Additive Manufacturing – Improve Print Quality with Powder Rheometry

Additive manufacturing, also known as 3D printing, is a highly efficient, potentially transformative manufacturing technique. The method involves ‘printing’ intricate components to a tight specification by gradually building up powder layers which are then selectively fused together.

For process efficiency and the quality of the end product, it is critical to control the performance of the powders. The way the powder flows and packs as the layers are formed are defining aspects of this performance. Variability in feedstock can result in non-uniform layering, inconsistent bulk density, poor surface finish, and low tensile strength.

The extent to which Additive Manufacturing will shape the industrial landscape depends on the development of high-speed, precision machinery, and on the identification and consistent supply of powders able to meet the exacting demands of these machines. This focus is increasingly turning to the powders themselves and how they can be optimized in a reliable and intelligent manner.

Powder characterization plays a critical role in supporting this process, and testing techniques that can reliably measure properties that directly correlate with AM performance are also essential.

By identifying the powder properties that lead to uniform, repeatable performance of powder, new formulations can be optimized, without the significant time and financial implications associated with running samples through the process to assess suitability. It also helps to reduce the occurrence of end products that are out of specification.

There are several existing methods such as flow through a funnel, bulk density measurements, and angle of repose testing that are well-documented. However, these techniques were developed without the advantages of modern technology, and they can sometimes be too insensitive to characterize the subtle differences between powders that behave differently in process.

A universal powder tester, the FT4 Powder Rheometer® offers comprehensive, automated, and reliable measurements of bulk material characteristics. When this information is correlated with process experience, it can improve processing efficiency and aid quality control.

The FT4 specializes in the measurement of dynamic flow properties, incorporates a shear cell, and has the ability to measure bulk properties like compressibility, permeability, and density.

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Quantifying Batch-to-Batch Variation in Feedstocks

The tight tolerances within which AM machines operate mean that differences between various batches of feedstocks can result in considerable variability in the quality and properties of the end product.

Any variation in performance can be avoided by screening each batch before it enters the process. However, the very subtle differences in properties that can lead to differences in performance are often undetected by conventional powder characterization techniques.

Three samples of stainless steel powder from a supplier demonstrated highly variable performance in an AM process. While Metal Powder A and Metal Powder B exhibited acceptable behavior, Metal Powder C regularly caused poor deposition and blockages, resulting in sub-standard final products.

Particle size distributions of all three samples were virtually identical and the samples demonstrated a similar response in hall flow and angle of repose tests.

However, evaluating the samples with the FT4 Powder Rheometer® highlighted a number of differences between the samples that correlated well with the process performance. The Specific Energy of the samples clearly differentiated Metal Powder C during dynamic testing, with the higher value indicating increased particle-particle friction and mechanical interlocking.

This increased resistance to flowing over itself is a common reason for blockages and other flow problems in low stress environments.

During bulk testing, an even more differentiating result was generated by the permeability test. Compared to the other samples, Metal Powder C generates a significantly higher Pressure Drop across the powder bed. This indicates that Metal Powder C is considerably less permeable than Powders A and B.

Permeability is very influential in any operation where powder is moved from one position to another, especially when gravity is the motivating force. The space vacated by the particle must be replaced by gas, and the more easily the powder can transmit this gas through the bulk, the more freely it is likely to pour, and also to release any air entrained during the pouring process.

When depositing or filling consistent densities of powder during AM applications, and when low permeability increases the amount of air retained in the bulk on deposition, it will cause poor uniformity in the layers. This can lead to imperfections in the final product that may require the product to be scrapped.

Process-Relevant Differences Between Fresh and Used Feedstocks

Laser deposition and powder bed technology require considerable amounts of powder, not all of which becomes part of the finished component. Re-use of powder has the potential to considerably reduce the overall levels of waste and raw materials costs.

However, for reuse to be possible, a careful assessment has to be performed of the extent of which powders are altered by passing through AM machines, and whether additional processing is possible without affecting the quality of the finished component.

To determine if important characteristics of the used powder differed from those of the virgin material, a range of different feedstocks that contain differing proportions of used and fresh feedstock were evaluated with the FT4’s dynamic methodology. The evaluation is also necessary to determine what strategies might be successful in returning the powder to a condition that would enable its re-use.

When the results for the virgin and used powders were compared, they showed that processing has signicantly increased the powder’s flow energy. This shows that the used powder would not flow as freely as the virgin material. Consequently, it is also less likely to perform as well in the process.

Powder exiting an AM machine may have changed chemically by picking up contaminants on the powder surface or may contain splatter from the melt pool in the form of larger particles. Therefore, experiments were conducted to determine if sieving the used powder would return it to a state where its flow energy was acceptable.

In this case, while sieving improved powder flowability it did not return it to the original flow energy values that were measured for the virgin material.

