Keep Your Powder Fit: Characterizing Metal Powders Between Reuse Cycles in Additive Manufacturing

Using particle characterization and rheological techniques, we investigate the properties of metal powders between processing cycles in additive manufacturing. Identifying the impacts of the selective laser melting process on the powder helps to determine its suitability for reuse.

High-tech and high-end applications often require cutting-edge quality 3D-printed components. Currently, the most versatile and most commonly used 3D printing technology for metal components is Laser Powder Bed Fusion (L/PBF), also known as Selective Laser Melting (SLM). This additive manufacturing technique makes it possible to create metal components of nearly any complexity, and is used in a variety of industries such as the aerospace and automotive industries, machine building or medical implants.

A typical example of the benefits of additive manufacturing is the heating chamber of the Anton Paar CTD 600 MDR. Here, SLM enables the creation of tightly stacked hollow air channels within the 3D-printed shells. This results in an absolutely homogeneous temperature distribution within the oven.

In SLM, a thin layer (usually between 20 - 60 μm) of metal powder is evenly distributed over the build platform. A high-power laser beam is then guided across the plane and selectively melts the powder particles, fusing them together in order to create one solid layer. Afterwards, another layer of powder is added and the laser maps the area again. These two steps are repeated until a structure of bulk metal is formed. Finally, the unmelted powder is removed to yield the final part. This residual powder is usually sieved, dried and reused in order to maximize the economic yield of the 3D printing process.

The quality and durability of the final product is crucially dependent on the quality of the feedstock (1). Therefore, characterizing the microscopic and bulk properties of the metal powders both before use and during their reuse cycles is an essential part of the quality control process (2).

Anton Paar offers a versatile toolbox for characterizing the impact of reuse on metal powders: laser diffraction measures particle and aggregate sizes, powder rheology enables characterization of their flow and mechanical behavior, while the streaming potential technique tracks qualitative changes occurring at the particles’ surface.

Laser diffraction is well suited to determine the particle size distribution of powders in this size range. The Particle Size Analyzer (PSA) allows powder dispersion in a carrier liquid where agglomerates can be broken down by sonication, as well as with compressed air in dry mode. Very coarse particles of up to 2.5 mm can be measured in the free-fall mode. With its robust design and easy-to-use software, the PSA is ideal for metal powder quality control.

Powder rheology can characterize powders using two unique powder cells on rheometers: the powder flow cell and the powder shear cell. These two cells  provide the means for comprehensive characterization of powder properties such as flow behavior, fluidizability, cohesion strength, permeability, compressibility, tensile strength and more. With these parameters, the powder behavior can be characterized under the conditions closely reflecting every step of the application. In the powder shear cell, it is even possible to control the ambient conditions, with a relative humidity range from 5 to 95 %, and an extensive temperature range from -160 to +600 °C.

The high cost of metal-based additive manufacturing has so far limited its widespread adoption. Curbing the costs of raw materials by reusing metal powders offers an opportunity to both expand the technology’s applicative range and to improve its sustainability.

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