Transformation-mediated in-situ β recrystallization in laser powder-bed fusion additively manufactured Ti-6Al-4V

Abstract

Achieving equiaxed grain structure is highly sought after in metal processing for most critical structural applications but it is seldom realized in Ti-6Al-4V additively manufactured (AM) by laser powder-bed fusion (LPBF) due to its unique thermal environment in favor of columnar grain growth. To convert columnar grains into equiaxed, post heat treatment in the β phase field is often adopted to initiate recrystallization of prior-β grains. This study demonstrates the achievement of equiaxed prior-β grain structure in situ in LPBF Ti-6Al-4V via epitaxial recrystallization mediated by α → β phase transformation taking place in the rapid heating stage of LPBF thermal cycling. In this rapid reverse phase transformation process, new β nuclei of distinct variants to their parent prior-β grain nucleate directly on α matrix in accordance with Burgers orientation relationship and grow towards the surrounding α matrix while α is transforming to β within a specific prior-β grain. This hypothesis is strongly supported by the experimental evidence that a significant fraction of β/β grain boundaries are featured with misorientations near 60° in both as-built Ti-6Al-4V and those subjected to post rapid heat treatment. This essentially excludes conventional recrystallization as the mechanism responsible for the evolution of equiaxed prior-β grain structure in the as-built state. The new finding unveils the critical role of LPBF thermal cycling and associated phase transformation in microstructure evolution and paves the way for the development of high-quality AM Ti-6Al-4V with reduced performance anisotropy.