Titanium alloy is an alloy composed of titanium as the base and other elements added. Titanium has two types of isomorphic crystals: closely packed hexagonal structure below 882 ℃ α Titanium, body centered cubic above 882 ℃ β Titanium. Alloy elements can be classified into three categories based on their influence on phase transition temperature: ① Stable α The elements that increase the phase transition temperature are α Stable elements include aluminum, carbon, oxygen, and nitrogen. Aluminum is the main alloying element in titanium alloy, which has a significant effect on improving the strength of the alloy at room and high temperatures, reducing the specific gravity, and increasing the elastic modulus stable β The elements that reduce the phase transition temperature are β Stable elements can be divided into two types: isomorphic and eutectoid. The former includes molybdenum, niobium, vanadium, etc; The latter includes chromium, manganese, copper, iron, silicon, etc The elements that have little effect on the phase transition temperature are neutral elements, such as zirconium and tin.



Oxygen, nitrogen, carbon, and hydrogen are the main impurities in titanium alloys. Oxygen and nitrogen in α There is a high solubility in the phase, which has a significant strengthening effect on titanium alloys, but it reduces plasticity. The content of oxygen and nitrogen in titanium is usually specified to be below 0.15-0.2% and 0.04-0.05%, respectively. Hydrogen in α The solubility in the phase is very low, and excessive hydrogen dissolved in titanium alloys can produce hydrides, making the alloy brittle. Usually, the 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.



Classification of titanium alloys



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Titanium is an allotrope with a melting point of 1720 ℃ and a dense hexagonal lattice structure below 882 ℃, known as α Titanium; Above 882 ℃, it exhibits a body centered cubic lattice structure, known as β Titanium. By utilizing the different characteristics of the above two structures of titanium, appropriate alloying elements are added to gradually change its phase transition temperature and phase content, resulting in titanium alloys with different structures. At room temperature, titanium alloys have three types of matrix structures, which can be divided into the following three categories: α Alloy（ α+β) Alloy and β Alloy. China is represented by TA, TC, and TB respectively.



α titanium alloy



It is α The single-phase alloy composed of phase solid solution is α Phase, stable structure, higher wear resistance than pure titanium, and strong antioxidant ability. At temperatures ranging from 500 ℃ to 600 ℃, it still maintains its strength and creep resistance, but cannot undergo heat treatment strengthening, and its room temperature strength is not high.



β titanium alloy



It is β The single-phase alloy composed of phase solid solution has higher 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 structural stability, good toughness, plasticity, and high-temperature deformation properties. It can perform hot pressure processing well, and can be quenched and aged to strengthen the alloy. The strength after heat treatment is approximately 50% to 100% higher than that in the annealed state; High temperature strength, capable of long-term operation at temperatures ranging from 400 ℃ to 500 ℃, with thermal stability inferior to α Titanium alloy.



The most commonly used among the three titanium alloys is α Titanium alloy and α+β Titanium alloy; α Titanium alloy has the best machinability, α+ Titanium alloy takes second place, β Titanium alloy is the worst. α The titanium alloy code is TA, β The titanium alloy code is TB, α+β The titanium alloy code 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) according to their uses. The composition and properties of typical alloys are shown in the table.



Heat treated titanium alloys can obtain different phase compositions and microstructures by adjusting the heat treatment process. It is generally believed that fine equiaxed structures have good plasticity, thermal stability, and fatigue strength; Acicular structure has high rupture strength, creep strength and fracture toughness; The equiaxed and needle like mixed tissues have good comprehensive performance.



Properties of titanium alloys



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Titanium is a new type of metal, and its properties are related to the impurity content of carbon, nitrogen, hydrogen, oxygen, and other impurities. The purest titanium iodide impurity content does not exceed 0.1%, but it has low strength and high plasticity. The performance of 99.5% industrial pure titanium is: density ρ= 4.5g/cm3, melting point at 1800 ℃, thermal conductivity λ= 15.24W/(m.K), tensile strength σ B=539MPa, elongation δ= 25%, reduction of area ψ= 25%, elastic modulus E=1.078 × 105MPa, hardness HB195.



(1) Higher than strength



The density of titanium alloys is generally around 4.5g/cm3, only 60% of that of steel. The strength of pure titanium is close to that of ordinary steel, and some high-strength titanium alloys exceed the strength of many alloy structural steels. Therefore, the specific strength (strength/density) of titanium alloy is much higher than that of other metal structural materials, as shown in Table 7-1. It can produce components with high unit strength, good rigidity, and light weight. At present, titanium alloy is used for the engine components, framework, skin, fasteners, and landing gear of aircraft.



(2) High thermal strength



The usage temperature is several degrees higher than aluminum alloy, and it can still maintain the required strength at medium temperatures. It can work for a long time at temperatures ranging from 450 to 500 ℃. These two types of titanium alloys still have high specific strength in the range of 150 ℃ to 500 ℃, while aluminum alloy has a significant decrease in specific strength at 150 ℃. The working temperature of titanium alloy can reach 500 ℃, while that of aluminum alloy is below 200 ℃.



