Aircraft/Aerospace. (Casting Market Focus).

Metal castings have played a vital role in aircraft from the very dawn of the industry. After all, Orville and Wilbur Wright's first machine-powered airplane featured aluminum-copper block and crankcase castings when their craft lifted off the beaches of North Carolina in 1903.

From those modest beginnings, castings have been selected to serve a variety of power and structural needs on aircraft. Beyond internal and external engine components, castings also serve as the basis for applications such as thrust-reversers, cockpit controls, instruments stands and environmental systems, not to mention main cabin applications such as food trays and overhead storage frames. Furthermore, you also will find state-of-the-art casting technology on the Space Shuttle, which utilizes 17 cast metal components in its main engine alone.

Conversions

Despite castings' long history, William Perry, a longtime veteran of Howmet and currently executive director of the Investment Casting Institute, Montvale, New Jersey, sees continued potential for additional casting utilization. "Aircraft have forever been made of built-up, welded and studded assemblies, which can be good and cost-effective," said Perry. "But when subassemblies or machined forgings are costed against the casting process, engineers are finding that they can take thousands of dollars out of individual components. The effort made in converting to castings, for those who commit to it, has a huge, huge payback."

This is a point that Boeing Commercial Aircraft Group CEO Harry Stonecipher made in a videotape he sent to engineers in the late 1990s, encouraging more cast component use in place of forgings, assemblies and fabricated parts. Boeing, he stressed, could achieve a 25-35% cost reduction in tooling alone by converting an assembly to a cast component. With more than 3 million components (excluding rivets and fasteners) typically assembled into a single airplane, each casting consolidation offers the potential for significant savings in tooling, inventory, labor, rework, materials, design, testing and manufacturing.

As Howmet was awarded an $8 million contract to supply new side-of-body and aileron titanium casting contracts (including the all-new arresting gear) for the F-22 Raptor jet fighter in late January, William Lahavich, Howmet's director of airframe sales/marketing, said a growing number of airframe components are being converted to castings. "Airframers will not proceed with a conversion unless cost savings of 30% or more are realized, and it is typical to realize cycle-time advantages of 50%."

Cost and speed are only two factors accelerating the conversion to castings, he said. Others include advances in casting process control, inspection techniques and end-product predictability, all of which enhance product quality. "We are making inroads with customers on design allowables, which permit the use of castings in fracture-critical applications," said Lahavich.

While casting quality is at a high level today, that was not always the case. Memories of poor performance as far back as four decades ago still pose an obstacle to wider casting utilization today. Because of poor quality at the time (which has been resolved through process controls and newer computer modeling tools for melt chemistry, solidification control and optimum gating), a "casting factor" was written into aviation regulations in the 1960s. This conservative safety factor typically adds mass, posing a disincentive to casting use in an industry in which weight is of vital importance.

According to Tim Scoville, Boeing Commercial Aircraft Group's Product Development Div., the highly politicized casting factor appears to finally be trotting off into the sunset. "Advances in foundry practices over the last 20 years have shown a consistency of properties that rival wrought alloys and forged components. We believe that the casting factor is becoming obsolete because of the greater certainty in the output of the casting process today. The movement away from casting factors of 1.25-1.5 reflect the quality of today's foundry."

In addition to the well-publicized benefits of castings, Perry mentioned another significant point for the aircraft industry--temperature. "There are casting components in jet engines in which the operating temperature of a section exceeds the part's melting point. This is achievable through internal cooling--possible through casting's internal passageways--and the application of special coatings to the castings. Higher temperature applications dictate exotic materials, which favors metalcasting."

Landmark Applications

Scoville highlighted three applications as "tide-turners" in aircraft history (Fig. 1). One was Boeing's $1.6 billion Air Launch Cruise Missile (ALCM) program in 1980--an unlikely program for showcasing cast aerospace structures.

Designed for launch from a B-52 or B-1, this 21-ft missile body's original production concept was to machine 46 in. segments from forged block and weld them together. After it was clear that the concept was time-consuming, defect-ridden and wasteful, Alcoa found a way to cast the entire missile body. This resulted in a radically different approach to missile manufacture that allowed Boeing's necessary production rates at a low expense. The photo in Fig. 1 shows a cutaway of four castings making up the fuselage. The largest of the castings is 65 in. long with nominal wall thickness of 0.15 in.

On the success of the ALCM program and the 1700 missiles built, Boeing reported that the 65% curve improvement was unheard of at the time. "The fact that castings were considered for such a high portion of this cruise missile was very significant," said Scoville.

Another impressive casting was the EE access door for the Boeing 757, which eliminated 113 parts, 940 fasteners and reduced cost by 35% over the previous fabricated design. Produced by Hitchcock Industries for Vought Aircraft Industries the 28 x 22 x 4 in. D357 casting also featured an integral, monolithic cast skin.

According to Scoville, this is one of the most severe applications, as a failure would be catastrophic. The casting process proved through static, fatigue and damage tolerance tests that it could meet this critical application, however. It meets 50 ksi, 40 ksi and 4% elongation requirements in critical areas.

A third landmark, said Scoville, was the F-16 inlet lip casting that resulted in the only published static allowables for aluminum casting materials and the first use of a casting factor of only 1. Also produced by Hitchcock, the 57 x 21 x 14 in., 53-lb casting met 50 ksi, 40 ksi and 5% elongation properties in critical areas. It served as the leading edge of the modular common inlet duct for the F-16.

