Application Of High Speed Milling In Titanium Alloy Processingpplication Of High Speed Milling In Titanium Alloy Processing

Because the titanium alloy material has advantages of lightweight, high strength, high temperature resistance and other excellent properties, such as substitute TC18 titanium alloy for high strength structural steel used for landing gear, which can realize the plane structure weight loss is about 15%. Thus, the new type high strength titanium alloy are widely used as the main bearing parts in the foreign advanced aircraft, such as the structure of the material of b-1 bomber which the titanium alloy accounted for about 21%. Russia’s Il-76 aircraft uses 12.5% of weight in titanium in its structure. From the perspective of development trend, countries in Europe and the United States are gradually increasing the use of titanium alloy, which also indicates that a large number of titanium alloys, especially some new titanium alloys, have become the development direction of aviation design. However, thin-wall titanium alloy parts are mostly used in aerospace products, with relatively complex structure and high precision requirements. Due to the thin wall, the stiffness of the parts is poor. Under the action of cutting force, machining bending deformation is easy to occur, and the thickness of the wall is inconsistent up and down, resulting in out-of-tolerance. At present, the commonly used method in enterprises is repeated milling in the finishing. Due to the small thermal conductivity of titanium alloy, low elastic modulus (about 1/2 of steel), high chemical activity, small margin cannot be reduced by milling, which often produce “less cut” phenomenon. In order to ensure the size of the parts can only rely on manual grinding, which is greatly increasing the processing cycle of the parts and may cause the surface of the parts burn phenomenon.　　

1. Difficulties in cutting of structural parts of titanium alloy

1.1 High cutting temperature　　

Because the thermal conductivity of titanium alloy material is small (about 1/3 ~ 1/6 of steel), it is easy to produce high cutting temperature when processing titanium alloy. Under the same conditions, the cutting heat produced by machining titanium alloy is more than one time higher than that of the same steel, and the heat produced by machining is difficult to be released through the work piece. Because titanium alloy specific heat coefficient is small, the local temperature rises quickly during the processing. It is easy to cause the tool instantaneous temperature too high which leads to the tip will wear sharply, so that the phenomenon of over-burning.

1.2 Strong cutting resistance　　

The cutting force of titanium alloy is basically the same as that of steel, so the energy consumed during cutting is the same or slightly lower than that of steel. However, the stress near the main cutting edge is very high when cutting titanium alloy. This may be due to the fact that when cutting titanium alloy, the contact area of the cutting chip on the front knife surface is usually very small (about 1/3 of the cutting steel under the same conditions), and the large cutting stress leads to the phenomenon of cutting back-off occurs during the machining process and the work piece size is not coordinated.　　

　1.3 Tremor of weakly rigid structure

　　Vibration is an important problem to be overcome when machining titanium alloys with weak rigid structures, especially during finishing machining, the primary cause of vibration is the very low elastic modulus of titanium alloys. T he deformation of titanium alloys is twice as much as carbon steels when subjected to cutting forces. There is friction between the back cutter surface and the spring back of the machined surface, which produces vibration and high cutting temperature. The high dynamic cutting force is partly responsible for the tremor, which can reach more than 30% of the static force due to the plastic shearing process during the formation of titanium alloy chips. Due to the effect of cutting vibration, the surface quality of the work piece after milling is difficult to meet the precision requirement.

2. Cutting solutions for titanium alloy structural parts

The main factors affecting the machining of titanium alloy’s weak rigid structure are as followings: the rigidity of machine tool, choice of cutting tool, technological parameters, effective cooling and so on. In the process of machining, various factors interact with each other. Beside, the accumulation of deformation error causes the machining of weak rigid structure to be out of tolerance and the machining deformation is difficult to control.

2.1 Selection of machine tools　　

The rigidity of the machine tool – fixture – tool system is good in performance by clearance between the machine tool components to be adjusted. The spindle radial vibration is preferred to be small by using such machine tools.

2.2 Selection of cutting tools　

The improvement of cutting productivity is mainly the result of the development and application of new cutting tool materials. Over the past few decades, cutting tools have developed greatly, including hard alloy coating, ceramics, cubic boron nitride, polycrystalline diamond. These are effective for processing cast iron, steel and super alloys. However, none of the tools can improve the machinability of titanium alloy, because cutting tool material of titanium alloy requires very important properties, which include: 1) good thermal hardness to resist high stress; 2) Good thermal conductivity to reduce thermal gradient and thermal shock; 3) Good chemical inertness to reduce the tendency of chemical reaction with titanium; 4) Good toughness and fatigue resistance to adapt to the chip cutting process. WC/ CO cemented carbide tools are considered to have the best performance in almost all titanium alloy cutting processes. Some tests show that the wear rate of all carbide-coated tools is higher than that of uncoated tools. Although ceramic cutting tools have improved in quality and are increasingly used to process difficult-to-cut materials, especially super alloys (such as nickel-based super alloys), they have not replaced cemented carbide and high-speed steel because of their poor thermal conductivity, low fracture toughness and reaction with titanium. Super hard cutting tool materials (cubic boron nitride and polycrystalline diamond) exhibit good performance in cutting titanium alloys due to their low wear rate.S　　

The main problem in the milling process of titanium alloy with weak rigid structure is the deformation of thin wall. Due to the low elastic modulus of titanium alloy and the relatively large cutting force, the thin wall is easily deformed by the milling force during the milling process. The result is that the actual thickness of the thin wall is greater than the theoretical thickness. The solution to this problem should be to reduce as much as possible the force on the thin wall from the direction perpendicular to the processed surface that caused by the cutter back-off.

2.3 cutting fluid

Titanium alloy has advantages of high strength, oxidation resistance, high temperature resistance and other lifting points, which not only meet the requirements of high performance, but also bring a lot of problems to machining. In order to reduce the cutting temperature, a large amount of cooling-based cutting fluid should be poured into the cutting area when cutting titanium alloys, which takes heat away from the edge and to wash away the chip to reduce the cutting force. Therefore, the requirements for cutting fluid are large thermal conductivity, large heat capacity, fast flow rate and large flow rate. The best cooling method is the high-pressure cooling method, cutting fluid flow is not less than 15 ~ 20L/min. There are three types of cutting fluids commonly used, namely water or alkaline solution, water-based soluble oil solution and non-water soluble oil solution.