What is titanium alloy?
What is titanium alloy?
Titanium alloy is alloy based on titanium with other elements added. Titanium has two kinds of homogeneous heterogeneous crystals: alpha titanium with dense hexagonal structure below 882℃ and beta titanium with body-centered cubic structure above 882℃.
Alloy elements can be classified into three categories according to their effects on phase transformation temperature:
- (1) The elements that stabilize the alpha phase and increase the phase transition temperature are alpha stable elements, such as aluminium, carbon, oxygen and nitrogen. Among them, aluminium is the main alloy element of titanium alloy. It has obvious effect on improving the strength at room temperature and high temperature, reducing specific gravity and increasing elastic modulus of the alloy.
- (2) The stable beta phase and the decreasing phase transition temperature are beta stable elements, which can be divided into two types: isomorphic and eutectoid. The former includes molybdenum, niobium and vanadium, while the latter includes chromium, manganese, copper, iron and silicon.
- (3) Neutral elements, such as zirconium and tin, have little effect on phase transition temperature.
- Oxygen, nitrogen, carbon and hydrogen are the main impurities in titanium alloys. Oxygen and nitrogen have higher solubility in the alpha phase, which has a significant strengthening effect on titanium alloy, but reduces its plasticity. Oxygen and nitrogen contents in titanium are usually stipulated to be below 0.15-0.2% and 0.04-0.05% respectively. The solubility of hydrogen in the alpha phase is very small. The excessive hydrogen dissolved in the titanium alloy will produce hydride, which makes the alloy brittle. Usually hydrogen content in titanium alloys is controlled below 0.015%. The dissolution of hydrogen in titanium is reversible and can be removed by vacuum annealing.
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- Properties of Titanium Alloys
- Classification of Titanium Alloys
- Application of Titanium Alloys
- Heat Treatment of Titanium Alloys
- Production Technology of Titanium Alloy
- Cutting of Titanium Alloys
- Problems in Titanium Alloys
Titanium is a new type of metal. Its properties are related to the impurity content of carbon, nitrogen, hydrogen and oxygen. The purity of titanium iodide is less than 0.1%, but its strength is low and its plasticity is high. The properties of 99.5% industrial pure titanium are as follows: density p=4.5g/cubic centimeter, melting point 1725℃, thermal conductivity a=15.24W/(m.K), tensile strength_b=539MPa, elongation delta=25%, section shrinkage_=25%, elastic modulus E=1.078*105MPa, hardness HB195. (B363 WPT2 90LR Titanium Welded Elbow DN350)
The density of titanium alloys is generally about 4.51g/cubic centimeter.
Only 60% of the steel, the density of pure titanium is close to that of ordinary 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, which can produce parts with high strength, good rigidity and light weight. Titanium alloys are used in aircraft engine components, skeleton, skin, fasteners and landing gear.
High thermal strength
The specific strength of these two kinds of titanium alloys can be maintained for a long time at 450-500℃, but the specific strength of aluminium alloy decreases obviously at 150℃. The working temperature of titanium alloy can reach 500℃, while that of aluminium alloy can be below 200 C.
Good corrosion resistance
Titanium alloy works in humid atmosphere and sea water medium, and its corrosion resistance is far superior to stainless steel; it has strong resistance to pitting corrosion, acid corrosion and stress corrosion; it has excellent corrosion resistance to alkali, chloride, chlorine organic articles, nitric acid, sulfuric acid and so on. However, the corrosion resistance of titanium to reductive oxygen and chromium salts is poor.
Good cryogenic performance
Titanium alloy can maintain its mechanical properties at low and ultra-low temperatures. Titanium alloys with good Cryogenic Properties and very low interstitial elements, such as TA7, can maintain a certain degree of plasticity at – 253℃. Therefore, titanium alloy is also an important low temperature structural material.
High chemical activity
Titanium has high chemical activity, and has strong chemical reactions with O, N, H, CO, CO, CO, steam, ammonia, etc. in the atmosphere. When the carbon content is more than 0.2%, hard TiC will be formed in the titanium alloy; when the temperature is higher, TiN hard surface will be formed by interaction with N; when the temperature is above 600 C, titanium absorbs oxygen to form a hardened layer with high hardness; when the hydrogen content increases, brittle layer will also be formed. The depth of hard and brittle surface layer produced by gas absorption can reach 0.1-0.15 mm, and the hardening degree is 20-30%. Titanium has high chemical affinity and is easy to adhere to the friction surface.
