A technical and economic appraisal of shape memory alloys for aerospace applications

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Abstract

The adaptive serrated nozzle is one of the most promising concepts to help reduce the noise level generated by aero-engines. The serrations protrude into the jet stream at the nozzle exit and force mixing with the surrounding ambient air. One design solution for such nozzles is to employ shape memory alloy (SMA) as the actuation mechanism. Unlike conventional SMA designs, large sections of SMA are required for such serrations. This paper shows that there are two major technological/economic challenges in the implementation of SMA adaptive serrated nozzles. These are the needs for high temperature SMAs and large section sizes. On the technical front, the paper also shows that joining of NiTi SMAs to dissimilar alloys is the major risk factor in the development of the SMA adaptive serrated nozzle. The problem is that the shape memory performance is severely affected by temperature yet it is virtually impossible to achieve a high strength joint without using joining techniques with operating temperatures approaching or beyond the melting point of these alloys. A preliminary assessment of laser welding of thin sections of NiTi SMA is presented and has highlighted the challenges that need further investigation.

Section snippets

Background

Actuation by shape memory alloys (SMAs) has generated considerable interest and some applications have been developed across a wide range of industries. Successful applications are still largely restricted to niche markets, although the extent of their applications is growing rapidly. Two factors seem to have restricted growth into other markets. These are the early dominance by the medical market, where small sections of low temperature materials have been developed. This has partially led to

Adaptive SMA serrated nozzles

SMAs have been widely investigated to produce adaptive structures. Traditionally, engineers have used the common product forms available such as wires and strips to alter the shape of such structures. This approach is not feasible for serrated nozzles because of the aerodynamic forces on the protruding serrations. A more monolithic design is therefore required, involving thin plates of SMA joined to a ‘passive’ elastic component manufactured from aerospace grade alloys such as Ti–6Al–4V alloy.

SMA selection

Adaptive serrations can be employed in both the core and by-pass nozzles. However, application in the hotter core nozzle requires transformation temperatures in excess of the 150 °C currently available from commercial SMAs such as NiTi binaries and NiTi(Cu). A number of high temperature SMAs such as NiTi(Hf), NiTi(Nb), NiAl and Fe-based systems are under extensive research [3]. Until these alloys are fully developed and become commercially available, it is difficult to see further advances in

Proof of concept

Our proof of concept work has focussed on sub-components for a by-pass SMA adaptive serrated nozzle using conventional binary NiTi alloys. Large sections of SMAs are very difficult to obtain in the current commercial market. For this project, the required SMAs have therefore been manufactured by the Institute of Metals Research at the Chinese Academy of Sciences. They have been produced by a standard process involving vacuum induction melting, followed by forging at 820 °C, hot rolling at 850 °C

Joining of NiTi shape memory alloys

The functionality of a single adaptive serration has been proven using these NiTi materials combined with an elastic bias component manufactured from Ti–6Al–4V. However, such proof of concept demonstrators have, to date, been mechanically fastened. A production serrated nozzle will need to be integrally bonded. A risk assessment carried out at Rolls-Royce has identified that joining is the process that carries the highest risk and is the most costly to mitigate. The joining of NiTi SMAs to

Summary

The SMA adaptive serrated nozzle is one of the most attractive developments to aid noise reduction in next-generation gas turbine engines. However, the benefit can only be utilised if issues such as the manufacturing of monolithic SMAs (for both high and low temperature actuation) and the ability to join them can be resolved. The latter carries the highest risk factor in realising the adaptive serrated nozzle concept. Preliminary results further reinforce the need for optimised joining

Acknowledgements

The authors would like to thank Rolls-Royce for its financial support, Daniel Clark (Rolls-Royce, Derby) for his technical assistance and the Institute of Metals Research for the supply of SMA.

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