Titanium alloys refer to a variety of alloyed metals made of titanium and other metals. Titanium is an important structural metal developed in the 1950s. Titanium alloys have high strength, good corrosion resistance and high heat resistance. In the 1950s and 1960s, high-temperature titanium alloys for aero-engines and structural titanium alloys for airframes were mainly developed.
Corrosion-resistant titanium alloys were developed in the 1970s. Since the 1980s, corrosion-resistant titanium alloys and high-strength titanium alloys have been further developed. Titanium alloys are mainly used to make aircraft engine compressor parts, followed by structural parts of rockets, missiles and high-speed aircraft
Properties of titanium alloy
Titanium is a new type of metal. The performance of titanium is related to the content of impurities such as carbon, nitrogen, hydrogen, and oxygen. The purest titanium iodide has an impurity content of no more than 0.1%, but its strength is low and its plasticity is high. The properties of 99.5% industrial pure titanium are: density ρ=4.5g/cm3, melting point 1725℃, thermal conductivity λ=15.24W/(m.K), tensile strength σb=539MPa, elongation δ=25%, section shrinkage Rate ψ=25%, elastic modulus E=1.078×105MPa, hardness HB195.
The density of titanium alloys is generally around 4.51g/cm3, which is only 60% of that of steel. Some high-strength titanium alloys exceed the strength of many alloy structural steels. Therefore, the specific strength (strength/density) of titanium alloy is much greater than that of other metal structural materials, and parts with high unit strength, good rigidity and light weight can be produced. Titanium alloys are used in aircraft engine components, skeletons, skins, fasteners and landing gear.
The operating temperature of titanium alloy is several hundred degrees higher than that of aluminum alloy, and it can still maintain the required strength at moderate temperature. The specific strength of aluminum alloy decreases significantly at 150 °C. The working temperature of titanium alloy can reach 500 ℃, and the working temperature of aluminum alloy is below 200 ℃.
Titanium alloy works in humid atmosphere and seawater medium, and its corrosion resistance is much better than stainless steel; its resistance to pitting corrosion, acid corrosion, and stress corrosion is particularly strong; it is resistant to alkali, chloride, chlorine, organic substances, nitric acid, sulfuric acid etc. have excellent corrosion resistance. However, titanium has poor corrosion resistance to media with reducing oxygen and chromium salts.
Titanium alloys can still maintain their mechanical properties at low and ultra-low temperatures. Titanium alloys with good low temperature performance and extremely low interstitial elements, such as TA7, can maintain a certain plasticity at -253 °C. Therefore, titanium alloy is also an important low-temperature structural material.
Titanium has high chemical activity and produces strong chemical reactions with O2, N2, H2, CO, CO2, water vapor, and ammonia in the atmosphere. When the carbon content is greater than 0.2%, a hard TiC will be formed in the titanium alloy; when the temperature is high, a hard surface layer of TiN will also be formed when it interacts with N; when the temperature is above 600 ℃, titanium absorbs oxygen to form a hardened layer with high hardness ; Increased hydrogen content will also form an embrittlement layer. The depth of the hard and brittle surface layer produced by absorbing gas can reach 0.1 to 0.15 mm, and the degree of hardening is 20% to 30%. The chemical affinity of titanium is also large, and it is easy to adhere to the friction surface.
The thermal conductivity of titanium λ=15.24W/(m·K) is about 1/4 of nickel, 1/5 of iron, and 1/14 of aluminum, and the thermal conductivity of various titanium alloys is about 50 lower than that of titanium. %. The elastic modulus of titanium alloy is about 1/2 of that of steel, so its rigidity is poor and it is easy to deform. It is not suitable to make slender rods and thin-walled parts. times, resulting in severe friction, adhesion and bond wear on the flank of the tool.
Classification of titanium alloy
Titanium is an allotrope with a melting point of 1668 °C. When it is lower than 882 °C, it has a close-packed hexagonal lattice structure, which is called α titanium; when it is above 882 °C, it has a body-centered cubic lattice structure, which is called β titanium. Using the different characteristics of the above two structures of titanium, adding appropriate alloying elements to gradually change the phase transition temperature and phase content to obtain titanium alloys with different structures. At room temperature, titanium alloys have three matrix structures, and titanium alloys are divided into the following three categories: α alloys, (α+β) alloys and β alloys. In China, it is represented by TA, TC, and TB, respectively.