Low thermal conductivity elasticity
The thermal conductivity of titanium is about 1/4 of nickel, 1/5 of iron and 1/14 of aluminium. 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 easy to deform. It is not suitable for making slender rods and thin-walled parts. The rebound of machined surface is about 2-3 times of that of stainless steel, which results in severe friction, adhesion and bond wear of tool flank.
Titanium is an isomer with a melting point of 1668℃. Titanium is densely arranged in hexagonal lattice at temperatures below 882℃. Titanium is called alpha titanium, and it is a body-centered cubic lattice at temperatures above 882℃. Titanium is called beta titanium. Titanium alloys with different microstructures can be obtained by adding appropriate alloying elements to change the phase transformation temperature and phase content gradually. Titanium alloys have three kinds of matrix structures at room temperature. Titanium alloys can also be divided into three categories: alpha alloys, (alpha+beta) alloys and beta alloys. China is represented by TA, TC and TB respectively. (ASTM B862 GR.2 Weld Titanium Pipe 8 Inch SCH40)
α titanium alloy
It is a single-phase alloy consisting of alpha-phase solid solution. It is alpha-phase both at general temperature and at higher practical application temperature. It has stable structure, higher wear resistance and strong oxidation resistance than pure titanium. Its strength and creep resistance are maintained at temperatures of 500-600℃, but it can not be strengthened by heat treatment, and its strength at room temperature is not high.
β titanium alloy
It is a single-phase alloy consisting of beta-phase solid solution.
Without heat treatment, the alloy has higher strength. After quenching and aging, the alloy is further strengthened, and the room temperature strength can reach 1372-1666 MPa. However, the thermal stability is poor, so it is not suitable for use at high temperature.
α+β titanium alloy
It is a dual-phase alloy with good comprehensive properties, good structural stability, good toughness, plasticity and high temperature deformation properties. It can be processed under hot pressure and strengthened by quenching and aging. After heat treatment, the strength increases by 50%-100% compared with annealing state, and the high temperature strength can work for a long time at the temperature of 400-500℃ and its thermal stability is inferior to that of alpha titanium alloy.
Among the three kinds of titanium alloys, Alpha-titanium alloy and alpha+beta-titanium alloy are most commonly used; Alpha-titanium alloy has the best machinability, followed by alpha+beta-titanium alloy and beta-titanium alloy. Alpha titanium alloy code TA, beta titanium alloy code TB, alpha + beta titanium alloy code TC.
Titanium alloys can be divided into heat resistant alloys, high strength alloys, corrosion resistant alloys (Ti-Mo, Ti-Pd alloys, etc.), low temperature alloys and special functional alloys (Ti-Fe hydrogen storage materials and Ti-Ni memory alloys).
Different phase composition and structure can be obtained by adjusting the heat treatment process. It is generally believed that fine equiaxed structure has better plasticity, thermal stability and fatigue strength; acicular structure has higher endurance strength, creep strength and fracture toughness; equiaxed and acicular mixed structure has better comprehensive properties.
Titanium alloys have high strength, low density, good mechanical properties, good toughness and corrosion resistance. In addition, titanium alloy has poor technological performance and difficult cutting. It is easy to absorb impurities such as hydrogen, oxygen, nitrogen and carbon in hot working. There are also poor wear resistance and complex production process. The industrialized production of titanium began in 1948. With the development of aviation industry, the titanium industry is growing at an average rate of 8% per year. The annual output of titanium alloy processing materials in the world has reached more than 40,000 tons, and there are nearly 30 kinds of titanium alloy grades. The most widely used titanium alloys are Ti-6Al-4V (TC4), Ti-5Al-2.5Sn (TA7) and industrial pure titanium (TA1, TA 2 and TA3).
Titanium alloy is mainly used to make compressor parts of aircraft engine, followed by rocket, missile and high-speed aircraft. In the mid-1960s, titanium and its alloys have been used in general industry to make electrodes in electrolysis industry, condensers in power plants, heaters for petroleum refining and seawater desalination, and environmental pollution control devices. Titanium and its alloys have become a kind of corrosion resistant structural material. In addition, it is also used to produce hydrogen storage materials and shape memory alloys.
Titanium and titanium alloys were studied in 1956 in China, and industrialized production of titanium materials and TB2 alloys were developed in the mid-1960s.
