Variant selection in laser powder bed fusion of non-spherical Ti-6Al-4V powder

• Published in: Journal of materials science & technology Vol. 147; no. N/A
• Main Authors: Asherloo, Mohammadreza, Wu, Ziheng, Sabisch, Julian E.C., Ghamarian, Iman, Rollett, Anthony D., Mostafaei, Amir
• Format: Journal Article
• Language: English
• Published: United States Elsevier 08.12.2022
• Subjects: additive manufacturing; electron backscattered diffraction; hydride-dehydride powder; MATERIALS SCIENCE; phase formation; texture analysis; variant selection
• ISSN: 1005-0302; 1941-1162

The presence of α/α' on prior β/β grain boundaries directly impacts the final mechanical properties of the titanium alloys. The β/β 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 µm) 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, the results showed the presence of β/β grain boundary α' lath growth inside two adjacent prior β grains. Electron backscatter diffraction micrographs confirmed the presence of β/β 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 β/β grain boundary α' lath growth. The self-accommodation mechanism was the main variant selection driving force, and the most abundant α/α boundary variant was type 4 (63.26°//[ $\bar{10}$ 5 5 $\bar{3}$ ]). The dominance of Category II α lath clusters associated with the type 4 α/α boundary variant was validated using the phenomenological theory of martensite transformations and analytical calculations, from which the stress needed for the β→α' transformation was calculated.