Origins of Variability in Electron Beam Powder Bed Fusion of Ti6Al4V

Abstract

Additive Manufacturing (AM) processes for metals are advancing at a rapid pace. Among manyattractive qualities, AM relaxes design constraints, enables customization 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. Most critical is the reliability of the mechanical properties of the metal and an identification of contributions (i.e., the root causes) to the mechanical variability. In powder bed fusion (PBF) AM, powder reuse from previous build cycles is desired to improve process economics. However, there is limited understanding of the contributions from feedstock reuse to particle-powder-properties and part quality. The first half of this dissertation investigates this topic in electron beam melting (EBM) powder fusion AM of a titanium alloy(Ti6Al4V) over 30 cycles of build and powder reuse. Results show that nearly all aspects of the process are influenced by powder reuse. Specifically, the particle size distribution tightens with reuse, but particle damage increases, which includes surface deformation (reduction in sphericity), partial melting and/or particle fusion and fracture. While no changes in the concentration of Al, V, Fe, H and N occur, there is a significant increase in Oxygen with reuse, which exceeded the concentration limit (0.20%) in just 11 build cycles. Results showed that the strength of the metal increased with powder reuse, whereas the ductility decreased significantly, both relevant to the damage tolerance of the metal. Powder reuse covers inter-build variability but not address the intra-build variability. Establishing an understanding of the mechanical response variation within a build is also necessary for quality control in metal AM. A design of experiments approach was adopted to determine the relative effects of part location, geometry, and proximity on quasi-static and cyclic properties of the metal. Significant effects were found from the radial distance from the center of the build plate and from the thickness of the parts to the ductility and fatigue life. The highest ductility and fatigue life can be obtained from part placement near the center of the build plate and with increasing thickness. This knowledge makes a fundamental contribution and supports successful application of EBM additive manufacturing for stress-critical components in aerospace and beyond. Nevertheless, more expansive efforts are necessary to further mature the technology, such that it can be integrated into aerospace manufacturing as a reliable production process.