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Superalloys: the technological frontier of the metal industry

Studying metals at a mesoscopic scale is both a major scientific breakthrough and a competitiveness challenge for the aeronautics industry. A research team is involved in this change of scale which resulted in significant progress in terms of industrial control.

19
May 2017
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Studying metals at a mesoscopic scale is both a major scientific breakthrough and a competitiveness challenge for the aeronautics industry. A research team is involved in this change of scale which resulted in significant progress in terms of industrial control.

Paris Innovation Review – You are conducting your research within the framework of an ANR industrial chair co-funded by the Safran Group. Your goal is to optimize the properties of nickel-based superalloys. Are these superalloys already used in the industry?

Nathalie Bozzolo – Yes indeed. They are metallic materials used to manufacture specific parts of aircraft engines.

These parts must bear high stresses as engine hot parts can reach extreme temperatures of over 1,000°F. Hence, they need both enhanced strength and resistance. The operating temperature is a crucial parameter of these engines: the higher the temperature, the more energy-efficient. Developing materials that are perfectly reliable at very high temperatures responds to a twofold challenge: economic and ecological.

The superalloys we are interested in are polycrystalline, like most common metals and alloys, as opposed to monocrystalline grades. This concept comes from their microscopic structure: polycrystalline materials consist of a multitude of small crystals called “grains”, which vary by their orientation and sometimes by their chemical composition and crystallographic structure. The size of these grains and their properties – i.e. microstructure – determines the application properties of any polycrystalline material.

For instance, the microstructure of turbine discs used in aircraft engines must be as homogeneous as possible. The grain size must be fine (approximately 10 micrometers) and even as possible in all parts of the piece. But at a smaller scale, these materials present some form of heterogeneity. Their grains contain fine particles (approximately a few dozen nanometers) of different chemical nature, the so-called second phase precipitates, that allow the hardening of materials. The hardening of metals by addition of alloying elements and formation of precipitates is a fundamental and widely-used principle in metallurgy – think of cast iron as opposed to pure iron, for example.

More precisely, how do you obtain these materials?

Alloys result from fusion. Workpieces are produced by hot forging using high-capacity presses at temperatures above 1,800°F. At these temperatures, the hardening precipitates are dissolved and alloys become ductile and malleable enough to be forged without breaking.

Forging modifies the microstructure, the grains deform and regenerate. The accumulation of defects (mainly dislocations) triggers the recrystallization process. Another, relatively quick phenomenon of fine precipitation occurs inside the grains and changes their structure during the cooling phrase. Fine precipitation is an important feature of nickel-based superalloys.

These dynamic phenomena influence one another. Grain defects affect precipitation kinetics. Furthermore, the amount of defects but also the size and other properties of grains depend on forging conditions.

Our research focuses on these mechanisms at a very fine scale by implementing a range of advanced techniques and tools and by combining experimental analysis and modeling with the development of advanced numerical tools. This combination – with a proven effectiveness – is quite unique, both on the international academic scene and in the industrial world.

Identifying, describing and being able to predict metallurgical mechanisms at work at a fine scale leads to practical knowledge: we provide our industrial partners with knowledge and tools to improve their control over processes and final properties.

What properties do you specifically seek?

There are several properties involved and they need an optimal combination. Once the parts are put into service, their microstructure will change due to the specific constraints they are subjected to.

In the case of aircraft engines, we aim at two specific properties: fatigue and creep resistance.

Fatigue refers to cyclic changes in the engine regime, alternating between take-offs, cruise speed and landings. It is worth mentioning that cyclic stress can ultimately lead to material failure at lower stress levels than under monotonic conditions.

Creep is the deformation and subsequent damage of a material induced by prolonged exposure to a given temperature and mechanical stress level.

