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Titanium and Titanium Alloy Castings


Titanium and Titanium Alloy Castings


Titanium and titanium alloy with low density, high specificity, corrosion resistance, linear expansion coefficient is small, good biocompatibility and other excellent performance in aviation, aerospace, ocean transportation, chemical, metallurgy, health care and other industries are indispensable Structural materials. The initial application of titanium and titanium alloy parts are the deformation of the workpiece, with its increase in the amount and application of the expansion, deformation reflects the large processing capacity, low utilization of materials, high production costs and other disadvantages, so casting technology from This development. Titanium casting is a relatively economical and easy to achieve near forming process. Titanium and titanium alloy in the molten state with high chemical activity, with the commonly used refractory chemical reaction, melting and casting forming a great degree of difficulty, must have its own modeling materials and modeling process and special melting and casting equipment The

China's titanium casting more than 90% of smelting and casting equipment are used vacuum electrode electrode arc furnace shell plus centrifugal casting. Crucible with water-cooled crucible, titanium liquid maximum pouring amount of 500 kg.

Consumable electrode arc melting method is made of titanium or titanium alloy consumable electrode for the cathode to water-cooled copper crucible for the anode; high current smelting, titanium electrode melting rate is much greater than the condensation rate of titanium, the melting of the electrode In the form of droplets into the crucible, the formation of molten pool; molten pool surface is heated by the arc, always liquid, the bottom and the crucible in contact with the surrounding water forced cooling, resulting in bottom-up crystallization. This method has the advantages of simple structure, low maintenance cost, easy to enlarge and so on. The disadvantage is that the pouring temperature is difficult to adjust and control. After the arc is stopped, the molten metal must be poured out from the crucible in 3 to 5 seconds, otherwise the temperature drops sharply , Metal liquid superheat is not high, making the liquid flow and poor filling capacity. Consumable electrode arc melting of the electrode quality requirements are high, requiring the internal structure of the electrode dense. Smelting process is more dangerous, a little operation will appear in the arc damage to the crucible, causing the crucible outer wall forced cooling of the circulating water into the crucible, pollution of titanium, water vapor damage to the vacuum pump system.

Titanium alloy casting process are mainly metal type, machining graphite, metal surface ceramic shell, oxide ceramic shell.

Metal type In the field of titanium alloy casting, metal materials used for casting are mainly copper, steel, cast iron, tungsten, molybdenum, and graphite processing together collectively referred to as hard mold system. Due to the difficulty of the classification of the process, this method is difficult to produce complex shapes of titanium castings, and most of the use of specific castings only.

Machining graphite type of high strength, the concession is not good, the liquid titanium to produce chill, often the casting surface cracks and cold separation, high production costs, long production cycle. Graphite porosity larger, easy to absorb moisture, so the machine must be used before processing graphite degreasing, degassing treatment, or casting surface oxidation serious. Casting size is relatively large, thick wall (≥ 5mm), the shape is simple, the required number of only one or several pieces. Choose machined graphite.

Metal tiles ceramic shell with refractory metal tungsten powder as refractory material, metal tungsten high melting point, contact with titanium liquid chemical stability, but tungsten powder should have a high purity, impurity content can not exceed the prescribed standard, otherwise Will affect the quality of titanium casting. Tungsten surface melting mold shell must be solvent dewaxing, and in a special dewaxing tank, the human health has a lot of damage, but also pollute the environment. Tungsten surface shell shell high temperature roasting must be carried out in a reducing atmosphere, dewaxing in the shell after the appearance of the mold ash is difficult to burn, in the casting is easy to react with the liquid titanium, in the casting appearance of the formation of pores. Paint slurry process performance is poor, poor suspension, coating slurry life is short, difficult to save, expensive.

The oxide ceramic shell is an inert oxide as a shell-type shell refractory. The order of the various oxide materials according to their chemical stability to the molten titanium alloy is as follows: SiO2, MgO, Al2O3, CaO, ZrO2, Y2O3, ThO2. ThO2 has been essentially free of radioactive. CaO is easy to absorb moisture, so it hinders its application. Now, the material used for the investment casting shell layer and the adjacent layer is mainly Y2O3, ZrO2.

The ZrO2, which has not been stabilized, can not be used as a casting material for cast titanium because it undergoes allotrope transformation, monoclinic at room temperature, tetragonal crystals at high temperature, and higher temperature is converted into cubic crystals, Crystal into a tetragonal crystal, accompanied by about 9% of the volume changes, so that the shell cracking. Usually to the ZrO2 by adding 4% to 8% of the CaO, after high temperature fusing or calcination can be stable ZrO2 solid solution (also useful Y2O3 stability), most of the industrial use of fused ZrO2.

Y2O3, like ZrO2, must be subjected to high temperature stabilization before it can be used as titanium alloy molding materials. Y2O3 ceramic shell with low thermal conductivity, high strength, pouring out the casting surface quality, but Y2O3 price is more expensive, difficult source.

