Advantages, Disadvantages and Applications of Titanium Alloy Fasteners

Titanium is a very active metal. In the liquid state, it reacts very quickly with oxygen, nitrogen, hydrogen and carbon. Therefore, the smelting of titanium alloys must be carried out under the protection of higher vacuum or inert gas (ar or ne). Crucibles for smelting are all water-cooled copper crucibles. The specific smelting process mainly includes three methods: (1) Non-consumable electrode arc furnace smelting. Alloy smelting is carried out under the protection of vacuum or inert gas. The process is mainly consumable electrode smelting to prepare electrodes. (2) Vacuum consumable electrode electric arc furnace smelting The consumable electrode made of titanium or titanium alloy is used as the cathode, and the water-cooled copper crucible is used as the anode. The molten electrode enters the crucible in the form of droplets, forming a molten pool. The surface of the molten pool is heated by the electric arc and is always liquid, and the surrounding area where the bottom and the crucible are in contact is forced to cool, resulting in bottom-up crystallization. The molten metal in the molten pool becomes a titanium ingot after solidification. (3) Vacuum consumable electrode condensed shell-protected smelting The smelting device is shown. This furnace is developed on the basis of vacuum consumable electrode electric arc furnace. It is a furnace type for casting special-shaped parts that combines smelting and centrifugal casting. Its biggest feature is that there is a thin solid shell of titanium alloy between the water-cooled copper crucible and the molten metal, the so-called solid shell. This solid shell of the same material is used as the lining of the crucible to form a molten pool to store titanium liquid , To avoid the crucible pollution to the titanium alloy liquid. After pouring, the layer of condensed shell left in the increased loss can be used as the lining of the crucible.

In recent years, with the development of science and technology and the needs of production, new methods and equipment for smelting titanium alloys and other active metals have been researched and developed, mainly electron beam furnaces, plasma furnaces, vacuum induction furnaces, etc., and have achieved a certain degree of application. However, from the comparison of technical and economic indicators such as power consumption, melting speed, and cost, consumable electrode electric arc furnace (including condensing shell furnace) smelting is still the most economical and applicable smelting method at present. Due to the physical-chemical properties of titanium, the casting process of titanium alloys has its own unique requirements and characteristics, whether it is the molding material rate or the process method. One is a molding material that requires a very high refractoriness; the other is that the pouring must be carried out under the protection of a higher vacuum or inert gas, sometimes with centrifugal force. The material of the connecting shell is different, and the fusion shell is divided into three different systems.

(1) Pure graphite shell system. Graphite powders of different particle sizes are used as refractory fillers and sanding materials, and resins are used as adhesives. The shell has high strength, light weight, low cost, and a wide range of raw materials. Suitable for centrifugal or gravity pouring.

(2) Refractory metal surface layer shell system. It is a composite system, except that the surface layer requires special processes due to different modeling materials (tungsten powder and other refractory metals), and the back layer from the modeling material to the shell making process is the same as the investment casting of cast steel.

(3) Oxide ceramic shell system. The surface and back layer of the shell are made of oxide as the modeling material, so the shell strength is high, and the thermal conductivity is the smallest among the three kinds of shells. It is suitable for pouring thin-walled castings with complex shapes.

Titanium castings poured by the above three shell systems have little difference in chemical composition and mechanical properties; but there are obvious differences in surface quality. The shrinkage rate of the latter two shells is significantly smaller than that of graphite shells, so the dimensional accuracy of the castings.
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Precautions for processing titanium alloy parts during processing

The welding performance of titanium, titanium alloy, and titanium wire has many remarkable characteristics. These welding characteristics are determined by the physical and chemical properties of titanium and titanium alloy. When welding titanium and titanium alloys, the possibility of hot cracks in the welded joints is very small. This is because the content of impurities such as S, P, and C in titanium and titanium alloys is small, and the low melting point eutectic formed by S and P is not easy to appear On the grain boundary, coupled with the narrow effective crystallization temperature range, the shrinkage of titanium and titanium alloys is small during solidification, and the weld metal will not produce thermal cracks. When welding titanium and titanium alloys on time, cold cracks can appear in the heat-affected zone, which is characterized by cracks that occur several hours or even longer after welding, which is called delayed cracks. Studies have shown that this kind of crack is related to the diffusion of hydrogen bombs during welding. During the welding process, hydrogen diffuses from the high-temperature deep pool to the lower-temperature heat-affected zone. The increase in hydrogen content increases the amount of TiH2 precipitated in this zone, which increases the brittleness of the heat-affected zone. In addition, the volume expansion during the precipitation of hydrides causes greater structural stress. , Coupled with the diffusion and accumulation of hydrogen atoms to the high-stress parts of the region, resulting in the formation of cracks. The method to prevent this kind of delayed cracking is mainly to reduce the source of hydrogen in the welded joints. When invoices are issued, the fire suppression treatment is also carried out.

