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Posted on Aug 21, 2020
Forty years ago, aluminum dominated the aerospace industry. As a new kid on the block, it was considered light, inexpensive, and state-of-the-art aircraft components. In fact, as much as 70% of airplanes were once made of aluminum. Other new materials were also used, such as composites and alloys, including titanium, graphite and fiberglass, but only in very small amounts - here 3%, there 7%. Readily available aluminum was used everywhere, from the fuselage to the main engine components.
Times have changed. A typical jet under construction today consists of just 20% pure aluminum. Most of the non-critical construction materials - panels and aesthetic interiors - now consist of even lighter carbon fiber reinforced polymers (CFRP) and honeycomb materials. Meanwhile, for engine parts and critical components of precision aircraft components, there is a simultaneous emphasis on lower weight and higher temperature resistance for better fuel efficiency, bringing new or previously impractical-to-machine metals into the aerospace material mix.
Aerospace manufacturing is unique among other mass production sectors, especially in the production of aero engines. The engine is the most complex component of an airplane, it contains the most individual components and ultimately determines fuel consumption. The advent of lean-mix engines with temperature potentials of up to 3,800 ° F (2,100 ° C) has helped increase demand for these new materials. Given that current superalloys have a melting point of around 1,850 ° C (3,360 ° F), finding materials that can withstand higher temperatures becomes a challenge.
To meet these temperature requirements, Heat Resistant Superalloys (HRSA) are now being introduced into the material equation, including titanium alloys, nickel alloys, and some non-metallic composite materials such as ceramics. These materials appear to be more difficult to machine than traditional aluminum, which historically means shorter tool life and less process safety.
There’s also a high process risk in machining aerospace parts. Because margins for error are non-existent at 35,000ft cruising altitude, tolerances in aerospace are more precise than almost any other industry. This level of precision takes time. Longer machining times are required for each component, and more time per part makes scrap relatively expensive, when factoring in time investment. Also, compared to other industries, aerospace component orders often consist of short run quantities and long lead times, rendering scheduling for productivity, throughput, and profitability difficult.
Unlike any other industry but oil and gas, which also has high temperature, pressure, and corrosion requirements, aerospace materials themselves impact component design. Design for manufacturability (DFM) is the engineering art of designing components with a balanced approach, taking into consideration both component function and its manufacturing requirements. This approach is being applied more and more in aerospace component design and more in the design of aerospace components and precision aircraft components, as their components need to withstand certain loads and temperature resistance, and some materials can only hold that much. The designs of materials and components really drive each other, not one after another. This relationship between material and design is especially important when studying next-generation materials. For all these reasons, aviation manufacturers are different from each other. No wonder their range of materials is unique.
Standard aviation aluminum - 6061, 7050 and 7075 - and traditional aviation metals - nickel 718, titanium 6Al4V and stainless 15-5PH - are still used in aviation. However, these metals are now giving territory to new alloys designed to improve cost and efficiency. To be clear, these new metals are not always new, some have been around for decades.Rather, they are new to practical manufacturing applications as machine tools, tooling technology, and wafer coatings are advanced enough to deal with difficult-to-machine alloys.
Although the amount of aluminum in aircraft is declining, its use is not completely disappearing. In fact, aluminum does come back, especially in cases where the transition to CFRP has proved prohibitive or ineffective. But aluminum that appears again is not your father's aluminum. For example, titanium clays (TiAl) and lithium aluminum (Al-Li), which have been in existence since the 1970s, have only been gaining ground in aviation since the turn of the century.
Like nickel alloy in its heat-resistant properties, TiAl retains strength and corrosion resistance at temperatures up to 600 ° C (1,112 ° F). But TiAl is easier to machine, showing similar machinability properties to alpha-beta titanium,such as Ti6Al4V. More importantly, TiAl can improve the thrust-to-weight ratio in aero-engines because it is half the size of nickel alloys. For example, both low pressure turbine blades and high pressure compressor blades, traditionally made of dense nickel-based superalloys, are now machined from TiAl-based alloys. General Electric was a pioneer in this development and uses TiAl low pressure turbine blades in its GEnx engine, the first large-scale use of the material in a commercial jet engine - in this case, the Boeing 787 Dreamliner.
Another reintroduction of aluminum into the aerospace industry can be found in lightweight Al-Li, specially designed to improve the properties of 7050 and 7075 aluminum. Overall, the addition of lithium strengthens the aluminum with lower density and weight, two catalysts for the evolution of aviation material. The high strength of Al-Li alloys, low density, high stiffness, damage tolerance, corrosion resistance and weld-friendly nature make it a better choice than traditional aluminum in commercial jet airframes. Currently, Airbus uses the AA2050. Meanwhile, Alcoa uses the AA2090 T83 and the 2099 T8E67. The alloy is also found in the fuel and oxidant tanks of the SpaceX Falcon 9 space rockets, and is used extensively in NASA's rocket and shuttle projects.
Titanium 5553 (Ti-5553) is another relatively new metal in aviation, showing high strength, light weight, and good corrosion resistance. The main structural components, which must be stronger and lighter than the stainless steel alloys used previously, are the ideal application points for this titanium alloy. Known as the triple 5-3, it was a material that was extremely difficult to machine - until recently. Extensive research and development has been carried out to make the metal practical for machining, and the triple 5-3 has recently proved to be very predictable due to machining consistency similar to more traditional titanium alloys such as the aforementioned Ti6Al4V. The differences in the two materials require different cutting data to be used in order to achieve similar tool life. But when the operator has the appropriate parameters set, a triple number of 5-3 machines can be predicted. The key with the Triples 5-3 is a bit slower operation and optimization of the tool path and cooling system to achieve a good balance between tool life and safety.
Certain structural components, such as fasteners, chassis, and cylinders, require raw strength, and lightness is less of a priority. In such cases, Ferrium S53 alloy steel provides mechanical properties equal to or better than conventional ultra-high strength steels such as 300M and SAE 4340, with the added benefit of overall corrosion resistance. This can eliminate the need for cadmium coating and the subsequent related processing.
Composite materials are also making up a growing chunk of the aerospace material pie. They reduce weight and reduce fuel consumption while being easy to handle, design, shape and repair. Once considered only for lightweight structural members or cabin components, the range of aerospace composites now extends to truly functional components - wing and fuselage skins, engines and landing gear.
Also important, composite elements can be formed into complex shapes that would require machining and bonding in the case of metal parts. Pre-formed composite components are not only light and strong, but also reduce the number of heavy fasteners and joints - which are potential points of failure - on an airplane. In this way, composite materials help drive the global trend of reducing the number of components in entire assemblies, utilizing one-piece designs whenever possible.
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