The cost of metal powders for additive manufacturing (AM) is high and contributes an appreciable proportion to the manufactured cost of printed components.
While the capital investment associated with a printer is committed, and progressively offset over time, feedstock costs attract ongoing scrutiny.
Regular usage is a constant reminder that metal powder supplies attract a high price, particularly when compared to the price of the solid material (bar stock) and highlights the need to extract maximum performance from a feed.
A price worth paying?
It’s worth noting that there are sound reasons for the high cost of metal AM powders. It largely reflects the production method, most commonly water, gas (nitrogen, argon, or helium), or plasma-based atomisation, which influences vital properties such as particle size and shape. The highly spherical, largely satellite-free, morphology of particles produced by plasma processing, along with excellent purity, typically result in performance that commands the highest prices.
An increasingly competitive market is driving cost reduction but against a backdrop of exacting requirements in terms of particle morphology, chemical purity, and consistency. Particle size distribution (PSD) requirements are a major obstacle to cost reduction with, for example, only 10-50 % of the product from a gas atomisation process typically falling within the 20-150 µm size range required for AM. Narrowing PSD means a lower usable yield and higher costs.
This context underlines the value of choosing a powder supply that is optimally matched to a specific printing requirement, and certainly not over-specified. However, it also raises the question of whether the performance of a purchased powder can be enhanced and if so how?
Optimising powder performance
A collaborative study with Desktop Metal (Burlington, USA), a manufacturer of commercial binder jet printers for metal AM investigated this question. The aim was to determine whether the properties of metal AM powders could be enhanced by baking (under air or nitrogen) or by storage conditions. The results provide useful insight for optimising AM powder performance.
Baking
The figure below shows how baking a stainless steel AM powder with a D50 of 12 µm, under air or nitrogen (@200oC for 12 hours), changes flow properties, as quantified by measurements of Basic Flowability Energy (BFE) and Specific Energy (SE). These dynamic properties, which have demonstrated relevance to AM performance, were measured using an FT4 Powder Rheometer, applying standard protocols for the instrument.
Freeman Tech
Baking in air has the most significant impact, causing a marked increase in BFE and decrease in SE. Baking in nitrogen has a similar but less pronounced effect. BFE values are generated from measurements of the axial and rotational forces acting on the blade of the tester during a downward traverse. In a densely packed powder, force transmission is highly effective, and the flow zone associated with this compressive action is relatively large. Higher BFE values are therefore often associated with more efficient particle packing. SE values in contrast are measured during an upward traverse of the blade and are influenced by the level of mechanical interlocking and friction between particles.
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Taken together the results indicate that baking improves particle packing efficiency, while at the same time reducing particle-particle interactions; both effects suggest a change in surface properties.
The more interesting question is what this might mean for AM performance. Efficient particle packing is associated with better print quality since it reduces voidage in the powder bed, and by extension the finished part. Good flowability under gravity is beneficial with respect to powder dispersion across the build platform. All the data therefore suggest that baking will likely improve performance.
Storage
Freeman Tech
The figure above illustrates the impact of storage conditions. Storage with a desiccant (16.5g of calcium oxide per 1600 g of metal powder) increases both BFE and SE, relative to storage under ambient conditions. It is clear that the sample is able to retain moisture - otherwise the desiccant would have negligible effect – and that in this case moisture is actually beneficial to flowability, particularly SE. It is not uncommon for water to lubricate particle-particle interactions, notably interparticle friction, or minimise electrostatic charging, which would rationalise this observation.
Storage and treatment
Freeman Tech
This final figure shows the combined effect of baking and storage to elucidate the relative impact of the different conditions. Looking at all three sets of BFE data, it is observed that storage with a desiccant not only increases the measured values, as discussed, but erodes the difference induced by baking. This highlights how changes in flow properties induced by storage are significant relative to those induced by baking, and that storage conditions require careful consideration to preserve any gains from pre-treatment.
This study shows that both baking and storage conditions can have a significant effect on the flowability of metal AM powders, and by extension printing performance. The associated mechanisms are complex and cannot be predicted but they can be measured. Further studies with a second stainless steel powder showed that PSD is also a significant factor, a not unexpected result that underlines the importance of assessing optimised treatment and storage conditions for each individual powder.
In conclusion
Better performance can be extracted from a metal AM powder via appropriate pre-treatment steps such as baking, and carefully considered storage. These strategies can pay dividends and potentially enhance the value of a feedstock. Systematic study, using a process-relevant metric, is fundamental to optimising performance and improving production economics.
Read more from Freeman Technology's Jamie Clayton:
- Is there a better way to specify powders for additive manufacturing?
- Pushing for performance in polymer powders
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