The milling of titanium alloy parts is the same as other difficult-to-machine materials, because of the small increase in cutting speed, which causes faster wear of the cutting edge of the tool. The difference is that due to the high strength and high viscosity of the titanium alloy, it is easier to generate and accumulate heat in the cutting area during cutting, coupled with poor thermal conductivity, there is a danger of burning when milling with a large amount of cutting. This is why milling titanium alloy parts must not choose high cutting speeds. However, the processing speed of titanium alloy parts can still be increased. That is, when the cutting speed remains unchanged, the processing speed of the part is increased by increasing the metal removal rate. Achieving this goal does not include the use of more powerful or high-end machine tools, but the provision of tools that can fully utilize the cutting capabilities of existing machine tools. It can also compensate for certain machine tool deficiencies, such as poor rigidity. Kennametal is a well-known tool manufacturer focusing on the experimental research of titanium alloy milling process. There is a technical consultant and milling product manager Mr. Brian Hoefler who has received many consultations on titanium alloy milling technology users. This article focuses on his extensive experience in titanium alloy milling. Why does the milling of titanium alloy cause people's special attention? There are at least two reasons. First, titanium alloys are mainly used for high-end parts, not only for manufacturing aircraft fuselage and engine parts, but also for manufacturing many parts in medical equipment. Especially for some growing American manufacturing companies, they must transfer to high-end products and often encounter technical difficulties in milling titanium alloy parts. Another reason is that not every workshop can achieve high feed speed processing, so when titanium alloy milling is difficult to process materials, or when the cutting speed is not high during processing, what method can be used to achieve high efficiency processing becomes urgent. The problems solved have aroused great attention from manufacturers. When the cutting speed is limited, the use of plunge milling for rough machining of parts is the most effective method that can significantly improve the metal removal rate. Rough machining with plunge milling, the milling cutter feeds in the Z axis direction. This method can be used for the tools shown in the left picture. This method can not only ensure that more cutting edges are cut at the same time, but also can take advantage of the high-efficiency machining of high-rigidity machine tools. The example of rough machining CAM by plunge milling is a major advantage of Mastercam/CNC software. The use of high toughness tool carbide tool can be a correct choice, and the machine shop is often used to cutting the best carbide Tool materials, especially in almost all difficult machining, usually choose cemented carbide. For titanium alloy processing, a new generation of high-speed steel will be a good substitute for cemented carbide. It stands to reason that carbide cutting tools with good wear resistance can implement high cutting speeds at reasonable processing costs. But this reasonable processing cost is based on the premise that the tool must have a "high toughness" or be able to resist impact and fracture resistance. Unfortunately, the commonly used cemented carbides are far more brittle than high-speed steels. This is of great significance in milling titanium alloys. Generally speaking, the main reason for the failure of cemented carbide tools is not the wear of the cutting edge, but the fracture of the blade body. Secondly, the increase in cutting heat during the milling of titanium alloys also prevents cemented carbide tools from taking advantage of high cutting speeds. Because machining at a high cutting speed requires the addition of a large amount of coolant, under the alternating effect of heat and cold, a strong thermal shock is generated between the tool and the workpiece, which will quickly cause the brittle carbide tool cutting edge broken. The above two technical problems need to be solved by the inherent high toughness of the tool itself. But ordinary carbide tools are far from competent. The cutting test proves that using a high-toughness tool, such as milling a titanium alloy workpiece with a high-speed steel tool, there is no need to worry about causing impact and cutting edge cracking during cutting. Especially when machining on less rigid machine tools, high-toughness high-speed steel tools can achieve high metal cutting rates by increasing the cutting depth rather than by increasing the cutting speed. Not only that, but a wide range of high-toughness high-speed steel tool materials are also available for users to choose. Most workshops do not know this. They also don't know that the high-speed steel cutters on the market can also undergo some special processing procedures, such as