Basic Knowledge of Cemented Carbide Blade Performance
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Basic Knowledge of Cemented Carbide Blade Performance
An average machining workshop may consume thousands of cutting inserts every year. An operator may use many cutting blades every day, but he has never thought about the complex scientific knowledge behind these blades. Knowing some manufacturing technology of cutting inserts will be of great help to the correct use and performance optimization of cutting tools.
- The composition of cemented carbide blades
As with all man-made products, manufacturing cutting blades must first solve the problem of raw materials, that is, determine the composition and formula of the blade material. Most of the current blades are made of cemented carbide, whose main components are tungsten carbide (WC) and cobalt (Co). WC is a hard particle in the blade, and Co as a binder can make the blade shape.
The easiest way to change the characteristics of cemented carbide is by changing the grain size of the WC particles used. Cemented carbide materials prepared with larger particle size (3-5μm) WC particles have lower hardness and are easier to wear; smaller particle size (<1μm) WC particles can produce higher hardness and better wear resistance , But also brittle hard alloy material. When processing metal materials with very high hardness, the use of fine-grained carbide blades may obtain the most ideal processing results. On the other hand, coarse-grained carbide inserts have better performance in intermittent cutting or other processing that requires higher insert toughness.
Another way to control the characteristics of tungsten carbide inserts is to change the ratio of WC to Co content. Compared with WC, Co has much lower hardness but better toughness. Therefore, reducing the Co content will result in a higher hardness blade. Of course, this once again raises the question of overall balance—harder blades have better wear resistance, but they are also more brittle. According to the specific processing type, selecting the appropriate WC grain size and Co content ratio requires relevant scientific knowledge and rich processing experience.
By applying gradient material technology, a compromise between blade strength and toughness can be avoided to a certain extent. This technology that has been commonly used by major global tool manufacturers includes the use of a higher Co content ratio in the outer layer of the blade than the inner layer. More specifically, the Co content is increased in the outer layer (thickness of 15-25 μm) of the blade to provide a “buffer” effect, so that the blade can withstand a certain impact without breaking. This allows the blade body to obtain various excellent performances that can be achieved by using higher-strength cemented carbide components.
Once the technical parameters such as the particle size and composition of the raw material are determined, the actual manufacturing process of the cutting insert can be started. First, put the tungsten powder, carbon powder, and cobalt powder in the proportions into a mill that is about the size of a washing machine, grind the powder to the required particle size, and mix the various materials uniformly. Alcohol and water are added during the milling process to prepare a thick black slurry. Then the slurry is put into a cyclone dryer, and after the liquid in it is evaporated, agglomerated powder is obtained and stored.
In the next preparation process, the prototype of the blade can be obtained. First, the prepared powder is mixed with polyethylene glycol (PEG). PEG is used as a plasticizer to temporarily bond the powder together like a dough. The material is then pressed into the shape of the blade in a press mold. According to different blade pressing methods, a single-axis press can be used to press, or a multi-axis press can be used to press the blade shape from different angles.
After obtaining the pressed blank, it is placed in a large sintering furnace and sintered at high temperature. During the sintering process, PEG is melted and discharged from the blank mixture, and finally the semi-finished carbide blade remains. When the PEG is melted out, the blade shrinks to its final size. This process step requires precise mathematical calculations, because the shrinkage of the blade varies according to different material compositions and ratios, and the dimensional tolerance of the finished product is required to be controlled within a few microns.
- Preparation of Tungsten carbide inserts coating
At this point, the shape of the product is almost the same as the final finished blade. However, in order to obtain the best cutting performance, the insert must be surface coated. The most commonly used blade coating process is a chemical vapor deposition (CVD) process, that is, a certain metal target is ionized by high current and then deposited on the blade by evaporation and condensation. This process can be vividly compared to that when the temperature of the asphalt road becomes very low and the air is full of high concentration of water vapor, a thin layer of ice will form on the road. However, the difference is that although the temperature of the blade placed in the coating furnace is relatively low, the actual furnace temperature may exceed 480°C.
Another commonly used blade coating process is the physical vapor deposition (PVD) process. Compared with CVD process, the use of PVD technology can deposit a thinner coating, so that the cutting edge can be sharper, and it can be more excellent when cutting difficult-to-machine materials (such as hardened steel, titanium alloy and heat-resistant super alloy) Cutting performance.
In a typical blade CVD coating process, the first layer of coating on the blade is titanium carbide nitride (TiCN). This coating material can provide excellent wear resistance, but also has the advantage of being easy to bond with the cemented carbide substrate. Usually, aluminum oxide (Al₂O₃) is used as the second coating. This coating has excellent thermal and chemical stability, and can protect the blade from the adverse effects of cutting high temperature and chemical composition in the coolant.
The thickness of TiCN and Al₂O₃ coating mainly depends on the type of blade processing. For example, when turning hard materials, the blade needs to be fully protected, so the thickness of each coating may need to reach 10μm. For the finishing of soft materials, it may be more appropriate to coat a 5μm thick TiCN layer and a 2μm thick Al₂O₃ layer.