1. Aerospace and Aeronautics
Titanium is an indispensable “space metal” in aircraft fuselage, engine and rocket components. The application of titanium alloy materials in the development of high performance aircraft in the United States, such as X-31, X-30, has achieved remarkable results. The M coefficient of the aircraft has increased by about three times, while the overall quality of the aircraft has decreased by 80%. Russia produces titanium alloy sheets, forgings and other raw materials used in aircraft manufacturing. Titanium alloys have been widely used in the research and development of Shenzhou spacecraft in China. In addition, high strength titanium alloys Ti-5553 [Ti-5Al-5Mo-5V-3 chromium (Cr)], Ti-55531 [Ti-5Al-5V-5Mo-3Cr-1 zirconium (Zr)] have also been developed abroad. Ti-5553, a high strength near-beta titanium alloy, can replace BT22 and Ti-1023 titanium alloys in landing gear of aircraft. Flame-retardant titanium alloys have been successfully developed abroad to ensure the safety of aircraft engines. Their typical representatives are Alloy C (Ti-35V-15Cr) in the United States and Ti-25V-15Cr-2Al-0.2C in the United Kingdom. They have good flame retardancy and mechanical properties. Alloy C has been applied in military aircraft engines.
Titanium alloys with high temperature resistance, excellent creep properties and good mechanical properties have also been developed by Institute of Metals, Chinese Academy of Sciences, Northwest Institute of Nonferrous Metals and Beijing Institute of Aeronautical Materials. Among them, TB 8 (beta 21s) and TC18 (BT22) high strength titanium alloys are ideal Aeronautical Structural materials, which are widely used in aircraft fuel tank, hydraulic system and foil. In terms of flame retardant titanium alloys, Northwest Institute of Nonferrous Metals has developed Ti14 flame retardant titanium alloys with independent intellectual property rights, and Ti40 flame retardant titanium alloys with lower cost than AlloyC (Ti-35V-15Cr) flame retardant titanium alloys.
2. Shipping field
Ships and other equipment have been immersed in water for a long time, so the materials are required to have high corrosion resistance, safety, reliability and long service life. Titanium alloys are widely used on ships because of their wide strength range, unique physical properties, excellent mechanical properties, corrosion resistance and impact resistance. Titanium alloys are used in pumps, filters, supply pipes, fire extinguishing systems, pressure vessels and other equipment on Russian ships. Timetal 5111 developed jointly by US Navy and Timet Company has good fracture toughness, stress corrosion resistance, room temperature creep property and easy welding. These high-performance titanium alloys are used in marine systems, cooling systems, sewage treatment systems, electrical components and other equipment of American ships.
Near-alpha corrosion-resistant titanium alloys Ti 75, Ti 31, Ti-B19, Ti91, Ti70 and Ti80 with different strength levels have been developed in China. Ti75 alloy developed by Northwest Research Institute of Nonferrous Metals has independent intellectual property rights. It has good comprehensive properties with Ti31 alloy and has been applied in practice. In addition, Ti70 developed by Baoji Nonferrous Metal Processing Plant and Ti91 developed by Northwest Research Institute of Nonferrous Metals have good moderate strength, high plasticity, cold working performance, sound transmission and solderability. These titanium alloys are widely used in ships, submarines and submarines, such as titanium flanges, titanium pipes and titanium pipe fittings.
3. Biomedical field
Titanium alloys have been widely used in surgical implantation. The representative titanium alloy Ti-12Mo-6Zr-2Fe is a metastable alloy. It is suitable for orthodontic devices because of its high strength, fracture toughness, wear resistance, corrosion resistance and elastic modulus. The main way to develop bio-titanium alloys is to improve the mechanical properties, corrosion resistance and biocompatibility of titanium alloys by alloying elements Zr, Pd, Nb, Sn and Ta.
In recent years, various countries have developed new medical titanium alloy materials, such as Ti13Nb13Zr produced in the United States, Ti6Al7Nb produced in Switzerland and Ti15Zr4Nb4Ta2Pd produced in Japan. In the 1970s, China was committed to opening up biomedical titanium alloy materials. Northwest Research Institute of Nonferrous Metals has developed a new medical titanium alloy TAMZ, which is ahead of the world in biocompatibility, mechanical properties and technological properties. After that, the third generation medical titanium alloys TLE and TLM with better biomechanical compatibility were developed. Ti-2448 (Ti-24Nb-4Zr-8Sn), developed by Institute of Metals, Chinese Academy of Sciences, significantly improves the mechanical compatibility between bone and implant. Since February 2008, Ti2448 alloy medical devices have entered the stage of batch application.
