Paris Air Show

New aluminum alloys, processes to help Alcan battle composites

 - June 16, 2009, 1:58 AM

Aluminum specialist Alcan (Hall 2 Stand B19) is developing new alloys and new processes to better compete with composite materials, the proportion of which has been steadily increasing in airframes over the past decades. At Voreppe in France, Alcan Engineered Products (Alcan EP) has a major research-and-development  (R&D) center to devise and test these solutions. Officials at Alcan Global Aerospace, Transportation and Industry (Alcan Global ATI), a division of Alcan EP, want to change the perception that only composites have features such as reduced maintenance costs and integrated health and monitoring.

According to Alcan Global ATI president Christophe Villemin, proven metal solutions help aircraft programs stay on schedule. He was referring to the ongoing delays in the Boeing 787 program, which relies on composites for some 50 percent of its structure. “Our customers want delay-free machining and assembly,” he insisted, adding that Alcan is now working on alloys that will fly on the successors of the Boeing 737 and Airbus A320.

To better support airframers, Alcan is offering more integrated solutions. For example, it can pre-machine plates at its factories. This is a way to improve the buy-to-fly ratio–reducing the amount of scrapped metal at the airframer’s facility.

Integral parts will offer several thicknesses on a single part. Friction stir welding  provides such a benefit by creating a welded area that is almost as strong, mechanically, as the rest of the part. This enables post-welding machining and allows joining of two alloys that could not be welded with conventional techniques.
Friction stir welding was much touted in the defunct Eclipse 500 very light jet program, but it was used only to replace rivets. R&D engineer Gaëlle Pouget acknowledged that friction stir welding has been under development in Europe for years, notably by EADS, and he said it is getting closer to production.

Combined with a new shape of stringer, dubbed “top-hat,” friction stir welding can yield panels that are 15 percent lighter and 14 percent stronger than conventional panels, said Bruno Chenal, director for technology and innovation. Alcan research engineers also have found that adding a “step” between two conventional stringers makes crack propagation slower by half. This can be done without adding weight, simply by making a panel thicker at one location and slimmer in nearby areas, yielding lower maintenance costs, thanks to longer times between overhaul.

Structural health monitoring will not be exclusive to composites soon. To see how a material is withstanding the effects of age and cycles, sensors like optical fibers can be included in an aluminum part. Moreover, “aluminum has a predictable behavior,” Villemin insisted.

Another future possibility is tailored performance for aero-elasticity. In other words, the same part will have different behaviors in different areas. “The properties you need on a wing are not the same along the entire span,” Chenal explained.

The quest for lightness is continuing. The latest generation of aluminum-copper-lithium (often referred to as aluminum-lithium) alloys are 4-percent less dense and 30-percent stronger than incumbent alloys. This allows using less metal for a given performance. Therefore, the total weight reduction is now in the 15- to 20-percent range, according to Alcan’s numbers.

Another benefit of new low-density alloys, including aluminum-lithium, is better resistance to corrosion. This allows increasing times between overhaul. Hence lower maintenance costs, Chenal stressed.

Finally, Alcan has been working on alloys that are compatible with composites. Previously, using metal next to composites raised corrosion issues, so airframers often chose to use only composites in a given area. “Now they have the choice,” Chenal said.

He also mentioned the company’s involvement in recycling initiatives. It has  participated in the Airbus-led Pamela R&D project for end-of-life aircraft dismantling. Alcan also wants to increase its pre-machining activity. “Recycling scraps and chips on site is greener than having to transport them from the airframer’s factory,” he pointed out.