Electron beam additive manufacturing of Ti6Al4V: Evolution of powder morphology and part microstructure with powder reuse

Abstract

Additive Manufacturing (AM) processes for metals are advancing at a rapid pace. Among many attractive qualities, AM relaxes design constraints and can significantly reduce material waste in comparison to subtractive manufacturing processes. However, there are some fundamental issues that must be addressed for metal AM to become prevalent in aerospace. In powder bed fusion AM, powder reuse from previous build cycles is desired to improve process economics. However, there is limited understanding of the contributions from powder reuse to particle and part quality. The present study investigates this topic in electron beam melting (EBM) powder bed fusion AM of a titanium alloy (Ti6Al4V) over 30 build cycles (∼480 h of build time) and powder reuse. Results show that nearly all aspects of the process are influenced by powder reuse. Specifically, the particle size distribution tightens, largely due to fewer particles with small diameter. Particle damage increases with reuse, which includes surface deformation (reduction in sphericity), partial melting and/or particle fusion and fracture. In regard to the built metal, the microstructure exhibits increasingly finer basket weave strucutre with greater surface area to volume ratio of the part. While there are no apparent trends in α volume fraction with powder reuse, an increase in α-lath thickness is observed . In the analysis of composition, no substantial changes in the Al and V alloy content are apparent, nor in Fe, H and N. However, the O concentration of the powder increased significantly with reuse, in fact, it exceeded the concentration limit (0.2%) in just 11 build cycles. Overall, powder reuse should be considered carefully in the development of titanium parts for performance critical applications by EBM AM.