Blisks located on the cold compressor side of the engine are made of titanium, while the hot turbine side requires blisks made of heat-resistant super alloys (HRSAs).
All photos credit: Sandvik Coromant
Heat-resistant super alloys (HRSAs) in aerospace engine components face extreme performance demands. What makes them incredibly durable in an engine’s high pressures and temperatures also makes them notoriously difficult to machine. It’s critical that shops know how to successfully tool up for HRSA engine components, which can cost tens of thousands of dollars each, to avoid making highly expensive mistakes.
Ensuring process security is key to a shop’s success. To achieve repeatability and high quality when machining HRSA aerospace engine components, it’s essential that shops follow some best practices. While these relate to machining HRSAs in general, each type of HRSA, engine component, and feature has its own considerations, tools, and techniques.
Work with the material properties
HRSAs are difficult to machine because they resist heat, and to machine them by shearing generates heat. When shops machine a piece of steel, the chips that come off absorb heat from the machining process. In HRSAs, the chips resist rather than absorb the heat, sending it back into tools or the workpiece. The generated heat can turn the carbide of the cutting tool into a plasticized or sintered state, and inserts can fracture; damaging a tool or, even worse, the engine component.
To protect tools and workpieces, it’s important that the process to machine HRSAs produces as little heat as possible. One way is to use tools that cut and shear HRSAs rather than push material off. Another is to not take off too much material too quickly, like burying the cutter insert deep into the material and plowing through. Instead, a series of lighter, faster cuts is more effective and produces less heat. Most computer-aided manufacturing (CAM) packages offer this trochoidal, or dynamic, technique that makes it easier for shops to apply.
Some solid carbide end mills, such as Sandvik Coromant’s CoroMill Plura Gannet, have specifically designed geometries for plunging out material on deep, narrow slots.
General HRSA cutting best practices apply to the different material bases. For aerospace engines, HRSA can be classified under two base elements with completely different cutting conditions, nickel- and titanium-based. In most cases when turning, use uncoated tools to machine titanium as it’s chemically reactive, especially at elevated temperatures. As most coatings have titanium as well as oxygen, nitrogen, and carbon, it’s possible the titanium in the coating and workpiece can react. If they …….