Materials in Aerospace

Technological advances in aerospace engineering have forced a shift away from the paradigm of aluminum aircraft. With stronger, lighter-weight metal alloys and composites coming into play, aircraft companies are beginning to realize the reality of much more cost-effective, comfortable planes for customers – and they're putting that realization to practice.

Aluminum structures pose inconveniences and problems to the design of aircraft. For example, humidity is intentionally lowered during flight in order to help reduce aluminum oxidation. This leads to bodily dehydration, thereby creating an uncomfortable atmosphere for passengers.

Perhaps the biggest driver of material advancement in aerospace is the desire to reduce the weight, and therefore the cost, of an airplane. Lighter composites with better strength-to-weight ratios than aluminum are being incorporated into next-generation aircraft designs at a fast rate. In addition to the threat of oxidation, aluminum has a high rate of thermal expansion. With composites becoming standard in aircraft, placing a composite and aluminum side by side and cooling them down to -65°F (the outside temperature during flight) causes the aluminum to shrink much faster than the composite material, leading to thermal stress in the fuselage. Because of this and other factors, titanium and other alloys are rapidly replacing many aluminum structures.

Also, most composite material used in aerospace consist of a carbon fiber embedded in an epoxy resin that bonds it together. Because of the galvanic corrosion that occurs when carbon fibers are in proximity to aluminum, engineers are replacing many aluminum parts with titanium, particularly in applications where it touches the composite materials.

In an effort to create lighter-weight, stronger, more comfortable and cost-effective aircraft, engineers must look to a complex mix of materials that will not only serve a better purpose than previous designs, but will safely work together to ensure a secure future in aerospace manufacturing and design plans.

Composites/Plastics

Composite materials are being used more and more frequently to build aerospace structures and components. With the ability to mix two or more different materials together comes an opportunity for flexibility in applications. For instance, composites can be tailored to meet the needs of the engineer by changing the formula or direction of the materials. They can be molded into the necessary shape and the fibers can be custom oriented to allow for varied thicknesses to easily accommodate the stresses that will occur at different locations around the fuselage.

One of the biggest driving forces in creating new, effective composite materials is the need for lighter aircraft without jeopardizing the safety of the plane. Composites make for ideal candidates because they are lighter than steel by 80%, and lighter than aluminum by 60%, yet the fibers embedded into the matrix material can be up to 25 times stronger than steel and extremely thin. The combination of highstrength and light-weight is the epitome of what aerospace engineers are looking for in advanced materials.

"Composites are really the key to building aircraft," says John Newman, president and CEO of Laser Technology, Inc., Norristown, PA. "Composite materials help aerospace engineers design aircraft and spacecraft to meet the performance requirements with the absolute least amount of weight."

The most common aerospace composite materials are made up of a polymer matrix material reinforced with a highly-controlled orientation of carbon fiber. Cross layers of this material can be placed atop one another running in different directions for maximum strength. In all, there are a total of four main types of composites: polymer matrix (plastics), metal matrix, ceramic matrix and carbon matrix.

Polymer matrix composites are the workhorse materials of the aerospace industry. They include fiber-reinforced thermoplastics and thermosetting plastics, multi-functional composites and nanocomposites. The fibers embedded into the matrix are what give the materials their strength.

"Fibers give plastics the mechanical properties that can meet or exceed performance requirements and actually make them much more comparable to metal alloys," explains Andrew Melcher, military and aerospace sales specialist at Spectrum Plastics Group, Minneapolis, MN. "To be able to find solutions to replace some of these heavy alloys, the alloy's properties are compounded into plastic resins."

Aside from the light weight and high strength that composites have to offer, there are plenty of other benefits. Composites are generally more readily available than titanium and other specialty alloys, and certainly less expensive. The corrosion-resistant composite materials are also easier to manufacture in bulk, as molds can be created and secondary manufacturing processes are eliminated, as opposed to machining components one by one.

Composite parts are molded into the desired shape, followed by a limited amount of machining. When molding a composite, a tool is made in the preferred shape, and the composite material is laid in. A widely-used process uses preimpregnated fabrics and unidirectional fibers, which are laid out in patterns specified by the design engineer.

"Frequently, they use a laser to guide where these pre-cutout composite fabric pieces go, and it'll project right on the tool," Newman says. "The operator will place a triangle, or whatever the shape is, right on the laser outline of the part and build up the part in a tool. Then it gets vacuum bagged, put in a metal tool in the autoclave, and cooked under pressure."

This operation squeezes the air out, and then the heat cures the part. Operators can then trim the sides and edges, add on honeycomb materials, add skins, and build up panels in three dimensions. One plane that has revolutionized the manufacture and assembly of composite aircraft parts is the Boeing 787.