When welding titanium and titanium alloys, porosity is a common problem. The root cause of the formation of pores is the result of the influence of hydrogen. The formation of pores in the weld metal mainly affects the fatigue strength of the joint. The main technological measures to prevent pores are:

(1) The protective neon gas should be pure, and the purity should not be less than 99.99%

(2) Thoroughly remove organic matter such as oxide scale and oil stains on the surface of the weldment and the surface of the welding wire.

(3) Apply good gas protection to the molten pool, and control the flow and flow rate of argon to prevent turbulence and affect the protection effect.

(4) Correct selection of welding process parameters and increase the right to use the deep pool residence time to allow bubbles to escape, which can effectively reduce pores.

The gas shielding problem of titanium and titanium alloy welding is the primary factor affecting the quality of welded joints. When welding titanium and titanium alloys, the heat input should be as small as possible. The source of hydrogen should be strictly controlled to prevent cold cracks. At the same time, attention should be paid to prevent the generation of pores. As long as welding is carried out in strict accordance with the welding process requirements and effective gas protection measures are taken, high-quality welded joints can be obtained.
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Reasons and characteristics of the inner edge of the titanium tube

Titanium rod barrel burnishing is to grind the parts and abrasives in dilute acid or dilute alkali with emulsifier (or corrosion inhibitor) and follow the rotation of the drum to obtain a bright appearance. It is suitable for the disposal of small parts and parts that are difficult to polish and polish. Correct selection of roller burnishing standards can improve productivity and quality

(1) The shape of the roller

The shape of the roller is round, hexagonal and octagonal, among which the polygonal shape is good for practicality. This is because the radii of the barrel wall from the axis are not equal, and there is a certain point of view, so the rolling parts are easy to change the position, the chance of collision with each other increases, the grinding is uniform, and the efficiency is high, which can shorten the rolling time.

(2) The size of the drum

The direct alignment is 50mm-100mm, and the length varies with the number of grids. The length of the first pattern roller is 600-800mm; the second pattern is 800-1500mm. Usually the larger one is better. This is because the parts are subjected to high pressure and friction in the drum, so the amount of cutting is also increased. But for those parts that are afraid of being stressed and easily deformed, a small roller can be used and the length of the roller can be increased.

1) Data for roller

There is a direct relationship between the rotation speed of the drum and the scraping amount of the parts within a certain range, that is, the faster the rotation speed, the greater the scraping of the metal surface. However, when the speed exceeds a certain upper limit, it drops instead. Because when the rotating speed is too fast, the centrifugal force of the parts in the drum increases, which reduces the friction. Usually it is appropriate to control at 45r/min.

2) Abrasives and solutions for roller burnishing

Abrasives for rolling include pumice, quartz, granite skin angles, shells, iron filings and ceramic fragments. The size of the abrasive particles should be larger or smaller than each hole of the Cen part. The amount of content in the drum is also an important factor affecting the quality of the agricultural surface of the parts. The amount of people inside the drum is usually 70% of the drum volume. Regarding heavier parts or threaded parts, the load should be controlled at 80%-90%. The solution in the drum should be added to about 95% of the drum volume (note: when adding acidic solution to the drum, you should add enough water first, and then add acid to avoid corrosion of parts during the tumbling process, because of the increase in time. , The concentration of the built-in solution gradually decreases, or even loses its effect. Therefore, the concentration of the solution should be checked and replaced at intervals. The burn-in time should not be too long, otherwise the parts will be damaged. That is, use a brush to remove the surface of the parts. Burr, terbium, residual oil and corrosion sludge, etc., and make the parts have a certain luster. Be sure to point out that elastic, rigid or thin-walled parts should be taken out in time after the finishing of rolling, otherwise it will easily cause hydrogen seepage or local over-corrosion .
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Technical requirements for the production of titanium alloy sheets

In the application of various titanium alloy products, titanium alloy forgings are mostly used in gas turbine compressor discs and medical artificial bones that require high strength, high toughness and high reliability. Therefore, not only high dimensional accuracy is required for titanium forgings, but also materials with excellent characteristics and high stability are required. The following mainly introduces 6 problems in titanium alloy flaw detection.

1. Segregation defects

In addition to β segregation, β spot, titanium-rich segregation and stripe α segregation, the most dangerous is interstitial α stable segregation (I type α segregation), which is often accompanied by small holes and cracks, containing oxygen, nitrogen and other gases. , The brittleness is greater. There are also aluminum-rich α stable segregation (type II α segregation), which is also accompanied by cracks and brittleness, which constitutes dangerous defects.

2. Inclusions

Most of them are metal inclusions with high melting point and high density. The high melting point and high density elements in the titanium alloy composition are not fully melted and left in the matrix (such as molybdenum inclusions). There are also cemented carbide tool chips mixed in the smelting raw materials (especially recycled materials) or improper electrode welding processes ( Titanium alloy smelting generally uses vacuum consumable electrode remelting method), such as tungsten arc welding, leaving high-density inclusions, such as tungsten inclusions, and titanium inclusions.

The existence of inclusions can easily lead to the occurrence and propagation of cracks, so it is a defect that is not allowed (for example, the Soviet Union's 1977 data stipulates that high-density inclusions with a diameter of 0.3 ~ 0.5 mm must be found in the X-ray inspection of titanium alloys. recording).