It is a single-phase alloy composed of α-phase solid solution. Whether it is at ordinary temperature or at higher practical application temperature, it is α-phase, with stable structure, higher wear resistance than pure titanium, and strong oxidation resistance. At the temperature of 500 ℃ ~ 600 ℃, it still maintains its strength and creep resistance, but it cannot be strengthened by heat treatment, and the room temperature strength is not high.
β titanium alloy is a single-phase alloy composed of β-phase solid solution. It has high strength without heat treatment. After quenching and aging, the alloy is further strengthened, and the room temperature strength can reach 1372-1666 MPa. However, it has poor thermal stability and is not suitable for use at high temperatures.
α+β titanium alloy It is a dual-phase alloy with good comprehensive properties, good organizational stability, good toughness, plasticity and high temperature deformation properties, and can be well processed by hot pressure, and can be quenched and aged to strengthen the alloy. The strength after heat treatment is about 50% to 100% higher than that in the annealed state; the high temperature strength is high, and it can work for a long time at a temperature of 400 ° C to 500 ° C, and its thermal stability is inferior to that of α titanium alloy.
Among the three titanium alloys, α titanium alloy and α+β titanium alloy are the most commonly used; α titanium alloy has the best machinability, followed by α+β titanium alloy, and β titanium alloy is the worst. The code name of α titanium alloy is TA, the code name of β titanium alloy is TB, and the code name of α+β titanium alloy is TC.
Titanium alloys can be divided into heat-resistant alloys, high-strength alloys, corrosion-resistant alloys (titanium-molybdenum, titanium-palladium alloys, etc.), low-temperature alloys and special functional alloys (titanium-iron hydrogen storage materials and titanium-nickel memory alloys), etc. .
Heat treatment: Titanium alloys can obtain different phase compositions and structures by adjusting the heat treatment process. It is generally believed that the fine equiaxed structure has good plasticity, thermal stability and fatigue strength; the needle-like structure has high lasting strength, creep strength and fracture toughness; the equiaxed and acicular mixed structure has good comprehensive properties.
Titanium alloy has high strength and low density, good mechanical properties, good toughness and corrosion resistance. In addition, the process performance of titanium alloy is poor, cutting is difficult, and it is very easy to absorb impurities such as hydrogen, oxygen, nitrogen and carbon during hot processing. There is also poor wear resistance and complex production process. The industrial production of titanium started in 1948. The need for the development of the aviation industry makes the titanium industry develop at an average annual growth rate of about 8%. The world’s annual output of titanium alloy processing materials has reached more than 40,000 tons, and there are nearly 30 types of titanium alloys. The most widely used titanium alloys are Ti-6Al-4V (TC4), Ti-5Al-2.5Sn (TA7) and industrial pure titanium (TA1, TA2 and TA3).
Titanium alloys are mainly used to make aircraft engine compressor parts, followed by rockets, missiles and structural parts of high-speed aircraft. In the mid-1960s, titanium and its alloys have been used in general industry for making electrodes in the electrolysis industry, condensers in power stations, heaters in petroleum refining and seawater desalination, and environmental pollution control devices. Titanium and its alloys have become a corrosion-resistant structural material. In addition, it is also used in the production of hydrogen storage materials and shape memory alloys.
