S. Al-Bermani, I. Todd, Advanced Manufacturing Research Center with Boeing, Sheffield, England
Metallic materials produced by additive layer manufacturing (ALM) exhibit microstructures markedly different to those of their wrought counterparts. This is due, in large measure, to their mechanism of formation, which may differ radically from those developed by conventional processing routes. The present paper concentrates on the microstructure of Ti-6Al-4V components, developed via electron beam melting (EBM) and compares and contrasts the structures with wrought plate and laser ALM manufactured materials. Whilst wrought and cast products transform from β → α + β structures; EBM material undergoes an initial diffusionless transformation from β → α’ (hcp martensite) before decomposition/tempering of α’ → α + β; Laser deposited Ti-6Al-4V also forms the martenistic structure on cooling, but does not decompose, because of differences in build environment. In EBM this decomposition arises due to the temperature of the powder bed, maintained in excess of 650°C. Build duration and temperature determine the extent of martensite decomposition to a basket weave α + β structure whilst powder bed systems operating at low build temperatures, produce fully martensitic structures due to the lack of thermal energy necessary to decompose the martensite.
Summary: Metallic materials produced by additive layer manufacturing (ALM) exhibit microstructures markedly different to those of their wrought counterparts. This is due, in large measure, to their mechanism of formation, which may differ radically from those developed by conventional processing routes. The present paper concentrates on the microstructure of Ti-6Al-4V components, developed via electron beam melting (EBM) and compares and contrasts the structures with wrought plate and laser ALM manufactured materials.
