Running red hot
Challenge: To efficiently machine aerospace engine components from HRSA.
Solution: Develop a balanced overall solution that encompasses the machine, tools, geometries and tool materials as well as the machining strategy.
Heat-resistant super alloys (HRSA) are widely used in jet engine compressor and turbine components. The main grades used for these applications are nickel-based types such as Inconel, Waspaloy and Udimet.
The properties of HRSAs vary greatly depending on the composition and production process. Heat treatment is particularly significant — a precipitation-hardened, i.e. “aged”, component can offer double the hardness of a soft annealed or untreated workpiece
Ever tighter emission regulations require higher operating temperatures from new engine types and call for new materials for the hottest components. Furthermore, the total amount of HRSA in a jet engine is increasing compared with other materials.
However, the benefits of HRSAs present a manufacturing challenge: High temperature strength leads to high cutting forces. Low thermal conductivity and excellent hardenability result in high cutting temperatures. Work hardening tendencies give rise to notch wear.
HRSA shaft component.
The components — turbine disks, casings, blisks and shafts — make demanding workpieces, many of them thin-walled and all including complex shapes. The safety-critical engine components must comply with stringent quality and dimensional accuracy criteria.
The preconditions for success include a powerful machine, rigid tools, high-performance inserts and optimal programming. The prevalent methods vary. Usually disk, ring and shaft components are turned, while casings and blisks are often milled.
The machining of HRSA is generally divided into three stages. During first stage machining (FSM) a cast or forged blank is given its basic shape. The workpiece is usually in a soft condition (typical hardness around 25 HRC), but it often has a rough, uneven skin or scale. The main priority is good productivity and efficient stock removal.
Between the first and the intermediate stage of machining (ISM), the workpiece is heat-treated to the much harder aged condition (typically around 36–46 HRC). The component is now given its final shape, except that the stock allowance is left for finishing. The focus is again on productivity, but process security is also important.
First stage, intermediate stage and last stage machining of a HRSA turbine disk.
The final shape and surface finish is created during last stage machining (LSM). The emphasis here is on surface quality, accurate dimensional tolerances and avoiding deformation and excessive residual stress. In critical rotating components, fatigue properties are the most important criteria and leave no room for surface defects that could initiate crack formation. The reliability of critical parts is guaranteed by applying a proven, certified machining process.
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.
Coolant should always be applied when machining HRSA, except when milling with ceramic inserts. Ceramic inserts require a large volume, while the accuracy of the stream is essential for cemented carbide. When using carbide inserts, high coolant pressure offers further benefits, including longer tool life and efficient chip control.
Machining parameters vary, depending on the conditions and the material. During FSM, good productivity is mainly obtained through the use of high feed rates and large depths of cut. In ISM, ceramic inserts are often used for higher speeds. The final stages focus on quality and the depth of cut is small. Since a high cutting speed can impair the surface quality, carbide inserts are applied for finishing.
Turbine disks are critical components made of advanced materials, requiring high security machining with optimized cutting tools and solutions.
Plastic deformation and notching are the typical wear mechanisms in carbide inserts, while top slice wear is common in ceramics. Vulnerability to plastic deformation decreases by increasing the wear resistance and hot hardness. Positive geometry and a sharp edge are also important in reducing heat generation and cutting forces. Remedies to notch wear on the main cutting edge include a small entering angle, for instance by using a square or a round insert, or a cutting depth that is smaller than the nose radius.
PVD-coated inserts are more resistant to notching on the main edge, while a CVD-coated insert offers better resistance against notch wear on the trailing edge. In finishing, notch wear on the trailing edge can impair the surface finish.
Summary
Efficient machining of engine components from HRSA requires a well-balanced overall solution, particularly taking into account factors such as workpiece condition, tool material and the related cutting data recommendations, use of coolant and optimized machining strategies.
Tools for intermediate and last stage machining of HRSAs include sharp, highly positive geometries for finishing and medium machining as well as geometries for operations requiring more toughness.
