Quality Matters


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Six Points to Consider for Better Powder

By: Caitlin Oswald

Caitlin Oswald is an additive manufacturing specialist with LAI International, Inc., a contract manufacturer of precision-engineered finished components and subassemblies for the aerospace, defense, energy, medical and industrial markets: coswald@laico.com; 612/300-8722; www.laico.com.

Wednesday, February 20, 2019
 

Powder quality greatly influences final part quality; that’s a fact. However, myriad powder variables create challenges. While the challenges are complex, industry users must find ways to drive control consistency and measure results. The following provides a roadmap for doing just that.

Sourcing—Ask your supplier how it sources and controls its raw material (rod, wire, etc.), and how it traces back to original ingots. Suppliers often customize the handling, testing, storing and shipping of an order to meet any requirements above and beyond the powder specification. For example, ordering a large lot of powder and having it drop-shipped in smaller increments to your facility may reduce traceability concerns.

Handling—Controlling how your facility handles the powder once it is received is vital. A contamination-control plan helps prevent contamination and reduces the potential for accidental contamination. Personnel training can ensure a heightened awareness of the importance of powder cleanliness. Many newer machines on the market feature options for full powder-handling systems, minimizing or eliminating the potential for external contamination. While effective, these systems require access to all surfaces that the powder touches for cleaning or replacing.

Environment and storage—Temperature and humidity are key factors related to machine environment and powder storage. Lower humidity, especially when using oxygen-sensitive alloys such as titanium, minimizes the influence of water vapor in the air. However, in extremely dry environments, or with powder containing microparticles, a minimum humidity level may be necessary to reduce the risk of combustible dust.

Traceability—Imperative for meeting most aerospace and defense application requirements, traceability and documentation allow for the recall of products within a specific powder lot when unknown defects are found downstream. Most industry-standard and customer-specific audits require users to show objective evidence of traceability documentation throughout the process. Numerous alloy varieties, lot numbers and specifications tracked inside of one facility can make documentation a complex challenge. Not only must traceability be documented digitally, it also must be traceable on the shop floor. Traceability here can be implemented by using specific colored labels, two-check systems, UPC labels and other means.

Reuse—Though cost-effective, reusing powder multiple times introduces multiple issues. The reuse process, due to repeat powder handling, inherently increases the risk for contamination, disturbance of the particle-size distribution, changes in chemistry due to pick-up of interstitials such as oxygen or nitrogen, and a loss of traceability when the powder is moved from container to container. Systematic and effective ways to reuse the powder include adding virgin powder after each build, or sending all of the powder through the machine(s) and mixing a master lot for reuse. The economies and ease of reuse depend on the alloy, as well as on the technology and machine model.

Testing—Material testing provides evidence of conformance to the powder specification. The most common required powder-property tests include particle-size distribution by sieve analysis or laser diffraction, chemistry composition, tap and bulk density, and flowability. The ultimate goal: Find a test that describes how consistently the powder will flow (by recoating mechanism, gravity, etc.) and pack down across the build area. Significant investment in powder rheology methods has resulted in a better way to compare the physics of how the powder flows and moves inside the machine. While I have not yet seen a powder rheology requirement in a specification, this area will continue to evolve over the next few years.

Powder always is tested in its virgin state before shipping to the user. How often testing occurs after shipping depends on user/customer specifications. Some users build strictly by number of reuses—not testing the powder and scrapping it once it reaches a specific reuse number. Others employ an internal system that takes a powder sample before each build, testing the sample each time for specification conformance, which then gets reported back to the end user.

The powder-bed-fusion process, like other manufacturing technologies, presents many variables. The interaction of powder properties and handling methods, and the impact on final part quality are topics that will continue to evolve and stay relevant to all users of the technology. For now, the best way to maintain quality: purchase, handle, store, recycle and test the powder as consistently as possible. Everyone in the value chain—powder manufacturer, supplier, machine OEM and end user—has a shared responsibility in strengthening the knowledge base, identifying significant variables and controlling processes.

While the industry continues to push boundaries with new powder-manufacturing methods, modified test requirements and improved handling techniques, one constant remains: Powder quality, with all of its complexities, is a top priority as users focus on cost and the quality impact of powder on final-part production. 3DMP

 

See also: LAI International Inc


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