Reuse of Ti6Al4V Powder and Its Impact on Surface Tension, Melt Pool Behavior and Mechanical Properties of Additively Manufactured Components

Abstract

The quality and characteristics of a powder in powder bed fusion processes play a vital role in the quality of additively manufactured components. Its characteristics may influence the process in various ways. This paper presents an investigation highlighting the influence of powder deterioration on the stability of a molten pool in a laser beam powder bed fusion (LB-PBF, selective laser melting) process and its consequences to the physical properties of the alloy, porosity of 3D-printed components and their mechanical properties. The intention in this was to understand powder reuse as a factor playing a role in the formation of porosity in 3D-printed components. Ti6Al4V (15 μm–45 μm) was used as a base material in the form of a fresh powder and a degraded one (reused 12 times). Alloy degradation is described by possible changes in the shape of particles, particle size distribution, chemical composition, surface tension, density and viscosity of the melt. An approach of 3D printing singular lines was applied in order to study the behavior of a molten pool at varying powder bed depths. Single-track cross-sections (STCSs) were described with shape parameters and compared. Furthermore, the influence of the molten pool stability on the final density and mechanical properties of a material was discussed. Electromagnetic levitation (EML) was used to measure surface tension and the density of the melt using pieces of printed samples. It was found that the powder degradation influences the mechanical properties of a printed material by destabilizing the pool of molten metal during printing operation by facilitating the axial flow on the melt along the melt track axis. Additionally, the observed axial flow was found to facilitate a localized lack of fusion between concurrent layers. It was also found that the surface tension and density of the melt are only impacted marginally or not at all by increased oxygen content, yet a difference in the temperature dependence of the surface tension was observed.