In the aviation engine manufacturing industry, material properties are constantly improving, and CNC machining technology for difficult-to-machine materials (high-temperature alloys) has become a common concern in the industry. Ceramic cutting tool materials have the characteristics of high hardness, good wear resistance and heat resistance, excellent chemical stability, and are not easy to bond with metals. It has become one of the main tool materials for high-speed cutting of high-temperature alloys. Moreover, the optimal cutting speed of ceramic tools is 8 to 10 times higher than that of cemented carbide tools, which can greatly improve cutting efficiency. At present, new ceramic tools are constantly emerging, and ceramic tools will account for 15% to 20% of mechanical processing tools worldwide. Its development may cause another revolution in the field of cutting processing.
Disadvantages of Carbide Tools in Processing High-Temperature Materials
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 HRC35-47) and good toughness. However, compared with ordinary steel parts, its machining performance is poor, and the cutting process consumes more energy.
In the past 10 years, the application of carbide tools to process titanium-based, nickel-based and cobalt-based high temperature alloys has been widely popularized. The high hardness and high toughness of carbide materials at working temperatures below 600°C make it an ideal tool for cutting high temperature alloys and titanium alloys. However, carbide tools have a fatal weakness. Its melting point is about 1200°C. When the temperature in the cutting zone is higher than 800°C, the strength and hardness of the blade will drop significantly, and the wear will increase. It is even difficult to complete normal cutting.
Therefore, when using carbide tools to cut high temperature alloy materials, in order to avoid excessively high temperatures in the cutting zone, the linear speed can only be maintained at about 40m/min. For parts with large machining allowance, due to the slow cutting speed, the metal removal rate is very low, the machine time is very long, and the production cost is greatly increased. This makes the potential of modern CNC machine tools far from being fully utilized. With the continuous improvement of the performance of new engines and the emergence of new materials, carbide tools have been difficult to adapt. Therefore, finding a more ideal cutting tool has become a top priority.
Advantages of Ceramic Cutting Tools for Processing High-Temperature Materials
As early as 20 years ago, aircraft engine companies in developed countries (such as GE in the United States and Rolls-Royce in the United Kingdom) began to use ceramic tools to process high-temperature alloy materials. The biggest feature of ceramic materials is their high melting point (above 2000℃). The hardness does not drop much at 1200℃. It is an ideal material to replace carbide tools to achieve high-speed cutting. However, in my country, due to various reasons, the use of such tools has not been widely popularized.
Chip forming in cutting processing is a typical large deformation process, involving material nonlinearity, geometric nonlinearity and boundary nonlinearity. In the high-speed cutting process, it also involves thermal coupling problems.
The famous cutting experts Piisnen and Merchant pointed out in the chip formation mechanism proposed as early as 1945. Under the action of shear force (cutting force), the grain boundaries near the shear surface begin to be torn and deformed, separated from the matrix to form chips, and generate a lot of heat. In fact, about 80% of the cutting heat is generated by this.
he core of using ceramic tools to achieve high-speed cutting is to make full use of the high-temperature characteristics of ceramic materials. Increasing the cutting speed will cause the cutting heat to accumulate continuously, the temperature of the cutting zone will increase, the chips will be softened, and cutting will become very easy. Although the toughness and wear resistance of ceramic materials are much different from those of cemented carbide materials, their high temperature stability is far beyond the reach of cemented carbide tools. Therefore, increasing the linear speed is the most effective way to increase the temperature of the cutting zone. Theoretically, the cutting speed and metal removal rate of ceramic tools should be 5 to 10 times or even more than that of cemented carbide tools.
When promoting, tool manufacturers only proposed that ceramic tools are suitable for processing materials above HRC55, and there are no corresponding reports on materials less than HRC55. This article talks about my own experience in the processing of materials less than HRC55.
Due to the long-term use of cemented carbide tools, operators have become accustomed to low-speed cutting, and this processing method suitable for cemented carbide is precisely the biggest taboo in the processing of ceramic tools. When using ceramic tools, for safety reasons, operators are always afraid to increase the speed, and even hope to use ceramic tools on ordinary lathes. Most of the problems encountered in the use of ceramic tools in the past were caused by insufficient cutting speed.
Matters Needing Attention in Processing of Ceramic Cutting Tools
Speed is the key to the life of the blade. We must change our mindset and boldly increase the cutting speed. Ensuring that enough cutting heat is generated during the cutting process is the key to improving the life of the tool. However, the higher the cutting speed, the better. If the cutting temperature is too high, too much cutting heat cannot be taken away and remains in the matrix, causing the temperature of the parts to rise, and the parts to deform due to thermal stress. In addition, in the test, we found that once the speed exceeds a certain limit, the blade will wear very quickly.
The wear resistance of ceramic materials is not as good as that of cemented carbide. If multiple cuttings are made with equal cutting depth, groove wear perpendicular to the blade will inevitably appear at the contact point between the blade and the part. Therefore, it is necessary to constantly change the contact point between the blade and the workpiece. This method is very effective in extending the service life of the blade.
Compared with cemented carbide, ceramic materials are still relatively brittle. Therefore, vibration should be resolutely eliminated during the cutting process. This requires that the machine tool has sufficient power, the spindle rotates smoothly, the feed is uniform, and the cutting route is “push cutting”. Do not try to use ceramic tools on ordinary machine tools.
For materials of different hardness, reasonable cutting parameters and tool paths should be selected. Optimize the feed and cutting speed combination, only in this way can efficient cutting be guaranteed.
If local chipping occurs on the front of the blade, it is caused by the pressure generated by the increased wear on the side. This phenomenon usually does not affect the performance of the tool. In fact, after the front of the blade is chipped, a new sharp blade will be produced, and the cutting can continue with satisfactory cutting results. In fine machining, “chipping” will affect the finish and produce “burrs”. When “chipping”, sparks can be seen in front of the blade. This spark is caused by the high-temperature iron chips passing through the rough surface of the blade. The feed should be reduced to complete this cutting.
Before the next cutting, check whether the blade needs to be replaced. When rough machining, make full use of the “chipped” blade and do not hastily decide to abandon the blade. The “chipped” blade can continue to be used until it is indeed unable to cut.
Ceramic blades will not break seriously and cause accidents unless serious errors are performed. The main wear forms of ceramic blades are chipping and back face wear. The so-called flank wear is a progressive wear form, which exists in all kinds of tools. Its wear degree and corresponding cutting speed are indicators of tool life. For nickel-based alloy parts, the groove wear of ceramic blades occurs on the cutting depth line. The ideal application method should be that the groove wear reaches the maximum while the flank wear also reaches the maximum. Groove wear is allowed to extend to 1/3 of the thickness of the blade. Rapid groove wear or chipping often occurs in the cutting area, which is caused by insufficient heat in the cutting area. It can be corrected by increasing the cutting speed or reducing the feed or adjusting both at the same time.
The tool path for carbide tools should be different from that for ceramic blades. Ceramic blades will fail quickly due to groove wear. The programming method and tool path of ceramic tool cutting are not completely the same as those of carbide tools. Appropriate tool paths and cutting parameters must be used.
Ceramic tools are not suitable for finishing thin-walled parts with a wall thickness of less than 2mm. Carbide tools should still be used. Conclusion Ceramic materials are the most promising and competitive tool materials in the 21st century. Its development may cause another revolution in the field of cutting processing. Although we have gained some experience in the process of trying, we still need to conduct further experiments to expand the variety of processing materials. Only by mastering the performance of ceramic tools can we better apply them to the processing of high-temperature alloys.
