How To Effectively Solve Titanium Alloy Processing And Cutting Problems
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How to effectively solve titanium alloy processing and cutting problems
Utilizing high-pressure cooling to improve titanium alloy cutting efficiency
Not only can it effectively solve the processing problems of cutting titanium alloys, it can also be effectively used in the processing of difficult-to-cut materials such as nickel-based alloys (such as Inconel 718-Inconel 718), stainless steel, and stainless steel. Mild steel. Titanium and titanium alloys are widely used in aerospace, medical, chemical, petroleum and other industries due to their excellent comprehensive properties such as high strength, good corrosion resistance, light specific gravity, and good heat resistance. Among them, the aviation industry has become the largest market for titanium products such as structural parts, landing gear parts, and turbine structural parts, accounting for 70% of consumption. Despite the higher cost of titanium materials, the use of titanium continues to increase.
The good physical and mechanical properties of titanium alloys (see table below) are of great significance for aircraft components: high specific strength, similar to the strength of steel, but only half the weight; low thermal conductivity, which can prevent components from being damaged at particularly low temperatures becomes brittle and does not ignite at higher temperatures Significant expansion; higher temperature strength, high temperature resistance up to 550°C, no change in material properties; good corrosion resistance, therefore, titanium alloys can be used to manufacture connectors connected to carbon fiber material components to replace those prone to electrical Chemically etched aluminum and aluminum alloys. The connection of carbon fiber materials; and the lower elastic modulus, making the parts resist plastic deformation, etc.
The table below shows a comparison of the physical and mechanical properties of the three materials.
In the aircraft industry, for titanium alloy structural parts, the material removal rate must reach 90%. Large aircraft like the Boeing B-787 are made from over 90 tons of titanium alloy machined into many different components, with a total weight of approximately 11 tons. However, in order to keep processing costs as low as possible, it is worthwhile to pursue high material removal rates. However, titanium material removal rates have only doubled over the past decade, while aluminum material removal rates have increased fivefold. Currently, aluminum’s material removal rate has reached 10L/min or higher, while titanium’s cutting rate has just reached 0.5L/min.
In view of the increasing share of titanium alloy parts and titanium alloy/carbon fiber connecting parts in recent years, especially in the aircraft manufacturing industry, it is increasingly important to improve the production efficiency of cutting titanium alloy materials.
Titanium alloy is a difficult-to-cut material
However, these advantages of titanium alloy materials have become difficulties in its cutting processing. One of the main reasons why titanium alloy materials are difficult to cut is their poor thermal conductivity and high specific heat capacity. This hinders the transfer of cutting heat from the cutting area through the chips and workpiece. Most of the heat (approximately 75%) is transferred to the cutting edge. Extremely high temperatures promote diffusion and adhesion on the blade surface, forming built-up edges. At the same time, due to the high strength of titanium alloy materials, large cutting forces will be generated during cutting. Therefore, the cutting tools are subjected to high thermal and mechanical loads during machining. Secondly, the elastic modulus of titanium alloy is low, and the parts will deform and then rebound under the action of cutting force, thus affecting the machining accuracy of the parts.
It can be seen from here that the main problem of cutting titanium alloy is that the tool absorbs too much cutting heat, which accelerates the wear of the tool and forces the use of a lower cutting speed, which will significantly reduce the processing efficiency and increase the tool loss. Unit cost. . For example, 50% of the manufacturing cost of a turbocharger compressor impeller made of Ti6Al4V is machining cost.
It is not difficult to see that the key to solving the cutting problem of titanium alloy materials lies in the use of high-temperature-resistant carbide cutting tools and effective cooling of the cutting tools during the cutting process. In order to improve the cutting efficiency and processing reliability of titanium alloy materials, many tool manufacturers and universities have carried out fruitful research and experimental work. In Germany, universities such as Darmstadt University of Technology, RWTH Aachen University, Braunschweig University of Technology, Leibniz University of Technology Hannover, and TU Dortmund are all engaged in titanium alloy cutting mechanism and finite element model analysis. , simulation and other aspects of research. A series of studies were carried out on tool geometry, cutting experiments and the use of different cooling methods. Among them, the Machine Tool Laboratory (WZL) of RWTH Aachen University also cooperates closely with tool factories such as Iscar, Kennametal, Seco Seco Tools and Sandvik. Conducting research on technologies including high-pressure cooling, the Institute for Production Technology and Machine Tools (IFW) at Leibniz University Hannover is supported by Airbus Germany, Kennametal, Paul Horn and Lehmann Precision Tools and other company leaders, funded the research work project "Improving material removal rate in titanium material milling through tool development."
