Making light work

While conventional steel has ruled the roost for more than a century, the lightness and strength of aluminium, magnesium and titanium, and their alloys makes them attractive for firms developing next-generation transport. At the University of Manchester, a five-year, £6m research project is underway to make these materials more workable.

Moreover, according to project director George Thompson, these materials could potentially be more versatile, and cost less to manufacture, than steel and even composite materials.

The Light Alloys Project is classified by the Engineering and Physical Sciences Research Council as a ‘portfolio partnership’, combining several related projects. For automotive applications, the focus is on aluminium and magnesium alloys; for shipping and light transit systems, it’s aluminium alloys; and for aerospace, alloys of all three metals.

There is much industrial interest, and partners include Alcan, BAE Systems, Corus, Qinetiq, Pechiney, Airbus and The Welding Institute (TWI), all of whom are contributing with funds and research to various parts of the project.

The driving force behind the project is the environmental impact of transport systems. These alloys have a higher strength-to-weight ratio than steels use; this means lighter vehicles and less fuel.

However, with new materials come new problems. Their crystalline structures — the size and shape of the grains of different metals that make up the bulk of the material — can tend to make them brittle and difficult to form and join together.

Moreover, the researchers are now beginning to understand how the processes used to make the alloys also affects the structure of the metal near and at the surface — and this may be very different from the bulk material. Because of this, the alloys may need specialised surface finishes to prevent corrosion. According to Thompson, the Manchester team is the only light alloys research effort to consider the interrelationship between the bulk, near-surface and surface properties of the alloys.

There are three main themes to the project: joining and forming; nano and microscale process control looking at the mechanisms that occur during deformation and heat treatment of the materials; and surface engineering research.

One part of the research looks at new methods of joining the different alloys in aircraft wings, particularly fusion welding. This technique, already used in fuselage manufacture, uses a beam of electrons to heat the edges of metal sheets. It produces very little distortion in the joint; is carried out in a vacuum, so does not cause contamination; and can join dissimilar metals. However, when making a fuselage, the welds only need to penetrate 3mm of metal. But for wings, they need to penetrate up to 15mm.

Christopher Heason and Philip Pragnell are co-ordinating Manchester’s research on this, as part of a consortium called DEFUSE. The £1.6m project — other partners are Cranfield University, TWI, Qinetiq, Alcoa and Airbus — is looking at ways to weld wing spars and panels which stiffen the wings (known as stringers) to the structure’s aerodynamic skin. But the aluminium alloys used are not very weldable; they can liquefy and become porous, cracking on solidification.

DEFUSE is being co-ordinated by TWI’s Paul Hilton who is also supervising trials of welding 15mm-thick stringers to skin panels, and testing whether laser or hybrid-arc laser techniques are best suited to the task.

Heason is involved with metallurgical optimisation of two welding processes — electron beam welding for the spars, and hybrid laser/arc welding for the stringers. The research involves thermodynamic modelling of the process, and development of filler materials.

Airbus is hoping to use the techniques on its next generation of aircraft, due to enter service in 2012.