Alloys and surface treatment

Citation

(2002), "Alloys and surface treatment", Aircraft Engineering and Aerospace Technology, Vol. 74 No. 2. https://doi.org/10.1108/aeat.2002.12774bad.012

Alloys and surface treatment

Keywords: Alloys, Surface treatment, Aircraft industry

Improvements in alloys and surface treatments – these are what aircraft manufacturers are looking for. There are a number of advantages: safety of personnel, reduction in aircraft weight, increased lifetimes, simplification of procedures, production savings. French companies are reported to have developed innovative processes that have progressed to the industrial phase.

"In the future we will concentrate less on the material itself than on its relationship with its environment, as shown by the interest recently expressed in surface treatment. Currently, aeronautical research is centred on improvements to alloys (aluminium) by optimising their mechanical properties and corrosion resistance," explains Armand Coujou, a CNRS research engineer at the Materials Development Centre in Toulouse, which is studying materials (alloys and superalloys, Composites, ceramics and polymers) on the microscopic and even atomic scale. "in the past, researchers were primarily concerned with mechanical structure. Now, a part can no longer be understood without considering its treatment. As constraints apply to parts that are becoming smaller and smaller, lightweight and subjected to harsh environments (friction, movement, elevated temperatures), their surfaces are becoming much more stressed. A mechanical part is therefore required to have both surface strength (wear resistance) and low chemical reactivity (corrosion resistance).

"Materials are important because they are related to the safety of personnel," confirms Jean-Pierre Bonino, a CNRS research engineer at the Inter-University Materials Research and Engineering Centre (Cirimat). Formed from the amalgamation of two laboratories in the Paul Sabatier University for Sciences in Toulouse and the Materials and Interface Laboratory of the National Polytechnic Institute of Toulouse, Cirimat has 150 researchers in nine teams, four of which are specifically working in aeronautics on alloys, surface treatment and polymers. These areas are included within the framework of the AMSA research project (Laboratory for the Development of Structural Materials for Aeronautics), set up to act as a technology awareness programme for new applications. The research programme (1999-2000), which includes Airbus, university laboratories and SMES, has led to several innovative developments.

A specialist in profiled and thick laminates for wing stiffeners in aerospace applications (Airbus, Dassault, Bombardier, Embraer and Boeing), Pechiney Aerospace's factory at Issoire has developed an alloy which is lighter and more corrosion resistant than "2024", which has been traditionally used in structures and fuselage skin for civil aircraft. "The alloy 2024 cannot be welded or riveted. We have developed a weldable material, called 6056, with a metallic state that desensitises the metal to corrosion. It was qualified for a little over a year on the A318 and the A380," says Philippe Lequeu, technical development engineer. "Aircraft manufacturers, because of lighter structures, want higher performance alloys in terms of cost and weight. We started research in 1990 based on the macroscopic requirements that they expressed. Since then, we have worked at early stages on the definition of the products". The European leader in alloys, devoting $100 million in R&D for aerospace, 50 per cent of which goes to the development of new alloys for wide- bodied aircraft, has also developed, with its research centre in Voreppe, near Grenoble, a whole range of products to compete with alloys that have long been qualified. An alloy specifically for wings (7449), which is claimed to improve corrosion resistance by 10 per cent, had its first flight last April on the A340- 500/600 and will almost certainly be adopted for the A380. Also qualified is another material (7040) reportedly providing all its advantages (optimisation by 10 per cent of mechanical properties and therefore strength) in thicknesses between 150 and 240 mm. Finally, two other alloys for skins and lower wing stiffeners, claiming a 10 to 20 per cent gain in damage resistance, are undergoing qualification.

The desired improvements are also thought to be applicable for very small aircraft parts. One of the leading suppliers of rivets for aerospace applications, Atelier de la Haute- Garonne (4 to 5 million parts produced per year), located in Flourens near Toulouse, has developed a new rivet, made with improved alloys, that has just been qualified on all Airbus assembly lines. "Traditional rivets require treatment time. The technique we have developed is simplified. No tempering or surface treatment is required before use, unlimited storage time, no difficulties in placement," explains Frangoise Montsarrat, sales manager of the company, which has also developed another rivet, used at Bombardier, that fulfills several functions and makes it possible to reduce by a factor of four the number of items used.

