Materials Research Agenda for the Automobile and Aircraft Industries (1993)

General and specific research opportunities were determined for the civil aircraft industry using the HSCT (high-speed civil transport) as a basis for analysis. Advanced-materials developments within the context of military aircraft are also occasionally discussed in this chapter, because much of the advanced work is done in this area. Within classes of materials-matrix composites (e.g., polymer, metal, and ceramic), there are a variety of specific materials that could be listed. In general, these specific systems were not listed because the system of choice is not currently known. In the few cases where a specific system is favored, it has been identified and discussed.

Innovative materials research and engineering is essential to achieve the high-strength, heat-resistant, lightweight structures required in advanced subsonic and supersonic aircraft. However, as discussed in Chapters 1 and 2, the drivers for technology implementation are primarily economic, not scientific. The barriers to the introduction of new materials include their unusual characteristics that may demand new design methods, the absence of

an industrial base to supply the potential need (production applications require production material sources), their cost, and the learning curve necessary to establish their life-cycle characteristics.

Despite the conservative nature of the civil aircraft industry, new materials have been introduced, as demonstrated in applications during the past 20 years. Such materials include graphite-epoxy composites, aluminum-lithium alloys, engineering microtextured materials, and single-crystal turbine blades. These efforts have required teaming relationships among inventors, suppliers, and users. In terms of application, a phased approach has been shown to work best. However, if advanced materials are to be successfully introduced, a more comprehensive materials-system approach must be developed that incorporates materials modeling and processing considerations, and that allows designers to interact even more closely with materials developers to ensure the proper application of new materials.

Processing and fabrication play critical roles in the utilization of materials. There are also clear opportunities for research on processing in all categories of advanced materials. Indeed, the properties of the advanced material-system candidates are defined by the processes used in their manufacture. Changes in processes can redefine or open new avenues of materials advances as has happened through the application of rapid solidification and melt-spray technologies.

No advanced material will be used, no matter how desirable its properties, if processing and manufacture cannot be performed efficiently and economically. Less than 20 percent of product cost is attributable to design, development, and analysis in a successful aircraft program, while more than 60 percent of the cost is due to manufacturing. There is currently some disconnect between materials, design, and manufacturing operations that together are charged with providing reproducible, cost-effective products. To resolve these

disconnects, processing considerations must be included early in the design cycle and production planning of new aircraft.

Another cost and time savings approach that offers potential within the next 20 years to reduce significantly the cycle time required for the introduction of new materials is the use of high-speed computational analysis that has been experimentally validated. One could envision the use of expert systems to help the designer choose appropriate materials and processing techniques for the production of aircraft components.

The materials requirements of the airframe and engine systems of aircraft are sufficiently distinct to permit them to be discussed separately. Despite this separation, there are two unifying themes. The first is the need to focus on the mission requirement of future aircraft systems. By understanding customer requirements, materials solutions that will contribute to the mission requirement can be better devised. A second theme is addressing these goals through a team approach.

Anisotropic materials will increasingly become the construction materials of tomorrow's products. This is especially the case in the aircraft industry, where many of the candidate materials systems are composites. Increasing numbers of engineers are required who are properly educated to respond to the problems associated with the synthesis of such materials, their long-term thermostructural and thermochemical stabilities, their design, and their application. Because of the special challenge of the application of materials at extreme temperatures, specialists with expertise in high-temperature

thermochemistry and the synthesis of materials for such environments will be required.

In modern, large civil aircraft, aluminum still provides approximately 75–80 percent by weight of the structural materials. Composites have not made the advances into primary structures of civil aircraft that were anticipated 20 years ago, predominantly because of the higher cost of manufacture, the lack of a design base, and the absence of a generally accepted life-prediction methodology. The projections in materials applications in subsonic commercial airplanes are shown in Figure 4-1. Greater progress with composite materials has been made in military aircraft. For example, composites accounted for approximately 50 percent of the structural weight of the prototype advanced tactical fighter (F-22) built by a team from Lockheed and Boeing (Peterson, 1991). The aircraft is projected to contain 35 to 40 percent composites when it goes into production during the latter part of this decade.

The structural materials requirements for the Mach 2.4 HSCT airplane would require materials that could withstand long-term operation at 350°F and very short-term operation at 400°F. However, decreasing the speed from Mach 2.4 to 2.0 has a large effect in reducing surface temperature requirements. At the lower speed, the airplane would need materials able to survive 220°F long-term and 275°F for very short-term exposures. Over the service lifetime of 25 years, the materials would be subjected to some 35,000 thermal cycles.

