Understanding Metal Powders in Additive Manufacturing

The requirements for metal powders for use in Additive Manufacturing (AM) are manifold. To achieve a stable manufacturing process and good part quality, a high and stable powder quality is crucial. These specialized requirements make the process of comparing powders on the market an arduous task, further complicated by the lack of transparency in the powder market.

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Powder Properties

Metal powders have a wide variety of properties, several of which are important to ensure good powder quality in powder metallurgy and AM. In addition, the requirements for the printed component and thus the powder are partly determined by the sector. In this article, an overview of the key powder properties is given. For each powder property, there are also different measurement methods, which have different suitability. For AM, the ISO/ASTM 52907 standard provides the most relevant powder properties and the possible analysis methods.

Chemistry

The central property is the chemical composition of the powder with the contents of oxygen, nitrogen and hydrogen. The composition corresponds to the standardised bulk materials. In some cases, certain alloying elements are increased, in order to achieve specific improved material properties and to counteract possible loss of elements in the printing process.

Particle Size Distribution

In addition to the composition, the particle size distribution (PSD) is the central material-independent parameter. Typical PSDs for the laser powder bed fusion (LPBF) process are 15–45 µm or 20–63 µm. Fine particle fractions in the PSD can increase the packing density, however, this worsens the flow behavior and can be absorbed via respiration which is harmful for health. The upper limit of the PSD is largely determined by the layer thickness in the printing process. The relevant measurement methods used to determine the PSD are laser diffraction and static/dynamic image analysis.

Morphology

The shape of the particles, the so-called morphology, plays a major role in determining how good the flowability of a powder and its characteristic relative densities are. For AM processes, especially the LPBF process, spherical powders are used, which are produced in a gas atomization process. The more spherical the powder, the better the flow behavior and densities. Qualitative analysis is performed by means of SEM images. A quantitative determination is made possible by means of shape factors via static/dynamic image analysis.

Flowability

Flow behavior is an important property of metal powders in the LPBF process. It is important whether a powder can flow freely in the containers, tubes and feeder of the printer and does not clump. Sufficient flowability is also required during powder spreading so that a thin, smooth, homogeneous and dense powder layer can be created. Powders with poor flow properties lead to problems during powder spreading and cause inhomogeneities in the powder layer. The most common measurement method is the hall flow method. It is simple, but there are limits to its applicability.

Densities

The highest possible and most uniform density of the powder bed is a central requirement resulting from both the process and the component requirements. High powder bed densities, and thus a low volume of gaps in the powder bed, can achieve high component densities and more productive process parameters. Low powder densities can lead to inhomogeneous material coalescence due to inhomogeneous laser-powder interaction. Typical measurement methods are bulk density and tap density.

Overall, qualloy simplifies the complex powder market. Users in the market face several hurdles during the powder sourcing process, including but not limited to complex powder property profiles, lack of transparency around suppliers, and variety of analysis methods. qualloy opens up new possibilities for users to source the best quality powder for their needs, and at a fair price.