Abstract:
The adaptive serrated nozzle (ASN) is one of the most promising concepts to help reduce
the noise level generated by aero-engines. Shear between a hot air stream and ambient air
at the nozzle exit creates noise. The serrated nozzle is designed to protrude into the air
stream causing mixing between the two air streams reducing the noise level. Adaptive
control system using shape memory alloy (SMA) actuators deploy the protrusion only
when required in order to maximise fuel efficiency.
The successful joining of NiTi shape memory alloy (SMA) to the titanium parent structure
is critical to the development of the adaptive serrated nozzle. However, joining of SMAs
to dissimilar metals is widely known as extremely difficult if not impossible. This
research provides a preliminary study into the potential of using SMAs in large
engineering applications such as the ASN and the development of viable joining methods
for joining SMA to titanium based alloy. Five most favourable conventional joining
methods were selected for experimental investigation. Results proved that the successful
joining of SMA to dissimilar alloys was extremely difficult, joint failures were mainly due
to the formation of brittle intermetallics at joint interfaces. The formation of these
intermetallics occurs irrespective of the type of joining method and level of heat input
employed. However, it has been shown that the formation of these intermetallics can be
suppressed by the manipulation of the joint composition. A marked improvement in joint
performance has been achieved for joints that contained no more than 25 at% nickel. Joint
improvement has also been achieved through the addition of titanium at the joint, although
further research is necessary to investigate the effect of titanium addition to joint
performance.
