Titanium and titanium alloys with low density, high specific strength, corrosion resistance, small coefficient of linear expansion, good biological compatibility and other excellent properties in aviation, aerospace, ocean transportation, chemical industry, metallurgy, health care and other industries are indispensable structural materials. The initial industrial application of titanium and titanium alloy parts are deformed parts, with the increase in its dosage and the expansion of the scope of application, the deformation reflects a large amount of machining, low material utilization, high production costs and other drawbacks, so the casting technology thus developed. Titanium casting is more economical and easy to realize the near-forming process. Titanium and titanium alloys in the molten state has a high chemical activity, to be commonly used with a variety of refractory materials to occur chemical reactions, melting and casting molding is very difficult, it must have its own special modeling materials and modeling process as well as special melting and casting equipment.
(a) melting process:
More than 90% of China's titanium casting melting and casting equipment are used in vacuum self-consumption electrode arc condensing shell furnace and centrifugal casting. The crucible adopts water-cooled copper crucible, and the maximum pouring capacity of titanium liquid is 500 kg.
Self-consumption electrode arc melting method is to use the cathode made of titanium or titanium alloy as self-consumption electrode, and the water-cooled copper crucible as the anode; the melting speed of titanium electrode is much larger than the condensation speed of titanium in the high-current melting, the melted electrode is in the form of droplets entering into crucible to form a molten pool; the surface of the molten pool is heated by the electric arc and it always appears liquid, and the bottom and the contact with crucible are in contact with the liquid electrode. The surface of the pool is heated by an electric arc and remains in a liquid state, and the bottom and the surrounding area in contact with the crucible are forced to cool by circulating water, resulting in bottom-up crystallization. This method has the advantages of simple structure, low maintenance cost, easy to large-scale, etc. The disadvantage is that the pouring temperature is difficult to regulate and control, a stop arc, the liquid metal must be poured out from the crucible within 3~5 seconds, otherwise the temperature drops sharply, the liquid metal superheat is not high, making the liquid liquid mobility and make-up shrinkage capacity is poor. Self-consumption electrode arc melting requires high quality of electrode and dense internal organization of electrode. The melting process is more dangerous, a little careless operation will occur arc damage to the crucible, resulting in the outer wall of the crucible forced cooling of the circulating water into the crucible, contamination of titanium liquid, water vapor damage to the vacuum pump system.
(B) casting cavity process:
Titanium alloy casting molding process mainly metal type, machined graphite type, metal surface layer ceramic shell, oxide ceramic shell.
1) metal type
Metal type in the field of titanium alloy casting, used as casting metal materials are mainly copper, steel, cast iron, tungsten, molybdenum, etc., and graphite machining type together collectively known as the hard mold system. Due to the existence of process parting and other difficulties, this method is difficult to manufacture complex shapes of titanium castings, and most of them are only used in specific castings.
2) graphite type
Machined graphite type high strength, bad concessions, on the liquid titanium to produce radical cooling, often resulting in cracks on the surface of the casting and cold segregation, high production costs, long production cycle. Graphite porosity is large, easy to absorb moisture, so machining graphite type must be de-oiled and degassed before use, otherwise the casting surface oxidation phenomenon is serious. The casting size is relatively large, the wall is relatively thick (≥5mm), the shape is simple, and the required quantity is only one or several pieces. Choose machining graphite type.
3) ceramic type
(1) metal face layer ceramic shell refractory metal tungsten powder as refractory materials, metal tungsten melting point is high, and titanium liquid contact chemical stability, but tungsten powder should have a high purity, the impurity content can not exceed the required standard, otherwise it will affect the quality of titanium castings. The tungsten surface layer melting shell must use solvent dewaxing, and it is carried out in the special dewaxing tank, which is very harmful to human health and pollutes the environment at the same time. The high temperature baking of tungsten surface layer shell must be carried out under reducing atmosphere, and the ash of the molding material deposited on the outer surface of the shell after dewaxing is hard to be sintered, and it is easy to react with liquid titanium when pouring and form porosity on the outer surface of the castings. Paint slurry process performance is not good, poor suspension, paint slurry life is short, preservation difficulties, expensive.
(2) oxide ceramic shell is the inert oxide as the surface shell refractory materials. A variety of oxide materials according to its chemical stability of molten titanium alloy from low to high in the following order: SiO2, MgO, Al2O3, CaO, ZrO2, Y2O3, ThO2. ThO2 due to radioactivity has been basically not used. CaO is easy to absorb moisture, so it hinders the application. Now, used as investment casting shell surface layer and the neighboring surface layer of the material is mainly Y2O3, ZrO2.
ZrO2 without stabilization can not be used as cast titanium modeling materials, because it will occur isotropic isomerism transformation, monoclinic crystals at room temperature, tetragonal crystals at high temperatures, higher temperatures will be transformed into cubic crystals, monoclinic crystals transformed into tetragonal crystals, along with the volume change of 9%, making the shape of the shell to occur, the volume change, the shape of the shell to occur. When monoclinic crystals change to tetragonal crystals, it is accompanied by a volume change of about 9%, which cracks the shell. Usually take to ZrO2 to add 4% ~ 8% of CaO, after high temperature fusion or calcination can be stabilized ZrO2 solid solution (also used in Y2O3 stabilization), most of the industrial use of electro-melting ZrO2.
Y2O3 with ZrO2, the same, it must be stabilized at high temperatures to be used as titanium alloy modeling materials. y2O3 ceramic shells with low thermal conductivity, high strength, casting out of the shell, the advantages of low thermal conductivity, high strength, Y2O3 ceramic mold has the advantages of low thermal conductivity, high strength and good surface quality of castings, but Y2O3 is expensive and difficult to source.
