Analysis of processing technology of titanium alloy parts
Titanium alloys have excellent mechanical properties but poor process properties, which leads to the contradiction that their application prospects are promising but processing is difficult. By analyzing the metal cutting performance of titanium alloy materials, combined with many years of practical work experience, this paper discusses the selection of titanium alloy cutting tools, the determination of cutting speed, the characteristics of different cutting methods, machining allowances and processing precautions. It expounds my views and suggestions on the machining of titanium alloys.
Titanium alloy has low density, high specific strength (strength/density), good corrosion resistance, high heat resistance, good toughness, plasticity and weldability. Titanium alloys have been widely used in many fields. However, poor thermal conductivity, high hardness, and low elastic modulus also make titanium alloys a difficult metal material to process. This article summarizes some technological measures in the machining of titanium alloys based on its technological characteristics.
1 The main advantages of titanium alloy materials
(1) Titanium alloy has high strength, low density (4.4kg/dm3) and light weight, which provides a solution for reducing the weight of some large structural parts.
(2) High thermal strength. Titanium alloys can maintain high strength under the condition of 400-500℃ and can work stably, while the working temperature of aluminum alloys can only be below 200℃.
(3) Compared with steel, the inherent high corrosion resistance of titanium alloy can save the cost of daily operation and maintenance of aircraft.
2. Analysis of machining characteristics of titanium alloy
(1) Low thermal conductivity. The thermal conductivity of TC4 at 200 °C is l=16.8W/m, and the thermal conductivity is 0.036 cal/cm, which is only 1/4 of steel, 1/13 of aluminum and 1/25 of copper. In the cutting process, the heat dissipation and cooling effect are poor, which shortens the tool life.
(2) The elastic modulus is low, and the machined surface of the part has a large rebound, which leads to an increase in the contact area between the machined surface and the flank surface of the tool, which not only affects the dimensional accuracy of the part, but also reduces the tool durability.
(3) Hardness factor. Titanium alloys with a low hardness value will be sticky when machining, and the chips will stick to the cutting edge of the rake face of the tool to form a built-up edge, which affects the machining effect; titanium alloys with a high hardness value are prone to chipping and abrasion of the tool during machining. These characteristics lead to the low metal removal rate of titanium alloy, which is only 1/4 of that of steel, and the processing time is much longer than that of steel of the same size.
(4) Strong chemical affinity. Titanium can not only chemically react with the main components of nitrogen, oxygen, carbon monoxide and other substances in the air to form a hardened layer of TiC and TiN on the surface of the alloy, but also react with the tool material under the high temperature conditions generated by the cutting process, reducing the cutting tool. of durability.
(5) The safety performance during cutting is poor. Titanium is a flammable metal, and the high temperature and sparks generated during micro-cutting may cause titanium chips to burn.
3 Titanium alloy processing technology
(1) Use cemented carbide tools as much as possible. Tungsten-cobalt cemented carbide has the characteristics of high strength and good thermal conductivity, and it is not easy to chemically react with titanium at high temperature, so it is suitable for processing titanium alloys.
(2) Reasonable selection of tool geometric parameters. In order to reduce the cutting temperature and reduce the sticking phenomenon of the tool, the rake angle of the tool can be appropriately reduced, and the heat dissipation can be dissipated by increasing the contact area between the chip and the rake face; at the same time, the relief angle of the tool can be increased to reduce the rebound of the machined surface and the tool flank. The tool sticks and the precision of the machined surface is reduced due to the frictional contact between the surfaces; the tool tip should adopt a circular arc transition to enhance the tool strength. When machining titanium alloys, it is necessary to grind the tool frequently to ensure that the blade shape is sharp and the chip removal is smooth.
(3) Appropriate cutting parameters. To determine cutting parameters, please refer to the following scheme: low cutting speed - high cutting speed will lead to a sharp increase in cutting temperature; moderate feed - large feed will lead to high cutting temperature, and small feed will cause the cutting edge to increase In the hardened layer, the cutting time is long and the wear is accelerated; the larger cutting depth - the cutting of the hardened layer where the tool tip crosses the surface of the titanium alloy can improve the tool life.
(4) The flow and pressure of the cutting fluid should be large during machining, and the machining area should be fully and continuously cooled to reduce the cutting temperature.
(5) The selection of machine tools must always pay attention to improving stability to avoid vibration trends. Vibration can result in chipping of the blade and damage to the blade. At the same time, the rigidity of the process system for machining titanium alloys is better to ensure that a large depth of cut is used during cutting. However, the rebound of titanium alloys is large, and the large clamping force will aggravate the deformation of the workpiece. Therefore, auxiliary supports such as assembling fixtures can be considered for finishing. Meet the rigidity requirements of the process system.
(6) The milling method generally adopts down milling. The chip sticking and chipping of the milling cutter caused by up milling in titanium alloy machining is much more serious than that of the milling cutter caused by down milling.
(7) The common problems of grinding are the clogging of the grinding wheel caused by sticky chips and the burn of the surface of the parts. Therefore, green silicon carbide grinding wheels with sharp abrasive grains, high hardness and good thermal conductivity should be used for grinding; F36-F80 can be used according to the different grinding wheel particle sizes of the surface to be processed; the hardness of the grinding wheel should be soft to reduce abrasive particles and debris Adhesion to reduce grinding heat; grinding feed should be small, speed is low, and emulsion is sufficient.
(8) When drilling titanium alloys, it is necessary to grind the standard drill bit to reduce the phenomenon of knife burning and drill bit breakage. Grinding method: appropriately increase the vertex angle, reduce the rake angle of the cutting part, increase the rear angle of the cutting part, and double the inverse taper of the cylindrical edge. The number of retractions should be increased during processing, the drill should not stay in the hole, the chips should be removed in time, and a sufficient amount of emulsion should be used for cooling. Pay attention to observe the dullness of the drill and remove the chips in time. Replace grinding.
(9) Titanium alloy reaming also needs to modify the standard reamer: the width of the reamer margin should be less than 0.15mm, and the cutting part and the calibration part should be arc-transitioned to avoid sharp points. When reaming holes, a group of reamers can be used for multiple reaming, and the diameter of the reamer is increased by less than 0.1mm each time. Reaming in this way can achieve higher finish requirements.
(10) Tapping is a very difficult part in the processing of titanium alloys. Due to the excessive torque, the tap teeth will wear quickly, and the springback of the machined part can even break the tap in the hole. When using ordinary taps, the number of teeth should be appropriately reduced according to the diameter to increase the chip space. After setting aside a 0.15mm wide margin on the calibration teeth, the clearance angle should be increased to about 30°, and 1/2～1 /3 tooth back, the calibration tooth is retained for 3 buckles and then increases the number of inverse tapers. It is recommended to choose a skip tap, which can effectively reduce the contact area between the tool and the workpiece, and the processing effect is also better.