General Electric is about to begin engine testing of new nickel superalloys designed to reduce its use of rhenium, a rare metal that has increased 10-fold in price in recent years as demand has increased.
Engine testing is the final step towards approving the new materials for introduction into the GE90 and CFM56 turbofans later this year. Use in other commercial and military engines will follow, says GE Aviation.
Development of the new alloys is part of a two-pronged approach by GE to reduce its rhenium consumption. The other involves recycling high-pressure turbine blades made from nickel superalloys containing the element.
GE launched a scrap reclamation programme at its own service centres a year ago, reducing its need for new rhenium by 1% in 2007. The initiative is now being extended to customers, with Turkish Airlines the first to sign up.
First used in turbine blades 20 years ago, rhenium increases creep strength of superalloys, enabling them to withstand higher temperatures. The metal is a byproduct of copper production, and extremely rare.
"It's occurrence in the Earth's crust is two parts per billion. In association with copper it reaches hundreds of parts per billion," says GE's materials head Bob Schafrik. On average it takes 120t of copper ore to produce 30 grams (1oz) of rhenium, says GE. Prices are approaching $10,000/kg ($4,500/lb), up almost 10-fold from 2005 as demand has increased.
"There isn't a shortage at the moment, but as we look into the future we see engine production rates increasing and other uses coming on line," says Schafrik. Rhenium is used as a catalyst in the oil and gas industry, and also in rocket motors, he says.
The two new superalloys - one with half the rhenium of current materials and one with none, both with the same or better properties - is the result of a two-year effort. "We had a huge database of what is possible in superalloys, so we went back to that body of knowledge and combined it with our new design tools," says Schafrik.
The low-rhenium alloy is used for turbine blades, which must withstand high stress with high durability, while the rhenium-free alloy is used for static hardware such as nozzles and shrouds, says GE. Mechanical testing of properties and demonstration of manufacturing is complete and engine testing is the final qualification step.
The new alloys will be used first in the GE90-115B, which has the toughest thermal cycle, and also in the CFM56, which is GE's biggest user of rhenium, says Schafrik. They will be introduced into the GEnx and military engines in the future, he says.
The company's scrap-blade recycling programme, meanwhile, has reclaimed "hundreds of pounds" of superalloys at its seven service centres, says GE supply-chain "black belt" Larry Dening. This will increase to "thousands of pounds" this year, he says, as the programme is expanded to commercial and military customers.
Previously, scrapped blades were sold and recycled for use by the stainless steel industry, and the rare elements in superalloys were lost to aerospace. Now the material is cleaned and melted for re-use in manufacturing high-pressure turbine blades.
Schafrick says GE is looking at reclaiming other engine parts, and continues research into replacements for nickel superalloys in hot sections. These include ceramic matrix composites and high-temperature intermetallic materials such as niobium silicide.
Source: FlightGlobal.com
