Professor Körner, you study how heavy, stable metal can be made light. Will we have turbine blades made of metal foam in the next twenty years?
Prof. Dr. Carolin Körner: The answer is a definite yes and no. Metal foam is literally foamed metal, so it’s porous and light. But its mechanical characteristics make it unsuitable as a material for manufacturing complete turbine blades. However, locally porous blades are a conceivable option, perhaps with a cellular structure inside to act as a heat exchanger or with a porous surface for effusion cooling to replace the cooling holes method generally used today. So it’s a no to blades made of foam, I’m afraid, but engine manufacturing will be able to use concepts from the development of lightweight metals.
Generally speaking, what limits the application possibilities of materials in engine and aircraft manufacturing?
Körner: Materials have become a key lever in the development of new aircraft and above all new engines. When it comes to further developing materials for aircraft engines, various approaches are being pursued: developing components with integrated functionality—such as components with porous structures—improving tried-and-tested alloys, and using new materials such as intermetallic compounds, whose properties lie somewhere between those of metals and those of ceramics.
After all, if engines are to become more efficient, then the turbines must run faster, for example, and the materials used in the turbines must withstand constantly increasing mechanical and thermal stresses. Although compounds and intermetallics are suitable for this role, they are difficult to manufacture and process. They present entirely new challenges to production engineering: How do I cast or forge a blade out of an intermetallic alloy such as titanium aluminide? And how do I then join it to the disk? Generally speaking, the melting point is the maximum physical limit for the use of a material. If the material is to be used for industrial applications, another consideration is manufacturing, which can be very complex and expensive and can therefore also limit its use.
What application possibilities do you see for lightweight metals in general and why do we need them?
Körner: Lightweight construction is important wherever people are looking to save resources and to reduce fuel consumption and emissions. Nature is our model here: bones, for example, are lightweight components, with a compact external shell and a cellular, so to say foamed core. This makes bones light while also providing stability and good damping characteristics. We try to transfer these principles to metal construction. A problem with the use of lighter materials is the loss of rigidity and the associated vibrations and noise. So you have to stiffen the design while simultaneously increasing its damping properties—the material has to fulfill several functions. In the case of metal foam, for instance, this multi-functionality is present: the material offers rigidity, effective damping, high energy absorption and good permeability. By embedding piezo elements, the metal component can even become active. For example, piezo elements can apply vibrations, so a possible application is on the aircraft’s wing: instead of moving the wing with hydraulically controlled movable flaps as is done today, you could move the wing directly using actuators embedded in the material. Research into this technology is already under way.
We are able to grow metals like crystals, we create intermetallic compounds, and we can manufacture metal components in 3D printers—where are the limits to innovation in the development of metallic materials?
Körner: Creating alloys is an age-old practice, and you’d be forgiven for wondering if any great progress was possible at this stage. However, we are really just at the beginning. When it comes to development potential, we’re talking, perhaps, about a fifty-dimensional space. We’ve only explored tiny corners of it to date. We’ve been following the same principle probably for millennia now: you take a base element such as aluminum, iron or nickel and you add further elements in various proportions. The new alloys created in this way are then tested and empirically evaluated. In the case of nickel-base alloys, which are used primarily in modern aircraft engine manufacturing, we’ve already begun to move away from this template. We now practice combinatorial material development: based on thermodynamic models and using numerical algorithms, we search for promising combinations among the virtually endless number of possibilities, and then we analyze and evaluate only these specific candidates. This is much more efficient than the old empirical approach.
Homogeneity In this context, the term refers to the even distribution of atoms and molecules, for example in alloys. Because of rapid solidification, the homogeneity of workpieces manufactured using additive methods is much greater than that of cast workpieces.
Distribution of rhenium in an alloy (CMSX-4): left, in a cast workpiece; center, in an additively manufactured workpiece; right, additively manufactured with four minutes of heat treatment.
Intermetallic compound A homogeneous compound of two or more metals that, unlike alloys, possesses lattice structures that differ from those of its constituents. Because of the particularly strong bonding between their disparate atoms, intermetallic compounds are usually harder, more brittle and more creep and temperature resistant than their original metals. However, they are also difficult to process using conventional manufacturing methods. From a chemical perspective, intermetallics fall between metals and ceramics.
