Understanding Metal Powders in Additive Manufacturing — Part 5: Relative Density

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.

Relative density: A key characteristic

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

The powder bed density is the density of the powder bed in additive manufacturing processes and describes how efficiently the powder particles are arranged in the bulk particle network.
The powder bed density is significantly influenced by the two variables of theoretical packing density and flowability. The flowability determines how close the real relative density comes to the theoretical packing density during powder application and can therefore be seen as a kind of “compaction potential”.
Powders are not the close-packing of equal spheres. The particles are arranged statistically, have a polydisperse particle size and shape, are not able to move without resistance and form randomly arranged gaps.
A high and uniform density of the powder bed is a central requirement resulting from both the process and the component requirements.

In addition to the flowability, the powder density also depends on the morphology and the particle size distribution. Spherical particles lead to a higher density and a PSD with small and large particles increases the density, as the small particles can fill the gaps between the large particles.

How to measure the powder density?

As the powder bed density is difficult to determine, the bulk density and tap density are often measured as an approximation. Due to the slight compaction during powder spreading, the powder bed density is usually between the values of the bulk and tap density.

The bulk or apparent density (ρa) is the ratio of the weight to the volume of a defined loose powder sample. The bulk density can be determined using a funnel method such as the Hall flowmeter (ISO 3923–1). The powder flows through the funnel into a container of defined volume until it overflows. After scraping off the powder heap, the bulk density is calculated from the weight and the defined volume.
The tapped density (ρt) is the density of a powder bulk that is compacted by uniform, defined mechanical tapping. The powder is tapped according to ISO 3953 in a measuring cylinder of a tapping apparatus with a defined stroke height and tapping frequency until the volume no longer decreases. The tapping density is calculated from the volume achieved and the defined weight.