Powder Pioneer: Jean-François Archambault, senior laboratory expert, AP&CNovember 12, 2020
What the future of metal powder characterization laboratories might look like?
The use of metal powders in additive manufacturing processes continues to grow rapidly and the powders used are tested to a high degree of quality, so that they are compliant with the requirements of the powder user.
However, there are several traditional material tests that need to be adapted for powders, either through their calibration procedure or their application. This can create certain issues when customers are not best informed about these limitations to set specifications for the characterization of the powder.
Communication at the Core
At AP&C, a GE Additive company, we have a clear solution to this, and it is communication. Many of the analytical techniques used for powders have some limitations, and knowing these limitations enables our customers to get the right results for their specific powders and application.
Without this knowledge, or without communicating with an organization that has knowledge about these techniques, it might mean that customers will not get the right, or most appropriate results, for their need. By clearly communicating what the best material requirements are for each specific application and helping customers better understand the tests needed ensures that products are not only of a high quality, but also fit for purpose for their intended use.
Characterization methods for the metal powders used in additive manufacturing processes are advancing all the time and new approaches to make the characterization process more streamlined and precise are starting to be implemented.
And as best practices continually evolve, it is important to keep up to date with the latest advancements through efficient, regular communication with your powder supplier.
Let’s examine how and where the characterization of metal powders is likely to evolve in the coming years, as well as look at some of the gold standard methods in use today.
Types of Tests for Metal Powders
Broadly speaking, the tests performed on metal powders fall into two distinct categories; chemical and physical.
Chemical tests are typically the most important set of tests for metal powders—especially in additive-based applications—as small chemical impurities can have a large impact on the performance of the finished part.
Moreover, the elemental composition of a metal alloy is often essential for performing its intended function, as many elemental additives will provide the extra functional properties, such as corrosion resistance or durability, to the alloy.
Ensuring that these ratios are correct is also paramount during any series of tests. Nevertheless, a combination of chemical and physical tests is key to ensure a material is compliant in all areas.
Testing the chemical composition of a metal alloy powder is important to ensure that it meets the correct composition and specifications.
The three main testing methods are; inductively coupled plasma atomic emission spectroscopy (ICP-AES), surface analysis methods and elemental analysis methods through fusion and combustion process.
- ICP-AES is commonly used to determine the elemental composition of a powder, including identifying the types and concentrations of any elemental contaminants within the powder. It has become a widely used technique for powders used in aerospace and medical applications because it is sensitive, has a high sample homogeneity and a dynamic measurement range. This technique generally requires acid-digestion to produce aqueous solution for simultaneous multi-element determination.
- At the other end, there are techniques allowing surface analysis without prior acid-digestion methods such as Spark atomic emission spectroscopy (Spark-AES) or laser-ablation ICP-AES/Mass spectrometry (MS) and X-Ray fluorescence (XRF). These techniques are only used when they correlate well with ICP-AES or are inappropriate when sensitivity cannot be attained.
- The final common chemical tests are fusion and combustion elemental analysis methods, which are used to analyze the oxygen, nitrogen, hydrogen, carbon and sulfur concentrations of the metal alloy. This is an important technique as many of these elements cannot be identified by ICP, yet they can have a big impact on the performance of the alloy once it has been crafted into a part.
The use of some elemental analysis methods is relatively new as there hasn’t been much interest until now in the oxygen and hydrogen concentrations within metal alloys. However, because of the surface interaction with the environment in additive manufacturing, there is a greater need to know the exact chemical compositions of the alloy. One issue with this area, is that because the oxygen and hydrogen has never been a consideration, there is a lack of certified reference materials in this area, but work is currently underway in this area.
There are several physical tests which can be performed. Many of these are tailored to the specific materials and end-use application of these materials, so the types of physical tests implemented varies on a case by case basis.