The candidate polymeric composites that can survive the conditions of a Mach 2.4 HSCT airplane for certain structural applications include the carbon-fiber reinforced thermoplastics and thermosets. Cyanate esters and toughened bismaleimides are the

FIGURE 4-1: Material weight-distribution projections for subsonic commercial airplanes (Source: Boeing Commercial Airplane Group).

currently favored matrices. Polyimide systems offer the requisite temperature capability, but their application is seriously limited by their processibility. Candidate metal systems are more thermally resistant; powder metallurgy aluminum alloys and the alloys of titanium are the most likely candidates. Selective use of aluminum and titanium-base metal-matrix composites may be required, but applications could be limited because of high cost. Currently, for certain small-size components, ceramic-matrix composites are also successfully competing with polymer-matrix composites at temperatures of 600°F.

As suggested in Figure 4-2, the takeoff gross weight favors applications of polymeric composites; however, technical and manufacturing uncertainties prevent the elimination of the metal-alloy candidates. Detailed design considerations are required to rank

FIGURE 4-2: Airplane size projections for the HSCT (Source: Boeing Commercial Airplane Group).

materials and determine where research should be focused.

The ability to achieve lighter-weight and more fatigue-resistant aircraft structures by adhesive bonding in place of riveting was convincingly demonstrated in the Primary Adhesively Bonded Structures Technology program, conducted by McDonnell Douglas for the Air Force (Hart-Smith, 1981). For successful application, the system for adhesive bonding must be durable in the thermal and chemical environment to which it is exposed. Strong, structural adhesives that are suitable for the majority of the aluminum structures of subsonic aircraft have been identified. Systems with greater thermal capability, as required in supersonic aircraft, compromise

bond strengths at lower temperatures. Improved processing methods and adhesive systems are required for the metals and composites being considered for supersonic aircraft.

A chance to change radically the approach to the construction of aircraft is also provided by the application of welding as a replacement to riveted or bonded lapped joints. Such a change should lessen the weight of a metal airframe and improve the attractiveness of metal constructs versus polymer composites. While the current high-strength (e.g., 2024 and 7075) aluminum alloys used for skins are difficult to weld and have demonstrated poor joint efficiency, some of the newer lithium-containing aluminum alloys are weldable. Furthermore, the titanium alloys suitable for airframes of high-speed aircraft are quite weldable. However, the benefits of welding would be lost if additional material was necessary due to limitations imposed by the welding process. Therefore, the committee recommends focused research to address the technical problems, such as residual stress, joint efficiency and reliability, fatigue characteristics, and quality assurance, associated with the application of welding to advanced materials for airframes.

Another associated research opportunity is that of interface science. This is specifically identified since many of the candidate aircraft materials systems are composites. The opportunities of system improvements through adjustments that modify the nature of the interface between the reinforcing phase and the host matrix are immense. Application requires a high degree of thermochemical stability of such engineered microstructures.

In considering future materials needs for civil aircraft engines, the customer requirements for the first decade of the next century and the

leading edge technologies currently being tested by the defense industry must be examined. There will be thrust-improvement derivatives of current advanced engines for both military and subsonic civil aircraft. In addition to the powerplant for the twenty-first century air-superiority fighter, a new high-performance engine for the Navy's A-X aircraft may be developed. An advanced (ultra high bypass) subsonic engine is also anticipated for introduction by about 2010 (Williams, 1991).

By examining such system opportunities, the demands on high-temperature structural materials become apparent as suggested by Figure 4-3. Systems for subsonic aircraft have increased and will continue to increase their operating pressure ratios and their operating temperatures at the rear of the compressor (T3) and at the entry to the turbine (T41). The operational temperatures for rear-stage compressor components and first-stage turbine components of the HSCT are also indicated.

The materials technology required to improve engine efficiency can be summarized as follows:

• Thrust growth in existing engines can be achieved by the use of higher cycle temperatures (T3 and T41), which in turn requires materials with higher temperature capabilities.

• Improvements in the thrust-specific fuel consumption are sought through reduced cooling flow and higher thermal efficiency that directly translates into a requirement for higher-temperature-capability materials.

• Weight reductions that improve thrust to weight of existing or proposed engines can be sought through materials with superior strength-to-weight characteristics.

• Improved value materials that display lower overall life-cycle cost help to define an improved engine; these costs relate not only

FIGURE 4-3: Applications for high-temperature structural materials for aircraft engines (Source: Williams, 1991).

to the first cost but also to such issues as the durability of the material and its ability to be repaired.

