Abnormal grain growth in commercially pure titanium during additive manufacturing with electron beam melting

The microstructural evolution of commercially pure titanium (CP-Ti) was investigated during additive manufacturing (AM) with electron beam melting (EBM). Abnormal grain growth (AGG) was observed throughout all but the top of the specimen, with a mixture of fine and very coarse α (hexagonal close-packed) grains, resulting in relatively large scattering in the Vickers hardness at a given build height. AGG proceeded by the growth of some α grains to a few hundred micrometers in size at the expense of the surrounding substructured fine grains, which were created during EBM but prior to the occurrence of AGG. A small amount of β-phase particles were also produced during rapid cooling after solidification, probably contributing the occurrence of AGG by pinning the grain boundaries. The observations highlight the evolution of a unique, process-specific microstructure during EBM fabrication. Improvement of the scan strategy as well as modification of the chemical composition are needed to obtain a homogeneous microstructure.