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Maintenance and repair of light alloy components

Maintenance, aluminium and magnesium components generally require very long service life. This can be traced to the fact that these are employed in a wide and often demanding range of working conditions (working temperature and thermal cycling, fatigue stress, tribological wear and exposure to corrosive environment e.g. oil, external environment), where the capability to provide new, performing and cost-effective maintenance solutions is strategic.

Although there is existing technology that could somewhat restore these parts, they are often inappropriate due to excess porosity, oxidation and thermal damage. Consequently, very often, most of such repairs could not be reclaimed adequately.

As part replacement is the only option, the cost of aircraft maintenance has significantly increased over the years.

Among the actually used technologies for maintenance and repair applications, welding is used at foundry level in the repair of castings, when the induced distortions can be accepted and a full heat treatment after the repair can be applied.

However, typical drawbacks of welding include porosities, presence of oxides, hot cracking, occurrence of heat affected zone and loss of structural properties. For these reasons, welding cannot be used to repair machined and serviced, end-life components, mainly because of deformation and local loss of mechanical properties.

Advanced repairs on fan cases have been recently reported that using stir welding as a real solution can reduce the drawbacks of welding process. On the other side, in the case of aluminium and magnesium alloy components, thermal spray and plasma spray deposition are also frequently used to repair already machined and heat treated components.

Low Pressure Cold Spray must also be mentioned among the actually available technologies for maintenance and repair of aeronautical components. However, due to the relatively low performances in terms of maximum gas temperature and pressure of actually available LPCS systems, its use is limited to high ductility coating materials such as zinc, copper, CP-aluminium and soft aluminium alloys.

Moreover, the coating quality is lower with respect to High Pressure Cold Spray systems in terms of coating porosity, oxide content and mechanical properties. The use of LPCS is reported in literature for the deposition of pure aluminium and aluminium-based metallic coatings (e.g. for corrosive mitigation and restoration in substitution of aluminium shims, which are alternatively used and adhesively bonded over ground down areas affected by corrosion).

The use of common processes like TIG, MIG and thermal spray are quite critical because of the great sensitivity of Mg alloys both to temperature and oxidation.

In this sense the use of a cost-effective low temperature repair technology such as Cold Spray could have a great impact on the aeronautical industry.