After finishing the preparation of TiCN and Al₂O₃ coatings, the cutting insert is close to the finished product in terms of function. Unfortunately, the color of the Al₂O₃ coating is completely black, and it is difficult for the user to distinguish which working surfaces of the blade have been used and whether the cutting edge has been worn. In order to solve this problem, most tool manufacturers will finally apply a layer of titanium nitride (TiN) coating on the blade. This bright golden coating has good visibility, and users can easily evaluate the wear status of the cutting blade through the change of its color.
In the past, the completion of the TiN coating marked the completion of the manufacturing of the cutting insert. But in recent years, the last process has become more popular. In the CVD or PVD coating process, when the blade cools, the degree of shrinkage of different coating materials varies. Therefore, stresses are generated in each layer of the coating, and microcracks appear. In order to eliminate these stresses and minimize micro-cracks, people have adopted an advanced technology of sandblasting the blade with a mixture of alcohol, alumina and fine sand. After the sandblasting process is completed, the manufacturing of the cutting insert is complete.
- The role of cemented carbide blade geometry
When it comes to cutting insert geometry, most tool manufacturers will immediately begin to describe the macro geometry (physical shape) of the insert. And a research field that has developed rapidly in recent years-the optimization of the microscopic geometry of the cutting edge of the insert-deserves great attention.
At the macro level, the optimization of the blade geometry mainly involves the best shape possible for chip control. According to different workpiece materials and processing methods, using different blade shapes and angles can provide the best results for chip breaking and chip removal from the cutting area. The design and optimization of the macroscopic geometry of the blade is a fairly mature technical field, and most major tool manufacturers are proficient in this.
It is only in recent years that the development of technology has reached a level that can control the microscopic geometry of the blade. Using advanced processing technology, round, oval or angled cutting edges can be prepared on the cutting surface of the blade, and tiny chamfers or grooves can be introduced into the cutting edge of the blade. With the application of various innovative technologies, people can passivate and measure the blade on a tiny scale, which greatly improves the service life and processing stability of the blade. It can be expected with certainty that future technological advancements will further promote the development of this field and will achieve more significant results.
- Ceramic blade technology
Although most cutting inserts are made of cemented carbide, the number of inserts made of other materials is increasing. Among them, ceramic blades may be one of the most important non-hard alloy blades. With the increasing application of heat-resistant alloy materials (such as Inconel alloy) in parts of the aviation industry and other industries, ceramic blades have shown excellent cutting performance in the processing of these difficult-to-machine materials.
The manufacturing process of ceramic blades is very similar to that of cemented carbide blades. Since ceramics are not as easy to bond as other materials, much higher temperatures and pressures must be used during sintering.
Generally, the use of silicon carbide (SiC) whiskers in ceramic blades can increase its strength. These tiny fibers can play the same role as reinforcing concrete with steel bars. In the past, the strengthening effect of adding SiC to ceramics was relatively small, but technological breakthroughs in recent years have changed this situation. The new process allows the SiC whiskers to be oriented in a specific direction, thereby greatly improving the strengthening effect. Compared with other blade materials, ceramics are more brittle and often have defects. The addition of correctly oriented SiC whiskers can significantly slow down the fracture failure process of ceramic blades, because microcracks in the blade require more energy to pass through the neatly arranged whiskers. As this technology and other similar technologies continue to develop, ceramic blades will become a more feasible solution suitable for various processing.
- Get more benefits from cutting inserts
From a purchasing decision point of view, the most important thing to keep in mind for cutting inserts is to ignore the hard-to-observe aspects. If the cutting test is not passed, it may be difficult to distinguish the difference between a high-quality blade and a poor-quality blade even with careful inspection. Because the blades are similar in appearance, choosing cheap blades will inevitably increase the cost in future processing.
When selecting an insert grade, the ideal approach is to consult the technical experts of the tool manufacturer. In addition, some basic concepts can be used to narrow the range of blades available. Most tool manufacturers use a way that reflects the characteristics of the blades to number them. Take Sandvik Coromant’s products as an example. The first digit of a blade brand reflects the category it belongs to. For example, 4 represents the grade of processed steel, 3 represents the grade of processed cast iron, and 2 represents the grade of processed stainless steel. In each category, the last two digits indicate the hardness of the blade. A small number indicates higher hardness but greater brittleness; a larger number indicates lower hardness but better toughness. In order to find the type of blade needed, it is best for the workshop to start from the middle of the product catalog and look forward or backward according to its performance.
Finally, if a certain insert does not achieve the best cutting performance, some evidence can be found to help determine the solution. A careful observation of the cutting edge of the blade with a magnifying glass can reveal the essence of the problem. If the inspection shows that the cutting edge of the blade has obvious abrasive wear or slight deformation, it indicates that the hardness of the blade is low, and it is necessary to change to a higher hardness grade. If the blade is chipped and the cutting edge is missing small pieces, you may need to switch to a grade with lower hardness and better toughness. By understanding how cutting inserts are manufactured and how to customize different insert grades for specific processing, various targeted measures can be taken to improve processing efficiency and reduce processing costs.
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