4. Automobile Industry
Automobile lightweight is one of the most potential markets for titanium materials. Titanium alloys are mainly used in automotive engine parts, such as titanium connecting rod, titanium spring, titanium crankshaft, titanium fasteners, etc.
The commonly used heat treatment methods are annealing, solid solution and aging treatment. The purpose of annealing is to eliminate internal stress, improve plasticity and structural stability, and obtain better comprehensive properties. Generally, the annealing temperatures of alpha and (alpha+beta) alloys are 120-200 degrees below the transition point of (alpha+beta) beta phase. Solid solution and aging treatment are rapid cooling in the high temperature region to obtain martensite alpha’-phase and metastable beta phase, and then the metastable phase is decomposed by holding in the middle temperature region to obtain fine dispersed second phase such as alpha phase or compound. Particle, to achieve the purpose of strengthening the alloy. Generally, quenching of (alpha+beta) alloys is carried out at 40-100 below (alpha+beta) beta phase transition point, and quenching of metastable beta alloys is carried out at 40-80 above (alpha+beta) beta phase transition point. Aging treatment temperature is generally 450 – 550 C.
In conclusion, the heat treatment process of titanium alloys can be summarized as follows:
- (1) Stress relief annealing: The purpose is to eliminate or reduce the residual stress produced in the process of processing. Prevent chemical erosion and reduce deformation in some corrosive environments.
- (2) Complete annealing: The purpose is to obtain good toughness, improve processing performance, facilitate reprocessing and improve the stability of size and structure.
- (3) Solution treatment and aging: In order to improve its strength, Alpha-titanium alloy and stable beta-titanium alloy can not be strengthened by heat treatment, but only annealed in production. Alpha+beta titanium alloys and metastable beta titanium alloys containing a small amount of alpha phase can be further strengthened by solution treatment and aging.
In addition, in order to meet the special requirements of the workpiece, dual annealing, isothermal annealing, beta heat treatment, thermomechanical heat treatment and other metal heat treatment processes are also used in industry.
The production technology of titanium alloy is related to the quality of titanium alloy. Titanium alloy smelting, casting, forming and other technologies have been greatly developed in recent years.
Melting Technology of Titanium and Titanium Alloys
The smelting of titanium and titanium alloys can be divided into two categories:
Vacuum consumptive melting and vacuum non-consumptive melting.
In the industrial production of titanium and titanium alloys, the most commonly used technologies are vacuum arc remelting (VAR) and cold bed furnace melting.
VAR technology can refine ingot structure and improve product purity in titanium alloy smelting. The main developments of this technology in recent years are as follows:
- 1) Automatic VAR resolving process. The development of advanced computer technology is also applied to VAR process. The data collection system of automatic electronic control box can establish a good smelting mode for specific ingots and alloys. In addition, it can also analyze the problems in the smelting process and improve the metal yield.
- 2) Large ingot size. Large VAR furnaces can melt titanium ingots with a quality of 30 tons. Most of the vacuum consumable arc furnaces used for titanium melting abroad have tonnages of 8-15 tons. In recent years, China has also achieved 8-15 tons of titanium melting tonnage.
- 3) Coaxial power supply mode is adopted to offset the magnetic field and prevent segregation.
- 4) Development of numerical simulation technology. Scholars at home and abroad have made some progress in the study of VAR process by numerical simulation.
Domestic scholars have explored the distribution law of temperature field of ingot and established a model for predicting the morphology of solidification structure, composition of ingot and defect distribution.
Cold-bed furnace melting is based on plasma (Plasma Arc) or electron beam (Electron Beam) as the heat source, forming two processes of plasma Cold-bed furnace and electron beam Cold-bed furnace melting respectively. Compared with vacuum consumable arc smelting, electron beam cold bed furnace smelting has many advantages: firstly, many kinds of raw materials and economical raw materials can be used, such as residues, loose sponge titanium and titanium chips; secondly, high density impurities such as molybdenum (Mo), tungsten (W) and tantalum (Ta), low density impurities such as cyanide and volatile impurities can be removed. Quality is an important technology for purifying titanium alloy materials. Thirdly, the yield of metal can be increased by producing ingots with various cross sections. In recent years, the development of electron beam Cold-bed furnace smelting technology is mainly manifested in the following aspects:
- 1) direct smelting of titanium ingots by sponge titanium stack;
- 2) numerical simulation technology;
- 3) single alloy ingot smelting technology;
- 4) surface smelting technology of ingots;
- 5) production technology of large hollow ingots.