Titanium alloy is a new important structural material used in the aerospace industry. Its specific gravity, strength and service temperature are between aluminum and steel, but it is stronger than aluminum and steel and has excellent seawater corrosion resistance and ultra-low temperature performance. In 1950, the United States first used non-load-bearing components such as rear fuselage heat shields, wind deflectors, and tail covers on the F-84 fighter-bomber. Since the 1960s, the use of titanium alloys has shifted from the rear fuselage to the middle fuselage, partially replacing structural steel to manufacture important load-bearing components such as bulkheads, beams, and flap slide rails. The amount of titanium alloys used in military aircraft has increased rapidly, reaching 20% to 25% of the weight of the aircraft structure. Since the 1970s, civil aircraft began to use titanium alloys in large quantities. For example, the amount of titanium used in Boeing 747 passenger aircraft reached more than 3,640 kilograms. Titanium for aircraft with Mach numbers greater than 2.5 is mainly used to replace steel to reduce structural weight. Another example is the American SR-71 high-altitude and high-speed reconnaissance aircraft (flying Mach 3 and flying height of 26,212 meters), titanium accounts for 93% of the weight of the aircraft structure, and it is known as an “all-titanium” aircraft. When the thrust-to-weight ratio of the aero-engine increases from 4 to 6 to 8 to 10, and the compressor outlet temperature increases from 200 to 300°C to 500 to 600°C, the original low-pressure compressor discs and blades made of aluminum must Switch to titanium alloys, or use titanium alloys instead of stainless steel for high-pressure compressor discs and blades to reduce structural weight. In the 1970s, the amount of titanium alloy used in aero-engines generally accounted for 20% to 30% of the total weight of the structure. Case, bearing housing, etc. Spacecraft mainly use the high specific strength, corrosion resistance and low temperature resistance of titanium alloys to manufacture various pressure vessels, fuel tanks, fasteners, instrument straps, frameworks and rocket casings. Artificial earth satellites, lunar modules, manned spacecraft and space shuttles also use titanium alloy sheet welding.
Commonly used heat treatment methods are annealing, solution and aging treatment. Annealing is to eliminate internal stress, improve plasticity and organizational stability, and obtain better comprehensive properties. Usually, the annealing temperature of α alloy and (α+β) alloy is selected at 120～200℃ below the (α+β)-→β phase transformation point; solution and aging treatment is fast cooling from high temperature region to obtain martensite α′ phase and metastable β phase, and then decompose these metastable phases by heat preservation in the medium temperature region to obtain finely dispersed second phase particles such as α phase or compounds, to achieve the purpose of strengthening the alloy. Usually (α+β) alloys are quenched at 40～100℃ below the (α+β)─→β phase transition point, and metastable β alloys are quenched at 40～80℃ above the (α+β)─→β phase transition point. conduct. The aging treatment temperature is generally 450 to 550 °C.
In summary, the heat treatment process of titanium alloys can be summarized as:
(1) Stress relief annealing: The purpose is to eliminate or reduce the residual stress generated during processing. Prevent chemical attack and reduce deformation in some corrosive environments.
(2) Complete annealing: The purpose is to obtain good toughness, improve processing properties, facilitate reprocessing and improve the stability of size and structure.
(3) Solution treatment and aging: The purpose is to improve its strength. Alpha titanium alloys and stable beta titanium alloys cannot undergo strengthening heat treatment, and only annealing is performed in production. α+β titanium alloys and metastable β titanium alloys containing a small amount of α phase can be further strengthened by solution treatment and aging.
In addition, in order to meet the special requirements of the workpiece, the industry also adopts metal heat treatment processes such as double annealing, isothermal annealing, beta heat treatment, and deformation heat treatment.
In the middle of heat treatment and after heat treatment, surface treatment is mostly required to remove oxide scale and various pollutants on metal surface, reduce the activity of bare metal surface, and apply protective layer and various functional coatings on titanium and its alloy surface halogen. Surface treatment is also carried out before and during the coating process. The coating is applied to improve the properties of the metal surface, for example, to prevent corrosion, oxidation and wear.
The pickling conditions of titanium and its alloys are determined by the type (characteristics) of the oxide layer and the existing reaction layer, and the type of this layer is affected by the high temperature heating process and the increase in processing temperature (such as forging, casting, welding, etc.). At a lower processing temperature or about 600X: the following high temperature heating temperature conditions only generate a thin oxide layer, under high temperature conditions, an oxygen-rich diffusion zone is formed near a certain oxide layer, which must also be eluted by acid remove this oxygen-rich diffusion layer. A variety of different descaling methods can be used: mechanical methods for removing thick oxide layers and hard surface layers, descaling in molten salt baths, and methods for acid elution descaling in acid solutions.
In many cases a combination of methods can be used, for example, a combination of mechanical descaling followed by pickling, or a descaling method combining a salt bath followed by pickling. In the case of the oxide layer and the diffusion layer formed at a higher temperature, a special method is required, but the oxide layer formed in the case of heating at a high temperature to 600X: most of the oxide layers can be dissolved by ordinary pickling.