Perry noted two additional "bellwether" casting applications. One was the V-22 transmission adapter on the Osprey, which features cast-to-size gear teeth, the ability to handle a tremendous load and the replacement of hundreds of details previously machined and welded together. The result was a savings on the order of thousands of dollars--per component. "I have not seen another part of this class that so maximized the investment casting process," he said.

Another vital application was the side-of-body parts on the original F-22, he said. These components hold the wings of the plane together, demonstrating great confidence in castings as safety-critical parts.

Future Trends

Asked about the future of casting utilization in aircraft industry, Perry said, "Certainly the parts are getting larger, more complex and feature thinner walls than ever before. Engineering is beginning to push the envelope on the process and is utilizing aluminum and titanium in some very exciting applications."

Scoville summed up castings' promise. "Castings impose fewer restrictions on a designer's creativity than any other manufacturing process. There are many opportunities to use castings for a real benefit, and increased complexity doesn't necessarily mean higher costs."

He also added that Boeing engineers are looking for opportunities to convert hog-outs and assemblies on existing aircraft and then reviewing the business case for cost reduction.

This article was adapted from one that originally appeared in the Spring 2002 edition of Engineered Casting Solutions.

For a free copy of this article circle No. 344 on the Reader Action Card.

For More Information

"Casting Conversions Fly to the Forefront at Boeing," A. T. Spada, p. 27-32, Engineered Casting Solutions, Fall 2000.

"Castings Propel Space Shuttle Improvements," M.J. Lessiter, R.L. Rosmait, p. 26-30, Engineered Casting Solutions, Winter 1999.

Design and Procurement of High-Strength Structural Aluminum Components, Aluminum Div. Premium Casting Committee (2-D), American Foundry Society, Des Plaines, IL (1995).

RELATED ARTICLE: Facts & Figures

According to Stratecasts, Ft. Myers, Florida, 106,000 tons of castings were consumed by the aerospace products and parts manufacturing market (NAICS 3364) in 2001. Aluminum castings represented 83,000 tons, with investment cast blades/vanes, aircraft and missile components making up the balance. The value of casting shipments to this market sector in '01 is estimated at $3.01 billion.

Stratecasts expects the industrial growth pattern for aircraft/aerospace to grow 2% annually over the next 10 years from '01 levels. Aluminum casting usage is forecast to grow to 100,000 tons by 2011, though it will be slowed somewhat by substitutions from titanium and other investment castings. By 2008, the high-value investment cast blades and vanes (superalloys) are expected to reach a new plateau that is 50% higher than last year's shipments.

At 91 in. long, this 330-lb titanium thrust beam for a satellite launch vehicle is the longest titanium casting in aerospace history. Produced via investment casting by Pacific Cast Technologies, this part was converted from an aluminum forging without any additional casting factor. Its fracture toughness is 30% greater than the forging, and its weight savings (1000 lb per vehicle) allows bigger payloads (12.2 tons).

This landing gear uplock door support holds the main leading gear door closed on the Boeing 767. Converted from a sheet metal assembly to a one-piece 34-lb, D357 aluminum casting (produced at Hitchcock), it has wall thickness of 0.08 in. in some areas. Total component costs were reduced by 50% as 35 parts, 25 shims and 2 assembly jigs were eliminated.

This 3-lb, 10-in.-long volute housing, produced by Vestshell, is part of a turbo pump assembly for a military aircraft engine. A modified stainless steel investment casting was selected in order to meet requirements for extreme temperatures, high vibration, thin walls (0.060 in.) and zero defects.

The selective laser sintering rapid prototyping process helped Clinkenbeard & Assoc. and Solidiform create a single sand core (I) that allowed the delivery of the A356 aluminum fuel control system casting (r) to the Aircraft Controls Div. of Woodward Governor Co. According to WGC, which needed a production configuration to demonstrate the new fuel control concept, the part was received in one-half the time of conventional methods while saving more than 80% over traditional tooling costs.

Previously fabricated, this weapons pylon was converted to a one-piece casting by Hitchcock Industries. This 67-lb, D357 alloy casting (95 in. long) provided a three-fold cost savings while significantly reducing manufacturing leadtimes.

This outboard overhead stow bin end frame (with 0.085-in, nominal walls) was cast in A357 via the tilt-pour semi-permanent mold casting process at Progress Casting Corp. Designed to withstand a 9-6 impact force, this design replaced a 17-component stamp assembly.

Hitchcock Industries converted this in-flight refueling trough used on military aircraft from a fabrication to a casting. This 25-lb, 58-in. D357 alloy casting allowed a reduced part count, the elimination of drawings and parts lists, and dramatically reduced the use of fasteners. A 3:1 cost savings was realized over the previous fabrication.

This 52-in, titanium investment cast fan frame hub casting supports the front fan section of a General Electric engine on numerous Boeing aircraft. Produced by Precision Castparts Corp., this single casting replaced 88 stainless steel parts that previously required much machining and welding. In addition to a 55% weight reduction and better strength and dimensional control, the conversion allowed the integration of numerous unique details.

A new cockpit main instrument panel design for Boeing's 767 airplane reduced 296 parts to 11 A357 investment castings. The project reduced cost by 50%, assembly time by 160 hr, tool count by 90% and fastener counts by 600. Pictured are three of the castings in the assembly, produced by Citation Precision Casting.