Understanding Metal Powders in Additive Manufacturing — Part 3: Morphology

Welcome to the next installment of our comprehensive series exploring the complex world of metal powders for additive manufacturing (AM). As part of this series, inspired by our foundational article “Understanding Metal Powders in Additive Manufacturing”, we continue our journey into the world of powder properties that are critical to AM processes. The powder is the foundation of the AM process, determining its efficiency, quality and profitability.

Morphology: The particle shape matters

In this article, we focus on one fundamental aspect: Morphology and how to measure it. The morphology of metal powders holds pivotal importance in powder-bed-based additive manufacturing processes like laser powder bed fusion (LPBF).

Morphology refers to the shape of the particles. A distinction can first be made between the macroscopic geometry of the particles and the microscopic roughness, whereby a distinction is not clear for particles in the micrometer range. In principle, a powder particle can take on a wide variety of shapes. This makes a qualitative and above all quantitative description and measurement difficult. The morphology of the powder depends on the manufacturing process. Water atomized powders wich are used mainly in press and sinter technology have a much more irregular shape than gas atomized powders, which have a relatively high sphericity.

Nevertheless, gas atomized particles can have various shape defects. Typical defects are satellites, splat caps or elongated particles.
The goal in the production of metal powders for the LPBF process is to achieve a spherical shape. This assumption is widely recognized in the industry. The particle shape defects described above must be minimized. The shape of the powder particles significantly influences the flowability and packing density, both of which increase as the shape of the particles becomes increasingly round and uniform.

Even though powders for AM processes must be spherical, it’s typically sufficient for the process and component quality that the powders have been produced via gas atomization. The pursuit of maximum sphericity is usually a marketing aspect of powder manufacturers and not technically necessary.

How to measure the morphology of metal powders?

The shape of powder particles can take countless forms, making it challenging to measure directly. Describing particle morphology can be particularly difficult, especially with irregularly shaped particles. Qualitatively, optical and verbal descriptions of morphology can be achieved through image analysis. Quantitatively describing particle shape remains a challenge, with researchers resorting to deriving dimensionless ratios known as shape factors. These factors serve as correlative indicators of particle shape. Currently, aspect ratio (length/width ratio) and sphericity are the common shape factors in powder-based additive manufacturing. These factors are determined using image-based analysis techniques, where various measurements and parameters are extracted from particle images to calculate the factors. Aspect ratio (l/b3) indicates the length/width ratio of the particle, while sphericity (SPHT = 4πA/U²) quantifies particle shape based on particle surface area (A) and perimeter (U). Qualitative and quantitative determination of metal powder morphology primarily relies on static and dynamic image analysis methods.