Titanium has the advantages of low density, high specific strength, and good corrosion resistance. The use of titanium in automobiles can greatly reduce the quality of the car body, reduce fuel consumption, improve the efficiency of the engine, improve the environment and reduce noise. However, due to the high price, titanium alloys can only be used in luxury models and sports cars in the automotive industry, and are rarely used in ordinary cars. Therefore, research and development of low-cost titanium alloys that meet the needs of the market are the key to promoting its application in ordinary household cars. In recent years, with the rapid development of the automobile industry, the fuel consumption, environmental protection and safety issues generated by automobiles have attracted increasing attention. Looking forward to the future development direction of the automobile industry, light weight, low fuel consumption and low emissions are the development themes. According to statistics from international authoritative departments, 60% of automobile fuel combustion energy is consumed in its own quality, although high-strength thin steel plates, aluminum, magnesium, metal matrix composites and plastic resin materials have played a role in reducing vehicle weight Function, but the emergence of industrial titanium materials has made car manufacturing a better choice.
The processing cost of titanium alloy materials in the production process accounts for more than 60% of the total cost. Therefore, in terms of cost reduction, how to reduce the processing cost of titanium alloys has become a key research direction. The research in this area is mainly divided into two aspects: one is to improve the traditional casting and forging process, and the other is to use powder metallurgy near net forming technology. In the development of a new type of forging process, cold forging is one of the most promising methods for manufacturing automotive parts from titanium alloys. Beta titanium alloy has low deformation resistance at room temperature, good cutting and forming, and is a material that can be cold forged. Currently, three cold deformed beta titanium alloys have been developed in Japan. Beta titanium alloy also has some shortcomings, it is easy to produce uneven deformation during cold forging, and it is easy to adhere to the mold, so mass production of beta titanium alloy parts by cold forging technology needs further exploration and development.
Titanium alloy has the advantages of light weight, high specific strength, good corrosion resistance, etc., so it is widely used in the automotive industry. The most widely used titanium alloy is the automobile engine system. The use of titanium alloys to manufacture engine parts has many benefits, mainly manifested in:
- 1) The low density of titanium alloy can reduce the inertial mass of moving parts. At the same time, titanium valve spring can increase free vibration, reduce body vibration, increase engine speed and output power.
- 2) Reduce the inertial mass of moving parts, thereby reducing friction and improving the fuel efficiency of the engine.
- 3) The choice of titanium alloy can reduce the load stress of related parts and reduce the size of the parts, thereby reducing the quality of the engine and the vehicle.
- 4) The reduction of the inertial mass of the parts makes the vibration and noise weaker and improves the performance of the engine.
The application of titanium alloy on other components can improve the comfort of people and the appearance of automobiles. In the automotive industry, titanium alloys play an inestimable role in energy saving and consumption reduction. Titanium adopts plastic processing, and the size of the soil is not limited, and it can be produced in large quantities, but the yield rate is low, and a large amount of waste residues are generated during the processing. In response to the above-mentioned shortcomings of titanium plastic processing, powder metallurgy of titanium has been developed in recent years. The powder metallurgy process of titanium is the same as ordinary powder metallurgy, except that the sintering must be carried out under vacuum. It is suitable for the production of large-volume, small-size parts, especially for the production of complex parts. This method requires almost no further processing and has a high yield rate. It can make full use of titanium waste as a raw material and can reduce production costs, but it cannot produce large-sized titanium parts. Titanium alloys add alloying elements to industrial pure titanium to increase the strength of titanium. There are three types of titanium alloys: a titanium alloy, b titanium alloy and a + b titanium alloy. a b titanium alloy is composed of a and b two-phase. This type of alloy has stable microstructure, good high temperature deformation performance, toughness and plasticity, and can be quenched and aged to strengthen the alloy. The performance characteristics of titanium alloy are mainly manifested in:
- 1) High specific strength. Titanium alloy has a low density (4.4kg / dm3) and light weight, but its specific strength is greater than that of ultra-high strength steel.
- 2) High thermal strength. Titanium alloy has good thermal stability, and its strength is about 10 times higher than that of aluminum alloy at 300 ～ 500 ℃.
