High efficiency milling technology of titanium all

2022-06-19
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High efficiency milling technology of titanium alloy the milling of titanium alloy parts is the same as that of other difficult to machine materials, which will cause rapid wear of tool cutting edges due to a small increase in cutting speed

the difference is that due to the high strength and viscosity of titanium alloy, it is easier to generate and accumulate heat in the cutting area during cutting. In addition, due to the poor thermal conductivity, there is a risk of combustion during milling with large cutting amount. This is the reason why high cutting speed must not be selected for milling titanium alloy parts

however, the processing speed of titanium alloy parts can be improved. That is, when the cutting speed remains unchanged, the machining speed of parts can be improved by improving the metal removal rate. This goal does not include the use of higher power or high-grade machine tools, but the provision of cutting tools that can give full play to the cutting functions of the existing machine tools. At the same time, it can also compensate for some shortcomings of the machine tools, such as poor rigidity

kennametal is a famous tool manufacturer focusing on the experimental research of titanium alloy milling process. In the company, there is Mr. Brian Hoefler, a technical consultant and milling product manager who has received many users consulting titanium alloy milling technology. This paper mainly introduces his rich experience in titanium alloy milling

why do people pay special attention to the milling of titanium alloy? There are at least two reasons. First, titanium alloys are mainly used for high-grade parts, not only for manufacturing aircraft fuselage and engine parts, but also for manufacturing many parts in medical devices. Especially for some growing American manufacturing enterprises, they must transfer to high-end products, and often encounter the technical problems of titanium alloy parts milling

another reason is that not every workshop can achieve high feed rate machining. Therefore, when it is difficult to machine materials in titanium alloy milling or the cutting speed is not high in the machining process, how to achieve high-efficiency machining has become an urgent problem to be solved, which has attracted great attention of manufacturers

when the cutting speed is limited, it is the most effective method to select Insert milling for rough machining of parts, which can significantly improve the metal removal rate

rough machining with insert milling method, and the milling cutter feeds along the z-axis direction. This method can be used for all the tools shown in the left figure

this method can not only ensure that more cutting edges can be cut at the same time, but also give full play to the advantages of high-efficiency machining of high rigidity machine tools

the example of rough machining cam with insert milling method is a major advantage of mastercam/cnc software.

the correct selection of cutting tool materials using high toughness tools will be the first important issue to achieve efficient milling of titanium alloys, Mr. Hoefler said. Cemented carbide tools can be a correct choice, and machining workshops are often used to taking cemented carbide as the best cutting tool material, especially in almost all difficult machining, cemented carbide is usually selected. For titanium alloy processing, the new generation of high-speed steel will be a good substitute for cemented carbide

it is reasonable to say that cemented carbide tools with good wear resistance can implement high cutting speed under reasonable processing cost. However, this reasonable processing cost is based on the premise that the tool must have "very high toughness" or be able to resist impact and fracture. 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 cause of carbide tool failure is not the wear of cutting edge, but the breakage of tool body. Secondly, the increase of cutting heat in the process of milling titanium alloy also makes cemented carbide tools unable to give full play to the advantages of high cutting speed. Because machining at high cutting speed requires filling a large amount of coolant, under the alternating action of heat and cold, there is a strong thermal shock between the tool and the workpiece, which will soon cause the breaking of the cutting edge of the brittle cemented carbide tool. The above two technical problems need to be solved through the inherent high toughness of the tool itself. But ordinary cemented carbide tools are far from competent. Cutting tests show that using a high toughness tool, such as using high-speed steel tools to mill titanium alloy workpiece, there is no need to worry about the generation of impact in cutting and the fracture of cutting edge. Especially for machining on machine tools with small rigidity, high-speed steel tools with high toughness can achieve high metal cutting rate machining by increasing the cutting depth rather than by increasing the cutting speed

not only that, but also a wide range of high toughness high-speed steel tool materials are available for users to choose. Most workshops do not know this. They also don't know that the high-speed steel cutting tools sold on the market can be subject to some special treatment procedures, such as heat treatment (multi-stage quenching and tempering) by adding high-speed steel smelting with certain element composition (such as increasing cobalt content), or the high-speed steel material can be made into powder metallurgy high-speed steel with uniform metallographic structure through strict control of its manufacturing process. Therefore, high cobalt high-speed steel and powder metallurgy high-speed steel, which are expensive, are ideal tool materials for efficient milling of titanium alloys

BASF with high cutting temperature exhibited a variety of product solution control

sometimes hard alloy tools can also be selected. Cutting titanium alloy parts with a small diameter cutting method can achieve amazing high speed (see section 10% and 100%). In these cutting, the tool should not only solve the problem of wear resistance under general conditions, but also solve the problem of wear resistance under high cutting temperature. This is very important, and it needs to use coated carbide tools for machining

hsk quick change tool holder and thermal expansion and cold contraction tool holder can be used for high rigidity machining

they can reduce vibration during machining and greatly improve the removal rate of metal processing.

