Titanium alloy high-efficiency milling technology

       

Titanium alloy high-efficiency milling technology

The milling of titanium alloy parts is similar to other difficult-to-machine materials in that it will cause faster wear of the cutting edge of the tool due to a small increase in the cutting speed. The difference is that due to the high strength and high viscosity of titanium alloy, it is easier to generate and accumulate heat in the cutting area during cutting. In addition, the thermal conductivity is poor, and there is a danger of burning when milling with a large amount of cutting.

This is the reason why high cutting speeds must not be selected for milling titanium alloy parts. However, the processing speed of titanium alloy parts can still be improved. That is, when the cutting speed remains the same, increase the processing speed of the parts by increasing the metal removal rate. Achieving this goal does not include using more powerful or high-end machine tools, but equipped with tools that can give full play to the cutting functions of existing machine tools, and it can also compensate for certain deficiencies of the machine tools, such as poor rigidity. Kennametal is a well-known tool manufacturer focusing on the experimental research of titanium alloy milling technology. There is a technical consultant and milling product manager Mr. Brian Hoefler who has received many users of titanium alloy milling technology in the company. This article focuses on his extensive experience in milling titanium alloys. Why does milling of titanium alloys cause special attention? There are at least two reasons. First, titanium alloys are mainly used for high-end parts, not only for the manufacture of aircraft fuselage and engine parts, but also for the manufacture of many parts in medical equipment. Especially for some growing American manufacturing companies, which must shift to high-end products, they often encounter technical difficulties in milling titanium alloy parts. Another reason is that not every workshop can achieve high feed rate processing, so when titanium alloy milling is difficult to process materials, or the cutting speed during processing is not high, how to achieve high-efficiency processing becomes urgent The problem solved has attracted the attention of the manufacturer. When the cutting speed is limited, the use of plunge milling for rough machining of parts is the most effective method that can significantly increase the metal removal rate. With plunge milling method for rough machining, the milling cutter feeds along the Z axis. 

The tools shown in the left picture can all use this method. This method can not only ensure that more cutting edges are cut at the same time, but also can give full play to the advantages of high-rigidity machine tools and high-efficiency machining. The example of rough machining CAM with plunge milling is a big advantage of Mastercam/CNC software. The use of high-toughness tools and cemented carbide tools can be a correct choice, and the machining workshop is often used to the best cutting of cemented 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 cemented carbide tools with good wear resistance can achieve high cutting speeds at reasonable processing costs. However, this reasonable processing cost is based on the "high toughness" or the ability to resist impact and fracture that the tool must have. Unfortunately, the brittleness of the commonly used cemented carbide is much greater than that of high-speed steel. 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. Secondly, the increase in cutting heat in the process of milling titanium alloys also prevents cemented carbide tools from taking advantage of high cutting speed machining. Because processing at high cutting speeds requires a large amount of coolant, under this alternating action of heat and cold, a strong thermal shock occurs between the tool and the workpiece, which will quickly cause the cutting edge of the brittle hard alloy tool. broken. The above two technical problems need to be solved by the inherent high toughness of the tool itself. However, ordinary cemented carbide tools are far from competent. Cutting tests have proved that using a high-toughness tool, such as a high-speed steel tool for milling titanium alloy workpieces, does not have to worry about impact and cutting edge fracture during cutting. Especially for processing on less rigid machine tools, high-toughness high-speed steel tools can achieve high metal cutting rate processing by increasing the cutting depth instead of increasing the cutting speed. Not only that, but a wide range of high-toughness high-speed steel tool materials are currently available for users to choose from. Most workshops do not know this. They also don’t know that high-speed steel knives sold on the market can also undergo some special treatment procedures, such as the implementation of high-speed steel smelting (such as increasing cobalt content) with a certain element composition for heat treatment (multiple grading quenching and tempering), or The high-speed steel material undergoes strict control of its manufacturing process to produce powder metallurgical high-speed steel with uniform metallographic structure. 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 hard alloy tools can also be selected, using a small radial plunge method to cut titanium alloy parts, which can achieve amazing high speeds (see the section "10% and 100%"). 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 is necessary to use coated carbide tools for processing. Both HSK quick-change toolholders and thermal expansion and contraction toolholders can be used for high-rigidity machining. They can reduce vibration during processing and greatly increase the removal rate of metal processing. According to Mr. Hoefler, titanium aluminum nitride (TiAlN) coated carbide tools are usually the best choice for processing titanium alloys. Among 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 protects the coating. The aluminum molecules are released from the coating by the processing energy during cutting, forming a protective layer of aluminum oxide on the surface of the tool. 