Cutting & Machining for titanium alloys
When the hardness of titanium alloy is greater than HB350, it is particularly difficult to cut, and when it is less than HB300, it is easy to stick to the knife and it is also difficult to cut. However, the hardness of titanium alloys is only one aspect that is difficult to machine. Titanium alloys have the following cutting characteristics:
(1) Small deformation coefficient: This is a significant feature of titanium alloy cutting, and the deformation coefficient is less than or close to 1. The sliding friction distance of chips on the rake face is greatly increased, which accelerates tool wear.
(2) High cutting temperature: Due to the small thermal conductivity of titanium alloy (equivalent to 1/5 to 1/7 of No. 45 steel), the contact length between the chip and the rake face is extremely short, and the heat generated during cutting is not easily transmitted. It is concentrated in a small area near the cutting area and the cutting edge, and the cutting temperature is very high. Under the same cutting conditions, the cutting temperature can be more than doubled when cutting 45# steel.
(3) The cutting force per unit area is large: the main cutting force is about 20% smaller than that when cutting steel. Because the contact length between the chip and the rake face is extremely short, the cutting force per unit contact area is greatly increased, which is easy to cause chipping. At the same time, due to the small elastic modulus of titanium alloys, it is easy to bend and deform under the action of radial force during processing, causing vibration, increasing tool wear and affecting the accuracy of parts. Therefore, it is required that the process system should have better rigidity.
(4) The phenomenon of chilling is serious: due to the high chemical activity of titanium, it is easy to absorb oxygen and nitrogen in the air to form a hard and brittle outer skin at high cutting temperatures; at the same time, plastic deformation during cutting will also cause surface hardening. . Chilling not only reduces the fatigue strength of parts, but also increases tool wear, which is an important feature when cutting titanium alloys.
(5) The tool is easy to wear: After the blank is processed by stamping, forging, hot rolling, etc., a hard and brittle uneven skin is formed, which is easy to cause chipping phenomenon, making the removal of hard skin the most difficult process in titanium alloy processing. In addition, due to the strong chemical affinity of titanium alloys for tool materials, under the conditions of high cutting temperature and large cutting force per unit area, the tool is prone to bond wear. When turning titanium alloys, sometimes the wear of the rake face is even more serious than that of the flank; when the feed rate f<0.1 mm/r, the wear mainly occurs on the flank; when f>0.2 mm/r, the rake face The rake face will wear; when using carbide tools for finishing and semi-finishing, the wear of the flank face should be less than 0.4 mm when VBmax is less than 0.4 mm.
In milling, due to the low thermal conductivity of titanium alloy materials and the extremely short contact length between the chip and the rake face, the heat generated during cutting is not easy to be transmitted, and is concentrated in the cutting deformation zone and a small range near the cutting edge. During machining, extremely high cutting temperature will be generated at the cutting edge, which will greatly shorten the tool life. For titanium alloy Ti6Al4V, under the conditions allowed by tool strength and machine tool power, the level of cutting temperature is the key factor affecting tool life, not the size of cutting force.
Machining titanium alloys should start from two aspects: reducing cutting temperature and reducing bonding, and selecting tool materials with good red hardness, high flexural strength, good thermal conductivity, and poor affinity with titanium alloys. YG-type cemented carbide is more suitable. Due to the poor heat resistance of high-speed steel, tools made of cemented carbide should be used as much as possible. Commonly used carbide tool materials are YG8, YG3, YG6X, YG6A, 813, 643, YS2T and YD15.
Coated inserts and YT-type hard alloys will have a strong affinity with titanium alloys, aggravate the bonding and wear of tools, and are not suitable for cutting titanium alloys; for complex and multi-edged tools, high-vanadium high-speed steel (such as W12Cr4V4Mo) can be used. ), high-cobalt high-speed steel (such as W2Mo9Cr4VCo8) or aluminum high-speed steel (such as W6Mo5Cr4V2Al, M10Mo4Cr4V3Al) and other tool materials, suitable for making drills, reamers, end mills, broaches, taps and other tools for cutting titanium alloys.
