Ongoing developments in innovative materials improve aerospace components, simplify manufacturing, and reduce costs.
By KAREN WILHELM, ABD Online
New high-performance materials continue to make jet engines and related components of modern-day aircraft lighter in weight, stronger, and, in some cases, more effective. In addition to these benefits, related efficiencies — from parts manufacturing to the performance of the aircraft — also can result in substantial cost savings.
Ceramic matrix composites (CMCs), carbon fiber composites (CFCs), and new alloys like titanium aluminide (TiAl) have helped manufacturers of components such as jet engines, for example, to reduce fuel consumption, emit less carbon dioxide (CO2) and nitrogen oxides (NOS), and significantly decrease noise. In addition to new composites and alloys, manufacturers in the aerospace industry are beginning to tap into the substantial potential and benefits of additive manufacturing metallurgy and nano-materials.
In one new composite material technology, the creation of ceramic matrix composites, a silicon carbide (SiC) ceramic resin matrix is reinforced with SiC ceramic fibers. In the manufacturing process, chemical vapor deposition (CVD) is used to coat the silicon carbide fibers with resin, and a matrix impregnation process produces a tape-like composite material. The component shape is then fabricated from the tape in a manner similar to that of forming polymer matrix composites. A molten silicon infiltration process densifies the component, forming the ceramic matrix. Finally, a coating is applied to the finished part.
CMCs withstand temperatures as high as 2,400 degrees F (1,316 degrees C), a level of performance not matched by metal alloys currently used in jet engines. They also are much lighter — approximately one-third the density and weight of those metal alloys. Removing just 1 pound (about .5 kilograms) from a spinning part can allow bearings and support structures to be reduced by another 3 pounds (1.5 kilograms). Thus, CMCs potentially can save an estimated 200 pounds (90 kilograms) in the total weight of a midsized aircraft engine. An additional benefit is that CMC parts can be mass produced.
Carbon Fiber Composites
In carbon fiber composites, carbon fibers are integrated into a polymer, metal, carbon, or ceramic matrix. Because of their strength and durability, CFCs are being used to manufacture fan blades. Titanium is added to the blade tip edges to add strength and damage resistance.
Materials research is producing many innovative advanced nickel, cobalt, and iron-based alloys that combine the advantages of structural support with light weight and low cost. One new nickel-based alloy is 718Plus, which maintains its strength and oxidation resistance at temperatures of 100 degrees F (38 degrees C), a higher temperature tolerance than that of the widely used 718 alloy.
Advanced alloys provide good performance at high temperatures and durability, and they may weigh half of the weight of conventional metals. For example, the advanced alloy TiAl can be used in such aircraft applications as low-pressure turbines.
Additive manufacturing (AM) has progressed from making plastic prototypes to producing functional metal parts. Using powder metallurgy, “3-D printing” uses a process called fused deposition modeling (FDM) to “print” an object from a digital file. In FDM, powdered metal only microns thick is mixed with a binder. The toothpaste-like material is deposited in very thin layers, building up the desired object gradually, until it matches the digital solid model. Finally, the object is heated in an oven to fuse the materials and stabilize the part.
In other types of AM processes, lasers or electron beams sinter powdered metal to build up the part. Metal powders used include such alloys as Ti6Al4V and Inconel 718. The net shapes, built with little or no wasted material, promise to rival castings in their performance. These forms of AM can be used to produce complex parts, with internal features not readily manufacturable by traditional processes. The technology also allows manufacturers to consolidate a number of parts into one, eliminating assembly time and part proliferation.
Above: NIST characterizes material properties of metal powders to help improve additive manufacturing equipment and processes. (Image courtesy of the National Institute of Standards and Technology)
AM currently is being used to make fuel nozzles and blades. Future applications may include engine combustors and cooling components. For the aerospace industry, with annual usage of fewer than 100 units of some part numbers, there is the additional advantage of being able to run a lot size of one component on demand.
New aerospace nanocoatings can replace chrome plating, as well as thermal barrier, icephobic, and protective coatings. Applied by physical vapor deposition (PVD), nanocoatings protect parts against particle erosion, fluid erosion, and corrosion. They have been used in the compressor sections of gas turbine engines and for erosion-resistant airfoils.
Another type of nano-scale materials, carbon nanotubes (CNTs) are tubular cylinders of carbon atoms with diameters ranging from less than 1 nanometer up to 50 nanometers and tube lengths ranging from microns to millimeters. Advantages of CNTs are that they can be made into materials with up to 200 times the strength of and five times the elasticity of steel. They also outperform copper with up to five times the electrical conductivity, fifteen times the thermal conductivity, 1,000 times the current capacity, and nearly 2,000 times the fracture toughness. By enhancing or replacing heavier, more fatigue-prone materials, CNTs promise to save fuel, while providing better structural, electrical, and thermal performance.
CNTs may be formed into wires, yarns, sheets, and mats. They have been used in wiring, shielding, heating, and composite structures. Wires and yarns can be used for data and electricity conduction and for structural wraps. Thin sheets (20 to 30 microns thick) have been used in electro-magnetic interference shielding and as current collectors for lightning strike protection.
Above: The fuel efficient, quiet, and low emissions GEnx jet engine. Layers of an isotropic braid provide improved impact and fatigue properties and reduce manufacturing time.
Developing the Innovative Aerospace Materials of the Future
Manufacturers, materials engineers, and scientists conducting research and development are exploring the endless potential uses of such advanced materials to transform the aircraft that fly us through air. The use of such new materials undoubtedly will propel manufacturers and military and civilian aircraft operators into a more profitable and sustainable future.