Aircraft engines are composed of a multitude of materials, with the design being led by the aim
to reduce the engine weight while at the same time increasing its performance. Light materials
such as titanium and carbon fibre are used in colder parts of the engine, while nickel-based
superalloys are used in the hot sections due to their good temperature resistance and high
strength. Traditionally, structural components such as the engine housing have been produced
as a single, large cast part. This approach limits the possibility of weight savings because of
lower strength of cast parts. Wrought material on the other hand has higher strength which
would enable a reduced component weight. Complex shapes can however only be realised by
extensive machining, if at all possible.
The so-called assembly approach uses small cast and wrought parts that are joined together
by welding. Component weight can be reduced by using wrought material where high strength
is required, while cast parts are used in places where strength requirements are lower but
complex shapes are needed instead. Such a manufacturing concept requires a good
weldability of the used materials. Welding of nickel-based superalloys is however more
complicated as for example the joining of construction steel. Their complex microstructure
requires the careful selection and control of welding parameters, and cracking often occurs
during welding. Weld cracking can furthermore occur during the post weld heat treatment, an
operation carried out to obtain uniform mechanical properties in the whole component after
welding. This provides the background for research on the weldability of nickel-based
superalloys. The aim of this work was to study what type of weld cracks occur during welding
and how the microstructure of the material affects their formation.
For this, welding tests were carried out using manual tungsten inert gas welding. Investigating
actual welds enables looking at conditions that are close to real application. The analysis
showed that the microstructure before welding has a large effect on crack formation during
welding. The presence of the Laves phase, which is typically considered detrimental for the
properties of the material, reduced the formation of cracks during welding. To further study
how the microstructure of the material can cause weld cracking, another part of the work was
focussed on developing a test method that can simulate the thermal cycle during and after
welding. The results contribute to better understanding the interrelationship between
microstructural changes during heat treatments and the likelihood of crack formation.
Knowledge about how microstructure changes affect the susceptibility towards weld cracking
can be used to adapt production processes and could ultimately help to avoid cracking
problems during welding.