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 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.“
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.