Forty years ago, aluminum dominated the aerospace industry. As the new kid on the block, it was considered to be lightweight, inexpensive, and state-of-the-art. In fact, as much as 70% of an aircraft was once made of aluminum. Other new materials such as composites and alloys were also used, including titanium, graphite, and fiberglass, but only in very small quantities – 3% here and 7% there. Readily available, aluminum was used everywhere from the fuselage to main engine components.
Times have changed. A typical jet built today is as little as 20% pure aluminum. Most of the non-critical structural material – paneling and aesthetic interiors – now consist of even lighter-weight carbon fiber reinforced polymers (CFRPs) and honeycomb materials. Meanwhile, for engine parts and critical components, there is a simultaneous push for lower weight and higher temperature resistance for better fuel efficiency, bringing new or previously impractical-to-machine metals into the aerospace material mix
Standard aerospace aluminums – 6061, 7050, and 7075 – and traditional aerospace metals – nickel 718, titanium 6Al4V, and stainless 15-5PH – still have applications in aerospace. These metals, however, are currently ceding territory to new alloys designed to improve cost and performance. To be clear, these new metals aren’t always new, some having been available for decades. Rather, they are new to practical production application, as machine tools, tooling technology, and insert coatings have sufficiently advanced to tackle difficult-to-machine alloys.
Even though the amount of aluminum is declining in aircraft, its use is not completely disappearing. In fact, aluminum is coming back, especially in cases where the move to CFRP has been cost prohibitive or unsuccessful. But the reappearing aluminum is not your father’s aluminum. Titanium aluminide (TiAl) and aluminum lithium (Al-Li), for example, which have been around since the 1970s, have only been gaining traction in aerospace since the turn of the century.
Similar to nickel alloy in its heat-resisting properties, TiAl retains strength and corrosion resistance in temperatures up to 1,112°F (600°C). But TiAl is more easily machined, exhibiting similar machinability characteristics to alpha-beta titanium, such as Ti6Al4V. Perhaps more importantly, TiAl has the potential to improve the thrust-to-weight ratio in aircraft engines because it’s only half the weight of nickel alloys. Case in point, both low-pressure turbine blades and high-pressure compressor blades, traditionally made of dense Ni-based super alloys are now being machined from TiAl-based alloys. General Electric was a pioneer in this development and uses TiAl low-pressure turbine blades on its GEnx engine, the first large-scale use of this material on a commercial jet engine – in this case in the Boeing 787 Dreamliner.
Another re-introduction of aluminum to aerospace is found in weight-saving Al-Li, specifically designed to improve properties of 7050 and 7075 aluminum. Overall, the addition of lithium strengthens aluminum at a lower density and weight, two catalysts of the aerospace material evolution. Al-Li alloys’ high strength, low density, high stiffness, damage tolerance, corrosion resistance, and weld-friendly nature make it a better choice than traditional aluminums in commercial jetliner airframes. Airbus is currently using AA2050. Meanwhile, Alcoa is using AA2090 T83 and 2099 T8E67. The alloy can also be found in the fuel and oxidizer tanks in the SpaceX Falcon 9 launch vehicle, and is used extensively in NASA rocket and shuttle projects.
Titanium 5553 (Ti-5553) is another metal that is reasonably new to aerospace, exhibiting high strength, light weight, and good corrosion resistance. Major structural components that need to be stronger and lighter than the previously used stainless steel alloys are perfect application points for this titanium alloy. Nicknamed triple 5-3, this has been a notoriously difficult material to machine – until recently. Extensive research and development has been devoted to making the metal practical to machine, and triple 5-3 has recently proven to be very predictable with machining consistency similar to more traditional titanium alloys like the aforementioned Ti6Al4V. The variances in the two materials require the use of different cutting data to obtain similar tool life. But once an operator has proper parameters set, triple 5-3 machines predictably. The key with triple 5-3 is to run a bit slower and optimize the tool path and coolant system to achieve a good balance of tool life and tool security.
Some structural pieces, like fasteners, landing gear, and actuators, require raw strength, with lightweight properties being less of a priority. In such cases, Carpenter Technology Ferrium S53 steel alloy has provided mechanical properties equal to or better than conventional ultra-high-strength steels, such as 300M and SAE 4340, with the added benefit of general corrosion resistance. This can eliminate the need for cadmium coating and the subsequent related processing.
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