The plasma beam cold bed smelting technology utilizes centralized, controllable and stable plasma arc to provide heat source. In plasma beam cold bed melting of titanium alloys, the volatilization of manganese (Mn), tin (Sn) and other volatile elements can be prevented, and the element content of titanium alloys can be precisely controlled; high-speed and rotating ion beams can be produced, thus making the composition of titanium alloys more uniform. In addition, the working atmosphere of plasma is close to atmospheric pressure, so it will not be restricted by raw materials; the bath of plasma cooling furnace bed is large and deep, so that the solution can be fully diffused. Titanium alloys produced by plasma cold melting furnace technology are widely used in engines of military aircraft in the United States. The research on plasma beam cold bed melting has also been carried out in China. The Beijing Institute of Aeronautical Materials has installed the ion beam cold bed furnace produced by Retech Company in the United States. TC4 and TIAL ingots produced by this equipment have achieved great success in controlling impurity element content, inclusion and alloying element content of alloys. Developing a single electron beam cold bed furnace melting control technology for high homogeneous titanium alloy ingots is the trend of future development of electron beam cold bed furnace melting technology for titanium alloy in China.
When the hardness of titanium alloy is greater than HB350, cutting is particularly difficult. When the hardness of titanium alloy is less than HB300, it is easy to stick knife and difficult to cut. However, the hardness of titanium alloy is only one aspect that is difficult to be machined. The key is the comprehensive influence of chemical, physical and mechanical properties of titanium alloy on its machinability. Titanium alloys have the following cutting characteristics:
- (1) Small deformation coefficient: This is a remarkable feature of titanium alloy cutting, and the deformation coefficient is less than or close to 1. The sliding friction path of chips on the rake face is greatly increased, which accelerates tool wear.
- (2) High cutting temperature: Because the thermal conductivity of titanium alloy is very small (only 1/5-1/7 of steel 45), the contact length between chips and rake face is very short, the heat generated during cutting is not easy to transmit, which is concentrated in a small range near the cutting area and cutting edge, and the cutting temperature is very high. Under the same cutting conditions, the cutting temperature can be more than twice as high as that of 45 steel.
- (3) The cutting force per unit area is large: the main cutting force is about 20% less than that of steel cutting. Because the contact length between chip and rake face is very short, the cutting force per unit contact area is greatly increased, which is easy to cause edge collapse. At the same time, because of the small elastic modulus of titanium alloy, bending deformation is easy to occur under the action of radial force, which causes vibration, increases tool wear and affects the accuracy of parts. Therefore, the process system should be rigid.
- (4) The phenomenon of cold hardening is serious: because of the high chemical activity of titanium, it is easy to absorb oxygen and nitrogen in the air to form a hard and brittle skin at high cutting temperature; at the same time, plastic deformation in the cutting process will also cause surface hardening. Hardening not only reduces the fatigue strength of parts, but also aggravates tool wear, which is an important feature in cutting titanium alloy.
- (5) Tools are easy to wear and tear: after blanks are processed by stamping, forging and hot rolling, hard and brittle uneven skin is formed, which easily causes the phenomenon of edge collapse, making the removal of hard skin the most difficult process in titanium alloy processing. In addition, because of the strong chemical affinity of titanium alloy to tool materials, the tool is prone to bond wear under the conditions of high cutting temperature and large cutting force per unit area. When turning titanium alloy, sometimes the wear of rake face is even more serious than that of flank face; when feed f < 0.1 mm/r, the wear mainly occurs on flank face; when f > 0.2 mm/r, the wear of rake face will occur; when finishing and semi-finishing with carbide tools, the wear of flank is more appropriate with VBmax < 0.4 mm.
In milling process, because of the low thermal conductivity of titanium alloy material and the very short contact length between chips and rake face, the heat generated during cutting is not easy to transmit, which is concentrated in a small range near the cutting deformation zone and cutting edge. During milling, the cutting edge will produce extremely high cutting temperature, which will greatly shorten the tool life. 。 For Ti6Al4V titanium alloy, cutting temperature is the key factor affecting tool life, not cutting force, under the condition of allowable tool strength and machine power.
