Heat Treatment Delivers More-Consistent Powder

October 14, 2019

The growing global metal additive manufacturing (AM) market is expected to reach $3.05 billion by 2025, according to a recent report by Grand View Research, Inc. To take full advantage of this growth and efficiently produce high-quality parts, powder producers, 3D printer manufacturers and others must ensure the consistent, repeatable quality of the metal powders used in the process.

AM applications range from aerospace, defense, and automotive to medical and jewelry. The metals involved include aluminum, titanium, stainless steel, cobalt-chrome, copper or nickel alloys, and precious metals such as gold, silver, platinum or palladium.

In the 3D printing process, parts are created from digital specifications by laying down successive layers of metal powder and using a laser to fuse the particles until the part is complete. Like ink-based printers, 3D printing has its own consumables—in this case, metal powder in extremely fine, sub-micron sizes.

Complementing the wide range of metal powder suppliers, contract powder processors provide sophisticated heat treatments to improve powder quality. As 3D printing techniques and equipment continue to advance, optimizing the powders with such heat treatments can improve powder flowability to prevent clogging, thus speeding the process and producing a higher-quality part.

Uniform Powder Flow a Must

Most metal powders used in 3D printing, such as iron, nickel, cobalt, aluminum and titanium alloys, are produced by gas atomization. In this process, feedstock melted in a crucible ejects from a nozzle into a high-pressure gas stream. This breaks the molten metal into fine particles, typically less than 50 to 150 microns in diameter. While this process typically produces spherically shaped powders, surface porosity must be addressed in order to improve flowability. Otherwise, the powder can clog or slow during the process, affecting the speed and quality of printing.

“For reliable 3D printing production, the powder must flow continuously and smoothly,” explains George Paffendorf, director of operations at Advanced Powder Solutions (APS), a contract manufacturer with extensive powder expertise and the inhouse testing/engineering arm of Gemco, a manufacturer of tumble blending and vacuum tumble drying equipment. “However, uniform powder flow can be inhibited if the particles are rough or porous, which creates more friction.”

The attractive force between tiny, sub-micron size particles also increases as the particles become smaller. So, finer powders are typically less free flowing anyway, says Paffendorf.

From a quality standpoint, metal-particle porosity also can reduce a finished part’s load-bearing, fracture-toughness and fatigue properties. Under cyclic stress conditions, porosity also can lead to cracks and part failure.

Optimizing metal powder via heat treatment can improve flowability to prevent clogging, thus speeding the process and producing a higher-quality part.

To eliminate porosity and enhance flowability, a growing number of powder suppliers and AM-machine manufacturers add a heat treatment step in the manufacturing process that involves tumble dryers to achieve a more consistent, high-quality product, says Paffendorf.

For example, APS utilizes advanced vacuum tumble dryers that provide sparging (gas injection) in addition to heat application. With this approach, a perforated tube positioned under the bed of material distributes a flow of inert gas, such as nitrogen, to help circulate heat evenly amid the powder.

“A raw metal powder may have particles with a porous, craggy outer surface,” says Paffendorf. “When we heat treat it in the tumble dryer, it closes up the pores to make the particle more spherical and flow more smoothly.”

The gas purge also provides a solution for protecting oxygen-sensitive or volatile powders, which otherwise can compromise certain alloy chemistries. Using a blanket of inert gas to cover the material bed provides a protective barrier that prevents the powder from being exposed to atmospheric oxygen. This also improves operator safety. Because 3D printing with metal powder can involve working with some materials such as nickel, potentially dangerous to operators in powder form, containing these may be necessary to eliminate fire or explosion risk.

Reblend to Save on Costly Powder

Reblending enables metal printing companies to extend the life of costly powders, with micro and macro mixing together providing an improved reblended product.

Given the cost of metal powder, end users must reuse any left over at the bottom of the tray. Here, vacuum tumble blenders can reblend the powder with new material. Advanced tumble blenders are designed to apply even turbulence in all corners of the mix. This is accomplished through a combination of macro and micro blending that produces a better distribution.

Macro blending, achieved by rotating the shaped vessel, allows the material bed to fall away from the vessel’s walls. The blender moves at a precise speed, with the vessel wall at a precise angle, so that the material cascades over itself. No additional force is supplied by paddles, plows or spiral ribbons—just gravity.

While this occurs, micro mixing (if needed) simultaneously proceeds via agitator blades located in the center mixing zone of the vessel, where fine processing in the material transpires. Together, the macro and micro mixing evenly expose each particle to six times more active blending per revolution than traditional mixers.

Beyond metal powders, adding a heat treatment step also can improve part quality and manufacturing productivity for applications involving plastic or resin-based powders. 3DMP

Article supplied by Advanced Powder Solutions, the inhouse testing/engineering arm of Gemco, Middlesex, NJ; 800/654-3626; www.okgemco.com.

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See also: Gemco