High pressure cooling is an effective solution
Research shows that cooling tools is an effective way to solve titanium alloy cutting problems. At present, there are two main development paths for the technological development of high-efficiency cooling tools. One is to use high-pressure cooling and lubrication, and the other is to use cold air cooling, that is, liquid nitrogen (-196°C) or liquid carbon dioxide (CO2) (-65°C) cooling, especially liquid nitrogen, which is suitable for cooling milling cutters. It is a cooling method with great application prospects. It should be pointed out that nitrogen cooling or carbon dioxide cooling-assisted cutting is a type of dry cutting. This dry cooling not only cools the tool, helps with rapid chip breaking, and extends tool life, but it also has many advantages of dry cutting. Economic, technical and ecological benefits.
At present, considering the good cooling effect of high-pressure cooling and the fact that existing machining centers and turning centers are equipped with cooling and lubrication equipment, many tool manufacturers can provide such high-pressure cooling tools and have accumulated a lot of experience. . Judging from actual use experience (whether turning or milling), therefore, using high-pressure lubricant to cool the spindle is undoubtedly the first choice.
With traditional high-flow cooling, the cooling lubricant cannot reach the cutting area between the cutting edge and the chips, and the cutting edge cannot be effectively cooled. In order to achieve effective cooling of the tool, the supply of cooling lubricant should be precisely targeted at the contact area between the cutting edge and the chip with high pressure and sufficient flow. A high-energy impact wedge is formed in this contact zone, thereby shortening the contact time between the chip and the cutting edge, reducing the temperature of the cutting zone, and embrittlement of the chip. Through the combined effects of cooling and mechanical impact, the chip quickly becomes brittle. The chips are broken off and discharged reliably, which greatly improves the reliability of processing and is also conducive to the automation of the cutting process.
High-pressure cooling helps improve production efficiency
Practice has proven that through high-pressure cooling, tool durability can be increased by 50%. By adjusting the pressure of the cooling lubricant, the shape of the chips can be influenced, thereby improving chip breaking. According to information from ISCAR, we can understand the chip formation under different cooling lubricant pressures. When 2MPa pressure is used for large-flow external cooling, the chips grow into long coiled chips; when 8MPa pressure is used for internal cooling, the chips are broken into fine arc-shaped chips under the high-pressure impact. If 30MPa ultra-high pressure is used for internal cooling, the chips will become needle-shaped chips. It is not difficult to see from these three examples that the formation of chips can be controlled through high-pressure cooling, thereby improving the reliability of the cutting process and increasing the cutting volume of titanium alloy processing.
What needs to be pointed out here is that when the pressure of the cooling lubricant is lower than 7MPa, the coolant vaporizes in front of the cutting edge, forming bubbles and hindering heat conduction. When using a coolant pressure greater than 7MPa, this bubble can be eliminated and the coolant can be sprayed directly to the cutting part. In addition, it should be pointed out that when using traditional mineral oil lubricants, a large amount of air is easily sucked into the oil during high-pressure cooling cutting, resulting in poor heat dissipation efficiency. To this end, Flowserve Lubricating Materials Europe of Germany has developed a synthetic grease-based cooling lubricant (Ecocool TN2525 HP) with exhaust properties, which can improve the heat dissipation and cooling effect of the cooling lubricant.
When processing titanium alloys, mechanically clamped indexable plate tools and solid carbide tools are mainly used. Traditionally, the cutting speed during rough machining is generally about 50m/min, and the cutting speed during finishing is (200~300)m/min. After using high-pressure cooling, the cutting speed can be increased by 20%. At this time, the cutting speed will not increase. As the cutting speed increases, the temperature rises. If ultra-high pressure cooling is used and CBN tools are used, the cutting speed can be further increased. However, the ultra-high-pressure cooling and lubrication device used requires special equipment. Because the pressure of the cooling and lubrication devices equipped with machining centers, turning centers and multi-functional compound machine tools is generally only (7-10) MPa.
