The physical properties of metal powders for additive manufacturing include loose density, tap density, fluidity, etc. If the changes in these properties are ignored, the same process parameters may be completely different from the performance of the parts printed by the powder particle size distribution.

This article discusses the testi ng methods of the physical properties of metal powders following the previous powder chemical composition testing methods.

2. Testing of physical properties of metal powder

(1) Loose density

The powder falls into a cup of known volume in a loose state, and the measured mass per unit volume is called the bulk density of the powder. The bulk density of the powder is a comprehensive performance, which is affected by factors such as powder size, particle size distribution, particle shape, and intra-particle pores. Here are two more convenient methods for measuring loose density.

Funnel method: The powder falls freely from the funnel hole at a certain height to fill a container with a certain volume, and then the mass of the powder in the container is measured. The measuring instrument is shown in Figure 1. This method is simple and fast, but it cannot measure powder with poor fluidity.

To

SCOT capacity meter: For metal powder that cannot flow freely through the funnel with a diameter of 5mm in the funnel method, the SCOT container method should be used. The standard GB 1479.2-2011 has a reference to the use of this equipment (Scott capacity See Figure 2). The powder flows through the screen from above to the column below. The inclined baffle in the column can fully disperse the powder and prevent the powder from agglomeration. The measured loose density is more accurate. It is especially suitable for ultrafine powder, wet powder, Powders with poor fluidity, such as magnetic powders, can be used for routine production testing of metal powders for 3D printing.

(2) Tap density

The tap density of metal powder is to pack a certain amount of powder in a container and vibrate through a vibrating device (as shown in Figure 3) until the volume of the powder no longer decreases. The mass of the powder is divided by the tapped volume to get its tap density.

The national standard GB/T 5162-2006 has specific regulations on the measurement of the tap density of metal powders. This method can be used for metal 3D printing powders. The final result is reported by measuring three times and taking the arithmetic average value. Generally, the tap density is 20%~50% higher than the loose density. For powders for additive manufacturing, because of the "less bridging" between spherical metal powders, the bulk density is generally higher than that of irregularly shaped powders.

Figure 3 Tap density tester

(3) Liquidity

The fluidity of the powder is the ability of the powder to fill a container with a certain shape, and the spherical powder can improve the fluidity of the powder to achieve the smoothness and smoothness of the high-quality powdered cloth. The influencing factors include particle shape, particle size combination, relative density and adhesion between particles. For example, the larger the particle, the more regular the shape, the smaller the proportion of fine powder in the particle size composition, the increased relative density, and the less water and gas adsorbed by the surface area, and the better fluidity. In addition, fluidity is also related to the bulk density of the powder. Generally speaking, the higher the bulk density of the powder, the better the fluidity.

There are usually two characterization methods for the fluidity of metal powders: Hall flow rate and angle of repose.

Hall flow rate

The Hall fluidity of metal powder refers to the time required for 50g of metal powder to flow through a funnel hole of a standard size, and the unit is (s/50g). The reciprocal is the mass of powder flowing out per unit time, called flow rate. The current domestic measurement of liquidity is carried out in accordance with the standard GB/T 1482-2010, and the device is shown in Figure 4.

Angle of Repose

For powders that cannot pass the standard funnel smoothly, the angle of repose is usually used to characterize the fluidity of the powder.

The measurement of the angle of repose of metal powder refers to "GB/T 16913 Dust Physical Property Test Method Determination of Angle of Repose". The powder is injected from the funnel to the horizontal tray by the injection limited bottom surface method, and the acute angle between the dust accumulation slope and the bottom horizontal plane is measured. Generally, the angle of repose of metal powder is required to be ≤45°.