Productive Grinding of Superalloys
Since the early days of metallurgy, alloys have evolved to meet performance requirements.
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.
Today's grinding machines themselves feature a variety of productivity-boosting systems including process monitoring and automatic loading that further enhance productivity. Metal removal rates achieved with modern grinding techniques can be relatively high, providing an economical way to process superalloy parts compared to EDM and other techniques.
To achieve maximum productivity, however, grinding methods and technology must be matched to the specific manufacturing situation at hand. By outlining the primary variables in the grinding process, then analyzing the benefits and limitations of different grinding technologies relative to those variables, you can get a picture of which methods are best for certain applications.
The five key variables in the grinding process to consider are:
- Investment: The most conspicuous variable of any grinding operation is the capital investment required. A shop must determine whether it needs a full five-axis machine with tool changer, or if a simple three-axis machine will suffice. The coolant system, critical to the grinding process, is another major investment. Even floor space is a cost factor, taking into consideration the size of parts and machining equipment involved.
- Strategy: Strategic approaches may include a lean production or single piece flow, where a part moves from machine to machine; or an automation approach where tool changing and other technologies permit completing a part on one machine; or a job/prototype shop focus, tailored to provide flexibility to meet constantly changing customer demand.
- Environment: The environmental variable in grinding is increasingly important. Part material and configuration as well as the grinding process itself determine whether oil- or water-based coolant systems will be applied. The choice involves weighing benefits and limitations including disposability, cleanliness and fire precautions. The amount of material removed and grinding wheel consumption also are environmental considerations.
- Design and tolerances: Issues include part geometry, material and possibly coating, and tolerance and surface finish requirements. Engineers often design complex superalloy parts and then expect a shop to find a way to produce the design. Some delicate components require special attention to avoid distortion from excessive grinding or clamping forces. And of course, part tolerance and surface finish requirements heavily influence the grinding process and abrasives that are chosen.
- Workholding: Choices here can include methods from hard point tooling to encapsulation of the part being ground. Fixturing costs can sometimes outweigh the cost of the part due to complexity and extra support requirements.
The above variables can be used to evaluate the benefits and limitations of four different combinations of grinding methods and technologies.
The four grinding methods and technologies are:
- Conventional creep feed grinding
- Creep feed continuous dress (CFCD) techniques
- Using vitrified CBN (cubic boron nitride) abrasives
- Using plated CBN wheels
Combining Grinding Technologies
Beyond the familiar technologies discussed above, grinding industry suppliers are continually developing new tools and methods to further improve productivity.
Machine manufacturers offer equipment that can enable a shop to combine a variety of grinding methods. Each grinding process offers one or two capabilities that make it excel in certain applications. Modern grinding machines with tool changing systems, integrated dressing systems and CNC coolant nozzle positioning can now mix abrasives to employ the optimum tool for each part feature. One machine can perform conventional creep feed and CFCD, apply vitrified or plated abrasives and even carry out other operations like milling. The advantages of each process or abrasive can be applied to specific materials and part geometries.
Multi-tasking: Combining All the Best
This multi-faceted grinding approach is excellent for complex parts with features that demand different grinding processes and abrasives. Such machines can even automate part loading and finishing in one clamping. An example of such a machine is the Mägerle MFP-50 nine-axis CNC grinding machine with a double axis CNC overhead dresser. It permits continuous dress, creep feed grinding and conventional grinding with ceramic, vitrified CBN and electroplated CBN wheels, all on the same machine. With integrated pallet systems, tool changer and CNC coolant nozzles, it can run automatically and adjust itself to handle different parts.
Linear motor drives represent another machine tool technology being developed for grinding. Linear motors have been employed in milling and EDM machines for more than 15 years. The process is very fast, offering advantages similar to those of high-speed milling, where high speed metal removal causes the heat generated by cutting to be carried away in the chip.
The light cuts and high speeds benefit wheel consumption by lowering cutting forces and reducing the heat generated. Reduced use of coolant and increased productivity also result. An example of such a machine is the Blohm Prokos five-axis grinder that that employs linear motors for three of the axes. It incorporates a tool changer, CNC coolant nozzle, and an advanced diamond dressing system for multi-axis part grinding.
There's no really easy solution to grinding superalloys. It depends on all the variables in your particular situation. By working closely with the wheel, the dressing, and the machine system there is a solution that will optimize productivity in your particular application.