Cutting titanium alloy should start from reducing cutting temperature and bonding. YG cemented carbide is suitable for cutting tools with good red hardness, high bending strength, good thermal conductivity and poor affinity with titanium alloy. Because of the poor heat resistance of high speed steel, the cutting tools made of cemented carbide should be used as far as possible. Commonly used cemented carbide tool materials are YG8, YG3, YG6X, YG6A, 813, 643, YS2T and YD15.
Coated blades and YT cemented carbides have strong affinity with titanium alloys, which aggravates the bond wear of cutting tools and is not suitable for cutting titanium alloys. For complex and multi-edged cutting tools, high-vanadium high-speed steel (such as W12Cr4V4Mo), high-cobalt high-speed steel (such as W2Mo9Cr4VCo8) or aluminum high-speed steel (such as W6Mo5Cr4V2Al, M10Mo4Cr4V3Al) can be selected. Material, suitable for the production of cutting titanium alloy drills, reamers, end milling cutters, broaches, taps and other cutting tools.
Using diamond and cubic boron nitride as cutting tools to cut titanium alloys can achieve remarkable results. For example, the cutting speed of natural diamond cutter can reach 200 m/min under the condition of cooling with emulsified fluid; without cutting fluid, the allowable cutting speed is only 100 m/min at the same wear rate.
Matters needing attention
In the process of cutting titanium alloy, attention should be paid to:
- (1) Because of the small elastic modulus of titanium alloy, the clamping deformation and stress deformation of the workpiece in processing are large, which will reduce the processing accuracy of the workpiece; the clamping force should not be too large when the workpiece is installed, and auxiliary support can be added when necessary.
- (2) If the cutting fluid containing hydrogen is used, it will decompose and release hydrogen at high temperature, which will be absorbed by titanium and cause hydrogen embrittlement. It may also cause high temperature stress corrosion cracking of titanium alloys.
- (3) When chloride in cutting fluid is used, it may decompose or volatilize toxic gases. Safety precautions should be taken, otherwise it should not be used. After cutting, parts should be thoroughly cleaned with chlorine-free cleaning agent in time to remove chlorine residues.
- (4) It is forbidden to use lead or zinc-based alloy-making tools and fixtures to contact with titanium alloys, and copper, tin, cadmium and their alloys are also forbidden to use.
- (5) All workpieces, 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 salt (sodium chloride) stress corrosion may occur in the future.
- (6) In general, when cutting titanium alloy, there is no danger of ignition. Only in micro-cutting, the fine chips can ignite and burn. In order to avoid fire, besides pouring a large amount of cutting fluid, chips should also be prevented from accumulating on the machine tool, the tool should be replaced immediately after blunting, or the cutting speed should be reduced, and the feed rate should be increased to increase the chip thickness. In case of fire, fire extinguishers such as talc powder, limestone powder and dry sand should be used to extinguish it. Carbon tetrachloride and carbon dioxide fire extinguishers should not be used, nor water should be used, because water can accelerate combustion and even lead to hydrogen explosion.
Deoxidation and pickling of titanium alloys
Surface treatment is usually required during and after heat treatment in order to remove oxide scales and contaminants on metal surfaces, reduce the activity of bare metal surfaces, and before and during the coating of protective coatings and functional coatings on titanium and its alloys with surface halogen. It is to improve the performance of metal surface, for example, to prevent corrosion, oxidation and wear.
The pickling conditions of titanium and its alloys depend on the type (characteristics) of oxide layer and existing reaction layer, which is affected by the high temperature heating process and the increase of processing temperature (e.g. forging, casting, welding, etc.). A thin oxide layer is formed at a lower processing temperature or at a heating temperature of about 600X: below. An oxygen-enriched diffusion zone is formed near an oxide layer at high temperatures. The oxygen-enriched diffusion layer must also be removed by acid washing. Various methods can be used to remove oxide scales: mechanical methods to remove thick oxide layer and hard surface layer, methods to remove oxide scales in molten salt bath and acid elution to remove oxide scales in acid solution.
In many cases, a combination of several methods can be used, such as mechanical removal of oxide scales and subsequent acid washing, or salt bath and acid washing combined removal of oxide scales ^ In the case of oxide and diffusion layer formed at higher temperature, special methods should be adopted. However, when heated to 600X: at high temperature, most of the oxide layers formed can be dissolved by general acid pickling.