Additional experiments were conducted to see if the virgin and used powders could be blended together to form an acceptable feed for subsequent processing A ratio of 25% used to 75% virgin powder provided a flowability most similar to that of the fresh powder, and this blend also exhibited relatively good performance.

Of all the blended samples, the 50:50 blend had the highest Basic Flowability Energy, indicating that flowability does not change linearly with respect to the volume of fresh powder present.

The results show the capability of dynamic testing to detect subtle changes in powders that are directly relevant to their performance in AM machines. Consequently, dynamic testing can support successful optimization and lifecycle management of metal powders for AM in a manner that is not possible for other powder flow testers.

The Influence of Different Suppliers and Manufacturing Methods

Different methods, each of which can generate powders with similar PSD and D50, can be used to manufacture powder feedstocks for additive manufacturing, and each manufacturer will have their own grades and acceptance criteria.

However, several other properties of the powder can also be influenced by the manufacturing method, leading to different performance in the overall process that the manufacturer’s own acceptance testing may not be able to identify.

In order to evaluate whether the different manufacturing methods or suppliers were likely to influence the performance of the powder in process three feedstock powders with the same D50 and PSD (two from the same supplier made using different manufacturing methods and two from different suppliers made using the same method) were evaluated using the FT4 Powder Rheometer®.

Differences caused by the change in manufacturing method were identified by shear cell tests, with Method 1 generating lower shear stresses than Method 2. This demonstrates the impact of variables potentially beyond a customer’s control and highlights the need for close, regular evaluation of raw materials. However, the samples produced by two different suppliers using the same method were characterized as identical.

The variation caused by changing the manufacturing method was reinforced by the dynamic tests, which also identified the differences in the samples from the two suppliers. The sample from Supplier 2 has a higher SE and higher BFE than the sample from Supplier 1. This is indicative of cohesive behavior in dynamic applications such as layering and filling.

From the variation in properties, it can be deduced that changing suppliers can significantly influence the process performance. This must be considered along with logistical or financial benefits to making the change.

These results demonstrate the need for comprehensive and relevant powder characterization using a multivariate approach, especially in AM processes that largely rely on accurate and precise layer deposition that can only be assured using powders with consistent and suitable properties.

The Effect of Additives on Feedstock Properties

To provide useful properties, such as improved flowability, pigmentation or specific functionality in the final product, feedstocks are often treated with different additives. However, each of these additives will have a different influence on the properties of the feedstock, and over its eventual performance in the application.

By quantifying the extent to which different additives will affect the properties of the formulation, both the formulation and the process can be optimized to accommodate the additive features and maintain an acceptable level of performance.

In an SLS operation, three samples of Polyoxymethylene (POM), two of which contained different additives (a flow additive and a pigment) were used. The three formulations were seen to flow differently from the storage hopper into the sintering machine, resulting in variation in the quality and properties of the end product. Although a range of traditional characterization techniques were used, they failed to offer differentiation between the samples.

When compared to the other two samples, the sample containing the flow additive generated a higher BFE, and required more energy to move the FT4 blade through the powder bulk. In this case, higher BFE is usually associated with the uniform structure of a more efficiently packed bulk. This causes more particles to be displaced by the movement of the blade than in a more poorly packed powder.

The sample that contained the flow additive generated the highest pressure drop across the powder bed at a low consolidating stress. This is indicative of reduced permeability and reflects the denser packing state of this sample. However, when consolidating stress increased and the Pressure Drop increased, that of the pure sample and the sample containing pigment changed to a far greater degree than the sample containing flow additive.

As there were fewer air voids for the particles to collapse into when compressed, low sensitivity to changes in consolidation stress further indicates a more efficiently packed bulk. The greatest change was observed in the permeability of the sample containing the pigment, and is consistent with the sample having the greatest volume of entrained air within the bulk. This is indicative of high cohesivity in this mode of flow.

Conclusion

Powder flowability is not an inherent material property. It has more to do with the powder’s ability to flow in a desired manner in a specific piece of equipment. For successful processing, the powder and the process must be well-matched.

Within an AM process, a powder can perform well in one unit operation, but poorly in another. Therefore, several characterization methodologies are required, the results from which can be correlated with process ranking to identify which parameters have the greatest influence on performance, and produce a design space that corresponds to acceptable process behavior.

These studies highlight the capability of the FT4’s multivariate approach to detect subtle changes in powders that are directly relevant to their performance in AM machines. Therefore, the FT4 Powder Rheometer® can support successful optimization and lifecycle management of metal powders for AM in a manner that is not possible by other techniques.

It also shows how even more modern techniques such as shear cell testing and particle size analysis may not always be able to consistently characterize process-relevant differences between these sample types. This reinforces the fact that more than one method is essential to fully describe a powder’s properties for a given process.

This information has been sourced, reviewed and adapted from materials provided by Freeman Technology.

For more information on this source, please visit Freeman Technology.

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