Application range of titanium and titanium alloy bellows

Titanium alloy has high strength, low density, good mechanical properties, toughness and corrosion resistance. In addition, titanium alloys have poor processing performance and difficult drilling production and processing. During heat treatment, it is very easy to absorb residues such as hydrogen, nitrogen, and carbon. There are also poor wear resistance and complicated production processes. Industrial production of titanium started in 1948. The development of the aviation industry is necessary for the titanium industry to develop at an average annual growth rate of about 8%. At present, the world's annual output of titanium alloy production and processing materials has reached more than 40,000 tons and nearly 30 types of titanium alloys. The most commonly 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 to make aircraft engine compressor parts, followed by structural parts for rockets, cruise missiles and high-speed aircraft. In the mid-1960s, titanium and aluminum alloys were used in general industry to make electrical grades for electrolysis industry, coolers for power plants, electric heaters for crude oil refining and desalination, and environmental pollution control equipment. Titanium and aluminum alloys have become a kind of corrosion-resistant structural materials. In addition, it is also used to produce hydrogen storage raw materials and shape memory alloys.
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Titanium fittings features:

When the hydrogen content in the titanium tube is too much, the impact toughness and notched tensile strength will drop sharply due to brittleness. Therefore, it is generally stipulated that the hydrogen content in the titanium tube should not exceed 0.015%. In order to reduce the amount of hydrogen absorption, fingerprints, rolling mill marks, grease and other residues should be removed before the parts are heat treated. There is no moisture in the atmosphere of the heat treatment furnace. If the hydrogen content of the titanium tube exceeds the allowable value, it must be removed by vacuum annealing. Vacuum annealing for dehydrogenation is generally maintained at 538-760°C and a pressure lower than 0.066Pa for 2-4 hours.

When the temperature does not exceed 540°C, the oxide film on the surface of the titanium tube will not be significantly thickened. At higher heat treatment temperatures (above 760°C), the oxidation rate will rapidly increase, and oxygen can expand into the material to form a diffusion layer— Pollution layer. The high brittleness ratio of the oxygen contamination layer leads to cracks and damage on the surface of the part. There are mechanical processing methods (such as sandblasting, house cutting, etc.) or chemical methods such as acid washing and chemical milling to remove the oxygen pollution layer. During the heat treatment, the heating time should be as short as possible under the premise of ensuring the heat treatment of the meteorite. It is carried out in a vacuum furnace or an inert gas (argon, nitrogen, etc.) heating furnace. The appropriate application can also avoid or reduce the pollution caused by the titanium tube parts being heated in the air furnace.
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Process for controlling heat equipment in titanium reactor

At present, there are only two industrial methods: the HDH method and the sodium method. However, the sodium method titanium plants have been closed down one after another, so far only the HDH method is still used in the industrial production of titanium rod manufacturers. The centrifugal atomization method and the gas atomization method are mainly used to produce (spherical titanium powder or) spherical titanium alloy powder, but the production volume is not large and can be considered as mass production. The various production methods are summarized below.

In the reduction process of magnesium tetrachloride, there is always a by-product-the outer titanium powder. This kind of titanium powder accounts for a few per cent of sponge titanium products, and its quantity is considerable. Titanium powders with a particle size of less than 0.83mm can be used directly. The sponge titanium produced by the magnesium reduction-vacuum distillation process is poor in quality due to long-term high-temperature sintering, and the proportion of titanium alloy powder is correspondingly small. Therefore, the titanium-magnesium tetrachloride reduction method is not a good method for preparing titanium powder, and only the by-product-the outer titanium alloy powder is obtained.

Grinding is a key step in the production of powdered titanium by the sodium reduction method. The sodium reduction product is a mixture of metallic titanium and NaCl. On the surface of gold and sodium-an extremely covering-layer of NaCl, titanium will not be contaminated by impurities during the grinding process, and most of the heat generated during grinding will be absorbed by NaCl and will not cause Titanium overheats and catches fire, thereby avoiding oxidation of the product. In addition, NaCl can also promote the pulverization of titanium and act as a pulverizing medium. Various mills can be used as grinding equipment.

The process of leaching, washing and drying is basically the same as the process of producing sponge titanium. Because powdered titanium has greater activity, the acid concentration in the leaching solution should be lower. Generally, 0.5%-1.0% hydrochloric acid concentration aqueous solution can be used for leaching. When drying, the temperature should be as low as possible while preventing fire.
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Titanium has excellent corrosion resistance

