In this interview, Dr. Peter Paplewski discusses the importance of ONH analysis in additive manufacturing and the development of a future-proof analyzer that can be operated by technicians.
Why is elemental analysis important in additive manufacturing?
The initial quality and the further treatment of the metal powder used for additive manufacturing plays an important role in the quality and properties of the final product. Parameters like particle size, distribution and particle morphology are established, but these represent only the “mechanical” part of the characterization, neglecting the metallurgical/chemical part.
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Although metals have fantastic properties, we all know from our daily lives that they can be prone to corrosion. Regardless whether we call it degradation, oxidation, aging or corrosion, the symptom is always the same: metal is oxidized at the interface between solid (metal) and gas phase (ambience).
There are certain factors that determine the speed of this chemical reaction and in this context, it is important to realize that additive manufacturing is related to welding (layer by layer) but has nothing in common with traditional manufacturing. During the additive manufacturing process, metals are in a molten state for a short period of time.
Various metals like Ti, Al, Mg are extremely reactive in this state and react even with trace amounts of non-metallic impurities (O, C, H, N, etc.). In fact, this principle is applied in commercial gas purification systems to produce ultra-high purity carrier gas, needed for laboratory analysis e.g. in gas chromatography.
For the powder metallurgic process there will be oxygen uptake at each process step: beginning with the powder production, its storage, the conditioning of the powder bed in the machine, the additive manufacturing process itself, followed by powder recycling steps. Without a constant quality/process control that includes oxygen determination users are “flying blind”, without the necessary data to optimize: build-to-build consistency, yield and final product quality.
What factors can alter the composition of metal powders during the manufacturing process? What impact does this have on the final product?
Before I answer this question, we need to define clearly what is meant with “composition” and underline one important fact that is often overlooked. The stable alloying elements (the metallic composition) will not be altered during an additive manufacturing process.
For instance, an INCONEL® 718 grade will not change its Ni, Cr, Mo, etc. content just because it was melted and re-solidified. The change of the composition happens solely on the light, volatile non-metal elements. These can be introduced or leave the metal matrix through the gas phase.
One major “ingredient” or source of contamination that accelerates the powder degradation process is water. In the form of humidity, water is present everywhere. It is well-known that the ambient moisture level makes the difference in terms of the dangerous hydrogen induced embrittlement or delayed fractures in the welding industry.
It is also common sense that corrosion happens much faster and severe in an environment with high humidity (e.g. offshore or coast) than in a dry desert although the oxygen content in both atmospheres is the same. It is also important to state that moisture behaves like a film that coats surfaces and is slow to remove even by vacuum systems.
Any surface represents a potential target for oxidation, moisture adsorption and growing hydroxide layers. Since the particle size of the powders used for additive manufacturing is small, the surface is huge and this makes the entire process susceptible for oxidation.
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The factors and process parameters that determine the speed and yield of this alteration are:
Particle size: The smaller the size, the higher is the specific surface, the higher is the initial oxygen level (provided identical powder production parameters are applied) and the higher will be the tendency for oxygen uptake during the process.
Temperature: The higher the temperature, the faster will the reaction. From chemical kinetics it is known this relation is exponential, not linear. The print parameters, the orientation of the part in the powder bed (build layout and thus the heat affected zone) but also storage and conditioning temperature will have an impact.
The concentration of impurities like moisture, oxygen, hydrocarbons in the shielding gas. Metals like Ti and Al are very reactive since they react with nitrogen at elevated temperature. For these metals even impurities of nitrogen present in the shielding gas (usually argon) can led to nitrogen uptake. In this context: Any form of leakage (e.g. on the doors) of an additive manufacturing machine can have a huge impact.
And finally, exposition time.
Generally, the impact of oxygen, hydrogen and nitrogen is negative. These elements form stable inclusions (metal oxides or nitrides) with the metal and degrade the mechanical properties, tensile strength, ductility, and create porosity (increasing porosity and impurity content). The most negative affect has hydrogen but the damaging mechanism and its relevance for additive manufacturing depends on the base metal of the metal powder.
Why is it important to be able to monitor not just O/N/H but carbon and sulfur too?
Carbon is known to be the alloying element that dominates the mechanical properties of steel. Its influence is less on other metals like Ni, Ti but still the carbon level can change during the additive manufacturing process and carbon can leave the material in form of CO or CO2.
Carbon is similar to water that is present in the ambience and substances like ambient dust contain carbon and can easily contaminate the powder bed. Thinking about binder jetting or other ways of AM, other than direct metal printing, the situation is different: you can monitor and control the usual de-binding step by the carbon level of the final product.
