Weld Cracking of Precipitation Hardening Ni-based Superalloys - Investigation of repair welding characteristics and susceptibility towards strain age cracking

Doctoral thesis, 2020

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.