How to solve the problem of cutting titanium alloy effectively

High pressure cooling is used to improve the cutting efficiency of titanium alloy

In the aircraft industry, for the structure of titanium alloy, the material removal amount has to reach 90%. Large planes like Boeing's b-787 are made from more than 90 tons of titanium alloy into many different parts that weigh about 11 tons. However, in order to minimize processing costs, it is worthwhile to pursue a higher material removal rate. In the last decade, however, the rate of removal of titanium has only doubled, while that of aluminum has increased fivefold. At present, the material removal rate of aluminum has reached 10L/min or higher, while the cutting rate of titanium has just reached 0.5L/min.

In view of the recent years, especially in the aircraft manufacturing industry, the share of titanium alloy parts and titanium alloy/carbon fiber joint components is increasing, improving the production efficiency of cutting titanium alloy materials is becoming more and more important.


It is hard to cutting titanium alloy materials

However, these advantages of titanium alloy materials have become the difficulties in cutting. One of the main reasons why titanium alloy is difficult to cut is its poor thermal conductivity and high specific heat capacity. This prevents the transfer of cutting heat from the cutting area through chips and workpiece. Most of the heat (about 75 percent) goes to the cutting edge. High temperature promotes the diffusion and bonding on the blade surface to form chip nodules, and at the same time, due to the high strength of titanium alloy material, greater cutting force is generated during cutting. Therefore, the cutting tool is subjected to high thermal and mechanical loads in the process of machining. Secondly, the elastic modulus of titanium alloy is low. Under the action of cutting force, components will be deformed and then spring back, which will affect the machining accuracy of components.

As can be seen here, the main problem with cutting titanium alloys is that the cutting heat absorbed by the tool is too much, leading to faster wear of the tool, forcing the use of a lower cutting speed, which will obviously reduce processing efficiency and increase the cost of a single piece. For example, 50% of the manufacturing cost of a Ti6Al4V turbo compressor impeller is spent on cutting.

It is not difficult to see that the solution to the problem of cutting titanium alloy material is the use of high temperature resistant carbide cutting tools and effective cooling of the cutting process. In order to improve the cutting efficiency and processing reliability of titanium alloy, many cutting tool manufacturers and universities have carried out fruitful research and experiments. In Germany, especially such as Darmstadt university of technology, RWTH aachen university, Braunschweig university of technology, leibniz university of Hanover and Dortmund university of technology and other colleges and universities in titanium alloy cutting mechanism, finite element model analysis, simulation, tool geometry angles, cutting test and different cooling ways and so on has carried out a series of studies, including machine tool of RWTH aachen university laboratory (WZL) with Iscar (Iscar), kenner metal (Kennmetal), Mountain high cutting Tools (Seco Tools) and Sandvik (Sandvik) tool factory in close cooperation to carry out the technology research, including high pressure cooling and leibniz production technology and machine tool research institute at the university of Hanover (IFW) by airbus Germany, kenner metal, Paul Horn and Lehmann precision cutting tool company for funding, carried out "by cutting tool development, improve the material removal rate of titanium milling" project research work.


High pressure cooling is an effective solution

The research shows that cooling tool is an effective method to solve the problem of cutting titanium alloy. At present, there are two ways to develop the technology of efficient cooling tool. One is the use of high-pressure cooling lubrication, the other is the use of cool air cooling, that is, the use of liquid nitrogen (-196℃) or liquid carbon dioxide (CO2) (-65℃) for cooling, especially liquid nitrogen, which is a very promising cooling method for cooling milling cutter. It should be pointed out that cutting assisted by nitrogen cooling or carbon dioxide cooling is a kind of dry machining, which can not only cool the cutting tool, help to break chip quickly and extend the life of the cutting tool, but also has many economic, technical and ecological benefits of dry machining.

At present, considering the good cooling effect of high pressure cooling, as well as the existing processing center and turning center are equipped with cooling and lubrication equipment, there are a lot of cutting tool manufacturer and can provide for this high pressure cooling of the cutting tool, and accumulated a lot of practical experience with (both turning and milling), therefore, is used by either by high pressure cooling lubricant is undoubtedly become a first choice.

With the conventional large flow cooling, the cooling lubricant cannot reach the cutting area between the cutting edge and the chip, so the cutting edge cannot be cooled effectively. To achieve effective tool cooling, the supply of cooling lubricant should be precisely aligned to the contact area between the cutting edge and the chip at a high pressure and adequate flow rate. In the contact area to form a high energy impact wedge, thus shortening the time of contact between the chip and the blade, reduce the temperature of cutting zone, at the same time make the chip brittle, by cooling the superposition and mechanical impact these two effect, quickly make reliable chip breaking and discharge, thus greatly improving the reliability of the processing, thus is advantageous to realize the automation of cutting process.


High pressure cooling helps increase productivity

The practice shows that the tool durability can be improved by 50% by high pressure cooling. By adjusting the pressure of cooling and lubricating fluid, the shape of chip can be affected to improve chip breakage. According to Iscar, you can see the chip forming process under different cooling and lubricating fluid pressures. When the chip is cooled with a large flow rate at a pressure of 2MPa, the chip grows into a wound chip. When internal cooling is applied at a pressure of 8MPa, the chips are broken into small curved chips under high pressure impact. If the use of 30MPa ultra-high pressure for internal cooling, when the chips become needle-shaped chips. From these three examples, it is not difficult to see that high pressure cooling can control the chip forming, thus improving the reliability of cutting process, and can improve the cutting amount of titanium alloy processing.

