Understanding Metal Powders in Additive Manufacturing — Part 4: Flowability

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.

Flowability: A controversal property

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

Good flowability of the powder is essential for its use in powder bed-based additive manufacturing processes such as the LPBF process. It is required when spreading powder layers so that a thin, smooth, homogeneous and dense powder layer can be created. Powders with poor flowability lead to problems during powder spreading and cause inhomogeneities in the powder layer. This results in a poorer powder bed density. In addition, sufficient flowability is in connection with powder transport in the AM system.

A general flowability according to the Hall flow method with a flow rate in the range of < 20–30 s/50g is often required for AM powders. However, studies show that a powder does not necessarily have to be free-flowing in order to be processed in powder bed-based AM processes. Some AM powders of certain alloy groups, such as aluminum powder, generally show no flow behavior. This property and the associated measurement methods are therefore the subject of controversial debate.

In order to achieve a high packing density, low layer thicknesses and high absorption, fine powders are often used, which reduces flowability. There is a trade-off between these properties, although it is not specified which “minimum flowability” must be maintained.
The flowability is a function of several factors. The most significant influences are the binding forces between the particles, which are mainly determined by the particle size (PSD), the moisture content, and the friction, which is largely determined by the particle morphology and surface oxidation. Moisture in the powder can lead to the formation of capillary bridges between the particles. These interparticle forces significantly impair the flow properties as the particles agglomerate.

How to measure the flow beahaviour of AM powders?

The flowability or flow behavior of a powder is not a directly measurable variable. The effort required to make a bulk material flow describes the flowability of a bulk material. There are numerous methods for measuring flowability. It is crucial to use methods for determining the flow behavior which correspond as far as possible to the mechanical conditions of the powder deposition or the powder transportation in the system. In the context of powder spreading in the additive process, flowability is also referred to as spreadability.

According to the ISO/ASTM 52907:2019 standard “Additive manufacturing - Feedstock materials - Methods to characterize metal powders ”, a funnel method, measurement of the angle of repose or an alternative method is required to characterize flowability in AM.

In the funnel method, the time required for a defined quantity of metallic powder to flow through a standardized outlet opening of a calibrated funnel is measured. The shorter the flow time, the better the flowability. These methods are often used in practice. Determination using the Hall flowmeter in accordance with ISO 4490 / ASTM B213 is the most widely used method. The Carney funnel (ASTM B964) and the Gustavsson funnel have different funnel geometries and are less common. The flowability measured in this ways does not necessarily correlate with the spreadability during powder deposition. The funnel techniques are primarily capable of mapping the powder flowability in relation to flow requirements in pipes and nozzles.

The angle of repose (AOR) of a powder is the steepest angle of dip relative to the horizontal plane on which the material can be piled without slumping.

The Hausner number is a widely used indicator of flowability and is calculated from the ratio between the tap density and the bulk density.

There are other more complex test procedures that automatically determine several physical parameters. Examples include the rotating drum with dynamic image analysis or powder rheometers. Such methods are currently the subject of standardization committees in order to improve the characterization of flowability in AM.