Titanium alloys are widely used in automotive industry because of their light weight, high specific strength and good corrosion resistance. The most widely used titanium alloys are automotive engine systems. Making engine parts from titanium alloys has many advantages.
The low density of titanium alloy can reduce the inertia quality of moving parts. At the same time, the titanium valve spring can increase the free vibration, reduce the body flutter, and improve the engine speed and output power.
The inertia quality of the moving parts is reduced, so that the friction force is reduced and the fuel efficiency of the engine is improved. Selection of titanium alloy can reduce the load stress of related parts and reduce the size of parts, thus reducing the quality of engine and vehicle. The reduction of inertia quality of components reduces vibration and noise and improves engine performance. The application of titanium alloy in other parts can improve the comfort of personnel and the beauty of automobile. Titanium alloys play an immeasurable role in energy saving and consumption reduction in automotive industry.
Although titanium alloy parts have such excellent properties, there is still a long way to go before titanium and its alloys are widely used in automotive industry. The reasons include high price, poor formability and poor welding performance.
The main reason that hinders the widespread application of titanium alloys in automotive industry is the high cost.
The price of titanium alloys is much higher than that of other metals in both initial smelting and subsequent processing. The cost of titanium parts acceptable to the automotive industry is $8-13 per kg for connecting rods, $13-20 per kg for valves, and less than $8 per kg for springs, engine exhaust systems and fasteners. It is 6-15 times as much as that of aluminium plate and 45-83 times as much as that of steel plate.
Defects of Titanium Alloys
The main limitation of titanium and titanium alloys is their poor chemical reactivity with other materials at high temperature. This property compels titanium alloys to be different from conventional refining, melting and casting technologies, and even often causes die damage. As a result, the price of titanium alloys becomes very expensive. As a result, they were initially mostly used in aircraft structures, aircraft, and high-tech industries such as petroleum and chemical industries. However, due to the development of space science and technology and the improvement of people’s living quality, titanium alloys are gradually used to make people’s livelihood goods for the benefit of people’s lives, but these products are still on the high side, mostly high-priced products, which is the biggest fatal injury that titanium alloys can not carry forward.
New Development of Titanium Alloys
Various countries are developing new titanium alloys with low cost and high performance, and striving to make titanium alloys enter the civil industry field with huge market potential. The new research progress of titanium alloy materials at home and abroad is mainly reflected in the following aspects.
High temperature titanium alloy
The first high temperature titanium alloy developed successfully in the world is Ti-6Al-4V, and its service temperature is 300-350 C. Subsequently, IMI550 and BT3-1 alloys at 400℃ and IMI679, IMI685, Ti-6246 and Ti-6242 alloys at 450-500 C were developed. New high temperature titanium alloys which have been successfully applied in military and civil aircraft engines include IMI829 and IMI834 alloys in Britain, Ti-1100 alloys in the United States, BT18Y and BT36 alloys in Russia, etc. Table 7 shows the maximum operating temperature of new high temperature titanium alloys in some countries.
In recent years, rapid solidification/powder metallurgy technology, fiber or particle reinforced composite materials have been used to develop titanium alloys as the development direction of high temperature titanium alloys abroad, which can raise the service temperature of titanium alloys to more than 650℃. A high purity and compactness titanium alloy has been successfully developed by McDonald Company of USA using rapid solidification/powder metallurgy technology. Its strength at 760 C is equivalent to that of the titanium alloy used at room temperature.
Compared with general titanium alloys, the greatest advantages of Ti3Al (alpha 2) and TiAl (gamma) intermetallics, which are based on sodium, are good high temperature properties (maximum operating temperatures are 816 and 982 C, respectively), strong oxidation resistance, good creep resistance and light weight (density is only 1/2 of that of nickel-based superalloys). These advantages make Ti3Al (alpha 2) intermetallics become nickel-based superalloys. The most competitive material for aeroengine and aircraft structural components in the future.
Two Ti3Al-based titanium alloys Ti-21Nb-14Al and Ti-24Al-14Nb- v-0.5Mo have been mass produced in the United States. Other developed Ti3Al-based titanium alloys include Ti-24Al-11Nb, Ti25Al-17Nb-1Mo and Ti-25Al-10Nb-3V-1Mo. TiAl(gamma)-based titanium alloys are of interest in the range of Ti-(46-52) Al-(1-10) M (at.%) where M is at least one element in v, Cr, Mn, Nb, Mn, Mo and W. TiAl 3-based titanium alloys, such as Ti-65Al-10Ni alloy, are beginning to attract attention.
