Microstructure and mechanical properties of Ti-6Al-4V alloy fabricated using powder bed fusion – laser beam additive manufacturing process: Effect of hot isostatic pressing

Abstract

Powder bed fusion laser beam (PBF-LB) is one of the most widespread and highly researched additive manufacturing (AM) methods, spanning multiple industries. Its feedstock material is metallic powder, where a conventional particle size range is 15–50 μm. The present study focuses on Ti-6Al-4V powder with a wider particle size distribution (15–90 μm). Two process themes are evaluated: one minimising porosity and one maximising build rate through a fast laser scanning speed. The effect of two hot isostatic pressing (HIP) heat treatments on mechanical properties, one below and one above the β-transus, are compared to those of as-built and stress relieved material. Room temperature impact toughness and tensile testing are used to compare the materials by determining UTS and Yield strength, elongation and reduction of area for the different process conditions and post build heat treatments. The minimal porosity theme reaches properties comparable to conventional manufacturing processes at all heat treatment temperatures (i.e., UTS >860 MPa, 0.2 % Yield >795 MPa). The high productivity theme treated below β-transus provides further improvement in overall reduction of area (>45 %) and elongation (>20 %) with respect to the minimal porosity theme, by showing a bi-modal microstructure that is the result of a recrystallisation process. This phenomenon is triggered by the closure of lack of fusion (LoF) defects via hot isostatic pressing, due to a higher dislocation density at the tip of these particular defects. Impact energy for this condition increases whilst hardness and texture become less pronounced. It is demonstrated that in those cases where a fast scanning speed creates LoF defects, those can assist in modifying microstructure during the consolidation process which has a positive effect on ductility.