Using diamond and cubic boron nitride as tools to cut titanium alloys can achieve remarkable results. If a natural diamond tool is used under the condition of emulsion cooling, the cutting speed can reach 200 m/min; if no cutting fluid is used, the allowable cutting speed is only 100 m/min under the same wear amount.
In the process of cutting titanium alloys, the matters that should be paid attention to are:
(1) Due to the small elastic modulus of the titanium alloy, the clamping deformation and stress deformation of the workpiece during processing will be large, which will reduce the machining accuracy of the workpiece; the clamping force should not be too large when the workpiece is installed, and auxiliary supports can be added if necessary.
(2) If a cutting fluid containing hydrogen is used, hydrogen will be decomposed and released at high temperature during the cutting process, which will be absorbed by titanium and cause hydrogen embrittlement; it may also cause high temperature stress corrosion cracking of titanium alloys.
(3) Chloride in the cutting fluid may also decompose or volatilize toxic gases. Safety precautions should be taken during use, otherwise it should not be used; after cutting, the parts should be thoroughly cleaned with a chlorine-free cleaning agent to remove chlorine-containing residues thing.
(4) It is forbidden to use tools and fixtures made of lead or zinc-based alloys that come into contact with titanium alloys, as are copper, tin, cadmium and their alloys.
(5) All tools, fixtures or other devices in contact with titanium alloys must be clean; cleaned titanium alloy parts should be protected from grease or fingerprint contamination, otherwise it may cause stress corrosion of salt (sodium chloride) in the future.
(6) Under normal circumstances, when machining titanium alloys, there is no danger of ignition. Only in micro-cutting, the fine chips cut off can ignite and burn. In order to avoid fire, in addition to pouring a large amount of cutting fluid, chips should also be prevented from accumulating on the machine tool, and the tool should be replaced immediately after it is blunt, or the cutting speed should be reduced, and the feed rate should be increased to increase the chip thickness. In case of fire, talcum powder, limestone powder, dry sand and other fire extinguishing equipment should be used to extinguish the fire. Carbon tetrachloride and carbon dioxide fire extinguishers are strictly prohibited, and water should not be used, because water can accelerate combustion and even cause hydrogen explosion.
Advantages and Disadvantages of titanium alloy
Titanium alloys have the advantages of light weight, high specific strength and good corrosion resistance, so they are widely used in the automotive industry, and the most widely used titanium alloys are automotive engine systems. There are many benefits to using titanium alloys to manufacture engine parts.
The low density of titanium alloy can reduce the inertial mass of moving parts, while the titanium valve spring can increase free vibration, reduce the vibration of the body, and improve the engine speed and output power.
Reduce the inertial mass of moving parts, thereby reducing friction and improving the fuel efficiency of the engine. Selecting 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 whole vehicle. Reduced inertial mass of components reduces vibration and noise, improving engine performance. The application of titanium alloys in other components can improve the comfort of people and the aesthetics of cars. In the application of the automobile industry, titanium alloys have played an immeasurable role in energy saving and consumption reduction.
Although titanium alloy parts have such superior properties, there is still a long way to go before titanium and its alloys are widely used in the automotive industry, due to problems such as high price, poor formability and poor welding performance.
The main reason preventing the widespread use of titanium alloys in the automotive industry is the high cost.
Whether it is the initial smelting of the metal or the subsequent processing, the price of titanium alloys is much higher than that of other metals. The cost of titanium parts acceptable to the automotive industry is $8-13/kg for connecting rods, $13-$20/kg for valves, and $8/kg for springs, engine exhaust systems and fasteners. It is 6 to 15 times that of aluminum sheets and 45 to 83 times that of steel sheets.
The main limitation of titanium and titanium alloys is their poor chemical reactivity with other materials at high temperatures. This property forces titanium alloys to be different from the general traditional refining, melting and casting techniques, and even often causes damage to the mold; as a result, the price of titanium alloys becomes very expensive. Therefore, they were initially mostly used in aircraft structures, aircraft, and in high-tech industries such as the petroleum and chemical industries. However, due to the development of space technology and the improvement of people’s quality of life, titanium alloys are gradually being used to make products for people’s livelihood to benefit people’s lives. However, the prices of these products are still high, and most of them are high-priced products, this is the root cause of titanium alloy not being widely applicable.