High Strength and High Toughness Type Beta
The Beta-type titanium alloy was originally B120VCA alloy (Ti-13v-11Cr-3Al) developed by Crucible Company in the mid-1950s. The Beta-type titanium alloy has good cold and hot working properties, easy forging, rolling and welding. It can obtain high mechanical properties, good environmental resistance and good combination of strength and fracture toughness through Solution-Aging treatment. The new type of high strength and high toughness Beta-type titanium alloys are most representative of the following:
Ti1023 (Ti-10v-2Fe-# al), which has the same properties as 30CrMnSiA high strength structural steel commonly used in aircraft structural parts, has excellent forging properties.
Ti153 (Ti-15V-3Cr-3Al-3Sn), which has better cold working properties than pure titanium, has room temperature tensile strength of over 1000MPa after aging.
Beta 21S (Ti-15Mo-3Al-2.7Nb-0.2Si), a new type of anti-oxidation and super-high strength titanium alloy developed by Timet Branch of Titanium Metal Company of America, has good anti-oxidation performance, excellent cold and hot working performance, and can be made into foil with thickness of 0.064 mm.
SP-700 (Ti-4.5Al-3V-2Mo-2Fe) titanium alloy developed by Japan Steel Tube Corporation (NKK) has high strength and superplastic elongation of up to 2000%. The superplastic forming temperature is 140 C lower than that of Ti-6Al-4V. It can replace Ti-6Al-4V alloy to manufacture various aerospace components by superplastic forming-diffusion bonding (SPF/DB) technology.
BT-22 (TI-5v-5Mo-1Cr-5Al) developed by Russia has a tensile strength of more than 1105 MPA.
Flame Retardant Titanium Alloy
Conventional titanium alloys tend to burn alkanes under specific conditions, which limits their application to a large extent. In view of this situation, various countries have launched research on flame retardant titanium alloys and made some breakthroughs. Alloy C (also known as Ti-1720) developed in the United States has a nominal composition of 50Ti-35v-15Cr (mass fraction). It is a flame-retardant titanium alloy insensitive to continuous combustion and has been used in F119 engines. BTT-1 and BTT-3 are flame-retardant titanium alloys developed in Russia. They are all Ti-Cu-Al alloys. They have good hot deformation properties and can be used to make complex parts.
Medical Titanium Alloy
Titanium is non-toxic, light, high strength and excellent biocompatibility. It is an ideal medical metal material and can be used as implants for human body. Ti-6Al-4v ELI alloy is still widely used in medical field. However, the latter can precipitate very small amounts of vanadium and aluminium ions, reduce their cell adaptability and possibly cause harm to human body. This problem has long attracted extensive attention in the medical field. Aluminum-free, vanadium-free and biocompatible titanium alloys have been developed in the United States as early as the mid-1980s for orthopaedic purposes. Japan and the United Kingdom have also done a lot of research work in this area, and made some new progress. For example, Japan has developed a series of alpha+beta titanium alloys with excellent biocompatibility, including Ti-15Zr-4Nb_4ta-0.2Pd, Ti-15Zr-4Nb-aTa-0.2Pd-0.20-0.05N, Ti-15Sn-4Nb-2Ta-0.2Pd and Ti-15Sn-4nb-2Ta-0.2Pd-0.20. The corrosion strength, fatigue strength and corrosion resistance of these alloys are better than those of Ti-6Al-4V-4ELI. Compared with alpha+beta titanium alloy, beta titanium alloy has higher strength level, better incision performance and toughness, and is more suitable for implantation into human body. In the United States, five beta titanium alloys have been recommended for medical applications, namely TMZFTM (TI-12Mo-^Zr-2Fe), Ti-13Nb-13Zr, Timetal 21SRx (TI-15Mo-2.5Nb-0.2Si), Tiadyne 1610 (Ti-16Nb-9.5Hf) and Ti-15Mo. It is estimated that in the near future, such Luti alloys with high strength, low modulus of elasticity, excellent formability and corrosion resistance are likely to replace Ti-6Al-4V ELI alloys widely used in medical field.
Source: China Titanium Alloy Pipe Fittings Manufacturer – Yaang Pipe Industry (www.steeljrv.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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