This article discusses the application and research status of titanium alloy materials, highlights the main characteristics of titanium and titanium alloys, processing performance and its application in aerospace, military industry and automobile manufacturing, and on this basis, prospects for titanium alloys Direction of development.
Keywords: Titanium alloy characteristic processing performance
Titanium alloy is an ideal manufacturing material for aircrafts and engines because of its high specific strength, good mechanical properties and good corrosion resistance. However, due to its poor machinability, it has long restricted its application to a large extent. With the development of processing technology, in recent years, titanium alloys have been widely used in the manufacture of aircraft engine compressors, hoods, exhaust devices and other parts and the manufacture of aircraft frame beams and other structural frame parts. The following mainly discusses the characteristics and processing properties of titanium alloys.
1. Characteristics of titanium and titanium alloys
Titanium and titanium alloys have many excellent characteristics, mainly reflected in the following aspects:
- High strength. Titanium alloy has high strength, its tensile strength is 686-1176MPa, and the density is only about 60% of steel, so the specific strength is very high.
- High hardness. The hardness HRC of the titanium alloy (annealed state) is 32-38.
- Low elastic modulus. The elastic modulus of titanium alloy (annealed state) is 1.078 × 10-1.176 × 10MPa, which is about half of steel and stainless steel.
- Excellent high and low temperature performance. At high temperatures, titanium alloys can still maintain good mechanical properties, their heat resistance is much higher than aluminum alloys, and the working temperature range is wide. At present, the working temperature of new heat-resistant titanium alloys can reach 550-600 ℃; at low temperatures However, the strength of the titanium alloy increases rather than that at room temperature, and has good toughness. The low-temperature titanium alloy can maintain good toughness at -253 ℃.
- Titanium has strong corrosion resistance. Titanium quickly forms a thin and dense titanium oxide film in the air below 550 ℃, so its corrosion resistance is superior to most stainless steels in oxidizing media such as the atmosphere, sea water, nitric acid, sulfuric acid, and strong alkalis.
Second, the processing properties of titanium and titanium alloys
- 1. Cutting performance
According to the nature of the titanium alloy and the characteristics of the cutting process, the following aspects should be considered during processing:
Use carbide tools as much as possible, such as tungsten-cobalt-based cemented carbides and titanium alloys have low chemical affinity, good thermal conductivity, and high strength. Intermittent cutting at low speeds can use impact-resistant ultrafine-grained cemented carbide, and high-speed steel with good high-temperature performance can be used for forming and complex tools.
Use a smaller rake angle and a larger rake angle to increase the contact length between the chip and the rake face, reduce the friction between the workpiece and the rake face, and use a circular transition edge to improve the strength and avoid burns at sharp corners And fall apart. It is necessary to keep the blade sharp to ensure smooth chip evacuation and avoid chipping of sticky chips. The cutting speed should be low to avoid cutting temperature too high; the feed rate is moderate, too large, easy to burn the knife, too small, because the cutting edge works in the work hardening layer and wears too fast; the cutting depth can be large, so that the tip of the tool is in the hardening layer The following work is conducive to improving the durability of the tool. Cooling fluid must be added to cool down during processing. When cutting titanium alloy, the resistance to knife eating is greater, so the process system needs to ensure that it has sufficient rigidity. Because the titanium alloy is easily deformed, the cutting clamping force cannot be large, especially in certain finishing processes, certain auxiliary supports can be used when necessary.
- 2. Grinding performance
Titanium alloys are lively, easy to adhere to and adhere to abrasives at high temperatures, block the grinding wheel, and lead to increased wear of the grinding wheel, reduced grinding performance, and difficult to guarantee the grinding accuracy. At the same time, the wear of the grinding wheel also increases the contact area between the grinding wheel and the workpiece, which deteriorates the heat dissipation condition, the temperature of the grinding area rises sharply, and a large thermal stress is formed on the grinding surface layer, causing local burns of the workpiece and grinding. crack. Titanium alloy has high strength and high toughness, which makes grinding chips difficult to separate during grinding, grinding force increases, and grinding power consumption increases accordingly. The thermal conductivity of titanium alloy is low, the specific heat is small, and the heat conduction during grinding is slow, which causes heat to accumulate in the grinding arc area, causing the temperature of the grinding area to rise sharply.
