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;

Industry-Related Terms: Metal additive manufacturing, Heat treat, Metal powder, Porosity, Additive manufacturing
View Glossary of 3D Metal Printing Terms


See also: Gemco

Technologies: Metal Powders, Post Processing


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