It would be great to think that that we could put in place specifications that define the performance of a given powder in a specific AM application. A specification that makes it possible to answer the question ‘Can I print with this?’ and that safeguards the defining attributes of a printed product. Such specifications would have considerable value for:
- supplier choice and supply chain optimisation
- the assessment of new or alternative feeds for an established printer or application
- the development of novel AM powder feedstocks
And, of course, simply for QC.
So, is that currently feasible? Is there already sufficient understanding of the interplay between powder properties, printing efficiency and product quality to set up such specifications? In my opinion the answer is a qualified yes, certainly if you look at the practice of those leading the way. That said, there is still some way to go with certain processes, and when it comes to securely predicting printed product quality. That makes it an interesting time to consider what has been learned, what is currently possible and the focus for ongoing activity.
Beyond particle size, lessons from binder jetting
ExOne, a global leader in binder jetting technology, has been open in sharing experience of learning how best to specify metal powders for use in its printers. This is a company that provides printers, printed products (via a parts-on-demand service) and powder supplies so developing a robust understanding of the properties that determine which powders will perform well is central to all activities. It can be argued that the sequential, as oppose to coincidental, nature of part-building and heating/sintering makes binder jetting one of the easier AM processes to investigate when it comes to specification setting. However, just like laser bed fusion or laser bed sintering the efficient dispersion of powder to rapidly form a powder bed just tens of microns thick is essential, increasing the applicability of any lessons learned.
Anyone who uses metal powder AM processes will be familiar with the importance of the particle size of the feed. In binder jetting finer metal powders facilitate the use of lower sintering temperatures reducing the risk of slumping and distortion and making it easier to achieve more precise dimensional integrity. Unfortunately, finer powders are often associated with poor flow characteristics which can compromise printing efficiency. Switching up to a powder with more spherical, regular particles is a common strategy for boosting flowability but this may not be feasible. And greater sphericity almost always comes at a higher price.
ExOne printers are capable of printing both angular water-atomised powders and more regular shaped, gas atomised feedstocks but some feeds perform far better than others. With a specification based on particle size, assessments of morphology by scanning electron microscopy (SEM) and a few other tests such as binder compatibility, ExOne could not differentiate materials with acceptable performance, other than by carrying out a printing trial, a potentially costly, time-consuming exercise. To remedy this situation the company set about identifying other properties with which to augment the existing specification to make it more useful.
Multi-faceted bulk powder characterisation
The bulk properties of powders, notably flowability but also packing behaviour, a critical characteristic for AM, are influenced by the properties of the constituent particles but not predictable from them. The quantification of bulk powder properties therefore relies on measurement. ExOne assessed the relevance of three different techniques, using a single instrument (FT4 Powder Rheometer, Freeman Technology): dynamic testing; shear cell analysis; and bulk property measurement – bulk density, compressibility, and permeability. Systematically correlating the properties measured by each technique with print performance identified six properties that could usefully be included in the powder specification:
- Stability Index (SI)
- Flow Rate Index (FRI)
- Wall Friction Angle (WFA)
SI and FRI are dynamic flow properties. Dynamic testing generates flow energy values from measurements of the axial force and torque acting on a blade as it rotates through a powder sample. SI quantifies how flow energy values change with repeat testing with values close to 1 indicating physical stability. FRI, on the other hand, quantifies how flow energy changes in response to changes in flow or shear rate.
Cohesion and WFA are both shear properties. Shear cell testing involves measurement of the forces required to shear one consolidated powder plane relative to another, or in the case of WFA measurement relative to a coupon of a potential material of construction or processing equipment surface.
Compressibility and permeability are both bulk powder properties. Compressibility quantifies how bulk density changes as a function of applied pressure while permeability is indicative of the how much resistance the powder presents to gas flow.
In summary, ExOne found that it was only by applying all three bulk powder testing techniques that they could generate the information needed to securely differentiate powder samples in a relevant way and identify poor performers. Here’s part of the specification that the company arrived at and now routinely uses:
The value of a better specification, detecting a sub-standard supply
Using this new specification, ExOne can confidently predict whether a customer powder or new supply will print well simply by testing. This is a valuable capability that enhances the efficiency of many operations. For example, the table below shows data for two supplies of 316L stainless steel powder, a material the company uses frequently for its parts-on demand service. The Alternate Supplier was offering a less expensive product, water atomised, just like the established supply, and with near identical particle size/particle size distribution (to within acceptable tolerances). A check of the morphology of the samples indicated close comparability.
However, bulk powder testing detects a difference between these powders. SI lies outside the acceptable range. The alternative supply does not appear to be physically stable and would be rejected on the basis of this specification.
Is that the right call?
Print trials indicate that it is. In trials the alternative supply initially performed well but part quality progressively degraded as the powder was recycled. After 4 to 5 cycles printing was no longer possible. The machine was cleaned out and the entire trial repeated, with the same end result. In contrast, the established supply maintained performance when subjected to the same conditions. Closer scrutiny of the alternative supply, to investigate this effect, revealed more flaky particles that became more prone to interlocking, with processing.
The experience of ExOne demonstrates that it is feasible to put in place secure specifications for metal powders for AM, specifically binder jetting, but only by embracing modern bulk powder testing techniques. The company now rarely encounters powders that deviate from predicted performance.
Users of AM, and powder manufacturers, continue to grapple with the concept of spreadability and how best to quantify it, and there is evidence here that dynamic, shear and bulk powder testing can do just that. There may be further work to do in stabling secure correlations for every process, and to accurately reflect all important attributes of the finished product, but there is a blueprint to follow that holds considerable promise.