Table 4-1 lists the material requirements for aircraft engines. This tabular summary progresses from advanced subsonic to military demonstrator to the HSCT engines, and from work that is evolutionary in nature and amenable to engineering program approaches through a set of goals that are a radical departure from industry experience. The latter goals require a suitably directed, highly speculative program of materials synthesis and development that might properly be sponsored on an industry-wide basis. The complexity of the materials innovations required for the HSCT engine will demand the attention of the most experienced and creative people in the profession, and even so, the outcome is uncertain. Worthy of special mention are the

ceramic-matrix composites. These composites may address certain of the requirements for materials that must operate in the medium-to-high temperature range, from 600 to 2600+°F. At the lower range of temperature, such materials may be competitive with the fiber-reinforced, polyimide-matrix composites, while at the upper range they compete with the capability offered by carbon-carbon composites.

In addition to these requirements that call for revolutionary materials for the HSCT engine, incremental developments must continue to support the upgrade of existing engines that are primarily dependent upon metals technology. Examples of such incremental approaches are modifications of nickel alloys to provide lower cost; columnar-grained turbine blades with superior oxidation resistance, single-crystal nickel-base alloys for air-cooled turbine-blade applications; dual-alloy turbine disks to permit growth in T3 and T41 temperatures; fine-grained and thin-walled castings for cost and performance benefits; and thermal barrier coatings for life extension and T41 growth. For select applications where thermal conductivity is important, as in turbine disks, metal-matrix composites may also provide useful solutions.

Process-development activities are quality and cost driven. Utilization and improvement of process controls that reduce the requirement for nondestructive evaluation are two such activities. Other examples are clean melting by plasma-arc and electron-beam processes, nickel-base forging billet preparation by spray deposition, and automated ply lay-up for polymer-matrix composites.

The increasing emphasis on using composites for solutions to the material requirements of advanced engines is shown in Figure 4-4. Advanced materials are viewed by domestic gas-turbine manufacturers as key technology discriminators. Failure by the U.S. commercial aircraft industry to achieve preeminence in this area would put the industry's competitive advantage at risk.

Materials development must be part of a comprehensive systems approach. Materials developers and designers must interact closely over a range of activities, including incorporation of processing considerations into the design process to mitigate the disconnects that occur during materials selection, design, and manufacture; greater

FIGURE 4-4: Engine-materials development activity trends showing increasing emphasis on composites (Williams, 1991).

emphasis on processing research for all categories of advanced materials to redefine or open new avenues of materials development; and development of materials-selection expert systems to help designers select appropriate materials and processing techniques for components.

Aluminum still accounts for 75–80 percent by weight of the structural materials in modern, large civil aircraft. To remain competitive, new advanced materials and techniques must be developed for airframes:

• Polymeric composites are prime candidates for airframe construction. Carbon-fiber reinforced thermoplastics and thermosets, involving the cyanate esters or toughened bismaleimides, are currently favored systems. Composite-research opportunities exist in the areas of (1) long-term thermostructural and thermochemical stabilization, synthesis, design, and application of anisotropic materials and (2) interface modifications between the reinforcing phase and the host matrix.

• The candidate, high-temperature metal systems are the (P/M) aluminum alloys and titanium alloys; selective use of aluminum-base and titanium-base metal-matrix composites may be warranted.

• Improved processing methods and systems for the bonding of metals and composites are needed. For instance, the replacement of riveting by welding in airframes requires further research of the technical problems of residual stress, joint efficiency and reliability, fatigue characteristics, and quality assurance. Candidate materials are the weldable lithium-containing aluminum alloys and the titanium alloys.

The materials research opportunities for aircraft engines are the development of materials with higher-temperature capabilities and thermal efficiency to withstand higher cycle temperatures and reduced cooling flow, superior strength-to-weight characteristics to permit weight reductions, and improved value to reduce overall life-cycle cost. The incremental research opportunities include the development of lower-cost nickel alloys, columnar grained turbine-blades with superior oxidation resistance, single-crystal nickel-base alloys for air-cooled turbine-blade applications, dual-alloy turbine disks for higher temperatures, fine-grained and thin-walled castings for cost and performance benefits, and thermal barrier coatings for life extension.

The ceramic-matrix composites offer a potential for revolutionary changes in propulsion systems. Fiber-reinforced ceramic-matrix composites potentially address a broad range of application temperatures: 600 to 2600+°F. Means to reinforce the ceramic matrices with fiber for higher temperatures is especially challenging, as is the approach to low-cost processing. The benefits afforded by these systems appear to justify the risk associated in their development.