Additive Manufacturing

Particle Testing Authority can provide the services needed to examine a diverse range of bulk powder and particle properties that can have a critical impact on AM processes:

Particle Size

One of the most critical attributes for control is particle size distribution.  As well as having a direct impact on powder flowability it also influences the ability to deliver a uniform, powder bed density. This in turn determines the energy input needed to sinter or bind particles and also affects the surface finish of the built part. Laser diffraction is one of the most well established and accepted analytical technique for the determination of particle size and particle size distribution.

 

Particle Shape

Particle shape or morphology also influences the packing and flow properties of a bulk powder feedstock. Spherical particles are expected to arrange and pack more uniformly than irregular particles. Such a shape is also known to facilitate the flowability of powders and can help ensure more uniform layers in powder bed systems. Shape also directly influences packing density of the powder bed and subsequently the final product’s apparent density. Irregular shaped particles are often associated with lower final component density and can lead to an increase in porosity.

Density

True density is an inherent property of a material, while apparent density takes account of occluded voids within a material. Knowledge of the true and/or apparent density of a material feedstock helps inform on powder bed formation and sintering kinetics as well as the porosity in the final product.

Bulk density of a powder is heavily influenced by physical properties of the particles but also by the quantity of air entrained within the bed. Bulk density can be important in establishing material specifications and complements other assessments of powder flowability and bed formation.

Envelope density is based on the geometric volume of a sample and is useful for evaluating the end-product as it can accurately measure intricate and irregular volumes. When combined with true density measurements, porosity can be quickly and easily determined.

Porosity

In AM porosity can indicate the final mechanical strength and quality of the finished component. Porosity is usually controlled to minimize its effect on material properties, hardness, and surface finish. However, porosity can actually be a designed parameter for the final product.

For example, artificial bone implants need to match the surrounding bone porosities or porosity may simply be specified in the design to achieve lightweight products with the desired mechanical strength.

Mercury intrusion is a proven technique for quantifying porosity characteristics of powders and formed product. This technique is based on the intrusion of mercury into a porous structure under stringently controlled pressures.

As well as offering speed, accuracy, and a wide measurement range, mercury porosimetry enables numerous properties to be assessed such as pore size distribution, total pore volume, total pore surface area, median pore diameter, bulk and skeletal density, and percentage porosity.

Surface Area

The surface area per unit mass of a powder is of great importance. Surface area indicates the quantity of a material that is available to react with other component particles and/or the surrounding environment. Particles with rough surfaces or internal porosity will generally exhibit higher specific surface areas. Surface area, therefore, is a critical tool in investigating the kinetics of the sintering process and end-product properties.

The specific surface area of a powdered material can be measured by gas adsorption using the well-established BET method. For this technique (typically) nitrogen gas is physisorbed at cryogenic temperatures and the quantity needed to form a monolayer on the surface determined by applying the BET method to the collected isotherm data.

Powder Flow

Manufacturing products from a powdered substrate is well-established within metallurgical industries and continues to develop in others.

Whether sintering powder densely packed into a mold or localized fusing layer-by-layer, the process will be sensitive to the flow properties and bulk behavior of the feedstock. Poor flow properties can lead to inconsistent density and non-uniform layering, fouling, blockages, and downtime – all of which result in low productivity and poor product quality.

Traditional techniques for quantifying flow, such as Angle of Repose and Hall Flow measurements are often acknowledged as being too insensitive to identify subtle differences between powders that can affect performance in an AM machine.

Powder rheology provides a comprehensive, multivariate assessment of dynamic, bulk and shear characteristics of feedstocks generating process relevant data that can be used to define materials suited to a process supporting process optimization and lifecycle management of powders.

Environmental Stability

When storing and handling feedstock materials, exposure to changes in temperature, humidity and other environmental conditions can affect how the materials will perform in process. It is therefore important to understand the impact of these changes and the environmental tolerances of the powder/process. This can be evaluated by testing powders that have been subjected to controlled changes in temperature and/or humidity via programmed studies using TGA, DSC, DVS or even inverse gas chromatography.

Surface Topography

Surface topography provides a visual and chemical assessment of surface textural properties. Scanning electron microscopy (SEM) is used to probe a surface to view microstructures such as surface voids, fissures/cracks, and edge dislocations. SEM is, therefore, an obvious use in component failure analysis.

SEM can also be used to analyze the raw material powders used in AM, for example, to detect agglomerations, assess surface roughness, and quantify the ratio of spherical to irregular shaped particles; all of which impact powder flowability and sintering.