F-22 Raptor Materials and Processes
Validating structural materials is especially important to the F-22 because new material technologies were incorporated to maximize aircraft performance. The overall percentage of composites in the F-22 (approximately 25%) is historically high, though not unprecedented. However, the extensive application of Resin Transfer Molding (RTM) technology and high temperature bismaleimide (BMI) composite materials directly resulted in the high weight/performance efficiency the Raptor demonstrates. The use of metallics technologies such as titanium Hot Isostatic Pressed (HIP) castings and electron beam welding allowed the airframe designers to incorporate complex features into a single component without the weight of fastened assemblies. The continuing challenge is to reduce material and component costs through a constant reassessment of emerging technologies. Recently developed machining technologies, for instance, have allowed the inlet canted frame lip to be changed from a casting to a lower cost machined component with no appreciable weight penalty.
Traditional aircraft materials such as aluminum and steel make up about 1/5 of the F-22's structure by weight. The high performance capabilities of the F-22 requires the significant use of titanium (42 % of all structural materials by weight) and composite materials (24 % by weight), which are both stronger and lighter weight than traditional materials, and offer better protection against corrosion. Titanium also offers higher temperature resistance.
(Current F-22 Weight Distribution)
Titanium 64 (Ti-64)
Titanium 62222 (Ti-62222)
* Other materials include coatings, paint, transparency, integrated forebody (radome), tires, brakes, sealants, adhesives, seals, actuators, gases, and fluids.
The types of titanium are different alloys and have different applications on the F-22. Ti-62222 is a very high-strength alloy that was introduced on the F-22.
On the F-22, the number of parts made from thermoset composites is approximately a 50/50 split between epoxy resin parts and bismaleimide (BMI) parts. The aircraft's exterior skins are all BMI, which offer high strength and high temperature resistance.
Thermoplastic composites are also highly durable materials but, unlike thermosets, thermoplastics can be reheated and re-formed. Thermoplastics proved more expensive and more difficult to incorporate in the F-22 than had been hoped in the early days of the program.
Thermoplastics are being used on the F-22 for items such as landing gear and weapons bay doors (which are opened frequently), where impact damage tolerance (to things such as small rocks that are kicked up from the runway, etc.) is required.
The Airmet 100 steel alloy used in the F-22's main landing gear is another innovation. It is one of the first applications of a special heat treatment of steel, which provides greater corrosion protection to the main gear piston axle.
Hot Isostatic Pressing (HIP) casting is a process where metallic castings are subjected to very high temperatures in a static pressure environment (more than 10,000 pounds per square inch). The effect is to collapse, or "heal", voids (gas pockets) that otherwise may be present. On the F-22, structural titanium castings are HIP'ed to eliminate any voids that are present from the casting process.
HIP casting is used on six large structures on the F-22: the rudder actuator housing (one for each rudder), the canopy deck, the wing side-of-body (SOB) forward and aft fittings (four total, two for each wing), the aileron strongback (one for each aileron, two total), and the inlet canted frame (one each for the left and right inlets).
Resin Transfer Molding (RTM)
The F-22 is the first aircraft to take advantage of Resin Transfer Molding (RTM) of composite parts. RTM is a method of composite parts fabrication well suited to economically fabricating complex shaped details repeatedly to tight dimensional tolerances.
Large composite parts traditionally are formed by applying and pressurizing hundreds of layers of fabric that contain a pre-embedded resin, and curing, or 'baking,' them in an autoclave. This is a very time consuming and labor intensive process.
The process employs fibrous "preforms" that are formed under vacuum from stacks of fabric and placed in metal tooling that matches the shape of the part. The tool is then injected with heated resin under pressure. The benefit of the matched metal tooling to RTM is a high level of part reproducibility, consistency in assembly operations, and consequently, economies of scale.
RTM is used to fabricate more than 400 parts for the F-22's structure ranging from inlet lip edges to load-bearing sine-wave spars in the fighter's wings. At Boeing, RTM has reduced the cost of wing spars by 20 percent and has cut in half the number of reinforcement parts needed for installing the spars in the wings. Both BMI and epoxy parts are fabricated using RTM.
The composite pivot shaft is an application of Automated Fiber Placement (AFP) technology, employed with unique tooling approaches to incorporate a composite structure in place of a titanium one in a flight-critical application - the F-22's horizontal stabilizers.
