Since the early days of metallurgy, alloys have evolved to meet performance requirements. Just as hard steels were developed for swords in ancient times, today tough, heat- and corrosion-resistant alloys are being developed for demanding applications such as nuclear power components, high-performance automotive parts and jet engine turbine blades, vanes, shrouds and disks able to withstand temperatures over 2,000° F.
Such “superalloys” exhibit excellent mechanical strength and creep resistance at high operating temperatures, with superior resistance to corrosion and oxidation. They generally are based on nickel or cobalt and feature a complex combination of other elements. Superalloys are known under a number of trade names including Inconel, Hastelloy, René and Haynes, and also exist as proprietary materials developed by the product manufacturers themselves.
High-performance materials usually present manufacturing challenges, and superalloys are no exception. Superalloys have the tendency to workharden at the surface and generate heat during machining. They are relatively poor conductors of heat, and accumulated high temperatures can interfere with the cutting process and/or deform or damage the part. A relative machinability comparison of selected alloys (considering cutting speed, surface finish, and tool life) places carbon steel 1212 at 100 percent, stainless steel 440 at 45 percent and Inconel 718 at only 19 percent.
Compounding manufacturing difficulty is the high-tolerance and complex nature of many superalloy components. The shape of the parts often makes them difficult to hold securely for machining. Finally, both the alloys and the parts they comprise usually are very expensive.
Advantages for Finishing
Progress continues in productive rough turning and milling of superalloys, but for finishing operations grinding is generally the process of choice. Although grinding is often thought of as expensive, dirty and relatively slow, it offers a number of clear benefits when handling superalloys.
Grinding processes can be customized to precisely match part requirements.
Varying the size of the wheel’s abrasive grains provides control of cutting forces and surface finish. The porosity of the grinding wheel can be manipulated to promote the flow of coolant into the cut and speed evacuation of chips. Diamond dressing enables the formation of highly accurate wheel shapes to produce complex part geometries meeting tolerances of 0.0001″ or better. Continual wheel dressing enables process control that is not possible with a cutting tool that becomes duller with each successive cut.
Read more: Productive Grinding of Superalloys
