Effects of Heat Treatment on The Microstructure and The Mechanical Properties of Electron Beam Additively Manufactured Ti6Al4V with Powder Reuse

Abstract

Metal additive manufacturing has many advantages such as low material waste, unlimited design complexity, and its cost-effectiveness as a near-net-shape process. It has been applied to many materials, but especially for the fabrication of Ti6Al4V components due to its excellent corrosion resistance, high strength to weight ratio, and biocompatibility. Among a variety of additive manufacturing (AM) techniques, especially powder bed fusion systems, electron beam melting (EBM) has been extensively investigated because of its lower thermal stress in parts compared to Selective Laser Melting (SLM), and high density of parts produced. However, there are limited investigations related to heat treatment as a post-processing technique with powder reuse in EBM AM of Ti6Al4V over consecutive builds for the comparison of microstructure and mechanical properties. Therefore, this research analyzed the effects of heat treatment with powder reuse over 30 build cycles to identify the changes in microstructures and mechanical properties of Grade 5 Ti6Al4V. EBM printed samples were analyzed by tensile testing and scanning electron microscopy. Heat treatment was conducted with sub (710°C), near (950°C) and super (1050°C) β-transus temperature. As a result of these heat treatments, the α lath thickness increased, which caused a reduction in strength and an increase in ductility. Metal subjected to the near β-transus temperature heat treatment (950°C/1hr) exhibited the best ductility among other heat treatments. The sub β-transus temperature did not show any difference when compared to the as-built samples, whereas the super β-transus heat treatment caused a decrease in both the strength and ductility. Through microstructural analysis, sub β-transus heat treatment showed the same microstructure as as-built samples (prior beta columnar grains with alpha lath). The near β-transus heat treatment caused coarsening of the α lath, whereas the super β-transus heat treatment caused recrystallization to uniform equiaxed structure with alpha grain boundaries. The super β-transus heat treatment and corresponding equiaxed grain microstructure caused a change in fracture mode from transcrystalline to intercrystalline. In addition to the use of the near β-transus heat treatment, the controlling cooling rate is an important factor in obtaining the optimum tensile properties. Regarding powder reuse, there was an increase in the oxygen content, which contributed to the α lath thickness and the tensile properties. Lastly, heat treatment and post-build machining the samples appeared to improve the reliability of the Ti6Al4V produced by EBM AM.