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

Compared with other metal materials, titanium alloy has the following advantages :① the specific strength (tensile strength/density) is high (see figure), the tensile strength can reach 100 ~ 140kgf/mm2, and the density is only 60% of steel. The medium temperature strength is good, the use temperature is several hundred degrees higher than the aluminum alloy, and the required strength can still be maintained at the medium temperature, and it can work for a long time at the temperature of 450 ~ 500 ° C. (3) Corrosion resistance is good, and a uniform and dense oxide film is immediately formed on the surface of titanium in the atmosphere, which has the ability to resist erosion by various media. Generally, titanium has good corrosion resistance in oxidizing and neutral media, and the corrosion resistance in seawater, wet chlorine gas and chloride solution is more excellent. However, in reducing media, such as hydrochloric acid and other solutions, titanium corrosion resistance is poor. Titanium alloys with good low temperature performance and very low gap elements, such as TA7, can maintain a certain plasticity at -253 ° C. ⑤ Low elastic modulus, small thermal conductivity, no ferromagnetism.

Alloying element titanium has two kinds of homogenous alloying crystals: close-packed hexagonal α titanium below 882℃, and body-centered cubic β titanium above 882℃. The alloying elements can be divided into three categories according to their influence on the phase transition temperature :① the elements that stabilize the α phase and increase the phase transition temperature are the α stable elements, including aluminum, carbon, oxygen and nitrogen. Among them, aluminum is the main alloying element of titanium alloy, which has obvious effect on improving the strength of alloy at room temperature and high temperature, reducing the specific gravity and increasing the elastic modulus. ② The elements that stabilize β phase and reduce the phase transition temperature are β stable elements, which can be divided into isomorphic and eutectoid types. The former has molybdenum, niobium, vanadium and so on; The latter has chromium, manganese, copper, iron, silicon and so on. ③ 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 of titanium alloys. Oxygen and nitrogen have a large solubility in the α phase, which has a significant strengthening effect on titanium alloy, but it reduces the plasticity. It is usually stipulated that the content of oxygen and nitrogen in titanium is below 0.15 ~ 0.2% and 0.04 ~ 0.05%, respectively. The solubility of hydrogen in the alpha phase is very small, and too much hydrogen dissolved in titanium alloys will produce hydrides, making the alloy brittle. Generally, 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.

Titanium alloys can be divided into three categories according to the composition of the phase :α alloy,(α+β) alloy and β alloy, China respectively to TA, TC, TB.

① α alloy contains a certain amount of stable α phase elements, and is mainly composed of α phase in equilibrium state. α alloy has small specific gravity, good thermal strength, good weldability and excellent corrosion resistance, the disadvantage is low room temperature strength, usually used as heat resistant materials and corrosion resistant materials. α alloys can also be divided into all-α alloys (TA7), near-α alloys (Ti-8Al-1Mo-1V), and α alloys with a small number of compounds (Ti-2.5Cu). ② (α+β) alloy contains a certain amount of stable α phase and β phase elements, and the microstructure of the alloy in equilibrium state is α phase and β phase. (α+β) alloys have moderate strength and can be strengthened by heat treatment, but the weldability is poor. (α+ β) alloys are widely used, of which Ti-6Al-4V alloy accounts for more than half of all titanium materials.

The β alloy contains a large number of stable β phase elements, and the high temperature β phase can be retained to room temperature. Beta alloys can also be divided into heat-treatable beta alloys (metastable beta alloys and near-metastable beta alloys) and heat-stable beta alloys. Heat-treatable β alloy has excellent plasticity in the quenched state, and can make the tensile strength reach 130 ~ 140kgf/mm2 through aging treatment. β alloy is usually used as high strength and high toughness materials. The disadvantages are heavy, high cost, poor welding performance, and difficult cutting.

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). The composition and properties of typical alloys are shown in the table.

Different phase composition and microstructure of heat-treated titanium alloys can be obtained by adjusting the heat treatment process. It is generally believed that fine equiaxial structure has good plasticity, thermal stability and fatigue strength. The acicular structure has high lasting strength, creep strength and fracture toughness. The equiaxed and acicular structures have good comprehensive properties.

Common heat treatment methods are annealing, solution and aging treatment. Annealing is to eliminate internal stress, improve plasticity and structure stability, in order to obtain better comprehensive properties. Generally, the annealing temperature of α alloy and (α+β) alloy is selected at 120 ~ 200℃ below the (α+β) -→ β phase transition point; Solid solution and aging treatment is fast cooling from the high temperature region to obtain martensitic α 'phase and metastable β phase, and then heat preservation in the middle temperature region to decompose these metastable phases, to obtain α phase or compounds and other small dispersed second phase particles, to achieve the purpose of strengthening the alloy. Usually (alpha + beta) alloy quenching in alpha + beta) -- - > beta phase transition point below 40 ~ 100 ℃, metastable beta alloy quenching in alpha + beta) -- - > beta phase transition point above 40 ~ 80 ℃. The aging treatment temperature is generally 450 ~ 550℃. In addition, in order to meet the special requirements of the workpiece, the industry also uses double annealing, isothermal annealing, β heat treatment, deformation heat treatment and other metal heat treatment processes.

Relevant criteria are:

GB/T 3620.1-94 Titanium and titanium alloy grades and chemical composition

GB/T 3625-95 Titanium and titanium alloy tubes for heat exchangers and condensers

TA1, TA2, TA3 are all industrial pure titanium, they have high mechanical properties, excellent stamping performance, and can be welded in various forms, the strength of the welded joint can reach 90% of the strength of the matrix metal, and the cutting performance is good. Titanium pipe has high corrosion resistance to chloride, sulfide and ammonia. The corrosion resistance of titanium in seawater is higher than that of aluminum alloy, stainless steel and nickel-based alloy. Titanium water impact resistance is also strong.

For the manufacture of condenser tubes, it can be used in contaminated seawater, water with high suspended matter content, and at high flow rates.

Titanium alloy can be divided into three categories according to the organization.(1 titanium adds aluminum and tin elements.2 titanium adds aluminum chromium molybdenum vanadium and other alloying elements.3 titanium adds aluminum and vanadium and other elements.) Titanium alloy has high strength and small density, good mechanical properties, toughness and corrosion resistance. In addition: the process performance of titanium alloy is poor, and the cutting process is difficult. In hot processing, it is very easy to absorb impurities such as hydrogen, oxygen, nitrogen and carbon. There are poor wear resistance and complex production process.

An alloy based on titanium with other elements. Industrial production of titanium began in 1948. The needs of the development of the aviation industry make the titanium industry develop at an average annual growth rate of about 8%. At present, the annual output of titanium alloy processing materials in the world has reached more than 40,000 tons, and 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, TA2 and TA3).

The order time for large quantities is 10 days.

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