The passivation of titanium depends on the existence of oxide film. Its corrosion resistance in oxidizing media is much better than that in reducing media, and high-rate corrosion can occur in reducing media. Titanium is not corroded in some corrosive media, such as seawater, wet chlorine, chlorite and hypochlorite solutions, nitric acid, chromic acid, metal chlorides, sulfides, and organic acids. However, in the medium that reacts with titanium to generate hydrogen (such as hydrochloric acid and sulfuric acid), titanium generally has a larger corrosion rate. However, if a small amount of oxidant is added to the acid, the titanium will form a passivation film. Therefore, in a mixture of sulfuric acid-nitric acid or hydrochloric acid-nitric acid, even in hydrochloric acid containing free chlorine, titanium is corrosion-resistant. The protective oxide film of titanium is often formed when the metal comes into contact with water, even in the presence of a small amount of water or water vapor. If titanium is exposed to a strong oxidizing environment with no water at all, it can quickly oxidize and produce violent, often spontaneous combustion reactions. This type of behavior has occurred in the reaction of titanium with fuming nitric acid containing excessive nitrogen oxides and titanium with dry chlorine gas. However, to prevent the occurrence of reactions in this state, a certain amount of water is necessary.
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Feasible method and process for reducing cost of titanium and titanium alloy materials

1. Consider the structure of the workpiece

When designing the structure of the workpiece, not only the use performance of the workpiece should be considered, but also the adaptability of this structure to the processing process. The processing methods corresponding to different workpiece structures are different, in order to ensure the processing of thin-walled parts. The accuracy of the workpiece structure is particularly important. Generally speaking, the application of thin-walled parts processed from titanium alloy plates has higher accuracy requirements and usage requirements. The deformation of the parts will not only cause difficulties in the process of installation, but also may not be able to complete the design parts. What needs to be done. Therefore, in order to avoid deformation of the workpiece during processing, first, consider designing the workpiece into a symmetrical structure. This structure enables the release of internal forces of each part of the workpiece during processing to be synchronized to avoid internal force distribution. Unequal conditions. Second, in the design of the thin plate, try to ensure that the thickness of the entire thin plate is as consistent as possible, and at some corners of the workpiece, due to processing or heat treatment, stress concentration may occur. The transition can be made by designing the corner to a circular arc structure. , Thereby reducing the deformation of the workpiece.

2. Consider from the perspective of workpiece clamping

The thin-walled parts themselves are thinner and only have lower stiffness, that is to say, the ability of the workpiece to resist elastic deformation is weak. Therefore, during the processing of the workpiece, the clamping will also affect the workpiece to a large extent. Deformed. The clamping is mainly used to fix the workpiece, and the clamping is used to locate the workpiece and ensure the stability of the workpiece during processing, as shown in Figure 3. Unreasonable clamping position and clamping force will cause the machining accuracy to decrease. Therefore, when selecting the clamping position, try to ensure that the clamping positions are in a symmetrical relationship, and the clamping force can be adjusted according to the rigidity of the workpiece. When the rigidity of the workpiece is high, a larger clamping force can be selected, but special attention should be paid to the fact that when the rigidity of the workpiece is low, an appropriate clamping force must be selected, otherwise it is easy to cause deformation of the workpiece during processing.

3. Consider from heat treatment

The general heat treatment of the workpiece is completed by quenching and artificial aging treatment, and the timing of the heat treatment of the workpiece is very important to reduce the deformation of the workpiece. Because when the workpiece is heat treated, the temperature stress and phase change stress will be generated inside the workpiece due to the change of the workpiece's own temperature, which is the main reason for the deformation of the workpiece. At the same time, heat treatment can not destroy the mechanical properties of the workpiece, so it is generally considered to arrange the timing of the heat treatment before the rough machining of the blank. Therefore, the timing of heat treatment should be rationalized as much as possible, so as to ensure the mechanical properties of the workpiece and reduce the deformation caused by the heat treatment of the workpiece.

4. Consider from the process method and cutting fluid

In the process arrangement of workpiece processing, firstly, according to the different composition and structure of different types of workpieces, the process arrangement should be carried out. Among them, special attention should be paid to the analysis of the easily deformed parts of the workpiece during processing, and whether Reduce the amount of deformation of the workpiece through some adjustments in the process. Secondly, when roughing the workpiece, it is necessary to reserve a large cutting margin at the beginning, and do a good job of positioning the reference surface. As the workpiece is processed, it is necessary to always pay attention to the correction of the reference surface, because the processing The reduction of the margin in the process will inevitably bring about a change in the reference level. The choice of cutting fluid is mainly based on the nature of processing and processing tools. The reasonable use of cutting fluid according to different process arrangements and tool usage will help improve the efficiency of workpiece processing.

5. Elimination of residual stress of thin-walled parts

The initial residual stress of thin-walled parts is generally determined by the heating factors of the blank material, and the processing residual stress is generally reflected after the processing of the thin-walled parts, so the research on the residual stress is worth paying attention to, how to predict The influence of residual stress and how to eliminate the influence of residual stress on the processing quality of parts.

Although the source of the residual stress in thin-walled parts is known, its influence on the deformation of thin-walled parts in processing can not be accurately determined, because the residual stress of thin-walled parts leads to deformation of thin-walled parts, which are generally caused by heating factors and mechanical forces. The result of the effect. At present, the control of residual stress generally uses the current more popular finite element analysis method to establish a finite element model of thin-walled parts and uses numerical analysis to predict the impact of residual stress. In addition, this method can not only simulate the results of deformation correction of thin-walled parts but also predict springback.