"From a surface treatment technique, we developed a material consisting of hollow metal spheres. After ten years of research, we obtained its qualification," states Delphine Ferrie, project manager at Mecaprotec Industries, a company based in the Toulouse region specialising in surface treatment. Said to be a serious competitor to honeycomb materials, providing protection to the cockpit, the new material reportedly has many other attributes: mechanical (energy absorption, low density, resistance to shock and vibration, isotropic), acoustic, inert to harsh chemical environments, resistance to temperatures from -100 to +1000°C, a variety of possibilities for shaping (adhesion, brazing). It can also be used in structures resistant to shock, impacts and crashes, also it claims a solution for noise reduction at the air intakes into aircraft nacelles, engines and turbojets, and can reportedly also be used as treatment solutions with the benefit of the three functions. The company, which has always responded with innovation to the economic vagaries of the sector, has just installed a production unit to manufacture under industrial conditions one cubic metre of hollow spheres per month.

"Manufacturers are looking for treatment processes without solvents. Polluting and harmful to the environment, chromates (chromium 6 and cadmium), used to treat parts for aircraft structures, have very useful anti-corrosion properties, but require manufacturers to treat their effluents, which represents a non-negligibie cost. All manufacturers are looking for processes to replace chromium 6, but there have been no developments until now with regard to chromated paint," explains Nadine Pebere, co- ordinator for the "Corrosion and Protection of Materials" project within Cirimat. A specialist aircraft paint subsidiary of the French company Peintures Maestria, Mapaero, which is based in Ariege, has developed and qualified a new generation of water-based paints. The research, which took seven years, led to qualification by Airbus in July 2000. Used since March 2001 on metal parts assembled in Saint-Nazaire, it will be used on all the other Airbus assembly sites. "This paint, intended for aircraft structures (wings, fuselage), contains 15 per cent of solvents compared with 70 per cent in currently used products, while guaranteeing equivalent performance. In this way, while reducing emissions of VOCs (volatile organic compounds), it at the same time enables a reduction in costs and higher production rates because it is slightly cheaper, requires smaller quantities and dries more quickly," explains Jean-Frangois Brachotte, managing director of Mapaero.

Are polymers the materials of the future? "In aeronautics, one tries to simplify the system by having materials that perform several functions. We have carried out research, with two partners, Hexcel Composites (Grenoble) and Metailisation Tarbaise (Soues, Hautes Pyernees), to replace the composites currently used in the composition of aircraft wings with a structure made entirely of polymer," explains Colette Lacabanne, a CNRS researcher. Very light, polymers have potential, but are not conductors of electricity and therefore can be damaged by lightning. Metailisation Tarbaise has developed a thermal sprayer that would allow metailisation of the material, but the technique, which has produced conclusive results, is not yet completed.

A renaissance for metals? At the request of Airbus, Snecma and Turbomeca, research centres in Toulouse and Poitiers are currently working on the characterisation of rhenium and powder metallurgy. A relatively rare metal, rhenium, which starts to melt at 3180 degrees, will be used in the combustion chambers of the engines for the A380. The last metal to be discovered (1925), whose production volumes are still microscopic, is experiencing a renaissance thanks to aerospace. As aircraft become larger and larger, and to improve their performance while reducing pollution, the propulsive gases have to be hotter and hotter. Rhenium becomes the ideal internal coating for combustion chambers as well as turbine blades. The widespread use of high temperature propulsion in aviation should encourage the producers of speciality alloys to adopt rhenium.

As for powder metallurgy, this could be a solution to make turbine discs homogeneous and so increase their lifetimes. Fortech, a subsidiary of Eramet Alloys based in Ariege and specialising in the forging and stamping of aircraft engine parts and aircraft structures, has been selected to lead this research.

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