The editor will take you to understand the application fields of titanium and titanium alloy castings

Titanium and titanium alloys have excellent properties such as low density, high specific strength, corrosion resistance, low coefficient of linear expansion, and good biocompatibility. They are indispensable structural materials in industries such as aviation, aerospace, ocean transportation, chemical, metallurgical, medical and health. In industry The initially used titanium and titanium alloy parts are all deformed parts. With the increase of their usage and the expansion of their application scope, deformation reflects the disadvantages of large mechanical processing volume, low material utilization rate, and high production cost. Therefore, casting technology has developed accordingly. Titanium casting is a relatively economical and easy to implement near forming process. Titanium and titanium alloys have high chemical activity in the molten state, and they need to undergo chemical reactions with various commonly used refractory materials. Melting and casting forming are very difficult, and specialized molding materials, molding processes, and melting and casting equipment are necessary.

More than 90% of the melting and casting equipment for titanium casting in our country adopts vacuum consumable electrode arc condensing furnace and centrifugal casting. The crucible adopts water-cooled copper crucible and titanium liquid The pouring amount is 500 kg.

The consumable electrode arc melting method uses a consumable electrode made of titanium or titanium alloy as the cathode and a water-cooled copper crucible as the anode; During high current melting, the melting rate of the titanium electrode is much faster than the solidification rate of titanium. The melted electrode enters the crucible in the form of droplets, forming a molten pool; The surface of the molten pool is heated by an arc and remains in a liquid state. The bottom and the surrounding area in contact with the crucible are forcibly cooled by circulating water, resulting in crystallization from bottom to top. This method has the advantages of simple structure, low maintenance cost, and easy scalability. The disadvantage is that the pouring temperature is difficult to adjust and control. After the arc is stopped, the metal liquid must be poured out of the crucible within 3-5 seconds. Otherwise, the temperature will drop sharply and the superheat of the metal liquid will not be high, resulting in poor liquid fluidity and shrinkage ability. The quality requirements for consumable electrode arc melting are very high, requiring a dense internal structure of the electrode. There is a high level of danger during the melting process, and even slight mishandling can result in arc damage to the crucible, causing circulating water that forcefully cools the outer wall of the crucible to enter and contaminate the titanium liquid. Water vapor can also damage the vacuum pump system.

The main molding processes for titanium alloy casting include metal molds, machined graphite molds, metal surface layer ceramic molds, and oxide ceramic molds.

In the field of titanium alloy casting, the metal materials used as molds mainly include copper, steel, cast iron, tungsten, molybdenum, etc., which are collectively referred to as hard mold systems along with graphite processing molds. Due to technical difficulties such as parting, it is difficult to manufacture titanium castings with complex shapes using this method, and most of them are only used on specific castings.

The metal surface layer ceramic shell uses refractory metal tungsten powder as the refractory material. Tungsten has a high melting point and good chemical stability when in contact with titanium liquid. However, tungsten powder should have high purity and impurity content should not exceed the specified standard, otherwise it will affect the quality of titanium castings. The tungsten surface layer melting model shell must use solvent dewaxing, and it is carried out in a specially designed dewaxing tank, which is very harmful to human health and also pollutes the environment. The high-temperature calcination of tungsten surface layer shell must be carried out under a reducing atmosphere. The ash content of the mold material deposited on the surface of the shell after dewaxing is difficult to burn, and it is easy to react with liquid titanium during pouring, forming pores on the surface of the casting. The process performance of the coating slurry is poor, the suspension is poor, the service life of the coating slurry is short, the storage is difficult, and the price is expensive.

Oxide ceramic shell is a refractory material that uses inert oxides as the surface layer of the shell. The order of chemical stability of various oxide materials to molten titanium alloys from low to high is as follows: SiO2, MgO, Al2O3, CaO, ZrO2, Y2O3, ThO2. ThO2 is basically not used due to its radioactivity. CaO is prone to moisture absorption, which hinders its application. Currently, the main materials used for the shell surface layer and adjacent surface layer in investment casting are Y2O3 and ZrO2.

ZrO2 without stabilization treatment cannot be used as a molding material for titanium casting because it undergoes allotrope transformation. At room temperature, it becomes a monoclinic crystal, at high temperature, it becomes a tetragonal crystal, and at higher temperatures, it transforms into a cubic crystal. When a monoclinic crystal transforms into a tetragonal crystal, it undergoes a volume change of about 9%, causing cracking of the mold shell. Usually, 4%~8% CaO is added to ZrO2, and stable ZrO2 solid solution (also stabilized with Y2O3) can be obtained by high-temperature melting or calcination. In industry, electric melting ZrO2 is mostly used.

Y2O3, like ZrO2, must undergo high-temperature stabilization treatment before it can be used as a titanium alloy molding material. Y2O3 ceramic shells have the advantages of low thermal conductivity and high strength, and the surface quality of castings cast is good. However, Y2O3 is relatively expensive and difficult to source.