There are three standard tests which are performed on powders to ensure that the powders behave how they are supposed to, including determining the particle size either through sieves and laser diffraction system, flow and density of the powder. The other physical test which is performed straight after collecting a powder and during final inspection is a visual test, and this is to ensure that there are no foreign objects in the powder.
Example: Erosion on Hall funnel
Procedure implementation for method improvement. Image credit: AP&C
Left: Old Hall funnel showing erosion that is affecting flow of powder with smaller particles size (< 45μm) but does not affect flow of calibration powder due to its size (~300μm).
Right: New Hall funnel showing clean orifice. Regular inspection is needed. Development of internal reference material to follow erosion and its effect on powder flow rate.
The AP&C Approach
At AP&C, we have specific procedures and ways of working that induces a high quality, performance and repeatability in all the tests we perform on metal powders. The basis for this is using two laboratories that are subject to strict requirements.
The use of more than one laboratory means that there is always capacity and time for an analysis to be performed, as well as ensuring that specific materials are only used with certain machines - reducing the chance of cross-contamination of the sample.
Given the challenges of limited or no reference materials for many of the tests for additive alloys, AP&C has developed its own internal reference materials. As well as both laboratories adhering to strict requirements and our own internal reference materials (IRM), every IRM is analyzed at least 10 times to provide a statistical-based analysis so it can be used for history purposes and all subsequent quality control checks will be compared to it. This approach also allows us to determine the life span of each IRM. The measures put into place ensure high quality performance and repeatability in all the tests we perform on metal powders.
The other aspect key to AP&Cs testing approach, is ensuring traceability at every stage of production and analysis. This is achieved through an extensive laboratory information management system (LIMS), enabling all information from which powder has been collected after atomization, to which wire was used as a precursor material, whether a rework on the product has been performed, to the different analyses performed on the product.
This open and transparent approach not only enables AP&C to access all the relevant information across the production chain, but also future proofs it, as many of these systems become automated and will be able to ‘speak with each other’.
Building AP&C’s foundations on best practice, quality, and open information systems enables us to easily integrate these next generation technologies that will make metal alloy powder analyses even more effective.
The Future of Metal Powder Characterization
There are several things that could help to improve the analysis of metal powders and become the testing laboratory of the future.
Given the shift of many manufacturing setups moving to Industry 4.0 protocols and the subsequent integration of the Internet of Things (IoT), it is only natural that the analysis stages will benefit in the future.
The integration of real-time analyses, either through in-line analyses (analyses within the manufacturing process) or at-line analyses (analyses requiring sampling within the manufacturing process but analyzed using a high-speed method/technique in the manufacturing zone) will not only help to ensure fast results, it will also enable the powder to be tailored in real-time to meet the specifications compared to finding out the results at the end of the manufacturing process.
This would be a big advancement, as the ability for engineers to analyze in real-time will prevent powders that do not meet the right specifications - for one reason or another - from being quarantined and potentially discarded. In the cases where the powder can be saved by extra processing steps, the integration of real-time analyses will enable these post-manufacturing steps to be negated by tailoring and changing the process during the initial manufacturing stages.
There are many testing methods which would be applicable to these systems, and could analyze the powder in real-time, including some of the key test performed today (such as analyzing the particle size). By analyzing key parameters and changing them while the transformation process is ongoing, it means that the whole of the sample can meet the desired particle size range (and properties)—rather than a portion in the desired size range that is recovered from a sieve—so less material gets wasted. Additionally, when timeframes are tight, the prevention of extra processing steps would be a keyway to saving valuable time.
Overall, the analysis methods used today enables organizations to obtain accurate results, and at AP&C, we pride ourselves on the level of quality our analyses provide. Nevertheless, shifting to real-time analysis in line with Industry 4.0 drivers could see production times shortened and less material wasted, while ensuring that the quality of the analysis remains high, as does the finished product. This paradigm shift will benefit everybody, from suppliers to end-users, and its manifestation is something to look out for soon.
To discuss which powder might be best for your application, or for like more information on our powder characterization, please get in touch.