Fundamentals of static crack growth in nickel-based superalloys after friction welding

Turbine sections of jet engines undergo significant and varying stresses, as well as operational temperatures beyond 750 degrees Celsius, requiring the use of materials capable of maintaining their mechanical properties at high temperatures. Nickel based super-alloys are the current material of choice for aeroengine discs, owing to their high resistance to creep, fatigue, and static loading in the temperature regimes required. If friction welding can be used to join discs into a turbine assembly then weight savings can result. Inertia friction welding (IFW) processes are favoured to join these large components as they do not require shielding environments and welding parameters are easily repeatable and controllable. During this process one component is attached to a rotating flywheel while the other is held fixed. The flywheel is rotated to a predetermined angular velocity (and hence energy), and the two components brought into contact under high axial pressure. Rotational energy is converted to heat via friction at the interface, the softened materials are expelled radially from the work-piece as flash, and a bond is formed between two components. IFW produces large changes in microstructure. High heat generation and deformation in the weld region promote dynamic recrystallisation, as well as dissolution of gamma ' precipitates and their subsequent re-precipitation in a much finer distribution. Unfortunately, such microstructures are highly prone to intergranular cracking if given threshold conditions are exceeded. This cracking is so rapid in air that in could jeopardise the integrity of the aero-engine and cannot be allowed to occur. This project will study the fundamentals of this intergranular crack growth mechanism to define limits to ensure cracks cannot grow under in-service conditions.