Variant selection in laser powder b e d fusion of non-spherical Ti-6Al-4V powder

The presence of alpha/alpha' on prior ,B/ ,B grain boundaries directly impacts the final mechanical properties of the titanium alloys. The ,B/ ,B grain boundary variant selection of titanium alloys has been assumed to be unlikely owing to the high cooling rates in laser powder bed fusion (L-PBF). However, we hypothesize that powder characteristics such as morphology (non-spherical) and particle size (50-120 mu m) could affect the initial variant selection in L-PBF processed Ti-6Al-4V alloy by locally altering the cooling rates. Despite the high cooling rate found in L-PBF, results showed the presence of ,B/,B grain boundary alpha' lath growth inside two adjacent prior ,B grains. Electron backscatter diffraction micrographs confirmed the presence of ,B/ ,B grain boundary variant selection, and synchrotron X-ray high-speed imaging observation revealed the role of the "shadowing effect " on the locally decreased cooling rate because of keyhole depth reduction and the consequent ,B/,B grain boundary alpha' lath growth. The self-accommodation mechanism was the main variant selection driving force, and the most abundant alpha/alpha boundary variant was type 4 (63.26 degrees//[ 10 5 5 3 over line ]). The dominance of Category II alpha lath clusters associated with the type 4 alpha/alpha boundary variant was validated using the phenomenological theory of martensite transformations and analytical calculations, from which the stress needed for the ,B->alpha' transformation was calculated.(c) 2023 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.