(3) Good corrosion resistance



Titanium alloys work in humid atmospheres and seawater media, and their corrosion resistance is much better than stainless steel; Strong resistance to pitting, acid corrosion, and stress corrosion; It has excellent corrosion resistance to organic substances such as alkali, chloride, chlorine, nitric acid, sulfuric acid, etc. However, titanium has poor corrosion resistance to reducing oxygen and chromium salt media.



(4) Good low-temperature performance



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 degree of plasticity at -253 ℃. Therefore, titanium alloy is also an important low-temperature structural material.



(5) High chemical activity



Titanium has high chemical activity and produces strong chemical reactions with O, N, H, CO, CO2, water vapor, ammonia, etc. in the atmosphere. When the carbon content is greater than 0.2%, hard TiC will be formed in the titanium alloy; When the temperature is high, the interaction with N will also form a TiN hard surface layer; At temperatures above 600 ℃, titanium absorbs oxygen to form a hardened layer with high hardness; An increase in hydrogen content can also form a brittle layer. The depth of the hard and brittle surface layer generated by gas absorption can reach 0.1-0.15 mm, and the degree of hardening is 20-30%. Titanium also has a high chemical affinity and is prone to adhesion with friction surfaces.



(6) Low thermal conductivity and elastic modulus



Thermal conductivity of titanium λ= 15.24W/(m.K) is about 1/4 of nickel, 1/5 of iron, and 1/14 of aluminum, while 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 it has poor rigidity and is prone to deformation. It is not suitable for making slender rods and thin-walled parts. During cutting, the rebound amount on the machined surface is large, about 2-3 times that of stainless steel, causing severe friction, adhesion, and adhesive wear on the back face of the cutting tool.



The Use of Titanium Alloys



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Titanium alloy has high strength and low density, good mechanical properties, good toughness and corrosion resistance. In addition, titanium alloys have poor processing properties and are difficult to cut. In hot working, it is very easy to absorb impurities such as hydrogen, oxygen, nitrogen and carbon. There is also poor wear resistance and complex production process. The industrial production of titanium began in 1948. The development of the aviation industry requires the titanium industry to grow at an average annual rate of about 8%. At present, the annual production of titanium alloy processing materials in the world has reached over 40000 tons, with nearly 30 titanium alloy grades. 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 alloy is mainly used for making aircraft engine compressor components, followed by structural components for rockets, missiles, and high-speed aircraft. In the mid-1960s, titanium and its alloys were widely used in general industries, such as making electrodes in the 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 corrosion-resistant structural material. In addition, it is also used for the production of hydrogen storage materials and shape memory alloys.



China began research on titanium and titanium alloys in 1956; In the mid-1960s, industrial production of titanium materials began and TB2 alloy was developed.



Heat treatment of titanium alloys



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The commonly used heat treatment methods include annealing, solid solution, and aging treatment. Annealing is used to eliminate internal stress, improve plasticity and structural stability, in order to obtain better comprehensive properties. usually α Alloy and（ α＋β） The annealing temperature of the alloy is selected at（ α＋β）— → β 120~200 ℃ below the phase transition point; Solution and aging treatment is to rapidly cool from high temperature zone to obtain martensite α′ Metastable β Phase, and then insulation in the intermediate temperature zone to decompose these metastable phases, resulting in α Fine dispersed second phase particles such as phases or compounds achieve the goal of strengthening the alloy. Usually（ α＋β) Quenching of alloys（ α＋β)— → β Conduct at 40~100 ℃ below the phase transition point, metastable β Alloy quenching（ α＋β)— → β Conduct at 40-80 ℃ above the phase transition point. The aging treatment temperature is generally 450~550 ℃.



In summary, the heat treatment process of titanium alloys can be summarized as follows:



(1) Stress relieving annealing: The purpose is to eliminate or reduce residual stresses generated during the processing. Prevent chemical erosion and reduce deformation in some corrosive environments.



(2) Complete annealing: The purpose is to achieve good toughness, improve processing performance, facilitate re processing, and improve the stability of size and structure.



(3) Solution treatment and aging: The purpose is to improve its strength, α Titanium alloy and stable β Titanium alloys cannot undergo strengthened heat treatment and only undergo annealing during production. α+β Titanium alloy and containing a small amount α Metastable phase β Titanium alloys can be further strengthened through solid solution treatment and aging.



In addition, in order to meet the special requirements of workpieces, dual annealing, isothermal annealing, and β Metal heat treatment processes such as heat treatment and deformation heat treatment.



