Lack of Fusion in Additive Manufacturing: Defect or Asset?

TL;DR

This paper demonstrates that intentionally introducing lack of fusion defects in additive manufacturing, followed by heat treatment, can create microstructures with superior mechanical properties compared to traditional methods.

Contribution

It introduces a novel processing regime that uses high-density lack of fusion defects and heat treatment to enhance microstructure and properties of Ti-6Al-4V.

Findings

Reduced texture and improved properties surpassing traditional alloys.

Formation of low aspect ratio α-grains around LoF defects.

Applicable to various AM processes and alloys.

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 pressure assisted heat treatment, we can print Ti-6Al-4V with reduced texture and exceptional properties surpassing that of wrought, cast, forged, annealed, and solution-treated and aged counterparts. Such improvement is achieved through the formation of low aspect ratio {\alpha}-grains around LoF defects upon healing, surrounded by {\alpha}-laths. This occurrence is attributed to surface energy reduction and recrystallization events taking place during healing of LoF defects. 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.