles and a textured pore structure. In-situ testing in a lab-scale PEMWE revealed that AM-produced PTLs exhibited improved performance compared to commercial press-sintered PTLs. This enhancement is attributed to the finer surface structure and favourable gas liquid transport properties of the AM-produced PTLs. These preliminary findings suggest that L-PBF is a promising method for manufacturing PTLs, offering potential advantages in design flexibility, material efficiency, and PEMWE performance. Further research is needed to fully optimize the AM process and comprehensively evaluate long-term PTL performance.