Archive — This September 2019 feature brings together explanations from LPW Technology, MIMETE and Linde. Prices and cost proportions are historical indications tied to the cases discussed.
Metal powders are the base materials for powder metallurgy operations such as uniaxial pressing and some types of additive manufacturing processes. These operations have already demonstrated that somehow, metal powders characteristics play a big role in the properties of the end-product. This role necessarily depends on the quality of the material, the printing process and costs (material cost vs cost of the printed part). This article will discuss these points.
Powder production influences particle shape, composition and size. Near-spherical particles can aid flow and spreading in some processes, but not all AM powders are perfectly spherical. Particle-size distribution and other characteristics must suit the machine, process and part requirements; they do not alone guarantee final properties.
Does the quality of the metallic powder impact its price?
Materials price is definitely an important issue for manufacturers because, as we said earlier, they constitute an essential part of powder metallurgy operations. So, their choice matters to achieve the desired finished part.
Evaluating a lower-priced powder requires comparison of specifications, traceability and process behaviour, with rejects and inspection included in the economic assessment.
Price varies with alloy, production process, volume and inspection requirements. A high price is not in itself proof of quality, nor is a low price proof of unsuitability.
For LPW Technology, a materials producer, quality “means consistency of supply, full traceability and reliable material performance. Consistency of supply and traceability are factors which are controlled by robust Quality Management Systems, optimised manufacturing methods and trusted supply chains”.
Characterisation can cover composition, morphology, particle-size distribution, flow, apparent or tapped density, and internal particle porosity. Measurement methods and acceptance criteria need to be specified.
According to Valentina Vicario, Technical Manager of MIMETE, producer of metal powders, among all of these features, “the first feature that impacts the powder price is obviously chemical composition. Usually Fe powders are less expensive than Ni, Co and Ti-base ones. Inside every “family” of alloys, there are expensive alloys, usually customized and/or characterized by special combination of elements, and other “commodities”, less demanding and more widely spread. In general, the more numerous and stricter are requirements on different powder properties (chemical analysis, granulometric range, density, etc.), the higher is the cost, because special customized processes are required and production planning can be heavily affected.”
Reuse should follow a qualified procedure for the alloy and application, with batch traceability and checks on powder changes. There is no universal cycle limit for every material and process.
Valentina Vicario from Mimete precises that “powder recycling is possible, but can be applied only for some alloys and for some applications. After a first usage, depending on printing parameters (i.e. protective atmosphere), powder can be affected by oxidation and agglomeration: a sieving is always needed to remove clusters and deformed particles before a new printing process, while higher oxygen content might theoretically reduce mechanical properties of the parts printed by the second cycle.”
Speaking of metal powders designed for additive manufacturing, LPW explains in a case study that: “Oxygen levels in powder are primarily driven by oxygen levels in the feedstock material, the process parameters and gas purity used during atomization. For this reason, it is typically more costly to produce powders with low oxygen levels. It may seem more cost effective to use powder with a higher oxygen content but at a lower cost, but this is only true if a user does not consider reusing the metal powder.”
These observations do not justify accepting out-of-specification oxygen content, even for a single use. Sieving does not reverse a change in chemical composition.
LPW & Mimete raise through these statements, another issue that impacts both the price of a metallic powder and the price of the printing process: gases.
The impact of metallic powders on the finished printed part
During an interview, Pierre Forêt, Head of the AM unit at Linde, told us that customers often worry about the purchase of metallic powders – especially titanium whose price can go up to 300€ per kg. Speaking of one customer that needed to buy a significant amount of powder, he explained that, “in addition to be expensive, the powder is very sensitive to humidity, which means that it is easy to lose the essence of the powder; and that will definitely impact the printing process. In order to avoid this waste of money, the ideal solution was to develop a system that could both spur the powder and remove humidity.”
MIMETE highlights relationships between powder characteristics, behaviour during manufacture and part properties. The diagram below brings them together; it is not a universal predictive rule.
Powder bed fusion can produce high-density parts with suitable parameters. This outcome is neither automatic nor exclusive to electron-beam melting; final porosity must be measured for the application.
Material density must be distinguished from the density of a volume of powder. Apparent powder density includes spaces between particles; tapped density also depends on the tapping procedure. Internal particle porosity is a separate characteristic.
These quantities help characterise the powder bed but do not by themselves predict part density. Energy input, process parameters and any subsequent treatments also matter. The NIST characterisation study examines several powder properties and how they change during reuse.
What about the final cost per part?
Several factors can influence the final cost per part. Additive manufacturing systems, machine
data, materials, construction job parameters, consumables data or even operational calculations are some of the factors that can influence the final cost per part.
At the materials level, it should be noted that AM technologies can be complementary. Indeed,
manufacturers can take advantage of the benefits brought by two technologies. Furthermore, the
increasing use of AM may result in a reduction in raw material cost through economies of scale.
According to Douglas Thomas’ research on “Costs, Benefits, and Adoption of Additive Manufacturing: A Supply Chain Perspective”, “the reduced cost in raw material might then propagate further adoption of additive manufacturing. There may also be economies of scale in raw material costs if particular materials become more common rather than a plethora of different materials.”
The economic reference is the study by Douglas Thomas. Economies of scale are considered as a possibility, not an assured price reduction.
As far as metal AM processes are concerned, Mimete believes that, “the cost of the powder might affect the final cost of the part from less than 10% to 20-30% depending on the alloy and the complexity of the printed component.”
These proportions depend on the calculation scope and are not a schedule applicable to every part.
LPW Technology on the other side, explains that two other key factors that influence the final cost per part are the number of viable builds that can be completed from a single batch of powder, and the number of parts that can be produced in each build. “The number of parts produced in a single build depends on the size of the part and the size of the available build volume. The number of builds that can be completed for a single batch is dependent on how quickly the powder breaches the specification limits. At this point we must consider the influence of powder evolution, how a material varies from initial specification.”
In a nutshell…
Cost per acceptable part should include feedstock, manufacture, treatment and inspection. A lower purchase price may be offset by more rejects or reduced powder reuse.
Control of powder characteristics and their consistency across batches contributes to manufacturing repeatability.
Selection should combine part requirements, material and process qualification, and an economic assessment of the actual case. Neither powder price nor a single characteristic is sufficient to decide.
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