Towards β-fleck defect free additively manufactured titanium alloys by promoting the columnar to equiaxed transition and grain refinement

Metastable beta-Titanium alloys containing a high content of segregated beta-stabilising elements such as Cr make segregation defects (known as beta-flecks or beta-freckles) difficult to avoid during solidification processing. This work investigates the formation of beta-flecks in the high strength Ti-3Al-8V-6Cr-4Mo-4Zr (Beta-C/ASTM Grade 19) produced by Wire Arc Additive Manufacturing and explores whether the introduction of grain refiner (La2O3) to the alloy can prevent the segregation defects from forming. Promoting the columnar to equiaxed transition and refining the grain size with La2O3 nucleant particles was found to significantly reduce the presence of beta-flecks by 99.9% compared to the un-refined (standard) alloy. Analytical and numerical modelling is used to provide insights into how grain refinement can achieve this significant defect reduction. (C) 2021 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Automated summary

This study investigates the formation of beta-flecks in the high strength Ti-3Al-8V-6Cr-4Mo-4Zr alloy and explores the use of grain refiner La2O3 to prevent segregation defects. The results show that the introduction of La2O3 significantly reduces the presence of beta-flecks. Analytical and numerical modelling provide insights into the mechanism of defect reduction through grain refinement.