Metal foam Umbrella term for foamed metallic materials. Metal foam has a low density and weight combined with high structural rigidity and strength. Among its various applications, metal foam is used as a crash absorber in rail vehicles and for damping vibrations in machine tools.
Die-cast aluminum component with foamed core.
Why can’t we make do with the alloys we already have?
Körner: To give an example, so-called 3D printing—the additive manufacturing of components layer for layer from metallic powder using laser or electron beams—presents us with completely new problems and opportunities. One advantage of additive manufacturing is the rapid solidification of the metal, which yields unprecedented homogeneity in the alloy. Now we need alloys that exploit this potential. This would allow us to avoid the separation processes that are an inevitable consequence of conventional casting methods. The inhomogeneities in a cast blade can be a few hundred micrometers in size, and the blade subsequently has to be heat treated for several hours. With additive manufacturing, the inhomogeneities are reduced to a few micrometers, and the heat treatment is accomplished in a matter of seconds.
Which requirements must future materials satisfy in the aviation industry?
Körner: They must become increasingly lighter while also becoming more and more stable. In particular, the requirements for creep resistance, oxidation resistance and weight will continue their upward curve. At the same time, the demands placed on manufacturing processes will also rise. Titanium aluminides, for example, are very light, temperature resistant and creep resistant, but they are very difficult to process in terms of casting or forging, and also in terms of ablation. New methods may be needed here.
Which new techniques for manufacturing metal components will we be using over the coming years? And which methods will they replace?
Körner: At the moment, additive manufacturing is positively booming. However, I believe enthusiasm will cool a little after a while. After all, just because something can be done doesn’t mean that it’s cost effective to do it, and that’s not even mentioning technical limits, some of which we may not even be aware of yet. Additive manufacturing opens up new design possibilities that we have to get to know before we can properly use them. Whereas production-oriented design was often the guiding principle in the past for the manufacture of components and workpieces, application-oriented design is now becoming possible (see “Additive manufacturing: Layer by layer”). In the course of this transition, optimization methods will also gain in importance, such as computational improvements of component geometries under stress, which is known as topology optimization. However, I don’t think additive manufacturing will completely replace another production method. If you can cast a component, then you should cast it: casting is still much more cost effective.
Why is innovation in materials so important? Have all the development possibilities in design been exhausted?
Körner: There’s always interplay between design and materials. If I fully exhaust the design possibilities, I’ll come up against the physical load limits of the materials. In a piston engine, for instance, if I optimize the fluid mechanics of combustion in the piston—that is, improve the topology of the piston such that it works more efficiently—this usually results in higher pressures and temperatures being generated in the piston. To withstand these, I will then most likely need to use other materials.
Could advances in material development one day deliver replacements for the rare metals currently used in aircraft manufacturing?
Körner: We certainly hope so—for elements such as scandium, which greatly increases the strength of aluminum alloys but is also extremely expensive. Thanks to the effective way they change the characteristics of other materials and alloys, even when added in very small quantities, rare earths have gained in importance, and people are searching systematically for alternatives with similar properties. Rhenium, for example, significantly increases creep resistance in nickel-base alloys, but it’s also very expensive and available only in limited quantities and from a handful of providers. Research is currently ongoing into whether and how it might be possible to replace it with a material such as tungsten. When the necessity is there to find something else, then people generally manage to find something.
Professor Dr. Caroline Körner Head of the Chair of Metals Science and Technology at the University of Erlangen-Nürnberg.
Professor Dr. Carolin Körner studied theoretical physics at the Friedrich-Alexander University of Erlangen-Nürnberg (FAU). She obtained a Ph.D. in 1997 at the FAU’s Faculty of Engineering under Professor H.W. Bergmann for her work on the interaction between ultra-short laser pulses and metals. She qualified as a university lecturer in 2007 with a thesis on the manufacture and simulation of lightweight metal foams for material sciences.
Since 2011, she has been Head of the Chair of Metals Science and Technology at FAU. In addition, she heads a working group at the Joint Institute of Advanced Materials and Processes (ZMP) in Fürth and at Neue Materialien Fürth GmbH (NMF). Her main areas of interest are additive manufacturing, light metal casting, cellular materials, composites and process simulation. She has been working closely together with MTU Aero Engines to develop new materials for many years.