China's cast titanium industry development is relatively fast, in recent years, a number of new cast titanium production plant. At present, the country's cast titanium production plants, research institutes have nearly 20, the new titanium foundry will be positioned in the titanium melt mold precision castings, Shaanxi Jinhan Rare Precious Metals Co., Ltd. perennial and Harbin Institute of Technology, Xi'an Jiaotong University, Northwestern Polytechnical University for technical exchanges and cooperation, is committed to titanium, nickel, zirconium and its alloys precision castings, the formation of a precision casting as the main, machined graphite-type as a complementary We have formed a production model which is mainly based on precision casting and supplemented by machining graphite type.
With the development and increasing maturity of titanium and titanium alloy casting technology, coupled with the birth of hot isostatic pressing (HIP) technology and the successful application of titanium alloy castings, a better solution to the quality of castings, improve the reliability of castings. From the 1980s onwards, titanium and titanium alloy castings in aviation, aerospace and other applications at a rate of 20% per year. Casting process, has been developed from single-piece casting to several or dozens of parts combined into a large whole casting. The scope of application has been developed from the early force is not critical to the development of non-critical static structure to become a component part of the aero-engine, completely replacing some of the deformation of titanium alloys, aluminum alloys, steel parts.
With the aero-engine thrust-to-weight ratio and stiffness requirements, some of the key titanium alloy components are required to be made into large and complex thin-walled precision castings. Some advanced aviation large turbine engine fan magazine, intermediary magazine, front magazine, compressor magazine, etc. began to use titanium alloy precision castings. Large airliner duct, insulation screen, bracket, frame, trunnion, support frame, brake housing, etc. are also titanium alloy castings to replace the original components.
Military aircraft, the use of titanium alloy castings are also gradually increasing, such as: supports, frames, brackets, brake hooks, wing force objects, rudder rotating device bracket, gear shells, hanger support attachments, etc., the practice has confirmed the application of titanium alloy castings in the aircraft is successful, reliable. Not only that, in the production cost, due to the use of titanium alloy castings, so that the aircraft of certain institutions of the design, processing, fastening, assembly, etc. have become more than the original did not use titanium alloy castings when the institutions simplified, thus greatly reducing the aircraft manufacturing costs. Titanium alloy castings in the aerospace field is mainly used for missiles, space shuttle spacecraft, artificial satellites. Its application parts are: missile shell, tail, rudder wings and connectors, etc., the space shuttle and airship brackets, frames, supports, accessories, shells, etc., due to the titanium alloy castings have a high degree of rigidity, light weight and coefficient of thermal expansion of the optical glass, but also used in artificial satellites and other optical instruments such as brackets, bases, connectors, and shells.
Titanium and titanium alloy castings also have a wide range of applications in daily industrial production. Because titanium and titanium alloy has good corrosion resistance, is the chemical industry and other corrosion-resistant industry irreplaceable material. They are widely used in chemical, paper making, petroleum, alkali making, metallurgy, pesticide and other industries. The main application products are cast titanium pumps and titanium fans made of industrial pure titanium and titanium-palladium alloys, and different types of valves, such as globe valves, ball valves, plug valves, gate valves, butterfly valves, check valves and so on.
With the improvement of people's standard of living and the enhancement of the quality of health requirements, titanium alloy with its high fatigue strength, and the human body's super affinity and many other advantages, but also more and more used in the field of medical and health care. Such as: casting titanium alloy hip joint repair parts, knee joint repair parts, human prosthesis, oral cavity repair and so on. The amount of titanium alloy precision castings in the field of sports equipment is very huge, such as: bicycle parts, golf head. Especially titanium alloy golf head market capacity is the most huge, but the casting process is more complex.
At present, the use of titanium and titanium alloy castings are still expanding, more applications are also being studied one after another, but there are still some problems: 1. Fewer varieties of alloys, fewer grades, basically the commonly used titanium alloys are industrial pure titanium castings and TC4 alloy castings. 2. casting application range is small, most of the castings are used in the petroleum and chemical industry (industrial pure titanium casting), aviation, space field Few applications, resulting in China's titanium casting industry, technology and technical level is difficult to improve. 3. Molding process is generally backward, most manufacturers are using graphite type molding process (machining graphite type and compacted graphite type), and few applications of investment casting. Castings casting surface is relatively rough. 4. Melting equipment are basically vacuum self-consumption electrode arc shell furnace, melting process is more dangerous, the molten metal liquid superheat is not high, resulting in the casting surface is easy to produce flow marks, cold segregation and other defects, thin-walled parts molding difficulties.
In order to improve the backwardness of China's titanium casting industrial production, to improve the overall technology and technical level of China's titanium casting industry, it is also necessary to carry out research in the following areas: 1. Improvement of the existing modeling process, the study of the new binder and modeling materials, simplify the process, shorten the production cycle, reduce production costs. 2. Research and development of new melting and casting equipment and its technology to improve the liquid metal superheat, improve and enhance the casting of parts. Heat, improve and enhance the fluidity of casting titanium liquid and filling and shrinkage capacity, to create favorable conditions for the development of large-scale complex thin-walled integral precision castings.3. Further expand the application of computer simulation of solidification technology in titanium alloy casting, in order to improve the quality of castings, reduce the casting scrap rate.4. Research and development of titanium alloy casting of a variety of heat treatment processes and thermo-chemical treatment technology, in order to improve the micro-organizational structure of the casting, and improve the mechanics of casting. Organizational structure, 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 molding process, improve the production capacity of titanium alloy castings.