Titanium and titanium alloy have many excellent properties, such as low density, high specific strength, corrosion resistance, small linear expansion coefficient, good biocompatibility, etc. The titanium and titanium alloy parts initially used in industry are all deformed parts. With the increase of their amount and application scope, the deformed parts reflect the disadvantages of large machining volume, low material utilization rate and high production cost, so the casting technology develops accordingly. Titanium casting is a relatively economic and easy to achieve near forming process. Titanium and its alloys have high chemical activity in the melting state. It is very difficult to melt and cast them in order to react with various refractory materials in common use.
More than 90% of the smelting and casting equipment of titanium casting in China adopt vacuum consumable electrode arc shell furnace and centrifugal casting. The crucible is water-cooled copper crucible, and the maximum pouring amount of titanium liquid is 500kg.
The consumable electrode arc melting method takes the consumable electrode made of titanium or titanium alloy as the cathode and the water-cooled copper crucible as the anode. In the high current smelting, the melting speed of titanium electrode is far greater than the condensation speed of titanium, and the melted electrode enters the crucible in the form of liquid drop to form a molten pool. The surface of the molten pool is heated by the arc and always in liquid state, and the bottom and the four sides contacting the crucible are forced to cool by circulating water, Produce bottom-up crystallization. This method has the advantages of simple structure, low maintenance cost and easy to be large-scale. The disadvantage is that the pouring temperature is difficult to adjust and control. Once the arc is stopped, the liquid metal must be poured out of the crucible within 3-5 seconds, otherwise the temperature will drop sharply, the superheat of the liquid metal is not high, which makes the liquid fluidity and feeding capacity poor. Consumable electrode arc melting requires high quality of electrode and compact internal structure of electrode. In the smelting process, there is a great danger. A little careless operation will cause the arc to damage the crucible, which will cause the circulating water forced to cool the outer wall of the crucible to enter the crucible, contaminate the titanium liquid, and water vapor will damage the vacuum pump system.
The main molding technologies of titanium alloy casting are metal mold, machining graphite mold, metal surface ceramic mold and oxide ceramic mold.
China’s titanium casting industry is developing rapidly. At present, there are nearly 20 titanium casting factories and research institutes in China. The new titanium casting factories also position their products on titanium investment castings. Shaanxi Jinhan rare and precious metal Co., Ltd. conducts technical exchanges and cooperation with Harbin University of technology, Xi’an Jiaotong University and Northwest University of technology all the year round, It is committed to the production of titanium, nickel, zirconium and their alloy precision castings, forming a production mode with precision casting as the main and machining graphite as the auxiliary.
With the development and maturity of titanium and titanium alloy casting technology, and the birth of hip technology and its successful application in titanium alloy castings, the quality problems of castings are solved and the reliability of castings is improved. Since the 1980s, the application of titanium and titanium alloy castings in aerospace and other fields has increased by 20% every year. In terms of casting technology, at present, it has developed from single casting to large-scale integral casting composed of several or dozens of parts. The application range has developed from the early non key static structural parts with little stress to the component parts of aeroengine, completely replacing some deformed titanium alloy, aluminum alloy and steel parts. With the improvement of thrust to weight ratio and stiffness requirements of aeroengine, some of the key titanium alloy components are required to be made into large and complex thin-walled precision castings. Some advanced large turbine engine fan case, intermediate case, front case, compressor case and so on have begun to use titanium alloy precision casting. Titanium alloy precision castings are also used to replace the original components of air ducts, heat shields, brackets, frames, trunnions, support frames, brake housings, etc. In military aircraft, the use of titanium alloy castings is also gradually increasing, such as: support, frame, bracket, brake hook, load-bearing objects on the wing, rudder rotation device bracket, transmission device shell, hanger support accessories, etc. the practice proves that the application of titanium alloy castings in aircraft is successful and reliable. Not only that, in terms of production cost, due to the use of titanium alloy castings, the design, processing, fastening and assembly of some mechanisms of the aircraft have become simpler than those without titanium alloy castings, thus greatly reducing the manufacturing cost of the aircraft. Titanium alloy castings are mainly used in missiles, spacecraft and artificial satellites in the aerospace field. The main application parts are: missile shell, tail wing, rudder wing and connecting seat, space shuttle and spaceship bracket, frame, support, accessory, shell, etc. titanium alloy casting has high rigidity, light weight and thermal expansion coefficient equivalent to optical glass, and is also applied to the bracket, base, connecting frame and shell of man-made satellite and other optical instruments.
Titanium and titanium alloy castings are also widely used in daily industrial production. Titanium and its alloys are widely used in chemical industry, paper industry, petroleum industry, alkali industry, metallurgy industry and pesticide industry because of their good corrosion resistance. The main application products are cast titanium pumps, titanium fans and various types of valves made of industrial pure titanium and titanium palladium alloy.
With the improvement of people’s living standard and the requirement of health quality, titanium alloy has been more and more used in the field of medical and health care and sports equipment because of its high fatigue strength and strong affinity with human body, but the casting process is complex.
At present, the application range of titanium and titanium alloy castings is still expanding, and more application fields are also being studied one after another, but there are still some problems: 1. There are few alloy varieties and brands, basically the commonly used titanium alloys are industrial pure titanium castings and TC4 alloy castings. 2. The application scope of castings is small, most of the castings are used in the petrochemical industry (industrial pure titanium castings), and the application in the field of aviation and aerospace is rare, which makes it difficult to improve the process and technical level of China’s titanium casting industry. 3. The molding technology is generally backward, most of the manufacturers use graphite mold molding technology (machining graphite mold and ramming graphite mold), while investment casting is rarely used. The surface of the casting is rough. 4. The melting equipment is basically vacuum consumable electrode arc shell furnace. The melting process is of high risk, and the superheat of molten metal is not high, resulting in defects such as flow marks and cold shuts on the casting surface, and forming of thin-walled parts is difficult.
In order to improve the backward production of titanium casting industry and improve the overall process and technical level of titanium casting industry in China, the following research should be carried out: 1. Improve the existing molding process, study new binder and molding materials, simplify the process, shorten the production cycle and reduce the production cost. 2. Research and develop new smelting and casting equipment and technology, improve the superheat of liquid metal, improve and improve the fluidity and filling and feeding capacity of liquid titanium, and create favorable conditions for the development of large-scale complex thin-walled integral precision casting. 3. Further expand the application of computer simulation solidification technology in titanium alloy casting, in order to improve the quality of castings and reduce the scrap rate of castings. 4. Research and develop various heat treatment and thermochemical treatment technologies of titanium alloy castings to improve the microstructure and mechanical properties of titanium alloy castings. 5. Investment casting can only produce small and medium-sized castings. It is necessary to seek a kind of molding technology to produce larger, cleaner and more efficient castings, so as to improve the production capacity of titanium alloy castings.