Cutting of titanium alloys



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Cutting characteristics



When the hardness of titanium alloy is greater than HB350, cutting is particularly difficult, and when it is less than HB300, it is prone to tool sticking and difficult to cut. However, the hardness of titanium alloys is only one aspect that is difficult to machine, and the key lies in the comprehensive impact of the chemical, physical, and mechanical properties of titanium alloys on their machinability. Titanium alloy has the following cutting characteristics:



(1) Low deformation coefficient: This is a significant feature of titanium alloy cutting, with a deformation coefficient less than or close to 1. The distance of chip sliding friction on the rake face is greatly increased, accelerating tool wear.



(2) High cutting temperature: Due to the small thermal conductivity of titanium alloy (only equivalent to 1/5-1/7 of 45 # steel), the contact length between the chip and the front cutting surface is extremely short, and the heat generated during cutting is not easily transmitted, concentrated in a small range near the cutting area and cutting edge, resulting in high cutting temperature. Under the same cutting conditions, the cutting temperature can be more than twice that of cutting 45 steel.



(3) The cutting force per unit area is large: the main cutting force is about 20% smaller than when cutting steel. Due to the extremely short contact length between the chip and the front cutting surface, the cutting force per unit contact area is greatly increased, which is prone to blade breakage. At the same time, due to the small elastic modulus of titanium alloy, it is easy to produce bending deformation 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 good rigidity.



(4) Severe cold hardening phenomenon: 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 the cutting process can also cause surface hardening. The phenomenon of cold hardening not only reduces the fatigue strength of parts, but also aggravates tool wear, which is a very important feature when cutting titanium alloys.



(5) Cutting tools are prone to wear and tear: After the blank is processed by stamping, forging, hot rolling, and other methods, it forms a hard and brittle uneven outer skin, which is prone to blade breakage, making cutting the hard skin the most difficult process in titanium alloy processing. In addition, due to the strong chemical affinity of titanium alloy to tool materials, the tool is prone to adhesive wear under high cutting temperature and high cutting force per unit area conditions. When turning titanium alloys, sometimes the wear on the front cutting surface is even more severe than on the back cutting surface; When the feed rate f<0.1 mm/r, wear mainly occurs on the rear cutting surface; When f>0.2 mm/r, there will be wear on the front blade surface; When using hard alloy cutting tools for precision and semi precision turning, VBmax<0.4 mm is more suitable for the wear of the rear cutting surface.



Tool material



Cutting titanium alloy should start from two aspects: reducing cutting temperature and reducing adhesion. Tool materials with good red hardness, high bending strength, good thermal conductivity, and poor affinity with titanium alloy should be selected. YG type hard alloys are more suitable. Due to the poor heat resistance of high-speed steel, cutting tools made of hard alloy should be used as much as possible. The commonly used hard alloy tool materials include YG8, YG3, YG6X, YG6A, 813, 643, YS2T, and YD15.



Coated blades and YT type hard alloys can have a strong affinity with titanium alloys, exacerbating tool adhesion wear and are not suitable for cutting titanium alloys; For complex and multi blade cutting tools, tool materials such as 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, suitable for making drill bits, reamers, end mills, broaches, wire taps, and other cutting tools for cutting titanium alloys.



Using diamond and cubic boron nitride as cutting tools to cut titanium alloy can achieve significant results. If natural diamond cutting tools are 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 100m/min at the same wear rate.



matters needing attention



During the process of cutting titanium alloy, the following precautions should be taken:



(1) Due to the small elastic modulus of titanium alloy, the clamping deformation and stress deformation of the workpiece during processing are large, which will reduce the machining accuracy of the workpiece; When installing the workpiece, the clamping force should not be too large, and auxiliary support can be added if necessary.



(2) If a cutting fluid containing chlorine is used, it will decompose and release hydrogen gas at high temperatures during the cutting process, which is absorbed by titanium and causes hydrogen embrittlement; It may also cause stress corrosion cracking of titanium alloy at high temperature.



(3) Chlorides in cutting fluid may also decompose or volatilize toxic gases during use. Safety precautions should be taken during use, otherwise they should not be used; After cutting, the parts should be thoroughly cleaned with a chlorine free cleaning agent in a timely manner to remove chlorine residues.



(4) It is prohibited to use tools and fixtures made of lead or zinc based alloys to come into contact with titanium alloys, and the use of copper, tin, cadmium, and their alloys is also prohibited.



(5) All tools, fixtures, or other devices in contact with titanium alloys must be clean; After cleaning titanium alloy parts, it is necessary to prevent grease or fingerprint contamination, otherwise it may cause stress corrosion of salt (sodium chloride) in the future.



(6) In general, when cutting titanium alloys, there is no risk of ignition. Only in micro cutting, the small chips cut can ignite and burn. In order to avoid fire, in addition to pouring a large amount of cutting fluid, it is also necessary to prevent chips from accumulating on the machine tool. After the tool is blunt, it should be replaced immediately, or the cutting speed should be reduced, and the feed rate should be increased to increase the chip thickness. If one