3. Residual shrinkage

In the central area of ​​the acid leaching test piece (in most cases), there are irregular wrinkle cracks or cavities, and there are often serious looseness, inclusions (slag inclusions) and component segregation on or near them.

4. Holes

The holes do not necessarily exist individually, but may also exist in multiple dense ones, which will accelerate the growth of low-cycle fatigue cracks and cause premature fatigue failure.

5. Cracks

Mainly refers to forging cracks. Titanium alloy has high viscosity, poor fluidity, and poor thermal conductivity. Therefore, during the forging deformation process, due to the large surface friction, the obvious internal deformation unevenness and the large internal and external temperature difference, it is easy to produce shear bands inside the forging ( Strain line), which leads to cracking in severe cases, and its orientation is generally along the direction of maximum deformation stress.

6. Overheating

Titanium alloys have poor thermal conductivity. In addition to overheating of forgings or raw materials caused by improper heating during hot working, the forging process is also prone to overheating due to thermal effects during deformation, causing microstructure changes and generating overheated Widmanstatten structures.
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Titanium alloy is more suitable for spacecraft manufacturing than steel

Titanium and titanium alloys have low density and high tensile strength. In the range of -253-600 degrees Celsius, its specific strength is almost the highest among metal materials. It can form a thin and hard oxide in a suitable oxidizing environment. The material film has excellent corrosion resistance. In addition, it is non-magnetic and has a small linear expansion coefficient. This makes titanium and its alloys first known as important aerospace structural materials, and then extended to shipbuilding, chemical industry, and other fields, and has been rapidly developed. Especially in the chemical industry, titanium and titanium alloy products are used in more and more products, such as petrochemical, fiber, pulp, fertilizer, electrochemistry, and seawater desalination industries, as exchangers, reaction towers, and synthesizers. Autoclave etc. Among them, the titanium plate is used as an electrolysis plate and an electrolytic cell in electrolysis and sewage desalination and is used as a tower body and a kettle body in the reaction tower and the reaction kettle.

With the development of science and technology, the application fields of titanium materials are becoming wider and wider, such as medical treatment, automobile, sports, etc. It also truly reflects that titanium, as light metal, has more and more excellent properties. It is recognized and determined by people, and it will replace other metals in our production and application fields, and even our body at the fastest speed.
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Three production processes of titanium rods

The specific strength of titanium alloy products is very high among metal structural materials. Its strength is equivalent to that of steel, but its weight is only 57% of that of rigid materials. In addition, titanium and titanium alloys have strong heat resistance, and can still maintain good strength and stability in the atmosphere at 500°C, and the short-term working temperature can even be higher. Titanium alloy has the characteristics of small specific gravity, high thermal strength, good thermal stability, and corrosion resistance, but the material is difficult to cut and has low processing efficiency. Therefore, how to overcome the difficulty of processing titanium alloys, and the difficulty of low efficiency has always been a problem for us.
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Titanium rod storage environment requirements

Titanium alloy materials for titanium alloy screws are closely related to the manufacturing process and uses of titanium alloy screws. On the one hand, the manufacturing process of titanium alloy screws mainly includes three parts: first, plastic deformation, such as upsetting, reducing and thread rolling, etc.; second, surface strengthening, such as strengthening the bolt bearing surface and the transition zone of the straight rod; finally, Mechanical processing, such as turning, milling and grinding. On the other hand, the purpose of titanium alloy screws is different, and the performance requirements of the required materials are also different, which requires the use of different titanium alloy materials. Taking rivets and bolts as examples, rivets need to be upset at one or both ends during the installation process, so the riveting process requires higher plasticity of the material. Bolts generally require high strength, and their strength level is close to that of 30CrMnSiA high-strength alloy steel, so high-strength titanium alloy materials are usually used. Based on the above two factors, the titanium alloy materials used for titanium alloy screws are also mainly divided into three types: industrial pure titanium, (α+β) type and β type titanium alloy, see Table 2 for details. It can be seen from Table 2 that the industrial pure titanium is mainly TA1 and TA2. (α+β) titanium alloys mainly include TC4, TC6, and Ti-662. β-type titanium alloys are mainly meta-stable β-type titanium alloys because the molybdenum equivalent of meta-stable β-type titanium alloys is generally about 10%. The near-beta titanium alloy with a molybdenum equivalent of less than 10% has insufficient heat treatment strengthening effect; the stable beta-type titanium alloy with a molybdenum equivalent greater than 10% will have high β-phase stability during the aging heat treatment process, and it is difficult to decompose, so meta-stable β-type titanium The strengthening effect of alloy materials is the most obvious. In addition, the metastable β-type titanium alloy has excellent cold formability and can be cold-headed, avoiding the use of professional heating equipment and gas protection media, high production efficiency, and material utilization, and the formed titanium alloy screw has high dimensional accuracy and surface Good quality. The (α+β) type titanium alloy titanium alloy screw can only be formed by hot upsetting, which requires special heating equipment and gas medium, low production efficiency and material utilization, and it is also prone to uneven heating temperature.
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