Titanium alloy investment casting process

Titanium alloy investment casting process

Introduction to titanium alloy investment casting

Titanium alloy has become an indispensable advanced structural material in modern industry due to its excellent comprehensive properties, such as low density, high specific strength, high temperature resistance and corrosion resistance. Especially in recent decades, with the rapid development of the aerospace and defense industries, the usage and application level of titanium alloys have been significantly improved. Taking civil aviation aircraft as an example, the amount of titanium used in France's Airbus A380 has soared from 6% to 10% in the fourth-generation A340; in the United States, Boeing's B787 has also increased from 8% to 15% in B777. Compared with foreign countries, although my country started late in the research and development of titanium alloys, it has shown strong development momentum. The titanium usage of my country's newly developed C919 large aircraft has reached 9.3%, which has exceeded the titanium usage of the B777 aircraft. However, due to its high chemical activity, small thermal conductivity and low plasticity, titanium alloys are difficult to form using traditional processing methods, especially for some complex components with thin-walled features. This undoubtedly limits the further application of titanium alloys. In order to break this dilemma, several industrialized countries in the world, such as the United States, Germany and Russia, have been committed to the research and development of titanium alloy casting processes since the 1950s. Among them, the investment precision casting technology of near net shape technology has achieved His achievements have attracted the most attention from the world.

Investment casting, also known as lost wax casting, is a method that uses soluble primary mold materials to make a shell and pour it into parts. Compared with traditional casting processes, investment precision casting has the following advantages: low surface roughness of castings (Ra=1.6~3.2μm); accurate dimensions; high raw material utilization rate (70%~90%); almost no restrictions on alloy types, and It can cast various castings with complex structures. Especially in the late 1970s, hot isostatic pressing (HIP) technology was widely used in the post-processing of castings, which allowed certain casting defects to be eliminated, thereby effectively improving the mechanical properties and stability of castings. Thanks to the above advantages, titanium alloy investment casting technology has developed into one of the irreplaceable basic processes in the world's manufacturing field. According to incomplete statistics, more than 98% of cast titanium alloy components currently used in the aviation industry are prepared and formed using investment casting technology. Objectively speaking, titanium alloy investment casting technology is to a certain extent an important reflection of the advanced level of national casting technology.

Castings produced by investment casting have low surface roughness, precise dimensions, and can cast castings with complex shapes, improving the utilization rate of metal materials. However, the investment casting process is complex and difficult to control, and the materials used and consumed are relatively expensive. It is usually suitable for the production of small parts with complex shapes, high precision requirements, or that are difficult to perform other processing, such as turbine engine blades.

Titanium alloy is one of the most difficult industrial metals to precision cast. Because titanium alloys have high chemical activity, they can chemically react with almost all refractory materials in the molten state, resulting in a decrease in melt quality. Therefore, the selection of shell material is the key in the titanium alloy investment casting process.

Molding materials used for titanium alloy casting should meet the following basic requirements:

1) No chemical reaction occurs when in contact with molten titanium alloy, and it has high chemical inertness;

2) It will not collapse or break under the high temperature of molten titanium alloy, and has high refractoriness and thermal shock resistance:

3) It has sufficient strength and is not easily damaged during transportation and furnace installation;

4) Small adsorption capacity for moisture and gas;

5) Low thermal conductivity to reduce defects caused by chilling of castings.

The casting materials currently used can only partially meet the above requirements. These materials can be roughly divided into the following categories.

1. Carbon refractory materials

The carbonaceous refractory material used for titanium alloy casting is mainly artificial graphite. Artificial graphite is made from petroleum coke and asphalt as the main raw materials, and is calcined at a high temperature of 2600-3000°C. Artificial graphite has high refractoriness under vacuum, a small thermal expansion coefficient, its strength increases with temperature, and it is inert to molten titanium alloys. However, artificial graphite also has the disadvantages of being easily oxidized, having strong gas adsorption capacity, high thermal conductivity, and easily causing micro-cracks on the surface of castings.

2. Oxide ceramic materials

Oxide ceramic materials are important materials used in titanium alloy precision casting. Commonly used oxide ceramics in industry include SiO₂, MgO, Al₂O₃, CaO, ZrO₂, Y2O3 and ThO₂. Their ability to react with molten titanium alloys gradually weakens.

3. Metal materials and other materials

The metal materials used for castings are mainly copper, steel, cast iron, etc. It is difficult to produce titanium alloy castings with complex shapes, and most of them are only used on specific castings. In addition, there are refractory metal powders such as tungsten, molybdenum, tantalum, and niobium. These materials have high melting points and good chemical stability when in contact with molten titanium alloys. They are often used as surface coatings for titanium alloy investment castings.

The metal utilization rate of titanium alloy investment casting can reach more than 90%, which can greatly reduce production costs. It is very likely to become one of the forming methods for a large number of titanium alloy castings suitable for civilian products, and is also the focus of future research.

Titanium and titanium alloy investment casting parameters