Shedding light on metal fatigue

A laser originally developed for fusion power research is being used by Rolls-Royce to strengthen aerospace components.

The laser is powerful enough to send a shockwave through the first 1-2mm of a metal surface, causing compressive residual stress.

Compressive stress is caused by the material’s atomic structure pushing inwards against itself, and is good for combating fatigue and inhibiting the initiation and propagation of cracks.

Rolls-Royce has been treating its Trent jet engine fan blade components for the past 21 months, although the work has only just been made public.

Metal Improvement, the company that has developed the technology, hopes to eventually offer it to the nuclear, oil and gas, medical and automotive industries.

David Francis, senior vice-president, laser operations, at Metal Improvement, said the technology is more effective in improving the strength of components than the existing shot peening technique.

‘Shot peening, which fires tiny little metal balls at a surface, would only achieve compressive stress in the top quarter of a millimetre of a material. With the laser, we can look at peening more complex structures. We could use this on an undercarriage, and expect to use it on advanced fighters with monolithic structures, such as the Joint Strike Fighter.’

At peak power the laser produces a billion watts in 20 nanosecond bursts, with five bursts per second. Between the laser beam and the component is a layer of de-ionised water and an ablative material – one that absorbs heat by melting – protecting the component.

The laser heats the ablative layer in nanoseconds, vaporising its surface and turning it into a plasma, or ionised gas. The water layer, through which the laser passes before reaching the ablative material, is used to contain the resulting plasma.

The rapid heating causes the ablative layer to expand, sending a kinetic energy ‘shockwave’ through its structure, which is transferred to the component. This shockwave alters the first millimetre or two of the component’s surface.

The laser is housed in a 3m x 6m clean room. A smaller transportable laser is in development for use on military bases and in aircraft hangars.

Laser peening was first developed in 1965 but has only recently been used commercially.

Since 1997, Metal Improvement has been developing the laser with the help of US government agencies.

The neodymium doped glass laser used in the process was designed for an inertial fusion power research programme run by the US government’s California-based Lawrence Livermore National Laboratory, the US Air force’s x-ray lithography research unit, and US military research agency Darpa.

In inertial fusion, high-powered lasers are used to repeatedly implode fuel pellet atoms inside a chamber, creating energy-generating plasma.