Effect of trace lanthanum hexaboride and boron additions on microstructure, tensile properties and anisotropy of Ti-6Al-4V produced by additive manufacturing

Abstract

In this work trace lanthanum hexaboride (LaB6) and elemental boron are alloyed with Ti-6Al-4V and their effects on the microstructure, tensile properties (including anisotropy) and melt pool shape during Additive Manufacturing (AM) are investigated. During the melting process, the LaB6 scavenges oxygen and decomposes into La2O3 and TiB. The presence of the rare earth element drastically changes the apparent surface tension and shape of the deposited layers. This is attributed to the Heiple-Roper effect and could have benefits during AM in producing components with unsupported overhangs. The formation of eutectic TiB during the final stages of solidification results in highly directional TiB needles in between columnar grains that are aligned with the build direction. The slow cooling rate during deposition of approximately 90–100 °C s−1 produces very large TiB particles which can exceed 50 µm in length. Although improving strength by up to 10%, under tensile stress the high aspect ratio TiB particles are sites for crack opening which leads to a decline in ductility in the longitudinal test direction and a corresponding increase in anisotropy over unmodified Ti-6Al-4V.