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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What types of corrosion of titanium pipe include?

The corrosion form of titanium includes two categories: general corrosion and local corrosion: general corrosion is a form that is completely and uniformly corroded; local corrosion is a form of local corrosion relative to general corrosion. It can be divided into: crevice corrosion, spot Corrosion, stress corrosion cracking, galvanic corrosion, abrasion, hydrogen absorption and hydrogen embrittlement.

1. Crevice corrosion: Corrosion that occurs between the flanges or flanges and the gaps near the deposits generally occurs in the narrow gaps.

2. Pitting corrosion: Corrosion that occurs in the process of opening holes, such as the corrosion phenomenon caused by the presence of halogen ions such as C1-, B-wide, and I-.

3. Stress corrosion cracking: In a specific environment, due to the effect of tensile stress, a kind of brittle failure.

4. Abrasion: The fluid promotes electrochemical corrosion due to mechanical action.

5. Galvanic corrosion: also known as dissimilar metal contact corrosion, it is a phenomenon that promotes the corrosion of the metal with the lower potential in the state of electrical short-circuit.

6. Hydrogen absorption and hydrogen embrittlement: Take hydrogen out of the material. When the amount of hydrogen taken out exceeds the solid solution limit of the material, brittle hydrides will be formed and hydrogen embrittlement will occur.
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Titanium plate manufacturing process and characteristics

TC4 titanium alloy has good mechanical properties at room temperature and high temperature, and is widely used in the manufacture of aviation structural parts and aero-engine fans, drums, etc., and has accounted for more than 50% of the total aviation titanium alloys. Many load-bearing structural parts of aircraft, especially variable cross-section structural parts, require the raw material bars to have good notched stress fracture performance. The purpose is to ensure that the parts have a strong ability to inhibit the initiation and propagation of cracks in a state of high-stress concentration. Without breaking and failing. The upper limit of the specifications of this type of TC4 titanium alloy bar in the relevant domestic technical standards is Ф220mm. At present, there are no public reports on the preparation process of TC4 titanium alloy bars requiring notched stress fracture performance at home and abroad. It is generally believed that the high hydrogen content or poor uniformity of the TC4 titanium alloy will reduce its room temperature notch stress fracture performance.

In industrialized mass production, it is also common that the notch stress fracture performance of such bars is unqualified due to improper process. The raw material procurement contract of a special forging requires the provision of ~b350mmTC4 titanium alloy bars in the range of beyond specifications in accordance with current technical standards. Under the premise of ensuring the same mechanical properties, the ultrasonic flaw detection level is required to be changed from GB/T5193 of the original smaller specifications of bars. The grade B in the medium is raised to grade A, so the relevant process research on the preparation of large-size TC4 titanium alloy bars with special needs has been carried out.

Titanium rod manufacturers use sponge titanium and master alloys to prepare 5t consumable electrodes. The 6720mm TC4 titanium alloy ingots are obtained through 3 times vacuum waste smelting through the ALD vacuum waste electric arc furnace imported from Germany. Because the microscopic concentration of hydrogen under high stress reduces the notch stress fracture performance J, certain measures have been taken to control the hydrogen content of the ingot in the experiment on the raw material selection and smelting process of the ingot. Samples are taken from the surface of the head, middle and tail of the ingot to test its chemical composition.

(1) By adopting the forging process of phase zone blanking and two-phase zone upsetting + straight drawing, it is possible to prepare Φ350mm large-size TC4 titanium alloy bars whose structure, performance and flaw detection level meet the technical requirements of the supply.

(2) The structure with good primary equiaxialization is beneficial to improve the notch stress fracture performance at room temperature, and the short rod-like α-phase structure with strong direction consistency will reduce the notch stress fracture performance.
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Analysis of Causes of Fracture of Titanium Alloy Screw Bolt

Titanium alloy has a high specific strength, a wide working temperature range and excellent corrosion resistance. It is widely used in the aerospace field to make compressor blades, discs, casings and fastening parts; however, it is different from structural steel and stainless steel. Compared with superalloys, it has greater sensitivity to surface damage and defects.

Any negligence in the processing, assembling and use of titanium alloy parts will cause unexpected adverse consequences for the use performance of titanium alloy parts, and even serious fracture accidents may occur.

There have been many fracture accidents in the use of titanium alloy screws, most of which are related to fatigue; therefore, fatigue resistance is one of the most important indicators to measure the reliability of titanium alloy parts. The fatigue resistance of titanium alloys is particularly sensitive to the surface integrity of parts. Many surface factors, such as pollution, scratches, inclusions, corrosion, etc., can induce the initiation and propagation of fatigue cracks, leading to parts fracture.

1. The cracking property of right-angle joints is fatigue cracking, and the reason for cracking is related to the large stress that the joint bears and the thick oxygen-rich α layer in the source area;

2. In view of the large force on the joint, it is recommended to appropriately increase the design margin under the premise of completely removing the oxygen-rich α layer.

3. In order to avoid fatigue fractures caused by wear and tear and destroy the surface integrity of the titanium alloy, the titanium screw manufacturer recommends that the single lug is in vertical contact with the bolt during the assembly process; at the same time, in order to better ensure the surface integrity of the titanium alloy bolt, it is recommended Increase the chamfer at the edge of the single ear hole, and increase the bushing structure at the bolt connection position.
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