Application of ceramic cutting tools in machining of aerospace parts

Ceramic materials are the most promising and competitive tool materials in the 21st century, and its development will likely cause another revolution in the field of cutting. Although we have gained some experience in the trial process, we need to further test and broaden the varieties of processed materials. Only by mastering the performance of ceramic tools can we better apply to the processing of superalloys.

In the aero-engine manufacturing industry, material performance continues to increase, and CNC machining technology for difficult-to-machine materials (high-temperature alloys) has become a common concern in the industry. Ceramic tool materials have the characteristics of high hardness, good wear resistance and heat resistance, excellent chemical stability, and difficulty in bonding with metals. They have become one of the main tool materials for high-speed cutting high-temperature alloys. Moreover, the optimum cutting speed of ceramic tools is 8 to 10 times higher than that of cemented carbide tools, which can greatly improve the cutting efficiency. At present, new ceramic tools are constantly appearing. Some experts predict that ceramic tools will account for 15% to 20% of the world's machining tools. Its development will cause another revolution in the field of machining.

High-temperature alloys (mainly nickel-based or cobalt-based alloys) have excellent stability and creep resistance at high temperatures. GH4169 has high hardness at room temperature (up to HRC 35~47) and good toughness. However, compared with ordinary steel parts, its machining performance is poor, and the cutting process needs to consume more energy.

At the beginning of the 21st century, our company began to purchase a large number of CNC equipment, and gradually eliminated the ordinary machine tools. The indexable carbide tools gradually replaced the traditional welding tools, and the production efficiency was improved as never before.

In the past 10 years, the application of carbide tools for the processing of titanium-based, nickel-based and cobalt-based superalloys has been widely used. The high hardness and high toughness of cemented carbide materials at operating temperatures below 600 °C make them cutting. A very ideal tool for superalloys and titanium alloys. However, cemented carbide knives have a fatal weakness with a melting point of about 1200 ° C. When the cutting zone temperature is higher than 800 ° C, the strength and hardness of the cutting edge will be greatly reduced, the wear will be intensified, and it is even difficult to complete the normal cutting. Therefore, when using a carbide tool to cut high-temperature alloy materials, in order to avoid excessive temperature in the cutting zone, the line speed can only be maintained at about 40m/min. For parts with large machining allowances, due to the slow cutting speed, the metal removal rate is very low, the occupation time is very long, and the production cost is greatly increased, which makes the potential of modern CNC machine tools far from being exerted. As new engine performance continues to increase and new materials continue to emerge, carbide tools have become difficult to adapt. Therefore, finding a more ideal cutting tool has become a top priority.

Aero-engine companies in developed countries (such as GE in the United States and Ronaldo in the United Kingdom) began processing high-temperature alloy materials with ceramic tools 20 years ago. The most important feature of ceramic materials is that the melting point is high (above 2000 °C), and the hardness does not drop much at 1200 °C. It is an ideal material to replace high-speed cutting of cemented carbide tools. In China, the use of such tools has not been widely used for various reasons.

Chip forming in cutting is a typical large deformation process involving material nonlinearity, geometric nonlinearity and boundary nonlinearity. In the high-speed cutting process, thermal coupling is also involved.

The famous cutting experts Pispenen & Merchant pointed out that the chip formation mechanism proposed in 1945 pointed out that under the action of shear force (cutting force), the grain boundary near the shear plane began to be torn. Deformation, separating from the substrate to form chips, and generating a large amount of heat. In fact, about 80% of the cutting heat is generated.

The core of high-speed cutting using ceramic tools is to make full use of the high-temperature characteristics of ceramic materials, increase the cutting speed, make the cutting heat accumulate, increase the temperature of the cutting zone, soften the chips, and make the cutting easy. Although ceramic materials have much different toughness and wear resistance than hard alloy materials, their high temperature stability is far beyond the reach of cemented carbide tools. Therefore, increasing the line speed is the most effective way to increase the temperature in the cutting zone. In theory, the cutting speed and metal removal rate of ceramic tools should be 5 to 10 times or more than that of cemented carbide tools.

In the promotion, the tool manufacturer only proposed that the ceramic tool is suitable for processing materials above HRC55, but there is no corresponding report for materials smaller than HRC55. This article talks about the processing of materials smaller than HRC55.

Due to the long-term use of cemented carbide tools, operators have become accustomed to low-speed cutting, and this is suitable for the processing of cemented carbide is the biggest taboo in the processing of ceramic tools. When using ceramic tools, the operator always dares not to increase the speed for safety reasons, and even wants to use ceramic tools on a common lathe. Most of the problems encountered in the use of ceramic tools in the past were caused by insufficient cutting speed.