Because the titanium alloy material has 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 foreign advanced aircraft main bearing a heavy use of new type high strength titanium alloy, such as: the b-1 bomber body structure of the material, the titanium alloy accounted for about 21%;The il-76 aircraft in Russia USES 12.5% of the body weight in titanium. From the perspective of development trend, the use of titanium alloy in European and American countries is gradually increasing, which also indicates that a large number of titanium alloys, especially some new titanium alloys, have become the development direction of aviation design.But the aviation aviation product mostly USES the thin-wall part, its structure is relatively complex, the precision request is high, because the wall thin causes the part rigidity is poor, in the cutting force action, easy to produce the processing bending deformation, the wall thickness size up and down is not consistent, causes the out-of-tolerance.Commonly used method in the enterprise is in finishing milling repeatedly, because of the titanium alloy small coefficient of thermal conductivity, low modulus of elasticity (about 1/2 of steel), high chemical activity, small allowance can't milling, tend to produce "less" phenomenon, in order to ensure parts size only by manual polishing, greatly increased the parts processing cycle, and has the potential to produce burnt parts surface.
1. Difficulties in cutting titanium alloy structural parts
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 generated by machining titanium alloy is more than one times higher than that of the same steel.The specific heat coefficient of titanium alloy is small, and the local temperature rises rapidly during processing. Therefore, it is easy to cause the instantaneous temperature of the tool to be too high, so that the tip of the tool will wear out rapidly, resulting in the phenomenon of overburning.
1.2 high cutting resistance
The cutting force when cutting titanium alloy is basically the same as that when cutting steel, so the energy consumed in the cutting process is the same or slightly lower than that of steel.However, when cutting titanium alloy, the stress near the main cutting edge is very high.This may be because when cutting titanium alloy, the chip contact area on the front cutting surface is usually small (about 1/3 of the cutting steel under the same conditions), the larger cutting stress leads to the workpiece in the processing of the phenomenon of the tool, the size of the workpiece is not coordinated.
1.3 tremors of weakly rigid structures
Vibration is an important problem to be overcome in the processing of weak rigid structural parts of titanium alloy, especially in the finishing process, the low elastic modulus of titanium alloy is the primary cause of vibration in the cutting process.Titanium alloys deform twice as much as carbon steels when subjected to cutting forces.Friction between the rebounding surface and the machined surface results in vibration and high cutting temperature.The high dynamic cutting force in cutting titanium alloy is part of the reason for the vibration, and the value can reach more than 30% of the static force, which is caused by the plastic shear process during the chip formation of titanium alloy.Due to the impact of cutting vibration, the surface quality of the workpiece after milling is difficult to meet the precision requirements.
2. Cutting solutions for titanium alloy structural parts
The main factors influencing the weak rigid structure of titanium alloy are: machine rigidity, tool selection, process parameters, effective cooling and so on.In the process of processing, the effect of various factors, mutual influence, the accumulation of deformation error resulting in the processing of weak rigid structural parts out of tolerance, processing deformation is difficult to control.
2.1 selection of machine tools
Machine tool - fixture - tool y rigidity is good, the clearance between the parts of the machine tool to adjust, the spindle radial runout to small, try to use such a machine tool.
2.2 selection of tools
The increase in 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 cemented carbide coatings, ceramics, cubic boron nitride, and polycrystalline diamonds.These are effective for processing cast iron, steel and superalloys.But no tool can improve the machinability of titanium alloy, this is because the cutting tool material requirements for titanium alloy 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 anti-fatigue ability to adapt to the chip segmentation process.Tungsten carbide (WC/co) carbide tools are considered to have the best performance in almost all titanium alloy cutting processes.Some tests have shown that the wear rate of all cemented carbide coated tools is higher than that of uncoated tools.Although ceramic tools have improved in quality and are increasingly used to process difficult cutting materials, especially superalloys (such as nickel-based superalloys), they do not replace hard alloys and high speed steels due to their poor thermal conductivity, low fracture toughness and reaction with titanium.Superhard cutting tool materials (cubic boron nitride and polycrystalline diamond) exhibit good properties due to their low wear rate in cutting titanium alloys.
The main problem in the milling process of weakly rigid structural parts of titanium alloy is the thin wall milling deformation.Because the elastic modulus of titanium alloy is low, and the cutting force is relatively large, so the thin wall in the milling process by the effect of milling force is easy to cause the tool deformation, the result is that the actual thickness of the processed thin wall is greater than the theoretical thickness.The solution to this problem should be to minimize the force on the thin wall that causes the tool to deform due to the vertical direction of the machined surface during milling.
2.3 cutting fluid
Titanium alloys have 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 cutting problems.When cutting titanium alloy, in order to reduce the cutting temperature, it is necessary to pour a large amount of cooling cutting fluid to the cutting area to take away the heat of the blade and wash away the chip, so as to reduce the cutting force.Therefore, the requirement of cutting fluid is high thermal conductivity, high heat capacity, fast flow rate, large flow rate.The best cooling method is high-pressure cooling, cutting fluid flow is not less than 15 ~ 20L/min.There are three types of cutting fluids commonly used, namely water or alkaline solutions, water-based soluble oil solutions and non-water soluble oil solutions.
This paper summarizes the machining difficulties of titanium alloy structural parts and the factors influencing the machining deformation of weak rigid structure, and puts forward the control method of machining deformation of weak rigid structural parts from the aspects of machine tool selection, tool selection and effective cooling.