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

喷气发动机的涡轮机部分经受显著的和变化的应力，以及超过750摄氏度的操作温度，需要使用能够在高温下保持其机械性能的材料。镍基超合金是目前航空发动机盘的首选材料，因为它们在所需的温度范围内具有高的抗蠕变性、抗疲劳性和抗静载荷性。如果摩擦焊接可以用于将盘连接到涡轮机组件中，则可以减轻重量。惯性摩擦焊（IFW）工艺有利于连接这些大型部件，因为它们不需要屏蔽环境，并且焊接参数易于重复和控制。在此过程中，一个部件连接到旋转飞轮，而另一个部件保持固定。飞轮旋转到预定的角速度（并因此获得能量），并且两个部件在高轴向压力下接触。旋转能量通过界面处的摩擦转化为热量，软化的材料以飞边的形式从工件径向排出，两个部件之间形成结合。IFW产生大的微观结构变化。焊接区域中的高热产生和变形促进动态再结晶，以及γ '沉淀物的溶解和它们随后以更细的分布再沉淀。不幸的是，如果超过给定的阈值条件，这种微结构非常容易发生晶间开裂。这种破裂在空气中非常迅速，可能会危及航空发动机的完整性，不能允许发生。本项目将研究这种晶间裂纹扩展机制的基本原理，以确定极限，确保裂纹在使用条件下不会扩展。

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.