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

Abstract

This study focuses on the microstructural evolution in additively manufactured (AM) β 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 α + β alloys such as Ti-6Al-4 V, and β 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 β 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 β -Ti alloys, Ti-12Mo and Ti-20 V, that have been processed using laser engineered net shaping (LENS™), a directed energy deposition technique for AM. Compared to Ti-V, which exhibited grains of only the β phase in the as-built condition, the less β stabilized Ti-Mo had extensive second-phase α precipitation within the build. The location within the LENS™ build played a pivotal role in determining the size scale, area fraction, and morphology of the α 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]β texture along the build direction. In the Ti-12Mo alloy, wherein second phase α precipitation takes place, there was no significant α texturing, with all twelve variants forming.