With the development and increasing maturity of titanium and titanium alloy casting technology, coupled with the birth of hot isostatic pressing (HIP) technology and its successful application in titanium alloy castings, the quality problems of castings have been well solved, and the reliability of castings has been improved. Since the 1980s, the application of titanium and titanium alloy castings in aviation, aerospace, and other fields has been increasing at a rate of 20% per year.

In terms of casting technology, it has evolved from single piece casting to large-scale integral castings composed of several or dozens of parts. The application scope has evolved from non critical static structural components with low stress in the early days to becoming a component of aviation engines, completely replacing some deformed titanium alloys, aluminum alloys, and steel parts.

With the increasing demand for thrust to weight ratio and stiffness in aviation engines, some key titanium alloy components are required to be made into large, complex, thin-walled precision castings. Some advanced aviation large turbine engine fan casings, intermediate casings, front casings, compressor casings, etc. have begun to use titanium alloy precision castings. The air ducts, heat shields, brackets, frames, ear shafts, support frames, brake housings, etc. of large passenger aircraft are also replaced with titanium alloy precision castings. In terms of military aircraft, the use of titanium alloy castings is gradually increasing, such as supports, frames, brackets, brake hooks, load-bearing objects on wings, rudder rotation device brackets, gearbox shell components, hanger support accessories, etc. Practice has proven that the application of titanium alloy castings in aircraft is successful and reliable. Moreover, in terms of production costs, the use of titanium alloy castings has simplified the design, processing, fastening, assembly, and other aspects of certain aircraft mechanisms compared to those without titanium alloy castings, greatly reducing the manufacturing cost of the aircraft. Titanium alloy castings are mainly used in the aerospace field for missiles, spacecraft, and artificial satellites. Its main application areas include missile shells, tail fins, rudder wings, and connecting seats, as well as space shuttle and spacecraft brackets, frames, supports, accessories, shells, etc. Due to the high rigidity, light weight, and thermal expansion coefficient equivalent to optical glass of titanium alloy castings, they are also used in the frames, bases, connecting frames, and shells of artificial satellites and other optical instruments.

Titanium and titanium alloy castings also have a wide range of applications in daily industrial production. Due to their excellent corrosion resistance, titanium and titanium alloys are irreplaceable materials in the chemical and other corrosion-resistant industries. Widely used in industries such as chemical, papermaking, petroleum, alkali production, metallurgy, and pesticides. The main application products are cast titanium pumps and fans made of industrial pure titanium and titanium palladium alloys, as well as various types of valves such as globe valves, ball valves, plug valves, gate valves, butterfly valves, check valves, etc.

With the improvement of people's living standards and the increasing demand for health quality, titanium alloys are increasingly being used in the medical and health field due to their high fatigue strength, strong affinity with the human body, and many other advantages. For example, cast titanium alloy hip joint restorations, knee joint restorations, human prosthetics, oral restorations, and so on. The amount of titanium alloy precision castings used in the field of sports equipment is very large, such as bicycle accessories and golf ball heads. Especially the market capacity of titanium alloy golf balls It is huge, but the casting process is relatively complex.

At present, the scope of use of titanium and titanium alloy castings is still expanding, and more application fields are being studied successively. However, there are still some problems: 1. There are few alloy varieties and grades, and the commonly used titanium alloys are industrial pure titanium castings and TC4 alloy castings. 2. The application scope of castings is small, and most castings are used in the petrochemical industry (industrial pure titanium castings), with few applications in the aviation and aerospace fields, making it difficult to improve the process and technical level of China's titanium casting industry. 3. The molding process is generally backward, and most manufacturers use graphite molding technology (machined graphite and compacted graphite), while the application of investment casting precision casting is rare. The surface of the castings cast is relatively rough. 4. Most melting equipment is vacuum consumable electrode arc condensing furnace, which poses a high level of danger during the melting process. The superheat of the molten metal is not high, resulting in defects such as flow marks and cold shuts on the surface of the castings, making it difficult to form thin-walled parts.

In order to improve the backward state of titanium casting industry production in China and enhance the overall process and technological level of China's titanium casting industry, the following research needs to be conducted: 1. Improve the existing molding process, study new binders and molding materials, simplify the process, shorten the production cycle, and reduce production costs. 2. Research and develop new melting and casting equipment and technologies, improve the superheat of molten metal, enhance the fluidity and filling and shrinking ability of cast titanium liquid, and create favorable conditions for the development of large complex thin-walled integral precision castings. 3. Further expand the application of computer simulation solidification technology in titanium alloy casting to improve casting quality and reduce the scrap rate of castings. 4. Research and develop various heat treatment processes and thermochemical treatment techniques for titanium alloy castings to improve the microstructure of titanium alloy castings and enhance their mechanical properties. 5. Investment casting can only produce small and medium-sized castings, and a molding process that produces larger, cleaner, and more efficient castings should be sought to improve the production capacity of titanium alloy castings.