the implementation of high-speed steel smelting (such as increasing the cobalt content) to increase certain element components for heat treatment (multiple grade quenching and tempering), or the High-speed steel materials are made into powder metallurgy high-speed steel with uniform metallographic structure through strict control of its manufacturing process. Therefore, expensive high-cobalt high-speed steel and powder metallurgy high-speed steel are ideal tool materials for efficient milling of titanium alloys. Control of high cutting temperature Sometimes carbide tools can also be selected, and a small radial cutting method can be used to cut titanium alloy parts, which can achieve amazing high speed (see "10% and 100%" section). In these cuttings, the tool must not only solve the problem of wear resistance in general, but also solve the problem of wear resistance of the tool at high cutting temperatures. This is very important, and it needs to be processed with coated carbide tools. Both HSK quick-change toolholders and heat-expandable toolholders can be used for high-rigidity machining. They can reduce vibration during processing and greatly improve the removal rate of metal processing. According to Mr. Hoefler, aluminum titanium nitride (TiAlN) coated carbide tools are usually the best choice for processing titanium alloys. In many basic tool coating types, TiAlN has a good effect on maintaining the comprehensive mechanical properties of the tool and maintaining the high-temperature cutting performance of the tool when the temperature increases. In fact, the high cutting temperature also has a certain protective effect on the coating. Aluminum molecules are released from the coating by the machining energy during cutting, forming a protective layer of alumina on the surface of the tool. This protective layer of alumina reduces the heat transfer between the tool and the workpiece and the diffusion of chemical elements. At the same time, shortly after the formation of this protective coating, more aluminum molecules will be continually added to keep this chemical reaction forming the protective layer of aluminum oxide continuing (see section "New aluminum-rich coating"). However, TiAlN coating is not suitable for applications with strong vibration. At this time, titanium carbonitride (TiCN) is used, which can prevent the coating from peeling off due to vibration. "When you are using interchangeable inserts and cutting on a machine with less rigidity, try TiCN may be the best choice." Mr. Hoefler said. More cutting edges participate in cutting Even if the cutting speed, the feed per tooth and the cutting depth of the milling cutter are kept constant during cutting, sometimes the production efficiency can be improved. The solution here is to make more cutting edges participate in cutting. For example, for a spiral milling cutter, choose a small-pitch tool (such as a spiral corn end mill) as much as possible. Using this kind of tool can make the high speed steel knife have more cutting edges. Since high-speed steel tools can provide more cutting edges than cemented carbide tools, the former are more commonly used. The illustrated tool is a large helix angle end mill with a cutting edge that has an axial rake angle different from the next cutting edge. This change can better suppress vibration and greatly improve production efficiency. The method for multiple cutting edges to participate in cutting is to take milling in different directions. Through the "plunge milling roughing" (sometimes called drilling roughing) method, using a set milling cutter, as if drilling along the Z axis, the end teeth and side teeth of the tool are jointly compiled according to the compiled machining program, Carry out lap processing. Therefore, the production efficiency is high and the chip removal is also convenient. This method can only be used for rough machining, because there are still some scallop-like unprocessed metals between each lap machining. However, because plunge milling has many cutting edges to participate in cutting, the feed rate per minute can be greatly improved when the feed per tooth of the tool is kept constant. In addition, the advantage of plunge milling roughing Z axis feed is that it can take advantage of the high rigidity of the machine tool, because the variety of connection mechanisms along the spindle (such as tool holder interface) are bound to be generated along the X or Y axis Deflection, and compression in the Z axis direction, so that the machine tool has a high rigidity in the Z axis direction. This means that the feed per tooth of the tool can be increased. Mr. Hoefler said, "Rough plunge milling is the best solution for efficient machining of high-strength metals. It is recommended that this machining scheme can be used in titanium alloy milling." Vibration elimination measures can cause tool deflection during cutting The reason and the research to eliminate the problem are also very important, because it will lead to a very important technical problem-vibration. Vibration