Promoting the columnar to equiaxed transition and grain refinement of titanium alloys during additive manufacturing

Preventing columnar grain formation during additive manufacturing has become a significant challenge. Columnar grains are generally regarded as unfavourable as their presence can impart solidification defects and mechanical property anisotropy, however, the thermal conditions experienced during additive manufacturing make columnar grains difficult to avoid. In this work the thermal conditions during solidification (cooling rate, temperature gradients) are characterised during wire based additive manufacturing. For the selection of deposition conditions that favour equiaxed grain formation, the role of alloy constitution is explored in three classical alloy design regimes: an alloy containing no grain refiners (Ti-6Al-4V); an alloy only containing grain refining solutes (Ti-3Al-8V-6Cr-4Mo-4Zr); and an alloy containing both grain refining solute and nucleant particles (Ti-3Al-8V-6Cr-4Mo-4Zr + La2O3). Substantial refinement and equiaxed grain formation is achieved in the latter case which is attributed to beta-Ti nucleation on La2O3. However, the thermal environment is dynamic during additive manufacturing and equiaxed grain formation is only achievable when temperature gradients decrease sufficiently to permit constitutional supercooling. (C) 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.