Session: 02-01-01: 7th Annual Conference-Wide Symposium on Additive Manufacturing: Metals I

Paper Number: 96806

96806 - Particle Flow and Packing Behavior of Electron Beam Melting Ti-6Al-4V Powder Under Atmospheric and Vacuum Conditions 

Electron beam melting (EBM) is a powder bed fusion process capable of manufacturing metallic components out of a variety of alloys. The process is unique in that it relies on the generation of a uniform, densely packed powder bed under vacuum conditions at high temperatures. Within the build chamber, the powder can behave as a solid, in the case of a sintered compact; a granular “liquid” during raking; or a coarse “gas” under severe electrostatic repulsion during detrimental “smoke” events. Therefore, understanding the behavior of the powder subjected to the build environment is thus crucial to developing the process further.

The performance of the powder is a function of the physical characteristics of the particles, such as size, shape, texture, and material composition. It is also a function of environmental factors, namely temperature and local humidity, which can modify the local cohesive forces between the particles. This performance is typically examined using benchtop measurements which include the angle of repose (AOR) and drainage time or
“flowability” using the Hall flowmeter. These simple, yet effective, measurements are intended to probe the physical behavior of the powder, prior to it entering the EBM machine, for manufacturing quality assurance. Given that the process is expensive both relative to the feedstock material as well as the time invested due to build pre- and postprocessing, the benchtop measurements are critical to elucidating any potential process risks.

Therefore, this study presents a method for comparing the flowability and packing arrangements of Ti-6Al-4V non-virgin powder under atmospheric and vacuum conditions. The powder particles were first characterized in terms of several parameters including diameter, sphericity, and compactness using optical microscopy. Next, baseline AOR and flowability measurements were taken over the course of a dryout cycle. After the dryout cycle, measurements were collected under ambient conditions. Finally, the two-dimensional particle packing behavior was acquired during the filling of a container constructed from microscope slides and double-sided adhesive. Data on the Netwon, contact, or “kissing” number was then analyzed using optical microscopy. To facilitate identical measurements within a vacuum environment, a device was designed to initiate flow within a vacuum chamber using a photoresistor switch and “mechanical finger”. The results show that the effects of vacuum have negligible effect on powder behavior. These results are important in understanding the connections between benchtop measurements and how they correlate to powder performance within the build chamber. Consideration for the temperature effects will also be discussed, however, will be more closely examined in a follow-up study.

Presenting Author: Garrett Kelley University of Washington

Presenting Author Biography: Garrett Kelley is a PhD student at the University of Washington where his focus is in experimental and computational characterization of the electron beam melting process for Ti-6Al-4V. He has 11 years experience in the aerospace tooling industry, as a project manager, designer, and stress analyst. He is also a registered professional engineer in the state of Washington.

Authors:

Garrett Kelley University of Washington
Ramulu Mamidala University of Washington