This protective layer of aluminum oxide reduces the heat transfer and the diffusion of chemical elements between the tool and the workpiece. At the same time, it can continue to replenish more aluminum molecules shortly after the formation of the protective coating to keep the chemical reaction that forms the aluminum oxide protective layer to continue (see the "New Aluminum-Rich Coating" section). However, TiAlN coating is not suitable for applications with strong vibration. At this time, titanium carbon nitride (TiCN) is used, which can prevent the coating from peeling off due to vibration. "When you are using interchangeable inserts and cutting hard on a less rigid machine, trying TiCN may be the best choice." Mr. Hoefler said. More cutting edges participate in cutting Even if the cutting speed, the feed per tooth of the milling cutter, and the depth of cut remain the same during cutting, production efficiency can sometimes be improved. The solution here is to make more cutting edges participate in the cutting. For example, for spiral milling cutters, small pitch cutters (such as spiral corn end mills) should be selected as much as possible. The use of this tool can make the high-speed steel knife have more cutting edges. Because high-speed steel tools can provide more cutting edges than carbide tools, the former is more commonly used. The tool shown in the figure is a large helix angle end mill with each cutting edge having an axial rake angle different from that of 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 different directions for milling. Through the method of "plunge milling roughing" (sometimes called drilling type roughing), a set milling cutter is used, as if drilling along the Z axis. The end teeth and side teeth of the tool are combined according to the compiled processing program. Perform lap joint 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 is still some scallop-shaped unprocessed metal between each lap process. However, because plunge milling roughing has many cutting edges participating in cutting, the feed rate per minute can be greatly improved when the feed per tooth of the tool remains constant. Furthermore, the advantage of the Z-axis feed for plunge milling rough machining lies in the high rigidity of the machine tool. This is because the diverse connection mechanisms along the spindle (such as the tool holder interface) are bound to occur along the X or Y axis. Deflection, and compression in the Z-axis direction, so that the machine tool has a high rigidity along the Z-axis direction. This means that the feed per tooth of the tool can be increased. Mr. Hoefler said, "Plug milling roughing is the best solution for efficient machining of high-strength metals. It is recommended that this machining solution can be used in titanium alloy milling." Elimination of vibration measures is very important for tool deflection during cutting. The research on the reason and its elimination is also very important, because it will lead to a very important technical problem-vibration.

 Vibration In titanium alloy milling, there are two unfavorable factors: one is that the generation and increase of cutting force can cause and increase vibration; on the other hand, the spindle speed of the machine tool seems to have nothing to do with vibration, so you can’t find it. A kind of "ideal" speed that can tune the vibration. In fact, vibration determines the production efficiency of most titanium alloy milling processes. A large number of cutting tests have proved that in the milling of titanium alloys, the maximum metal removal rate is obtained not when the machine tool outputs the maximum power, but when the extreme vibration starts. This is why it is necessary to establish and can also establish a program that can control the vibration in time. Mr. Hoefler suggested that in order to improve the production efficiency of titanium alloy milling processing, we must also pay attention to solving the following technical problems: The connection between the rigid 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 toolholders, thermal expansion and cold contraction, provide the best solution. For spindles, HSK quick-change toolholders provide the best rigidity compared with ordinary taper interfaces. Damping The tool is designed with an eccentric relief angle or a tool head structure with "edges", which can provide good damping to suppress vibration generated during cutting. When the tool flexes and deforms, the flank face of the tool with an eccentric relief angle will contact and rub against the workpiece. Not all materials can rub well with the workpiece, and aluminum alloy has a tendency to adhere. For titanium alloy milling, the "edge" ground on the cutting edge of the tool will also act as a good shock absorber. Changing the chip flute space between the cutting edges Many workshops may not be familiar with the tool design and anti-vibration measures of such a structure. During the high-speed rotation of the tool, the cutting edge regularly hits the workpiece, which causes vibration. If the chip groove space of the milling cutter is designed to be irregularly arranged, the cutting test proves that it will have a good vibration damping effect. For example, when the distance between the first and second cutting edges of a milling cutter is 72°, the distance between the second and third cutting edges should be 68°, and the distance between the third and fourth cutting edges is 75°, which is unevenly distributed . 

Another patented anti-vibration measure designed by Kennametal is to design the cutting edge of the milling cutter to have unequal axial rake angles, which can also achieve good vibration damping effects. The new aluminum-rich coating "Al" molecule is the most active among TiAlN coatings, and it has a great influence on the cutting performance of coated tools. It can form a layer of 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, thanks to the continuous improvement of the vapor deposition technology used to produce the coating, it can make the content of "Al" molecules in TiAlN continue to increase, and as a result, the newly formed TiAlN coating can be formed without sacrificing toughness. , It improves the red hardness of the coating (tool) extremely well. Kennametal has developed this new aluminum-rich TiAlN coated tool in the first half of this year. 10% and 100% At present, some advanced workshops have been able to use carbide-coated tools to cut titanium alloy parts with a small radial plunge method. The main purpose is to solve the high cutting temperature technology in titanium alloy processing. problem. The cutting principle is to select a radial cutting depth much smaller than the radius of the tool for radial cutting in the cutting process using the small radial cutting method. Due to the selection of a very small depth of cut, the cutting speed can be greatly increased. As a result, the cutting time of each cutting edge is greatly reduced, that is, the machining time of the cutting edge is reduced, and the non-cutting time is prolonged, that is, the cutting edge is increased. Cooling time, excellent control of cutting temperature. According to Mr. Brian Hoefler of Kennametal, cutting titanium alloy parts with a small radial plunge method can control the cutting temperature very well and achieve high-speed machining at the same time. Small radial depth of cut will not bring high metal removal rate, but using this method in the factory can improve machining accuracy. The cutting test conducted by Mr. Hoefler proved that in the milling of titanium alloy parts, the small radial plunge 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%.