- 3. Extrusion processing performance
When extruding titanium and titanium alloys, the extrusion temperature is high and the extrusion speed is fast to prevent the temperature from dropping too fast. At the same time, the contact time between the high temperature billet and the mold should be minimized. Therefore, a new type of heat-resistant mold material should be used for the extrusion die, and the conveying speed of the billet from the heating furnace to the extrusion barrel should be fast. In view of the fact that metals are easily contaminated by gases during heating and extrusion, appropriate protective measures should also be adopted. When extruding, proper lubricant should be selected to prevent sticking to the mold, such as sleeve extrusion and glass lubrication extrusion. Due to the large deformation thermal effect of titanium and titanium alloys and poor thermal conductivity, special attention should be paid to prevent overheating during extrusion deformation. The extrusion process of titanium alloy is more complicated than that of aluminum alloy, copper alloy and even steel, which is determined by the special physical and chemical properties of titanium alloy. When the titanium alloy is formed by conventional hot reverse extrusion, the mold temperature is low, the surface temperature of the blank in contact with the mold drops rapidly, and the temperature inside the blank rises due to the heat of deformation. Due to the low thermal conductivity of the titanium alloy, after the temperature of the surface layer drops, the heat of the inner layer blank cannot be transferred to the surface layer for supplementation in time, and a surface hardened layer will appear, which makes the deformation difficult to continue. At the same time, the surface layer and the inner layer will have a large temperature gradient, even if it can be shaped, it is also easy to cause deformation and uneven structure.
- 4. Forging processing performance
Titanium alloys are very sensitive to forging process parameters. Changes in forging temperature, deformation, deformation and cooling rate will cause changes in the structure and properties of titanium alloys. In order to better control the structure and performance of forgings, in recent years, advanced forging techniques such as hot die forging and isothermal forging have been widely used in the forging production of titanium alloys.
The plasticity of titanium alloy increases with the increase of temperature. In the temperature range of 1000-1200 ℃, the plasticity reaches the maximum value, and the allowable deformation degree reaches 70% -80%. The titanium alloy forging temperature range is narrow, and it should be strictly controlled according to the (α + β) / β transition temperature (except for ingot opening), otherwise the β grains will grow violently and reduce the room temperature plasticity; + β) Forging in the two-phase zone, too high a forging temperature above the (α + β) / β phase transition line will result in a β brittle phase. β transition temperature. The deformation resistance of the titanium alloy increases rapidly with the increase of the deformation speed, and the forging temperature has a greater influence on the deformation resistance of the titanium alloy. Therefore, the conventional forging must be completed with the minimum cooling in the forging die. The content of interstitial elements (such as O, N, C) also has a significant effect on the forgeability of titanium alloys.
- 5. Casting process performance
Due to the high chemical activity of titanium and titanium alloys, they are susceptible to violent chemical reactions with N, O, N in the air, and chemical reactions with refractory materials commonly used in casting. The casting of titanium and titanium alloys, especially investment precision casting, is much more difficult than that of aluminum and steel. Special methods are needed to achieve it. In the early stage of the development of cast titanium, because the development of casting technology lags behind the pressure processing technology, first select medium-strength titanium alloys with certain deformations, such as TiΟ6AlΟ4V, TiΟ5AlΟ2.5Sn, etc. as casting alloy materials. These alloys are still widely used today. However, with the development of the titanium casting process and the application field, the performance requirements of various aspects of casting titanium alloys are increasing, and the complexity of the casting structure is increasing. In the past, the argument that “all deformed titanium alloys are suitable as casting alloys” It should be revised. As the temperature and working strength of the alloy increase, the number and amount of elements added in the alloy increase accordingly, but at the same time, the casting performance of the alloy, the crystalline structure of the fluidization solidification zone, the mechanical properties, etc., that is, the alloy’s The chemical composition must be adjusted according to the requirements of the casting process.
In summary, titanium alloys have very wide applications in aerospace and other fields due to their excellent performance, but they are also constrained by their processing efficiency and production costs. Once there is a breakthrough in titanium smelting technology, its price will also be significantly reduced. With the development and development of titanium alloys, the increase of titanium materials and the reduction of prices, the application of titanium in civil industry will increase exponentially, especially in shipbuilding, automobile manufacturing, chemical industry, electronics, marine development, seawater desalination, geothermal power generation , Sewage and anti-corrosion and other civil fields will be widely used. At the same time, market demand will also accelerate the development of titanium industry and titanium processing technology.