Microstructure and micro-texture evolution of additively manufactured β-Ti alloys

This study focuses on the microstructural evolution in additively manufactured (AM) beta titanium alloys due to solid-state phase transformations occurring during the reheating of previously deposited layers, directly influencing the uniformity of microstructure across the entire build. During the AM of titanium alloys of a wide variety of compositions, including alpha + beta alloys such as Ti-6Al-4 V, and beta alloys, when the laser or electron beam hits the sample, the grains in the previously deposited topmost layers either re-melt or transform into the beta phase. Subsequently, during the cooling cycle, depending on the alloy composition, second-phase precipitation may occur within these layers via solid-state precipitation. The present study compares two binary beta-Ti alloys, Ti-12Mo and Ti-20 V, that have been processed using laser engineered net shaping (LENS (TM)), a directed energy deposition technique for AM. Compared to Ti-V, which exhibited grains of only the beta phase in the as-built condition, the less beta stabilized Ti-Mo had extensive second-phase alpha precipitation within the build. The location within the LENS (TM) build played a pivotal role in determining the size scale, area fraction, and morphology of the alpha precipitates. These changes have been attributed to the different thermal cycles experienced during the deposition process. Irrespective of the alloy composition, columnar grains were observed in the depositions with a strong [001]beta texture along the build direction. In the Ti-12Mo alloy, wherein second phase a precipitation takes place, there was no significant a texturing, with all twelve variants forming.