In titanium alloy milling, there are two unfavorable factors: First, the generation and increase of cutting force will cause and increase vibration; on the other hand, the spindle speed of the machine tool does not seem to be related to vibration, so it cannot be found. It produces an "ideal" speed that can tune vibration. In fact, vibration determines the productivity of most titanium alloy milling processes. A large number of cutting tests have proved that in the milling of titanium alloys, the maximum metal cutting rate is obtained not at the time when the machine tool outputs maximum power, but at the beginning of extreme vibration. This is why it is necessary to establish a vibration program that can be controlled in time. Mr. Hoefler suggested that in order to improve the production efficiency of titanium alloy milling, the following technical issues must also be solved: The connection between the rigidity tool and the tool holder, and the connection between the tool holder and the spindle must be made as much as possible To ensure sufficient rigidity. For the tool holder, the thermal expansion and contraction type provides the best solution. For the spindle, the HSK quick-change tool holder provides the best rigidity compared with the ordinary taper interface. Damping The tool is designed with an eccentric relief angle or a "edge" structure, which can provide good damping to suppress the vibration generated during cutting. When the tool is flexed and deformed, the flank of the tool with eccentric relief will contact and rub against the workpiece. Not all materials can rub against the workpiece better, and aluminum alloys tend to adhere. For titanium alloy milling, the "edge" sharpened on the cutting edge of the tool will also serve as a good shock absorber. Changing the chip flute space between cutting edges For such a structured tool design and anti-vibration measures, many workshops may not be familiar with it. During the high-speed rotation of the tool, the cutting edge regularly strikes the workpiece, thus generating vibration. If the chip flute space of the milling cutter is designed to be arranged irregularly, the cutting test proves that it will play a good role in damping vibration. For example, when the first and second cutting edges of the milling cutter are 72° apart, the second and third cutting edges should be 68° apart, and the third and fourth cutting edges should be 75° apart, which is unevenly distributed. . Another patented anti-vibration measure designed by Kennametal is that the cutting edge of the milling cutter is designed to have unequal axial rake angles, which can also achieve good vibration reduction effects. New aluminum-rich coating "Al" molecule is the most active in TiAlN coating, which has a great influence on the cutting performance of coated tools. It can form an aluminum oxide protective film on the surface of the tool. In the coating, the content of "Al" molecules increases, making this effect more effective. Of course, we should thank the continuously improved vapor deposition process technology used to produce coatings, which can continue to increase the content of "Al" molecules in TiAlN, and as a result, the newly formed TiAlN coatings without sacrificing toughness , Excellently improves the red hardness of the coating (cutting tool). Kennametal has developed this new aluminum-rich TiAlN coated tool in the first half of this year. 10% and 100% At present, some advanced technology workshops have been able to use cemented carbide coated cutting tools and use a small radial cutting method to cut titanium alloy parts. The main purpose is to solve the high cutting temperature technology generated in titanium alloy processing problem. The cutting principle is to select a radial cutting depth that is much smaller than the radius of the tool for radial cutting during the cutting process using the small radial cutting method. As a small depth of cut is selected, the cutting speed can be greatly improved. As a result, the cutting time of each cutting edge is greatly reduced, that is, the processing time of the cutting edge is reduced, and the non-cutting time is extended, that is, the cutting edge is increased. The cooling time is excellent to control the cutting temperature. According to Mr. Brian Hoefler of Kennametal, cutting titanium alloy parts by small radial cutting method can control the cutting temperature very well and realize high-speed machining. Small radial depth of cut will not bring high metal removal rate, but the use of this method in the factory can improve processing accuracy. The cutting test conducted by Mr. Hoefler proved that in the milling of titanium alloy parts, the small radial cutting method will follow the following rules: When the radial cutting depth is less than 25% of the diameter, the cutting speed can be increased by 50% (sfm ), generally exceeding the rated speed for heavy cutting. When the radial cutting depth is less than 10% of the diameter, the cutting speed (sfm) can be increased by 100%.