At present, the methods to eliminate residual stress of workpiece blanks include pre-stretching, vibration aging, aging annealing and cryogenic treatment. Among these methods, the cryogenic treatment application is the most successful. Cryogenic treatment can effectively reduce the residual stress in thin-walled parts. At the same time, the treatment can also increase the hardness and strength of the parts, improve the wear resistance of the workpiece, and increase the service life of the parts. In addition, cryogenic treatment can ensure the dimensional accuracy of the parts and improve the internal stress distribution in the parts. To reduce the impact of machining residual stress on the deformation of parts, it is still necessary to start from the aspect of reducing cutting heat.
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What is the difference between hot rolled titanium plate and cold rolled titanium plate?

Titanium alloy is an alloy composed of other elements based on titanium element. Titanium has two kinds of isomorphic crystals: Titanium is an allotrope with a melting point of 1668°C and a close-packed hexagonal lattice structure below 882°C, called α-titanium; it is body-centered cubic above 882°C Character structure, called β-titanium. Using the different characteristics of the above two structures of titanium, adding appropriate alloying elements to gradually change the phase transformation temperature and component content to obtain titanium alloys with different structures.

Oxygen, nitrogen, carbon and hydrogen are the main impurities in titanium alloys. Oxygen and nitrogen have greater solubility in the α phase, which has a significant strengthening effect on the titanium alloy, but it reduces plasticity. It is usually stipulated that the oxygen and nitrogen content in titanium should be below 0.15-0.2% and 0.04-0.05%, respectively. The solubility of hydrogen in the α phase is very small, and too much hydrogen dissolved in the titanium alloy will produce hydrides, which will make the alloy brittle. Generally, the hydrogen content in titanium alloys is controlled below 0.015%.
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Related processing requirements for titanium alloy rods:

In 1947, people began to smelt titanium in factories. That year, the output was only 2 tons. Production surged to 20,000 tons in 1955. In 1972, the annual output reached 200,000 tons. The hardness of titanium is about the same as that of steel, and its weight is almost half that of steel of the same volume. Although titanium is slightly heavier than aluminum, its hardness is twice that of aluminum. Now, in space rockets and missiles, a large amount of titanium is used instead of steel. According to statistics, the world's titanium used for space navigation every year has reached more than 1,000 tons of extremely fine titanium powder, which is also good fuel for rockets, so titanium is known as cosmic metal and space metal.

Titanium has good heat resistance, and its melting point is as high as 1725°C. At room temperature, titanium can lie in a variety of strong acid and alkali solutions. Even the most ferocious acid, aqua regia, cannot corrode it. Titanium is not afraid of seawater. Someone once sunk a piece of titanium to the bottom of the sea. Five years later, he took it up and took a look. There were many small animals and seabed plants stuck on it, but there was no rust at all, and it was still shiny.

Now, people are beginning to use titanium to make submarines-titanium submarines. Because titanium is very strong and can withstand high pressure, this submarine can sail in deep seas as deep as 4500 meters.

Titanium is corrosion-resistant, so it is often used in the chemical industry. In the past, stainless steel was used for the parts containing hot nitric acid in chemical reactors. Stainless steel is also afraid of the strong corrosive-hot nitric acid. This kind of parts must be replaced every six months. Now, titanium is used to make these parts. Although the cost is more expensive than stainless steel parts, it can be used continuously for five years, which is much more cost-effective to calculate.

The biggest disadvantage of titanium is that it is difficult to extract. The main reason is that titanium has a strong ability to combine with oxygen, carbon, nitrogen and many other elements at high temperatures. Therefore, no matter when smelting or casting, people are careful to prevent these elements from "invading" titanium. When smelting titanium, air and water are of course strictly forbidden. Even the alumina crucible commonly used in metallurgy is also forbidden to use because titanium will take oxygen from the alumina. At present, people use magnesium and titanium tetrachloride to interact with inert gas-helium or argon to extract titanium.

People take advantage of the extremely strong chemical ability of titanium at high temperatures. During steelmaking, nitrogen is easily dissolved in the molten steel. When the steel ingot is cooled, bubbles are formed in the steel ingot, which affects the quality of the steel. Therefore, the steelworkers add titanium metal to the molten steel to combine with nitriding to become slag-titanium nitride, which floats on the surface of the molten steel, so that the steel ingot is relatively pure.

When a supersonic aircraft is flying, the temperature of its wings can reach 500°C. If the wing is made of relatively heat-resistant aluminum alloy, one to two or three hundred degrees will be overwhelming. There must be light, tough, and high-temperature resistant material to replace the aluminum alloy ethyl titanium to meet these requirements. Titanium can withstand the test of more than one hundred degrees below zero. At this low temperature, titanium still has good toughness without being brittle.
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