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Powder Recyclability Factors Into Future of Metal Additive Manufacturing

By: Rutuja Samant

Rutuja Samant (rsamant@ewi.org) is responsible for managing EWI's AM portfolio at its Buffalo (NY) Manufacturing Works. She serves as interim director of the Additive Manufacturing Consortium (AMC), a collaborative group of industry, academia and national labs advancing metal-AM technology adoption and deployment. In addition, She represents EWI as the R&D lead for the ASTM Additive Manufacturing Center of Excellence, which aims to create a global innovation hub for advancing AM technical standards, related R&D, education and training. Samant has experience in various metal- and polymer-based AM processes. Her area of specialty at EWI is metal AM with a primary focus on electron-beam-melting technology. Her project portfolio includes work for aerospace, medical, defense and nuclear-industry clients, as well as AM material qualification research for multiple metal-powder producers.

Wednesday, February 20, 2019
 

Fact: The cost of metal additive manufacturing (AM) continues to increase, as manufacturers push conventional metal-powder producers to provide specialty powders designed for applications in the aerospace, defense and biomedical sectors. This reality places growing importance on powder-recycling efforts.

Understand Powder Degradation


Fig. 1—Powder degradation after multiple laser powder bed fusion builds.
First challenge: Understanding diminishing powder properties observed with different types of powders after one or more build cycles.

High-temperature powder alloys such as Inconel 718, generally chemically stable over many powder recycles, are limited by physical traits such as morphology and flowability when evaluating reusability. As these materials melt at higher temperatures, the material surrounding the melt becomes distorted and sintered together, which can make powder particles larger and unusable.

On the other hand, titanium powders are more susceptible to oxygen pickup, and, therefore, can be used only a couple of times before the powder falls out of specification due to high oxygen content.

Thus, understanding the degradation behavior of different types of powders is important for developing standards for powder recyclability that don’t exist today.

In a recyclability study performed with a high-temperature material at EWI, researchers used and analyzed powder through multiple, laser-powder-bed-fusion (L-PBF) builds to understand the effects on powder and part properties. Fig. 1 shows a scanning-electron-microscope image of degraded powder after multiple AM runs. Through the course of these builds, satellites attached to larger powder particles started separating out, creating smaller individual particles. Meanwhile, powder particles started fusing together to form agglomerates with particles fragmented into incomplete particles. All of this affected the flow and packing density of the bulk powder, with particle-size distribution widening and the powder’s oxygen content increasing. However, after 13 consecutive builds with the same batch of powder, the powder still fell within the composition specifications, making it a viable candidate for reuse.

Increase Reusability

Second challenge: Increase reusability of metal powders, identifying qualified techniques to recondition out-of-spec powders and bring them back into the AM ecosystem.


Fig. 2—Preplasma conditioning                           Post-plasma conditioning
One popular method: Blend virgin powder with re-used powder before each build. This reduces oxygen content of powders (such as titanium), known to be more susceptible to oxygen pickup. It also can control physical properties of bulk powder such as powder-size distribution, packing density, etc. However, no standard practices are identified for this method. Each service provider uses experience to develop best practices, which typically vary based on material used and type of build geometry.

Another plausible reconditioning technique: using an induction-plasma process, which consists of in-flight heating and melting of feedstock, followed by solidification under controlled conditions. This method can improve powder characteristics such as flow and packing density by producing spherical particles (Fig. 2; decreasing porosity by remelting and resolidifying the powder particles; increasing powder density; and improving powder purity through the selective/ reactive vaporization of impurities by increasing the plasma melting temperature and modifying the shielded gas. EWI has performed some early studies using this method but further testing must be performed to evaluate viability for bulk-powder reconditioning.

Develop Guidelines

Third challenge: Develop industry guidelines for improved traceability.

A lot of work remains when it comes to powder-reconditioning techniques. For example, while powder blending reduces the overall oxygen content of the bulk-powder lot, the powder batch still can contain particles with high oxygen content. If these particles end up in the final part, catastrophic part failure may occur. Several studies have shown that in Ti-6Al-4V powders, higher oxygen content leads to reduction in the ultimate tensile strength of the final components.

Currently, companies tend to use virgin powders for critical AM builds to maintain consistency in manufactured-part properties and minimize risk. This practice has produced thousands of pounds of out-of-spec powder, either stored or wasted. As AM production and powder consumption increase, so, too, will concern about powder traceability. This will result in higher costs and larger energy footprints associated with AM processes.

To keep AM materials affordable as the technology expands, the need to develop qualified methods to not only recondition powders, but also to requalify and bring them back into production, is paramount. EWI is working toward development of some of these guidelines through its Additive Manufacturing Consortium as well as through the ASTM AM Center of Excellence. Stay tuned. 3DMP

 

See also: EWI


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