Fragmentation and Branch Elimination of Primary α′ Martensite in Additively Manufactured Ti-6Al-4V Alloy

Abstract

Fragmentation and branch elimination are generally noticed in the conventionally processed Ti-6Al-4V. Such a key morphological change produces a positive change in certain mechanical properties. We, for the first time, observe fragmentation and branch elimination of the primary α′ martensite in additively manufactured as-built Ti-6Al-4V and the effect of scan speed on these is studied. Nanovoids inside and on the surface of the primary α′ martensite are assumed to be the starting points of the fragmentation and branch elimination, respectively. At a lower scan speed (250 mm/s) a relatively shorter branch length than that of a higher scan speed (500 mm/s) is observed. Such change in the morphology of primary α′ martensite has positively impacted hardness. This has been discussed in terms of additive manufacturing parameters. Such a fundamental morphological change will further help the understanding of laser powder bed fusion metal additive manufacturing. The hardness of the samples is measured and correlated with the fragmentation of the primary α′ martensite.

Keywords

• additive manufacturing
• Ti-6Al-4V
• primary α′ martensite
• fragmentation
• branch elimination
• laser scan speed