The G6 LEONARDO is Bruker’s latest ONH analyzer. How does this product address the growing need for analytical solutions in additive manufacturing?
The G6 LEONARDO applies the inert gas fusion principle, a fast, reliable and volumetric method analyzing the entire sample specimen, not only the surface. Sample preparation in additive manufacturing is easy, and it is possible to test the final part by producing test specimen or analyzing parts of the support structure manufactured with the same print parameters. This method is applied for QC/QA in every primary metal manufacturer but is said to be complicated and can only be applied by laboratory personnel at big metal production sites.
This change with the G6 LEONARDO, which has been designed without complicated “bells and whistles”, is that it can be operated by technicians, not scientists. The G6 LEONARDO applies the same proven detection system as Bruker’s high-end analyzer but works with less installation requirements, lower running costs and providing the robustness requires in the industry.
The biggest change is its new software package that allows factory calibrated methods and provides the full audit trail but is easy to operate. These characteristics make the G6 LEONARDO a very attractive solution for additive manufacturing.
What sets the G6 LEONARDO apart from other ONH analyzers on the market?
Apart from its ground-breaking software package, the G6 LEONARDO provides the same unique features than Bruker’s high-end analyzer G8 GALILEO.
It is available with FusionControl, a contact free pyrometer controlling the real
sample temperature; oxygen is measured directly in form of CO without prior chemical conversion into CO2 (and at the optimal temperature). Finally, the G6 LEONARDO comes with SampleCareTM providing additional reliability and robustness.
What effect does drifting have on ONH analysis and how did Bruker overcome this issue when developing the G6 LEONARDO?
Drifting detector baselines require frequent control of calibration validity or recalibration. A processes costing time and money with no availability for production samples. Detector drift on a shorter time scale affects the limits of detection.
To overcome drift to the maximum possible extent, Bruker uses a dedicated, physical reference channel on each and every detector and applies high precision temperature control on the detectors where necessary.
Its ambient pressure flow system makes it insensible against leakage or ambient pressure changes, without the need for error prone flow compensations.
Why did you choose an argon gas carrier instead of helium? What advantages does this provide?
Argon is much cheaper than helium, already available at almost every additive manufacturing plant and always available. The helium resources on this planet are limited: it is diffusing into outer space and not being refilled by a natural process and the helium demand for superconducting application like MRT are high. Thus, helium is expensive and sometimes even not available.
How does the temperature of the sample affect elemental analysis? How does the G6 LEONARDO prevent overheating and what benefits does this provide for the user?
Temperature is one of the most important parameters for ONH analysis. The temperature determines whether a chemical reaction is complete, not activated at all or another reaction route applies, and unwanted by-products are formed.
There is no common “one fits all” temperature and a brute force approach: “maximum temperature, always” is not applicable, because of side reactions. With its unique FusionControl, the G6 LEONARDO ensures the analysis is running at the right temperature, delivering trustable results with minimum maintenance or wear due to thermal stress.
How do you think the product will evolve over the next decade?
It is hard to say what the future will hold, however, thus far, the G6 LEONARDO has been well-accepted by those working in the metal industry. The product has been designed for use in many different settings, with additive manufacturing being just one example of this.
Over time, I hope that smaller metal producers and supply chains will implement this product into their workflow. For now, I can safely say that the G6 LEONARDO has all of the genes to be a success and I am very optimistic about the future.
Where can readers find more information?
- For more information about the G6 LEONARDO, click here.
- Bruker also provides application support and training by product specialists and maintains demo laboratories around the globe. There, you have the opportunity for a hands-on demonstration while analyzing your own samples.
- In addition, Bruker provides Lab Reports and specific guidelines for many applications. Feel invited to discuss your analytical challenges with our team of experts.
About Dr. Peter Paplewski
Dr. Peter Paplewski is the Product Line Manager for spark-OES and CS/ONH-analysis at Bruker AXS, Karlsruhe, Germany. He received his MSc and Ph.D. in inorganic chemistry from the University of Wuppertal with research emphasis on the synthesis and characterization of novel small, short-lived molecules in the gas phase.
After his academic research he started as R&D scientist for a Channel Partner of Agilent Technologies in 2001, followed by transition into Product Management for an atomic emission detector. He joined Bruker in 2010 as Product Manager for CS/ONH-Elemental analysis of metals and inorganic solids, especially metals.
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