MARTENSITIC TRANSFORMATION AND ITS EFFECTS ON MICROSTRUCTURAL EVOLUTION AND MECHANICAL PROPERTIES IN ELECTRON BEAM MELTING OF COMMERCIALLY PURE TITANIUM AND Ti-6Al-4V ALLOY

摘要

Electron beam melting (EBM), which is a powder-bed-fusion-type additive manufacturing technique, has received considerable attention from the biomedical, aerospace, and automotive industries. EBM is characterized by rapid solidification, but it is a completely hot process that maintains the fabricated materials at elevated temperatures after solidification. We investigated the microstructural evolution of commercially pure titanium (CP-Ti) and Ti-6Al-4V (Ti-64) alloys during the EBM process. The as-built CP-Ti components exhibited weak textures, which are quite difficult to form in conventional processes, due to the occurrence of the β → α' martensitic transformation after solidification. On the other hand, the Ti-64 alloy consisted of fine acicular α plates and β phase at interfaces between the α plates. These microstructures also developed via decomposition of the α' martensite formed from the columnar prior-p grains. The relatively high strengths of the as-built specimens originated from the evolution of the dislocation density over their complex thermal histories.