The hardness of a material can contribute to its resistance to fatigue. Conversely, it can make it indirectly more sensitive to creep. Different microstructures will present different properties. An important property of microstructure is the dislocation density i.e. the amount of linear defects, discontinuities in the organization of the crystal structure caused by plastic deformation. But these defects are responsible for other... qualities: they harden the material. But they can also lead to larger precipitates, whereas “fatigue resistance” requires fine precipitates. In short, compromises need to be found constantly.

How do you conduct this research?

The Opale Chair studies what is going on during forging i.e. the way processing affects microstructure. We work in close collaboration with a team at Institut P’, in Poitiers, which focuses on the relationship between microstructure and application properties. Our colleague Patrick Villechaise from Institut P’ is the deputy holder of the Opale chair. We also have a strong industrial partnership with the Safran group and its forging companies, notably Safran Aircraft Engineering (formerly Snecma) but also Safran Helicopter Engines and Safran Nacelles.

We use the Forge® NxT software for the finite-element simulation hot or cold deformation processes. This software solution could be seen as “mature” (it is almost 30 years old) but keeps being in constant development. It integrates the numerical developments realized in our laboratory and equips many industrialists all over the world. But Forge® provides information at the macroscopic scale, i.e. at the scale of the whole piece, and regarding only the thermomechanical conditions (evolution of temperature and strain rate throughout the piece and all along its deformation). We work on the mesoscopic scale, that of grains, between the macroscopic and the atomic scales.

For that purpose,, we perform lab experiments, by reproducing in a highly-controlled environment – while varying the conditions – everything that occurs during industrial processes, in order to observe the evolution of microstructure at the suitable scale. Lab samples used for our metallurgical analyses have sizes measured in inches. We can use the industrial forging presses of our partners in case we need larger samples – for example, in order to test mechanical strength.

The engineers from Safran and our research team are frequently in touch: it’s a real two-way exchange.

What do we share with them? Insight into the mesoscopic scale provides knowledge and a precise understanding of certain phenomena, that is complementary to their industrial know-how. Their empirical knowledge goes hand in hand with industrial process control : they know that changing the temperature from 1,750°F to 1,785°F – or changing the type of press – leads to different material properties, but they don’t necessarily know why. They cannot foresee everything and they need the support of researchers to understand what is exactly going on when, for contingent reasons or for the sake of experimentation, they change their forging conditions and come across unanticipated side effects. Knowledge of the mechanisms involved in physical metallurgy also allows them to gain efficiency in adapting their processes to new grades of alloys. Most parameters are specific to each alloy, but the underlying physics, trends, couplings, remain the same. In the aeronautics field, demand for new materials with improved properties is in constant growth. Quick adaptation is crucial in a context of global competition in this sector.

Conversely, when we start a research or experimentation, Safran also shares with us their knowledge, which saves us valuable time. Nickel-based superalloys have been developed specifically for aeronautics and manufacturers have an unmatched sectorial expertise.

turbine

Beyond contributions in terms of knowledge, what is the meaning of this partnership for Safran?

Industrial control of superalloys is a crucial competitive challenge for aerospace engine manufacturers. I evoked energy efficiency issues, with obvious economic aspects. Safety and security also represent a significant challenge: if a turbine disc breaks, fragments can pass through the crankcase and even perforate the cabin of the aircraft

– with obvious devastating consequences.

Forming a partnership with a leading research team is therefore a key element for engine manufacturers. Every four years, a international conference brings together the entire academic community working on these issues, as well as industrial companies. A joint presentation of innovative research may have a positive impact on clients, especially when most of them are sitting in the audience.

At the same time, we are talking about public research, and your results are intended to be published.

Of course, but between the results production and publication, there is a time gap that gives our partner a significant head start. Besides, publications never present the results of a study in all details. Let’s be clear: we are not a subsidiary of Safran R&D. The company might ask us questions, ask us to focus on a specific subject, benefit privileged access to our work... But if we explain to them that during the following three months, we will be focusing on a tiny detail, our partner will trust our judgment.

