Transformation-mediated in-situ β recrystallization in laser powder-bed fusion additively manufactured Ti-6Al-4V
Jianhao Chen, Daniel Fabijanic, Milan Brandt, Wei Xu
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-4 V 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-4 V 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-4 V 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-4 V with reduced performance anisotropy.