It should be noted here that at a cooling lubricant pressure of less than 7MPa, the cooling fluid vaporized in front of the cutting edge and formed a bubble, thereby impeding the heat transfer. When the coolant pressure is greater than 7MPa, the bubble can be eliminated and the coolant can be sprayed directly to the cutting site. In addition, it should be pointed out that the use of traditional mineral oil lubrication fluid, high pressure cooling cutting, oil easy to breathe a lot of air, resulting in poor heat dissipation efficiency. To this end, Fuchs Europe lubricant company of Germany developed a cooling lubricant (Ecocool TN2525 HP) with exhaust performance based on synthetic grease to improve the cooling effect of cooling lubricant.

In the processing of titanium alloy, the main tools are mechanical clamped indexable pieces and solid carbide tools. Generally speaking, the cutting speed of rough machining is about 50m/min, and the cutting speed of finishing machining is (200-300) m/min. After high pressure cooling, the cutting speed can be increased by 20%. At this time, the temperature will not increase due to the increase of cutting speed. If the use of ultra-high pressure cooling, while using CBN tool, cutting speed can be further improved. However, the ultra-high pressure cooling and lubrication devices used need to be specially equipped. Because of the machining center, turning center and multi-function complex machine equipped with cooling lubrication device pressure is generally only (7-10) MPa.


Comparison of machining effects by conventional cooling and high-pressure cooling (data from Sandvik)

It can be seen from the comparison of machining effects of different cooling methods that high-pressure cooling provides conditions for improving cutting parameters. High cutting parameters can significantly improve production efficiency and greatly reduce the cost of a single piece. Although tool durability can be increased by 50 per cent through high-pressure cooling, this can only be reduced by 1.5 per cent per unit as tool costs typically account for only 3 per cent of manufacturing costs.

Use high pressure cooling, pay attention to the accurate coordination of pressure, flow and nozzle aperture. According to Sandvik, for example, using a nozzle with a 1mm aperture on the tool requires a cooling and lubricating fluid flow rate of 5l/min to maintain pressure. Therefore, the nozzle aperture size should be selected to produce the highest pressure and the best use of cooling lubricant flow.

For milling processing, in the case of using more than one blade, there are corresponding number of nozzles, at this time requires a large flow of cooling and lubrication fluid, if the lubrication system flow is insufficient, it will affect the nozzle outlet pressure. At this point, consider using a nozzle with a small nozzle diameter to reduce the flow and maintain the spray pressure of cooling lubricant.


Use suitable cutting tools and machine tools

In the aircraft industry, most titanium alloy components are processed from rough to finished products with a large amount of material removed. The wall of component finished product is very thin, shape is very complex, often encountered in the process of milling deep groove. Therefore, it is very important to improve the material removal rate. The limiting factor for improving the material removal rate is tool wear. Research by the institute of production technology and machine tools (IFW) at the university of leibniz Hanover shows that milling titanium alloy (TiAl6V4) components with a smaller back Angle (alpha =6o) and a relatively large front Angle (gamma =14o) reduces tool wear.

Due to the low elastic modulus of titanium, vibration is easy to occur in milling. In view of this situation, the cutter design plans to use the milling cutter with unequal teeth, and the narrow brake blade belt with zero rear Angle. To improve chip removal, polish the front of the tool.

To improve the material cutting rate, often to use a higher back and side cutting amount, therefore, in the processing will produce a larger cutting load. Titanium is prone to vibration because of its low elastic modulus. For these reasons, the machine should have high rigidity, good damping performance, high spindle torque and high power feed drive device. For end milling and circular milling slot cavities or slots, reliable chip removal is particularly important, for this reason, the machine should use a horizontal spindle configuration

Currently, processing centers such as Ecoforce 2035 and 2060 processing centers of DST corporation, C 60U 5-axis processing centers of Hermle corporation and Makino T4 of Makino corporation can be used for processing titanium alloy components. Including Makino T4 is specially designed for the processing of titanium alloy, in addition to the machine tool has high rigidity, especially stability of machine tool structure, horizontal spindle configuration and high power spindle and efficient cooling system, the machine also has the active damping system, through the innovation of damping vibration of system can inhibit especially in rough machining. The system ACTS on the guide rail in proportion to the cutting force by friction, so as to achieve the balance of the cutting force by friction. Thus, Makino T4 can achieve deeper cutting depth, achieve higher material removal rate (about 500 cm3/min in rough machining) and reduce tool wear.


Conclusion

High pressure cooling technology has many advantages such as extending tool life, controlling chip formation, increasing cutting speed and improving workpiece surface quality, and thus increasing production efficiency.

At present, high pressure cooling technology has been a mature technology, in practical use, cooling lubrication fluid high pressure, sufficient flow and the formation of precise alignment of the cutting edge and chip contact area of high energy jet, which is the cutting tool for effective cooling and effective control of chip is a basic condition. In order to obtain the best results of processing titanium alloy components, it is necessary to combine the high pressure cooling with the rational selection and design of tool materials, coatings, geometric angles and cutting parameters.

Therefore, the choice of cutting tools suitable for titanium alloy processing, high rigidity, high damping performance and machine tool is another important condition to realize the economical cutting of titanium alloy components.