(1) Titanium alloy has high strength and low density (4.4kg/dm3) and light weight, which provides a solution for reducing the weight of some large structural parts.
(2) High heat strength. Titanium alloy can maintain high strength at 400-500 °C, and can work stably, while the working temperature of aluminum alloy can only be below 200 °C.
(3) Compared with steel, the inherent high corrosion resistance of titanium alloy can save the cost of daily operation and maintenance of the aircraft.
2 Analysis of processing characteristics of titanium alloy
(1) The thermal conductivity is low. TC4 has a thermal conductivity of 1 = 16.8 W/m? °C at 200 ° C, and the thermal conductivity is 0.036 cal/cm? sec? ° C, which is only 1/4 of steel, 1/13 of aluminum, and 1/25 of copper. The heat dissipation and cooling effects are poor during the cutting process, which shortens the tool life.
(2) The elastic modulus is low, and the surface of the machine has a large rebound, which leads to an increase in the contact area between the machined surface and the flank of the tool, which affects the dimensional accuracy of the part and reduces the durability of the tool.
(3) Hardness factor. When the titanium alloy with low hardness value is processed, it will be sticky, and the chip will form a built-up edge near the cutting edge of the rake face of the tool, which will affect the processing effect. When the titanium alloy with high hardness value is processed, the tool will be chipped and abraded. These characteristics result in a low removal rate of titanium alloy metal, which is only 1/4 of that of steel, and the processing time is much longer than steel of the same size.
(4) Strong chemical affinity. Titanium can not only chemically react with nitrogen, oxygen, carbon monoxide and other substances in the air, but also form TiC and TiN hardened layers on the surface of the alloy, and it can react with the tool material under the high temperature conditions generated by cutting, reducing the tool. Durability.
(5) Poor safety performance during cutting. Titanium is a flammable metal, and the high temperatures and sparks generated during processing during micro-cutting may cause the burning of titanium chips.
3 titanium alloy processing technology
(1) As far as possible, the cemented carbide tool is used. The tungsten-cobalt type hard alloy has the characteristics of high strength and good thermal conductivity, and is not easy to react with titanium at high temperatures, and is suitable for processing titanium alloy.
(2) Reasonable selection of tool geometry parameters. In order to reduce the cutting temperature and reduce the tool sticking phenomenon, the front angle of the tool can be appropriately reduced, and the contact area between the chip and the rake face can be increased to dissipate heat. At the same time, the back angle of the tool is increased to reduce the rebound of the machined surface and the tool back. The frictional contact of the surface causes the tool to be bonded and the surface precision of the machined surface to be reduced; the cutting edge should adopt a circular arc transition to enhance the tool strength. For the processing of titanium alloys, the tool should be ground frequently to ensure that the blade shape is sharp and the chip removal is smooth.
(3) Suitable cutting parameters. To determine the cutting parameters, please refer to the following scheme: lower cutting speed – high cutting speed will lead to a sharp increase in cutting temperature; moderate feed rate – high cutting temperature, high cutting temperature, small feed rate The hardened layer has a long cutting time and accelerated wear; a large depth of cut – cutting of the hardened layer over the surface of the titanium alloy improves the tool life.
(4) The flow rate and pressure of the cutting fluid during processing should be large, and the machining area should be cooled continuously to reduce the cutting temperature.
(5) The selection of the machine tool must always pay attention to improve the stability to avoid the vibration trend. Vibration can result in chipping of the blade and damage to the blade. At the same time, the processing precision of the titanium alloy process system is good to ensure a large cutting depth during cutting. However, the titanium alloy has a large rebound and the clamping force is large, which will aggravate the deformation of the workpiece. Therefore, it is possible to consider the use of auxiliary supports such as assembling fixtures during finishing. Meet the process system stiffness requirements.
(6) The milling method generally uses down milling. Milling cutters and chipping caused by up-cut milling in titanium machining are much more severe than those caused by down-cut milling.
(7) The common problem of grinding is that the swarf causes the grinding wheel to clog and the surface of the part to burn. Therefore, it is advisable to use a green silicon carbide grinding wheel with sharp abrasive grains, high hardness and good thermal conductivity during grinding; F36～F80 can be used according to different surface smoothness of the machined surface; the hardness of the grinding wheel should be soft to reduce abrasive grains and wear debris. Adhesion reduces grinding heat; grinding feed is small, speed is low, and emulsion is sufficient.
(8) Titanium alloys need to be ground to the standard drill bit to reduce the burning of the knife and the bit. Grinding method: appropriately increase the apex angle, reduce the rake angle of the cutting part, increase the back angle of the cutting part, and double the number of reverse taper of the cylindrical blade. During processing, the number of retractions should be increased. The drill bit should not stay in the hole. The chips should be removed in time. A sufficient amount of emulsion should be cooled. Pay attention to observing the bit becoming dull and remove the chips in time. A sufficient amount of emulsion should be cooled. Replace the grinding.
(9) Titanium alloy reaming also needs to be modified for the standard reamer: the width of the reaming blade should be less than 0.15mm, and the cutting part and the calibration part should be arc-transition to avoid sharp points. When reaming, you can use the set reamer to ream several times. Each time you increase the diameter of the reamer by 0.1mm or less, the spindle speed should be slightly slower and will not stop when retracting. Reaming in this way achieves a high finish requirement.
(10) Tapping is the most difficult part of titanium alloy processing. Because the torque is too large, the tap teeth will wear quickly, and the rebound of the processed part can even break the tap in the hole. When using ordinary taps, the number of teeth should be reduced according to the diameter to increase the chip space. After leaving 0.15mm width on the calibration teeth, the back angle should be increased to about 30°, and 1/2~1 should be removed. /3 tooth back, the calibration tooth retains 3 buckles and increases the reverse taper number. It is recommended to use the jumper tap to effectively reduce the contact area between the tool and the workpiece, and the processing effect is also good.
4 Processing precautions
(1) The tool is sharpened and kept sharp to ensure that as little cutting heat is generated during the machining process.
(2) Equipment, knives, work and fixtures should be kept clean and swarf removed in time.
(3) Transfer titanium shavings using non-combustible or flame-retardant tools. Store the processed debris in a non-combustible container and cover it.
(4) Wear clean gloves when handling cleaned titanium alloy parts to avoid corrosion corrosion of sodium chloride in the future.
(5) Fire protection facilities are available in the cutting area.
(6) In the case of micro-cutting, once the cut titanium chips are on fire, they can be extinguished with dry powder fire extinguishing agent or dry soil or dry sand.