- 3) Great chemical activity. Titanium can have a strong chemical reaction with oxygen, nitrogen, carbon monoxide, water vapor and other substances in the air, forming a hardened layer of TiC and TiN on the surface.
Poor thermal conductivity. The thermal conductivity of the titanium alloy is poor. The thermal conductivity of the titanium alloy TC4 at 200 ° C is 1 = 16.8 W / m · ° C, and the thermal conductivity is 0.036 calories / cm · sec · ° C.
1. Selection of tool material
Tool material selection should meet the following requirements:
- 1) Sufficient hardness. The hardness of the tool must be much greater than the hardness of the titanium alloy.
- 2) Sufficient strength and toughness. Because the cutting tool bears a large torque and cutting force when cutting titanium alloy, it must have sufficient strength and toughness.
- 3) Sufficient wear resistance. Due to the good toughness of titanium alloy, the cutting edge should be sharp during processing, so the tool material must have sufficient wear resistance, so as to reduce the work hardening. This is the most important parameter for selecting titanium alloy cutting tools.
- 4) The tool material has poor affinity with the titanium alloy. Because of the high chemical activity of titanium alloys, it is necessary to avoid the formation of alloying and diffusion of tool materials and titanium alloys to form alloys, resulting in sticking and burning.
2. Accuracy, conditions and correct cutting parameters
Tests on domestic commonly used tool materials and foreign tool materials show that the use of high cobalt tools is ideal. The main role of cobalt can strengthen the secondary hardening effect, improve red hardness and hardness after heat treatment, and have high toughness and wear resistance. , Good heat dissipation.
Geometrical parameters of milling cutter
The processing characteristics of titanium alloy determine the geometric parameters of the tool, which are quite different from ordinary tools.
Helix angle β Choose a smaller helix angle to increase the chip flute, easy chip evacuation, fast heat dissipation, and also reduce the cutting resistance during the cutting process.
Rake angle γ During cutting, the cutting edge is sharp and the cutting is light and fast, avoiding excessive cutting heat generated by the titanium alloy, thereby avoiding secondary hardening.
Back angle α reduces the wear rate of the blade, which is conducive to heat dissipation, and the durability is also greatly improved.
Cut parameter selection
Titanium alloy machining should choose a lower cutting speed, an appropriate large feed, a reasonable depth of cut and a finishing amount, and sufficient cooling.
Cutting speed Vc Vc = 30 ～ 50m / min
Feed amount f Take a larger feed amount during roughing, and a moderate feed amount for finishing and semi-finishing.
Depth of cut ap ap = 1 / 3d is suitable. Titanium alloy has good affinity, difficult chip removal, and too large cutting depth, will cause tool sticking, burning and breaking.
Finishing allowance αc is suitable for the hardened layer of titanium alloy surface is about 0.1 ~ 0.15mm, the allowance is too small, the cutting edge is on the hardened layer, the tool is easily worn, the hardened layer should be avoided, but the cutting allowance should not be too large.
Although titanium alloy parts have such superior performance, they are still far away from the general application of titanium and its alloys in the automotive industry. Reasons include high prices, poor formability, and poor welding performance. With the development of near-net forming technology of titanium alloys and modern welding technologies such as electron beam welding, plasma arc welding and laser welding in recent years, the forming and welding of titanium alloys are no longer the key factors restricting the application of titanium alloys and hindering titanium alloys The main reason for the widespread use in the automotive industry is the high cost. Whether it is the initial metal smelting or subsequent processing, the price of titanium alloys is much higher than other metals. The cost of titanium parts acceptable to the automotive industry is 8 to 13 US dollars / kg for connecting rod titanium, 13 to 20 US dollars / kg for valves, and titanium springs, titanium engine exhaust systems and fasteners are expected to be 8 US dollars / kg or less. At present, the cost of parts produced with titanium materials is much higher than these prices. The production cost of titanium plates is mostly higher than 33 US dollars / kg, which is 6 to 15 times that of aluminum plates and 45 to 83 times that of steel plates.