The development of all-ceramic milling cutters has opened up new prospects for the machining of nickel-based alloy materials. Compared with hard metal tools, all-ceramic tools can increase machining efficiency by eight times.
Technological progress and continued development in the field of materials science continue to promote the ability and efficiency of machining difficult workpieces. The use of nickel-based alloys can greatly improve the overall effectiveness of steam turbines. Single-crystal steam turbine bucket wheels made of nickel-based alloys are equipped with a complex cooling groove and ceramic insulation layer system and will be put into use under temperature conditions of up to 1450°C. This unique mechanical and heat-resistant properties place great demands on machining. In the machining of a jet propulsion unit alone, about 3,000 indexing inserts are required, while in contrast, only two indexing inserts are required on average for the manufacture of a car.
Enable Higher Cutting Speeds
Nickel-based alloys have high heat resistance and poor thermal conductivity, which can cause high temperatures on the cutting surface. This causes the cutting material to soften. Due to the presence of easily abrasive carbides in their microstructure, the tool tends to fail under temperature and mechanical overload conditions. Coated hard metal tools only work stably at cutting speeds below 20m/min. Various tests have shown that cutting speeds can be increased by 30 to 50 times by cutting with ceramic materials. The key factor is the excellent heat resistance of ceramics.
Therefore, the temperature can be raised to a high enough level during the cutting process to soften the workpiece material and make it easier to cut. This allows entry into the field of high-speed cutting (HSC) technology. Milling cutters that match the indexable inserts made of ceramic materials are already available on the market, and such tools can also be used for rough machining of turbine bucket wheels. However, for design reasons, the minimum size of the tool is still limited. The diameter of the smallest commercial tool is currently 32mm. For machining tasks that require smaller tool diameters or complex cutting contours, in addition to hard metal tools and HSS tools, grinding and wire cutting can also be used.
A comparison of commonly available coated and uncoated hard metal tools on the market with the developed ceramic tools in terms of standard travel and cutting volume per unit time shows that productivity can be increased eightfold using all-ceramic milling cutters.
Applying the cutting capabilities of modern ceramic cutting materials to these applications has become a research focus of the Fraunhofer-Institut für Produktionsanlagen und Konstruktionstechnik (IPK) in Berlin. As early as 2006, the first ceramic milling cutter test samples came out in the “Cercut” project of Fraunhofer-Allianz High Performance Ceramics. These tools have been successful in experimental applications. In view of the positive response from tool manufacturers and users, the joint institute will continue to push forward the development work that has already been carried out.
Promoting the Development of All-ceramic Cutting Tools
In January 2008, the project “Tech-Volk” was launched. Since then, the project team has been working hard on the development of fully ceramic milling cutters with cutting profiles that are suitable for the application. Since the various partners in the project have their own strengths and work focuses, the entire process can be considered comprehensively. From the production of ceramic blanks, the application of grinding strategies and the processing of grinding tools to the cutting of specific workpieces in modern HSC machining centers, the workpiece materials range from nickel-based forging alloys such as Nimonic 90 to cast alloys such as MAR M247.
In order to be able to adapt to the load conditions of the complex HSC milling process, the cutting materials must meet certain special requirements. Interruptions in the cutting process can lead to high load changes and temperature fluctuations at the cutting edge. During certain time intervals, the cutting edge may not be in the cutting state, and the surface temperature of the cutting edge cools down more easily than the temperature inside the core layer. Due to the difference in thermal expansion states, tensile stresses are formed in the tool edge area, which can easily lead to cracks.
Ceramics are more sensitive to tensile stresses and are therefore particularly vulnerable to this mechanism. Dry machining becomes necessary for this cutting material, as cooling lubrication will increase the cooling effect of the tool and have an additional negative impact on the tool’s operating state. The internal composition and structure of Al2O3 and SiAlON reinforced with silicon carbide whiskers determine that these materials have the characteristics of preventing crack formation and increasing fracture toughness. These two cutting ceramic materials have already been put on the market as indexing inserts and perform well in use.
The most promising follow-up development is the production of so-called graded ceramics. In this regard, the strength characteristics of this material can be specifically changed by subsequent processing. Just like steel through quenching, ceramics can also be made into wear-resistant edges and non-fracture-resistant core areas.
Before the full ceramic development process, the use characteristics of these two ceramic materials as indexing inserts were analyzed. In the meantime, the wear resistance of the materials was also studied and evaluated. A detailed analysis of the tool design can be achieved by mechanical measurements on the workpiece combined with FEM simulations. The knowledge gained from practical tests is complemented by the numerically determined load limits. This allows the identification of load conditions that are detrimental to the strength characteristics of the ceramic. At the same time, the different self-vibration characteristics of different tool shapes are taken into account. Through targeted settings of tools and processes, not only can tool wear be significantly reduced and tool service life be increased, but also the geometric quality of the workpiece surface is significantly improved.
For a research institute, it is crucial that it has the unique conditions to manufacture tools on its own high-precision grinders, subsequently measure them on tool measuring equipment, and test them on powerful HSC machining centers. This allows for very short development cycles and in-depth knowledge of the process.
