Pratt, Alcoa Pioneer Use of Aluminum Fan Blades
Pratt & Whitney and Alcoa recently have revealed that the fan blades of the PW1000G family of geared turbofans will consist mainly of aluminum alloy–an industry first. Although the PW1500G for the Bombardier CSeries–the first member of the engine family–won certification last year, Pratt & Whitney had so far only loosely referred to a “hybrid-metallic” fan blade. Metal engineering company Alcoa now calls using aluminum in a fan blade “revolutionary” due to the material’s ability to combine light weight with affordability. The two companies have just announced a 10-year, $1.1 billion agreement for “key engine parts,” including aluminum fan blades.
Pratt & Whitney designed the geared turbofan to run the fan and the low-pressure turbine each at its aerodynamically optimal speed. The fan turns more slowly than the LP turbine, at about one third of the rotation speed of a conventional fan. In turn, the tensile strength requirement decreases sharply, from 200,000- to 400,000 psi to 100,000- to 150,000 psi, allowing engineers to reconsider the material used in the fan.
“This range of tensile strength is compatible with our new, high-strength, very stiff and fatigue-resistant alloys,” Alcoa COO Eric Roegner told AIN. Previously, titanium and carbon-fiber composites presented the only options. Carbon fiber weighs less that titanium but costs much more, noted Roegner.
Compared with carbon fiber, Alcoa’s new aluminum alloys are significantly cheaper, he continued. Designers also found a 2-percent benefit in aerodynamic efficiency, thanks to the thinner trailing edge. Alcoa’s alloys also cost and weigh considerable less than titanium, which is 56 percent more dense than aluminum.
“Combining Alcoa’s proprietary alloys and unique manufacturing processes with Pratt & Whitney’s design, we cracked the code on forging an aluminum fan blade,” Alcoa chairman and CEO Klaus Kleinfeld said. Roegner explained that the manufacturing process “has developed the most advanced microstructure in an aluminum forging,” resulting in higher tensile strength and better fatigue resistance.
Alcoa provides various alloys depending on the application (five manufacturers have chosen the PW1000 family for their new airplanes). Some belong to the relatively conventional 7000 series, while others use aluminum-lithium. All fan blades use titanium in the leading edge to cope with water erosion due to cloud droplets, Roegner explained.
Pratt & Whitney and Alcoa have worked on the new fan blade material since 2009. “Pratt intentionally kept it quiet,” Roegner noted. Every certification and flight-test engine for Bombardier, Airbus and Mitsubishi has used Alcoa’s forged blades.