AFP technology makes possible the exact fiber positioning required to achieve the complex geometry of the pivot shaft, which is a 10-inch diameter cylinder at one end; and a rectangular spar at the other approximately four inches wide; with a offset in the transition area. Its shape can be likened to that of an oversized hockey stick.
Alliant Techsystems is the contractor for the composite pivot shaft, which is laid out using computerized fiber placement machines. The pivot shaft is composed of more than 400 plys (layers) of composite tow tapes ranging from 1/8 of an inch wide to 1/2 inch wide.
The shaft is cured in stages to prevent internal cracking and no wrinkles, as there is no allowances for voids in the shaft. After layup, the shafts are nondestructively inspected and tested.
The composite pivot shafts take up to 60 days to produce, but they save 90 pounds per shipset (two shafts) over titanium, which is an extremely large amount of weight to take out of an aircraft at one time. Also, because of the high temperatures in the engine bay area of the fighter, it is constructed mostly of titanium, and any weight is difficult to engineer out of that area.
When the first F-22 was rolled out in April 1997, four shipsets of flightworthy composite pivot shafts had already been produced. A plan is in place to use thicker tow tapes, which should greatly reduce production time for the shafts.
An automated process called electron beam (EB) welding is helping Boeing and Aerojet, its supplier, build lighter-weight titanium assemblies for the aft fuselage. EB welding takes place in a vacuum chamber and uses a stream of electrons to weld titanium parts together.
Performing the welding in a vacuum prevents exposure to oxygen, which can create an undesirable brittle surface during the process. Electron beam welding is able to weld thick titanium parts (i.e., more than an inch) considerably better than other methods.
Electron beam welding reduces the need for fasteners in some fuselage components by up to 75%, which reduces weight, simplifies the assembly process, and avoids the costs associated with fasteners. The reduction in the number of fasteners also means fewer openings for possible fuel leaks.
F-22 HAZARDOUS MATERIALS (HAZMAT) PROGRAM
The F-22 is one of the first weapons development programs to incorporate contractual requirements for hazardous materials (HAZMAT) use and pollution prevention in manufacture, operation, maintenance, support, and disposal over the life cycle of the weapon system.
The F-22 Weapon System Hazardous Materials Analysis Report (WSHMAR) was developed to provide a basis of understanding between the contractors and the Air Force to ensure that adequate consideration was and continues to be given to the elimination, minimization, and mitigation of hazardous materials, as well as environmental, safety, and health (ESH) concerns and the compliance of hazardous materials.
The F-22 HAZMAT Program approach consists of these processes:
Identification and Tracking: Hazardous materials are identified and selected hazardous materials are targeted for elimination, minimization, or mitigation efforts.
Materials Evaluation and Materials Decision: Hazardous materials selected for use by the various Integrated Product Teams (IPTs) are first evaluated through the HAZMAT Program (which included coordination with Materials & Processes and Corrosion Control) before inclusion, and the use of these materials is continuously monitored.
Reporting and Documentation: Hazardous materials data is collected and recorded.
Information Dissemination: Hazardous materials emergency, disposal, handling, storage, repair, and transportation information is collected and reported.
Included in the Hazardous Materials Database (HMDB) is information on materials delivered as part of the F-22 weapons system (which includes the air vehicle, training system, and support system) and end items that create a hazard in post-delivery operations and/or are disposed of as hazardous waste, as well as materials used in operation, maintenance, and support of the entire weapons system.
Once it is determined which HAZMAT materials will be used on the F-22, mitigation factors are introduced into design and maintenance procedures to reduce exposure risk and maintain the risk at an acceptable level.
The HAZMAT program has been very successful, eliminating or greatly minimizing a significant number of hazardous materials on the F-22.
Ozone Depleting Substances: All ozone depleting substances except one has been eliminated. The remaining substance, Halon 1301, is used in the aircraft's fire protection system. The Air Force is working on finding an acceptable substitute for this substance.
Cadmium: Cadmium, a material long used for corrosion protection, is being minimized on the F-22's landing gear. There is a small amount on the gear now, and by the time the tenth aircraft (the first production aircraft) is built, cadmium will be significantly reduced.
Other Substances: Other substances such as Volatile Organic Compounds (VOCs) and isocyanates (used in the aircraft's topcoats) have been greatly reduced, as has chrome (used in sealants). Work continues on minimizing methyl ethyl ketone (used in wipe solvents) and methylene dianiline (used in adhesives).
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