In pursuit of achieving net-zero greenhouse gas emissions by 2050, the aerospace industry requires new engine types that utilize Heat-Resistant Super Alloys (HRSAs). However, machining HRSAs poses a significant challenge. Sébastien Jaeger, Product Manager Turning – Europe at Sandvik Coromant, highlights that a balanced approach is crucial to ensure efficient HRSA machining in aerospace engines. This approach involves considering various factors such as machines, tools, geometries, and materials. By carefully addressing these factors, the aerospace industry can overcome the difficulties associated with machining HRSAs and achieve their goal of reducing greenhouse gas emissions.
HRSAs are the primary materials used in compressor and turbine components of jet engines, with nickel-based grades like Inconel, Waspaloy, and Udimet being the most widely used for these applications. However, the properties of HRSAs can vary considerably depending on their composition and production process. Heat treatment plays a crucial role, as a precipitation-hardened (aged) component can exhibit twice the hardness of an untreated or soft annealed workpiece. Thus, selecting the appropriate grade of HRSAs and implementing the right heat treatment process is critical to ensure the desired properties and performance of the component in jet engine applications.
As emission regulations become stricter, new engine types are required to operate at higher service temperatures, necessitating the use of new materials for the hottest components. HRSAs are increasingly being used in jet engines due to their high-temperature capabilities. However, the benefits of using HRSAs are counterbalanced by the challenges involved in their manufacturing. Firstly, the high temperature strength of HRSAs results in high cutting forces during machining. Secondly, their low thermal conductivity and excellent hardenability lead to high cutting temperatures. Finally, their tendency to work harden results in notch wear. Therefore, it is important to address these challenges to effectively utilize HRSAs in jet engine manufacturing.
Turbine discs, casings, blisks, and shafts are complex components that require high precision and comply with stringent quality criteria due to their safety-critical nature in engine operations. These demanding workpieces often feature complex shapes and thin walls, making them challenging to manufacture. Achieving successful production requires a powerful machine, rigid tools, high-performance inserts, and optimal programming. The choice of machining methods may vary depending on the specific component. For instance, disc, ring, and shaft components are typically turned, while casings and blisks are often milled. Regardless of the method, precision manufacturing of these critical engine components necessitates the application of advanced machining techniques and equipment.
The machining process for HRSAs typically involves three stages. The first stage machining (FSM) involves shaping a cast or forged blank into its basic form. In this stage, the workpiece is usually in a soft condition, with a Rockwell hardness of approximately 25 HRC, but may have a rough, uneven skin or scale. The primary focus of FSM is to achieve efficient stock removal and maximize productivity. The goal is to remove excess material and create a smoother surface for subsequent machining stages. By prioritizing efficiency and precision during FSM, manufacturers can streamline the overall HRSAs machining process and produce high-quality components for aerospace applications.
After the first stage machining (FSM), the workpiece is heat-treated to achieve a much harder aged condition, typically ranging from 36-46 HRC, before proceeding to the intermediate stage machining (ISM). In this stage, the component is given its final shape, leaving a stock allowance for finishing. While productivity remains a critical focus, process security is also essential in ensuring the dimensional accuracy and quality of the finished component. With the workpiece in a much harder condition, the ISM stage requires careful consideration of cutting tools and techniques to maintain the desired tolerances and surface finish. By prioritizing both productivity and process security during the ISM stage, manufacturers can successfully produce high-quality components with the desired hardness and dimensional accuracy.
The last stage machining (LSM) is the final stage in the HRSAs machining process where the component is given its final shape and surface finish. The focus in this stage is on achieving accurate dimensional tolerances, high surface quality, and avoiding deformations and excessive residual stress. In critical rotating components, fatigue properties are of utmost importance, leaving no room for surface defects that could initiate crack formation. To ensure the reliability of critical parts, manufacturers must apply a proven and certified machining process that guarantees the desired surface quality and dimensional accuracy. By prioritizing precision and reliability during LSM, manufacturers can successfully produce HRSAs components that meet the stringent quality criteria required for aerospace applications.
General requirements for indexable inserts include good edge toughness and high adhesion between the substrate and the coating. While negative basic shapes are used for high strength and economy, the geometry should be positive.
Machining HRSAs typically requires the use of coolant, except for milling operations that use ceramic inserts. For ceramic inserts, a copious volume of coolant is necessary, while accurate coolant stream delivery is critical when using cemented carbide inserts. When utilizing carbide inserts, applying high coolant pressure provides additional benefits, such as longer tool life and effective chip control. By utilizing the appropriate coolant application based on the type of insert, manufacturers can optimize their machining processes for improved tool life, chip control, and overall machining efficiency.
Machining HRSAs requires varying machining parameters based on the conditions and material being used. During the first stage machining (FSM), manufacturers typically aim for high productivity by utilizing high feed rates and large depths of cut. In the intermediate stage machining (ISM), ceramic inserts are often used to achieve higher speeds. However, in the final stages, the focus is on achieving high-quality surface finish, and thus the depth of cut is kept small. Additionally, since high cutting speeds can negatively impact surface quality, finishing is often done using carbide inserts instead of ceramic inserts. By tailoring the machining parameters based on the stage of machining and the desired outcome, manufacturers can optimize their machining processes and produce high-quality HRSAs components.
Carbide inserts often experience wear mechanisms such as plastic deformation and notching, while ceramics commonly face top slice wear. To decrease susceptibility to plastic deformation, wear resistance and hot hardness can be increased. Additionally, reducing heat generation and cutting forces can be achieved through the use of a positive geometry and sharp edge. To address notch wear on the primary cutting-edge, a smaller entering angle can be employed, such as a square or round insert, or a cutting depth that is shallower than the nose radius.
When it comes to reducing notch wear, Physical Vapor Deposition (PVD)-coated inserts are more effective on the main edge, while Chemical Vapour Deposition (CVD)-coated inserts perform better on the trailing edge. Notch wear on the trailing edge can negatively impact the surface finish in finishing processes.
To achieve efficient machining of HRSAs for engine components, it is essential to have a comprehensive solution that takes into account various factors such as workpiece condition, tool material, recommended cutting data, coolant usage, and optimized machining strategies. By addressing these factors, manufacturers can contribute to the aerospace industry’s objective of achieving net-zero greenhouse gas emissions by 2050.