China's titanium industry development is relatively fast, in recent years, a number of new cast titanium production plant. At present, the country's titanium production plant, the Institute has nearly 20, the new titanium foundry products will also be positioned in the titanium casting precision castings, Shaanxi Jin Han rare precious metals Co., Ltd. perennial and Harbin Institute of Technology, Xi'an traffic University, Northwestern Polytechnic University for technical exchanges and cooperation, is committed to titanium, nickel, zirconium and its alloys precision casting production, the formation of precision casting, machining-based graphite production model.

With the development of titanium and titanium alloy casting technology and increasingly mature, coupled with the birth of hot isostatic pressing (HIP) technology and titanium alloy casting in the successful application of a better solution to the quality of the casting, improve the casting reliability. Since the 1980s, titanium and titanium alloy castings in aviation, aerospace and other applications every year to 20% rate of increase. Casting process, has been developed by a single piece of casting to a few pieces or dozens of parts into a large group of integrated castings. The scope of application has been from the early force of the non-critical static structure to develop into a component of the aircraft engine components, completely replace some of the deformation of titanium alloy, aluminum alloy, steel. With the aircraft engine on the thrust ratio and stiffness requirements of the increase, which requires some of the key titanium alloy components made of large complex thin wall of the whole casting. Some of the advanced aviation large-scale turbo engine fan casing, intermediary casing, the former casing, compressor casing and so began to use titanium alloy casting. Large passenger air duct, insulation screen, bracket, frame, trunnion, support frame, brake shell, etc. are also titanium alloy casting to replace the original components. Military aircraft, the use of titanium alloy casting is also gradually increased, such as: bearings, frames, brackets, brake hooks, the wing of the object, the rudder rotation device bracket, transmission shell parts, hanger support accessories, Practice has proved that the application of titanium alloy castings on the aircraft is successful and reliable. Not only that, at the cost of production, due to the use of titanium alloy castings, so that some of the aircraft design, processing, fastening, assembly and so have become less than the original titanium alloy castings when the mechanism is simplified, thus greatly reducing The cost of manufacturing the aircraft. Titanium alloy castings in the field of space is mainly used for missiles, spacecraft spacecraft, satellite. The main parts of the application are: missile shell, tail, rudder wing and connecting seat, space shuttle and spacecraft bracket, frame, bearing, accessories, shell, etc., because the titanium alloy casting with high rigidity, light weight and optical Glass is equivalent to the thermal expansion coefficient, also used in artificial satellites and other optical instruments of the frame, base, frame and shell and so on.

Titanium and titanium alloy castings in the daily industrial production also has a wide range of applications. Titanium and titanium alloy has good corrosion resistance, chemical and other corrosion-resistant industries irreplaceable material. Widely used in chemical, paper, oil, alkali, metallurgy, pesticides and other industries. The main application of the product is industrial titanium and titanium - palladium alloy for the casting of titanium pump, titanium fan, a variety of different types of valves, such as: globe valve, ball valve, plug valve, gate valve, butterfly valve, check valve.

With the improvement of people's living standards and the improvement of health requirements, titanium alloy with its high fatigue strength, and the human superior affinity and many other advantages, are also increasingly used in the medical and health fields. Such as: casting titanium alloy hip repair parts, knee repair parts, human prosthesis, oral repair and so on. The amount of titanium alloy precision castings used in the field of sports equipment is very large, such as: bicycle accessories, golf head. In particular, titanium alloy golf ball market capacity is the most huge, but the casting process is more complex.

At present, the use of titanium and titanium alloy casting range is still expanding, more applications are also studied, but there are still some problems: 1. alloy varieties less, less grades, basically commonly used titanium alloy are pure industrial Titanium castings and TC4 alloy castings. 2. Casting application range is small, most of the castings are used in the petrochemical industry (industrial titanium casting), aviation, aerospace applications rarely, resulting in China's titanium casting industry, the level of technology and technology is difficult to improve. 3. Molding process is generally backward, most manufacturers are using graphite-type modeling process (machining graphite and tamping graphite type), and casting precision casting applications rarely. Casting the casting surface is rough. 4. Smelting equipment are basically vacuum consumable electrode arc shell furnace, the smelting process is more dangerous, molten metal liquid superheat is not high, resulting in casting surface easy to produce flow marks, cold separation and other defects, thin-walled parts forming difficult.

In order to improve the backward state of China's titanium casting industry and improve the overall process and technical level of China's titanium industry, the following aspects are needed: 1. To improve the existing modeling process, to study the new binder and molding materials , Simplify the process, shorten the production cycle, reduce production costs. 2. Research and develop new smelting and casting equipment and technology to improve the superheat of molten metal, improve and improve the flow of molten titanium and filling capacity, for the development of large complex thin-walled whole casting to create favorable conditions The 3. To further expand the computer simulation of solidification technology in the application of titanium alloy casting, in order to improve the quality of castings, reducing the scrap rate of castings. 4. Research and development of titanium alloy castings of various heat treatment process and thermal chemical treatment technology to improve the microstructure of titanium alloy castings to improve the mechanical properties of castings. 5. Investment casting can only produce small and medium-sized castings, should seek a production of larger, more net shape, more efficient casting of the modeling process to improve the production capacity of titanium alloy castings

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