Exploiting lack of fusion defects for microstructural engineering in additive manufacturing

Abstract

Rapid cooling rates and stochastic interactions between the heat source and feedstock in additive manufacturing (AM) result in strong anisotropy and process-induced defects deteriorating the tensile ductility and fatigue resistance of printed parts. We show that by deliberately introducing a high density of lack of fusion (LoF) defects, a processing regime that has been avoided so far, followed by hot isostatic pressing (HIP), we can print Ti-6Al-4V with reduced texture and combinations of strength (TS=1.0 ± 3E-2 GPa) and ductility (ε failure =20 ± 1%) surpassing that of wrought, cast, forged, annealed, solution-treated and aged counterparts. Such improvement is achieved through the formation of low aspect ratio α-grains around LoF defects upon healing, surrounded by α-laths. This occurrence is attributed to surface energy reduction and recrystallization events taking place during the healing of LoF defects via HIP post-processing. Our approach to design duplex microstructures is applicable to a wide range of AM processes and alloys, and can be used in the design of damage tolerant microstructures.