"The molds for the 787 are huge molds and they'll lay up the fabric on the inside, pressurize it from the inside and then cure it. It's just an amazing engineering marvel, what they've done. To make something that size is truly revolutionary," Newman says.

Many composites may be engineered to have high energy absorption rates or high energy conductivity, based on the designers' choice. These are highly important traits when considering conditions such as lightning strikes or evading enemy radar, in the case of military aircraft.

"Right now, when you have a carbon structure on an aircraft, the typical approach is to use a copper mesh on the surface to dissipate lightening strikes," explains Dr. Carl Zweben, industry consultant, Devon, PA. "The hope is that since carbon fibers are electrically-conductive and carbon nanotubes are electricallyconductive, you can make the inherent structure electrically-conductive enough to eliminate the copper mesh."

As for avoiding enemy detection in military aircraft, composites are normally visible to electromagnetic radiation, but can be seeded with appropriate materials to absorb radiation and divert its energy away from the source.

Four main types of problems may occur with the use of composites: delamination, disbond, debond and porosity. Delamination is a splitting apart of the plies in the composite material. Disbonding is when a core material has become detatched in an area from a face sheet, while debonding occurs when the material was never bonded in the first place. Porosity leads to tiny bubbles in the material. Each of these defects could lead to serious problems if not addressed.

Shearography testing reveals defects in carbon composite materials.

To keep these problems from occuring, a process called shearography testing has been developed at Laser Technology, Inc. This testing method uses a common path interferometer to image the first derivative of the out-of-plane defect in response to a change in load.

Current common test procedures include ultrasonics, where the operator scans a part with a single probe, building up the image over time. Ultrasonic systems for aerospace applications generally run at about 10ft2 per hour, according to Newman. With shearography, operators are able to run at about 500ft2 per hour.

"Shearography non-destructive testing helps by being able to provide an inspection capability for a lot of unique applications where conventional technologies don't work," Newman says. "It really brings a very important capability for inspection of composite materials at a lower cost and much higher throughput, allowing operators to test parts during manufacturing and prepare them or scrap them at the least amount of cost."

This method can help manufacturers to drive down costs and find problems with processes, enabling them to improve that process and further lower costs.

Another area of composite material development is toward what are called multifunctional composite structures and materials, which incorporate electronic sensors, actuators or in some cases, microprocessors. Currently, all electronics are kept in a box, and the box is screwed or bonded to a structure.

"Now the trend is to incorporate the electronics into the structure, and that saves the weight of the box, it saves the volume of the box, and it is just a more efficient way to do business," Zweben says.

While polymer matrix composites are the standard, ceramic matrix composites are an up-and-coming development for high-temperature aerospace applications, but use of these materials is plagued with many challenges, according to Zweben.

"There is a lot of interest in ceramic composites for hightemperature jet engine applications," Zweben explains. "Ceramic composites are the most complex and problem-filled materials because you get chemical reactions, and basically if you want to use them in high temperatures, you have to worry about oxidation and reactions between the environment, fibers, matrix and coatings."

Ceramic composites boast a lower density than most other composites, but unreinforced ceramics are extremely brittle and susceptible to mechanical and thermal shock.

Europe currently leads the way in R&D and application of ceramic composites. The U.S. is ramping up efforts and research to perfect the utilization of ceramic composites.

"Ceramics are too brittle to use as structure materials, but if you reinforce them with continuous fibers, which is what they do in the case of a ceramic matrix composite, then you can have a useful ceramic structure material, which is kind of the Holy Grail for jet engine applications," Zweben continues.

As aircraft such as the Boeing 787 and the Airbus A380 find innovative ways to use fuel-saving, cost-effective composite materials, they are setting the standard for the future of aircraft design. Composites are proving to be reliable, efficient and weight-saving, and show improved performance over metals in many applications.

Metals/Alloys

To safely interact and supplement the new composite materials that are quickly taking over the aerospace industry, aluminum structures are being replaced with metal alloys such as titanium and nickel-based superalloys in areas where composites are not appropriate, such as applications requiring extremely high strength-to-weight ratios.

Alloys are metals formulated for specific applications. Nickel-, cobalt- , and iron-based superalloys provide high strength and corrosion resistance at elevated temperatures, making them ideal for hot applications in jet engines. Certain alloys have great strength and stiffness and are used for landing gear and frame structures.

Most of the alloys used in aerospace applications are nickel-based, although some have cobalt or iron as the predominant element. These alloys are used in high-temperature environments when extremely high strength is necessary. Applications also include compressor blades, vanes, spacers and discs; shafts; casings and rings; and sheet components such as combustion chambers, ducting, exhaust systems, afterburners and thrust reversers.

