The machining difficulties of titanium alloy structural parts and the factors affecting the deformation of weakly rigid structures are proposed. From the aspects of machine tool selection, tool selection, and effective cooling, control methods for the deformation of weakly rigid structural parts are proposed. Titanium alloy materials have excellent properties such as light weight, high strength and high temperature resistance. For example, using TC18 titanium alloy instead of high-strength structural steel for landing gear can reduce the weight of the aircraft structure by about 15%, so it is the main bearing of advanced aircraft in foreign countries. A large number of new high-strength titanium alloys are used in the force parts. For example: in the body structure materials of the B-1 bomber in the United States, titanium alloys account for about 21%; the amount of titanium used by the Russian Il-76 aircraft reaches 12.5% of the weight of the body structure From the perspective of development trends, the use of titanium alloys in Europe and the United States is gradually increasing, and it also shows that a large number of titanium alloys are used, especially some new titanium alloys have become the development direction of aviation design. However, most aviation and aviation products use thin-walled parts, which are relatively complex in structure and require high precision. Due to the thin wall, the rigidity of the parts is poor. Under the action of cutting forces, it is easy to produce bending deformation, and the thickness of the wall is inconsistent up and down, resulting in excessive . At present, the commonly used method in enterprises is repeated milling in finishing. Due to the small thermal conductivity of titanium alloy, the low modulus of elasticity (about 1/2 of steel), the high chemical activity, and the small margin cannot be milled at all, often The phenomenon of “less cutting” is generated. In order to ensure that the size of the parts can only be polished by hand, the machining cycle of the parts is greatly increased, and the surface of the parts may be burned.
Titanium alloy structural parts machining solutions
1.The main factors affecting the machining of titanium alloy weak rigid structure
Main factors: machine tool rigidity, tool selection, process parameters, effective cooling, etc. In the process of machining, various factors act, interact, and the accumulation of deformation errors causes the weak rigid structural parts processed to be out of tolerance, and the machining deformation is difficult to control.
2.1 Selection of machine tools
The machine tool-fixture-tool system has better rigidity, the gap between the machine tool parts should be adjusted, and the radial runout of the spindle should be small. Try to use such a machine tool.
2.2 Selection of cutting tools
The increase in cutting productivity is mainly the result of the development and application of new tool materials. In the past few decades, cutting tools have developed greatly, including cemented carbide coatings, ceramics, cubic boron nitride, and polycrystalline diamond. These are effective for machining cast iron, steel and high temperature alloys. But none of the tools can improve the machinability of titanium alloys, because the cutting tool materials for cutting titanium alloys are required to have very important properties, these 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. In almost all titanium alloy cutting processes, tungsten carbide (WC / co) cemented carbide tools are considered to have the best performance. Some tests have shown that all carbide coated tools have a higher wear rate than those of uncoated tools. Although the quality of ceramic tools has been improved and more and more are used to process difficult-to-cut materials, especially those high-temperature alloys (such as nickel-based superalloys), due to their poor thermal conductivity, low fracture toughness and titanium Reaction, so they did not replace cemented carbide and high-speed steel. The ultra-hard cutting tool materials (cubic boron nitride and polycrystalline diamond) have a low wear rate when cutting titanium alloys and therefore show good performance.
The main problem in the milling process of titanium alloy weakly rigid structural parts is the problem of thin-wall milling deformation. 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 after machining is greater than the theoretical thickness. The solution to this problem should be to reduce as much as possible the force that the thin wall is subjected to during the milling process from the direction perpendicular to the surface being processed, causing the thin wall to deform the knife.
2.3 Cutting fluid
Titanium alloy has high strength, oxidation resistance, high temperature resistance, etc., while meeting the requirements of high performance, it also brings many problems to cutting. When cutting titanium alloys, in order to reduce the cutting temperature, a large amount of cooling-based cutting fluid should be poured into the cutting area to remove the heat of the blade and wash away the chips 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 method of cooling is high-pressure cooling, the cutting fluid flow is not less than 15 ~ 20L / min. There are three types of cutting fluids generally used, namely water or alkaline solutions, water-based soluble oily solutions and non-aqueous soluble oily solutions.
Titan alloy structural parts cutting difficulties
1.1 High cutting temperature
Because the thermal conductivity of titanium alloy materials is small (about 1/3 to 1/6 of steel), it is easy to produce high cutting temperatures when machining titanium alloys. Under the same conditions, the cutting heat generated by machining titanium alloys is more than double that of the same steel, and the heat generated by machining is difficult to release through the workpiece. The specific thermal coefficient of titanium alloy is small, and the local temperature rises quickly during machining, so it is easy to cause the instantaneous temperature of the tool to be too high, so that the tool tip will be sharply worn, so that over-burning occurs.
1.2 High cutting resistance
The cutting force when cutting titanium alloy is basically the same as the cutting force when cutting steel, so the energy consumed during cutting is the same or slightly lower than steel. However, when cutting titanium alloys, the stress near the main cutting edge is very high. This may be due to the small chip contact area on the rake face when cutting titanium alloy (about 1/3 of the steel under the same conditions), and the large cutting stress causes the workpiece to appear during the machining process. Phenomenon, the size of the processed workpiece is not coordinated.
1.3 Tremor of weak rigid structure
Tremor is an important problem that needs to be overcome when machining titanium alloy weakly rigid structural parts. Especially when finishing, the very low elastic modulus of titanium alloy is the primary cause of vibration during cutting. When subjected to cutting forces, the amount of deformation of the titanium alloy is twice that of carbon steel. The amount of springback between the flank face and the machined surface causes friction and vibration, and also generates a high cutting temperature. The high dynamic cutting force when cutting titanium alloy is part of the cause of tremor, and its value can reach more than 30% of the static force, which is caused by the plastic shearing process during the formation of titanium alloy chips. Due to the influence of cutting tremor, the quality of the workpiece surface after milling is difficult to meet the accuracy requirements.
