Understanding Metal Powders in Additive Manufacturing — Part 2: Particle Size Distribution (PSD)

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.

Particle Size Distribution: A Vital Metric

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

How does the PSD influence the AM process?

The PSD is typically defined by the lower and upper limit values of the particle sizes. The minimum particle size has an influence on the powder bed-based AM process. Small powder particles fill the gaps between larger particles in the powder bed, which leads to a higher packing density. However, small powder particles under 10 or 20 μm also lead to lower flowability. Particles with a size of less than 10 μm are also particularly harmful to health. Fine powder particles below 10 or 20 μm should be avoided, primarily because of the negative effect on flowability.
The maximum particle size determines the minimum layer thickness in the powder bed-based additive manufacturing process. This results in recommendations in the literature for the maximum particle size as a function of the layer thickness: d90 ˂ layer thickness.
Smaller layer thicknesses enable smoother surfaces or better mechanical properties. Greater layer thicknesses, on the other hand, enable higher build rates. The standard layer thicknesses are between 20–50 μm.

The PSD is matched to the process parameters of the printing process, so it is important to use powders with a defined PSD to ensure stable, good print quality. However, practice and literature also show that certain deviations in the PSD in a stable process window of the printing process have no measurable influence on the component quality. The process has a relatively wide process window and is therefore resilient to slight changes in PSD. In the LPBF process PSDs such as 15–45 µm, 15–53 µm or 20–63 µm are predominantly used.

It is important to understand the PSD as a dynamic parameter, subject to variations during deposition in the powder bed or powder usage cycles in the AM machine.

How is the PSD defined?

The PSD is typically displayed as a graph. Beside the lower and upper limit of the particle size, the characteristic parameters D10, D50 and D90 can be seen as typical values to describe the PSD. For example, D10 = 20 μm means that 10% by volume of the powder sample has a particle diameter of ≤ 20 μm. To defin the lower and upper limits of the PSD the minimum and maximum particle size is often defined as follows: Max 5 vol% < 15 µm, Max 5 vol% > 53 µm.

How to measure the PSD?

The PSD can be determined using different measurement methods. Different physical principles apply to the different methods. Therefore, measurement results from different measurement methods can be compared with each other to a limited extent. However, possible differences are so small that they have no influence on the use in AM.
In addition to the different measurement methods, there are different definitions of the size of a particle. This results from the non-ideally round shape of the powder particles, so that the simple specification of the diameter of the particle is not trivial.

In dynamic image analysis, the optical recording of particles and the evaluation of shadow projections is carried out using software-supported calculation algorithms. The powder sample is guided past one or more optical measuring sensors. The measured values of the individual particles are divided into size classes. In contrast to sieve analysis, the resolution is almost linear.

In the static image analysis, the particles are dispersed on a plane surface. Light optical microscopes (LOM) or scanning electron microscopes (SEM) can be used for image capturing. Equipped with an automated analysis system, the particle size of the individual particles can be calculated using the projection surface as in dynamic image analysis. The image analysis method (dynamic and static) is widely used in the AM industry due to the possibility of simultaneous measurement of PSD and morphology.

The laser diffraction method for determining the PSD is based on the phenomenon of the dependence of the intensity distribution of light scattered by particles on the particle size. The powder sample is dispersed in a liquid or gas and passes through a monochromatic light beam. The light radiation scattered by the particles is detected by photosensors. The particle size of the individual particles is determined from the scattering data using mathematical concepts.

In sieve analysis, the particle size is determined using the mesh size of the sieve.
In vibration sieving, the test material is filled into a sieve tower consisting of several sieves arranged according to mesh size. A three-dimensional sieving movement divides the powder into its size fractions. The powder fractions in the individual sieves are weighed after the sieving process. Dry sieving in accordance with the standards cannot be used for powders whose particle size is completely or predominantly below 45 μm.