The importance of characterising metal powders
The physical properties of metal powders affect their ability to be processed in Additive manufacturing machines and significantly affect the quality of the final product. Poor metal powder quality can produce defects in the final component such as pores, inclusions, residual stresses, cracks and surface roughness. In addition, poor quality powder affects the process by compromising the uniformity and flowability of the powder bed. There is a direct correlation between the physical properties of the powder on the one hand and the performance of the process and the properties of the final components on the other. Therefore, characterisation of the powders allows for process optimisation and improves the finished product.
The characterisation of metal powders is important at different stages of the process:
- Production of Additive manufacturing powders: quality control of powders produced with different technologies
- Choice of the best performing powders: definition of the ideal physical characteristics of the powders according to the application field
- Incoming raw material: Check of the specifications declared by the manufacturer and monitoring of constancy of supplies
- Reuse of waste powders: characterisation of the powders after process in order to maximise reuse, lowering production costs
Physical properties of metal powders for Additive manufacturing
In addition to the chemical composition of the powder, which is fundamental for the quality of the final component, the physical characteristics of the powder determine its performance: these characteristics include both the batch properties and the properties of the individual particles. The fundamental properties of powders are its ability to flow and compact in the best possible way to allow consistent dosing and layering. These properties are directly, though not exclusively, influenced by particle size and shape, as well as surface roughness and electrostatic charge. Flowability, compaction density, particle size distribution and particle shape can be quickly and effectively assessed by powder rheology, density, laser diffraction and image analysis.
Figure 1. SEM images of the particles of two metal powders
Particles size and shape
The particle size of a powder for Additive manufacturing must be within a size range suitable for the process. Moreover, the relative proportions of fine and coarse particles are essential in controlling and predicting the density of compaction of the powder. Maximum compaction density is achieved with a distribution that includes the right ratio of coarse to fine particles, where the finer particles fill the gaps left by the larger particles.
The shape of the particles influences both the compaction capacity of the powder and its flowability. Generally, smooth, regular shape particles flow more easily and tend to compact efficiently, creating a denser powder bed than those with a rough and/or irregularly-shaped surface. Rougher surfaces create more inter-particle friction, which limits the flowing properties, while irregularly shaped particles are more prone to mechanical interlocking which, in addition to limiting flowability, results in lower powder compaction efficiency.
Particles with satellites often represent a problem in the use of metal powders, since the presence of satellites also influences the flowability of the powder and their compaction capacity.
Figure 2. Particle classification based on morphological parameters for the characterisation and comparison of recycled powders (18 processing cycles in green and 24 cycles in blue) and virgin powder (in red).
Figure 3. Stability and Variable Flow Rate chart obtained from the rheology analysis of three batches of metal powders supplied by different manufacturers, for the characterisation of their “flowability”.
Reuse of metal powder
At the end of each production cycle, unmelted powders can be reused in the next cycle. However, it must be kept in mind that an excessive amount of powder reuse can cause a deterioration of their morphological characteristics, lowering their quality. For this reason, powder characterisation is fundamental in investigating the changes they have undergone after use and in verifying that the new characteristics are still compatible with further processing. The reuse of powders involves the addition of virgin powder to the mixture in proportions determined by the characteristics of the recycled powders, in terms of surface oxidation and morphology.
Recycled powder shows more elongated particles, more satellites on the surface and more agglomerates with a higher percentage of surface oxides that may compromise the quality of the next cycle. The powders must therefore be reconditioned before being used again. Powder recovery generally involves sieving the powder before reuse to remove contaminants such as large particles and agglomerates. This treatment, however, does not guarantee an accurate control of the particle size due to the limits of sieving, so a morphological analysis, in terms of particle size and shape, is essential to ensure the quality of the powder.
Alfatestlab, your dedicated technological platform for Additive manufacturing
Alfatestlab offers a metal powder analysis service dedicated to additive technologies, thanks to a state-of-the-art technological platform able to characterise the particle size, shape, flowability and density of the powder, as well as providing images of electron microscopy and EDX chemical analysis.