according to Mr. Hoefler, aluminum titanium nitride (TiAlN) coated cemented carbide tools are usually the best choice for machining titanium alloys. In many basic tool coatings, TiAlN plays a good role in maintaining the comprehensive mechanical properties of the tool and the high-temperature cutting performance of the tool when the temperature increases. In fact, high cutting temperature can also protect the coating. Aluminum molecules are released from the coating through the machining energy during cutting, forming an alumina protective layer on the tool surface. This aluminum oxide protective layer reduces the heat transfer between the tool and the workpiece and the diffusion of chemical elements. At the same time, more aluminum molecules can be added soon after the protective coating is formed, so as to keep the chemical reaction of forming the aluminum oxide protective coating continuing (see the section of new aluminum rich coating)

however, TiAlN coating is not suitable for occasions with strong vibration. At this time, titanium nitride (TiCN) is used, which can prevent the coating from peeling off due to vibration. "When you use replaceable blades and perform heavy cutting on a less rigid machine tool, trying TiCN may be the best choice." Mr. Hoefler said

more cutting edges participate in cutting

even if the cutting speed, feed per tooth and cutting depth of the milling cutter remain unchanged during cutting, sometimes the production efficiency can be improved. The solution here is to make more cutting edges participate in cutting

for example, for spiral milling cutters, select small pitch cutters (such as spiral corn end mills) as far as possible. Using this tool can make the high-speed steel cutter have more cutting edges. Because high-speed steel tools can provide more cutting edges than cemented carbide tools, the former is more widely used

the tool shown in the figure is a large spiral 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.

another way to make more cutting edges participate in cutting is to milling in different directions. Through the method of "plunge milling rough machining" (sometimes also known as drill in rough cutting), a suit of milling cutter is used, just like drilling along the Z axis. The end teeth and side teeth of the cutter are used to carry out lap joint machining according to the assembled machining program. Therefore, the production efficiency is high and the chip removal is convenient

this method can only be used for rough machining, because there is still some scallop like raw metal between every two lap machining. However, because there are many cutting edges involved in insert milling rough machining, the feed rate per minute can be greatly improved when the feed rate per tooth of the tool remains constant. Moreover, the advantage of z-axis feed for insert milling rough machining is that it can give full play to the high rigidity advantage of the machine tool. This is because the various connecting mechanisms along the spindle (such as tool holder interface) are bound to produce deflection along the X or Y axis and compression in the z-axis direction. In this way, the machine tool automatically measures the high rigidity along the z-axis direction. This means that the feed per tooth of the tool can be increased

hoefler said, "insert milling rough machining is the best solution for efficient machining of high-strength metals. It is suggested that this processing scheme can be used in titanium alloy milling."

measures to eliminate vibration

is also very important for the research on the causes of tool deflection in cutting and its elimination, 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 seems to have nothing to do with the vibration, so it is impossible to find an "ideal" speed that can tune the vibration

in fact, vibration determines the productivity of most titanium alloy milling. A large number of cutting tests have proved that in titanium alloy milling, the maximum metal cutting rate is obtained not when the machine tool outputs the maximum power, but at the beginning of great vibration. This is why it is necessary to establish and can also establish a vibration control program in time. Mr. Hoefler suggested that in order to improve the production efficiency of titanium alloy milling, the following technical problems must also be solved:

the connection between the rigid tool and the tool holder, and the connection between the tool holder and the spindle must be as rigid as possible. For the tool holder, the thermal expansion and cold contraction type provides the best solution. For the spindle, the HSK quick change tool holder provides the best stiffness compared with the ordinary taper interface

damping the tool is designed with an eccentric back angle or a tool head structure with "edges", which can provide good damping to suppress the vibration generated in cutting. When the tool deflects, the flank of the tool with eccentric back angle will contact and rub with the workpiece. Not all materials can rub well with the workpiece, and aluminum alloys tend to adhere. For titanium alloy milling, the "edge" on the cutting edge of the tool will also play a good shock absorber role

change the space of chip removal groove between cutting edges. Many workshops may not be familiar with the tool design and anti vibration measures of such a structure. When the tool is rotating at high speed, the cutting edge impacts the workpiece regularly, resulting in vibration. If the space of the chip removal groove of the milling cutter is designed to be irregularly arranged, the cutting test shows that it can play a good role in vibration reduction, including bumper, door panel, seat back cover, cylindrical molded parts, door trim panel, instrument panel, head and side impact area, fender and horn, tail lamp housing and fairing. example

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