Challenge: Efficient aerospace machining of engine components from HRSA .
Solution: Develop a balanced overall process that encompasses the machine, tools, geometries and tool materials as well as the machining strategy.
Heat-resistant super alloys (HRSA) are the dominant materials in jet engine compressor and turbine components. The foremost grades used for these applications are the nickel-based types such as Inconel, Waspalloy and Udimet.
The properties of HRSAs vary greatly depending on the composition and production process. Heat treatment in particular has great significance; a precipitation-hardened – i.e., “aged” – component can show double the hardness of a soft annealed or untreated workpiece. Ever tighter emission regulations require higher service temperatures from new engine types and call for new materials for the hottest components. Furthermore, the total amount of HRSA in a jet engine compared with other materials is increasing.
The benefits of HRSAs present a manufacturing challenge, however: 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.
The components – turbine discs, 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 disc, ring and shaft components are turned; 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 receives 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 machining (ISM), the workpiece isheat-treated to the much harder aged condition (typically around 36–46 HRC). The component now receives its final shape, except that the stock allowance is left for finishing. The focus is again on productivity, but process security is also important.
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 deformations 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 for milling with ceramic inserts. When turning with ceramic inserts a copious volume of coolant is important, while the accuracy of the stream is essential when turning with cemented carbide.
Machining parameters vary, depending on the conditions and the material. During FSM, good productivity is mainly realized through the use of high feed rates and large depths of cut. In ISM, ceramic inserts are often used for higher speeds. 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.
Plastic deformation (PD) and notching are the typical wear mechanisms in carbide inserts, but top slice wear is common in ceramics. Vulnerability to PD decreases by increasing the wear resistance and hot hardness. A 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 lower than the nose radius.
PVD-coated inserts are more resistant to notching on the main edge; a CVD-coated insert has a better resistance against notch wear on the trailing edge. In finishing, notch wear on the trailing edge can impair the surface finish.
Originally published in Metalworking World 2.2010, a business magazine published by Sandvik Coromant.
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