Comparison of the processing effects of conventional cooling and high-pressure cooling (excerpted from Sandvik company information)
From the comparison of the machining effects of different cooling methods, it can be seen that high-pressure cooling provides conditions for improving cutting parameters. Using high cutting parameters can significantly increase production efficiency and significantly reduce unit costs. Although high-pressure cooling can increase tool durability by 50%, since tool costs generally only account for 3% of manufacturing costs, this can only reduce the cost of a single piece by 1.5%.
When using high-pressure cooling, care must be taken to accurately coordinate the relationship between pressure, flow rate and nozzle aperture. For example, according to Sandvik, using a 1mm bore nozzle on a tool requires a cooling lubricant flow of 5l/min to maintain pressure. Therefore, the nozzle hole size is selected so that it generates the highest pressure and optimally utilizes the flow of cooling lubricant.
For milling, when multiple blades are used, there are correspondingly multiple nozzles. At this time, a larger flow of cooling lubricant is required. If the flow rate of the lubrication system is insufficient, the nozzle outlet pressure will be affected. At this time, you can consider using a nozzle with a smaller nozzle diameter to reduce the flow rate and maintain the injection pressure of the cooling lubricant.
Use the right tools and machines
In the aircraft industry, most titanium components require significant material removal to be machined from blank to finished product. The finished parts have thin walls and complex shapes. A common process is milling deep grooves. Therefore, it is particularly important to improve the material removal rate in milling. The limiting factor in improving material removal rates is tool wear. Research from the Institute for Production Technology and Machine Tools (IFW) at Leibniz University in Hannover shows that when milling titanium alloy (TiAl6V4) parts, a smaller relief angle (α=6o) and a relatively large rake angle (γ=14o) are used ) can reduce tool wear.
Due to the low elastic modulus of titanium material, vibration is easily generated during milling. In view of this situation, it is proposed to use milling cutters with unequal teeth, narrow braking edges, and zero relief angle in the tool design. To improve chip evacuation, the front of the tool is polished.
In order to increase the material removal rate, it is usually necessary to use higher back and side cutting amounts. Therefore, a large cutting load will be generated during machining. Because titanium has a low elastic modulus, it easily causes vibration. Therefore, the machine tool should have high rigidity, good damping performance, high spindle torque and high-power feed drive. Reliable chip evacuation is particularly important for face and circumferential milling of pockets or pockets. For this purpose, the machine tool should adopt a horizontal spindle configuration.
Currently, DST’s Ecoforce 2035 and 2060 machining centers, Hermle’s C 60U 5-axis machining center and Makino’s Makino T4 can all be used to machine titanium alloy parts. Among them, Makino T4 is specially designed for processing titanium alloy. In addition to its highly rigid and particularly stable machine structure, horizontal spindle configuration, high-power spindle and efficient cooling system, the machine also features active damping. System, this innovative damping system suppresses vibrations that occur especially during rough machining. This system acts on the guide rail proportionally through friction and cutting forces to achieve a balanced effect of friction on cutting forces. This enables the Makino T4 to achieve deeper cutting depths, higher material removal rates (roughing: approximately 500 cm3/min) and reduced tool wear.
in conclusion
The many benefits of high-pressure cooling technology include extended tool life, controlled chip formation, higher cutting speeds, and improved workpiece surface quality, thereby increasing productivity.
At present, high-pressure cooling technology is a mature technology. In actual use, the cooling lubricant has a high pressure, sufficient flow rate and a high-energy jet precisely aimed at the contact area between the cutting edge and the chip. This is very important for cutting tools. Effective cooling and effective chip control are basic conditions. In order to obtain the best results in the processing of titanium alloy parts, high-pressure cooling must be combined with the reasonable selection and design of tool materials, coatings, geometric angles, cutting volumes and other factors.
Therefore, selecting cutting tools suitable for titanium alloy processing and machine tools with high rigidity and high damping performance is another important condition for achieving economical cutting of titanium alloy components.