An industrial chair allows to develop an ambitious research program – Opale supports nine PhD theses and the work of five post-doctoral fellows – combining upstream studies and studies more directly related to industrial problems. There is much talk about the structuring of research; the principle of ANR industrial chairs fits into this logic and operates at a level that really makes sense.

Moreover, this industrial environment is an ecosystem where knowledge is shared. Researchers who work with several companies can take advantage of this transverse position to suggest new collaborations.

This is precisely what we did with another ANR industrial chair, Digimu, held by the Marc Bernacki, also member of Opale and expert in computational metallurgy. Opale aims to understand metallurgical phenomena; Digimu, to model and simulate them. Just like the Forge® software, but at another scale, Digimu simulates phenomena at a microstructural scale.

The development of adapted numerical methods is a challenge for the dissemination of our research. But there is always a necessary delay. We have performed advanced calculations over the last ten years. These are both original and powerful calculations but also very resource-intensive. The purpose of the Digimu chair is to optimize the numerical methods and thus enable their dissemination, first to our partners and ultimately to other clients of Transvalor – exactly like Forge®. There is a gap of several years between what we do in the lab and what Digimu can do.

Marc Bernacki could gather around him an industrial consortium formed by six partners: ArcelorMittal, Areva, Ascometal, Aubert & Duval, CEA, Safran. They all agreed to work together to develop generic tools. Each company will then determine its own set of model parameters, that are specific to the processed material and to the process itself. Individually, they wouldn’t necessarily have the resources or skills to develop these digital tools on their own. The chair is a competitiveness enhancer at the scale of the ecosystem.

Your lab is at the technological frontier. It is a challenge for your partners, but also a challenge for competitiveness at a national scale. Does this aspect play any role for you?

Yes, for several reasons. First, let’s be clear: our partner stands, just like us, on this technological frontier. Hence, we have access to far more resources than a purely academic lab without any direct contact with the industry.

At the same time, our recognition in the academic world attracts foreign groups who wish to discuss with us or request our services. The Japanese Mitsubishi, Toyota, Nippon Steel, the Germans Otto Fuchs KG and VDM Metals International GmbH have already paid us a visit.

Loyalty towards our partners and their subcontracting ecosystem obviously marks a boundary between what we can do and what we cannot. Transparency is paramount: when we are approached, I first talk with my partners and we assess together whether the project is consistent with our work. If so, we can for example go on for a PhD thesis or a post-doctoral research.

It isn’t always as simple and one needs to recognize the full picture. If it’s simply a matter of presenting your work, easy. But in the ensuing discussion, you will necessarily have to share some ideas in response to the industrial problems that have been raised. Some groups have specialized in browsing the research conducted by other teams to pick up promising ideas and work on them on their own. A researcher is not necessarily trained to deal with these issues and will need a certain sensitivity, a set of skills that is more common to the industrial world than to the academic world. Even more so if beyond the competition between firms, the issue at stake involves competing countries!

But don’t you researchers run the risk of being dragged out of the logics of academic world and caught in issues involving both competition and competitiveness?

I would put things differently: as a researcher, it is a source of motivation to know that my work opens industrial prospects, with a direct impact on corporate life and national competitiveness.

These objectives are not contradictory with the service of science: in the case of NASA, for instance, their roadmap is at the service of US power. The question never arises whether it is also good research... For a researcher, serving one’s country or participating more closely to the economic life can be a source of satisfaction, motivation, and recognition. It is another way of being useful.

It can also prove a valuable asset on the academic level. The industrial aim provides a perspective on research by deepening its purpose. Exchanges with our industrial counterparts often shed a light on aspects that we wouldn’t have explored otherwise: their input enriches our activity as researchers. Besides, the academic and the industrial worlds are both subject to powerful network effects. Networks may intertwine and mutually activate one another. There is a form of leverage effect. Doors open, granting access to knowledge, equipment and questions that help research move forward.

Nathalie Bozzolo
Professor in Physical Metallurgy at Mines ParisTech PSL Research University, holder of ANR Industrial Chair Safran Opale