Inconel, a trademarked group of alloys used in a variety of industries, is renowned for its structural integrity in high-temperature atmospheres, as well as its resistance to oxidation. Inconel is used in applications that require a material that won't give in to caustic corrosion or stress-corrosion cracking.

Inconel 625, specifically, is a material used in aerospace applications because it is made up of large amounts of nickel, chromium and molybdenum, plus an addition of niobium. This makeup ensures high levels of strength without having to go through a strengthening heat treatment. The material is also effective at resisting crevice corrosion.

Some companies are dedicated to creating alloys based on customers' needs, continuously adapting to the industry and adjusting the makeup to produce next-generation materials for aerospace applications. For example, ATI Allvac, an Allegheny Technologies company, produces nickel-base and cobalt-base superalloys, titanium-base alloys, and specialty steels for the aerospace industry. One example of their materials is the ATI 718Plus alloy.

ATI 718Plus is a nickel-based alloy that is precipitation-hardened. It displays extremely high-temperature properties and good fabricability, making it ideal for jet engine rotating components. The high temperature capability and thermal stability is accomplished by double- or triple-vacuum melting, depending on the application. This process also ensures micro-cleanliness and tight compositional control. Triple melting then further reduces the risk of macrosegregation.

The most common of these alloys in aerospace applications is the titanium group. The aerospace industry is the largest single consumer of titanium. The main types of titanium alloys in aircraft are Ti-6Al-4V (Ti 6-4), Ti-10V-2Fe-3Al (Ti 10-2-3), and, increasingly, Ti-5Al-5V-5Mo-3Cr (Ti5553).

"Ti 6Al4V is the workhorse of the titanium industry," explains Dan Cooper, account executive at MAG Maintenance Technologies. "Probably 75% to 80% of the titanium is 6-4. The whole idea with Ti5553 is that it's stronger material pound for pound than the 6-4, so you can reduce the weight by having smaller walls, thinner and lighter parts, because you can use less material and get the same strength as the 6-4."

Ti 6-4 is really a general-purpose titanium in the aerospace industry. Ti 10-2-3 is an alloy that is used on landing gears and highstress areas. Ti-5553 is a patented material from VSMPO that is for high-strength areas and places where weight reduction is necessary. Although Ti-5553 seems to be the solution to titanium needs, it comes with some setbacks.

Titanium's coefficient of thermal expansion is around half that of stainless steel and copper, and 1/3 that of aluminum. Its density is 60% of steel's, half of copper's and 1/7 of aluminum's. The modulus of elasticity is half that of stainless steel, making it durable and shock resistant.

Ti-5553 is notorious for being difficult to cut efficiently. The material is tough, and has a tendency to develop high amounts of chatter, leading to extremely short tool life when run at high speeds. Therefore, titanium parts must be run at low speeds on a rigid, tough machine with specialty tools.

"The problem with Ti-5553 is that the inserted carbide tools don't respond well; it chips very easily with Ti-5553," Cooper says. "If you use carbide you get very poor tool life. A solution we offer at MAG Maintenance Technologies are high-speed cobalt cutters, which are much tougher at the cutting edge."

Another disadvantage of using Ti-5553 is the cost. The material is only produced in Russia; therefore, there is a run-up in price of four to five times the tooling cost of most machined materials.

Five-spindle machines are one way to efficiently machine titanium. The problem is finding titanium spindles in the industry, as most are for aluminum parts. Photo courtesy of MAG Cincinnati.

"The greatest advantage of Ti-5553 is strength per pound," Cooper says. "You can make an aluminum component close to what titanium is, but it's the weight difference. Titanium is the strongest material in the industry compared to aluminum and steel in terms of fatigue strength."

Titanium and other specialty alloys are addressing the issues of corrosion resistance when coupled with composites, high strength-to-weight ratios, high thermal and high stress capabilities, and fatigue strength. All of these things lead to less expensive, safer and more efficient aircraft.

"The two most important things in the aircraft industry are, number one, to make it more comfortable for the passenger by decreasing the cabin pressure and increasing humidity, and number two, reducing cost," Cooper says. "If you can save money by getting fuel costs down, it pays for itself and they're going to do that."

The future of the aerospace industry lies with advanced materials such as composites and alloys that maintain structural integrity as well as a high strength-to-weight ratio. As these materials become available, the manufacturing industry must answer the call by adapting new practices and new equipment, as well as educating themselves in the latest methods of machining or producing parts from these materials. To ignore these "trends" is to deny the future of aviation.