Titanium casting is the process of melting and pouring titanium materials into castings under vacuum or protective atmosphere. It is one of the titanium material preparation processes.
Titanium is very active in the molten state. For a long time, no suitable melting and casting method and molding materials have been found, resulting in the lagging of cast titanium behind deformed titanium alloy for more than 20 years. From 1956 to 1962, Beal et al. in the United States developed vacuum consumable arc shell melting and casting technology, which officially entered the industrial production of titanium castings. In the 1970s, it began to be applied in the aerospace field. Since the 1970s, large-scale thin-wall precision casting technology of titanium alloy has been developed and applied. Under the premise that its material performance is close to or equal to that of aviation titanium forgings, the cost is reduced by about 50%, so the casting titanium technology has developed rapidly. It is expected that in the near future, it will achieve the same important status as deformed titanium alloy.
Melting and casting equipment
The vacuum consumable arc shell furnace is the main equipment for producing titanium castings. Its principle is: in the furnace body, titanium ingots or forged rods are used as the parent material electrode (negative electrode), and the water-cooled copper crucible acts as the positive electrode. In a vacuum atmosphere, a low-voltage (25-40V) high current is input. After the two poles are close to the arc, the end of the consumable electrode of the titanium material is melted and dripped into the crucible to form a molten pool. Under the action of water cooling, a layer of shell is formed between the copper crucible wall and the molten pool to protect the crucible from erosion and the titanium liquid from pollution. When the molten pool in the crucible grows to a sufficient amount, the power is turned off and the arc is broken, the electrode is quickly lifted, the crucible is turned over, and the molten titanium water is injected into the static or centrifugal rotating casting mold. The advantages of the shell furnace are high production efficiency and good control of alloy composition. The disadvantage is that the metal superheat is low, and a fast pouring and a large pouring system are required. The pouring capacity of the largest shell furnace in China is 500kg. The United States and the former Soviet Union both have furnaces of about 1000kg. Other titanium casting methods that are in the experimental research stage include electron beam, plasma and cold wall crucible induction casting .
Molding methods
Includes graphite mold casting and investment casting.
Graphite mold casting Graphite mold is the main molding process for producing civilian titanium castings. It is divided into processed graphite mold and rammed graphite mold. Processed graphite mold is made by hand or machine processing of high-quality artificial graphite blocks. Depending on the complexity of the casting, a combination of multiple live blocks can be used. This type of casting can be used multiple times. Rammed graphite mold is made by rolling artificial graphite sand and carbon organic binder into a mixture, and then molded by hand or by a molding machine in a wooden mold or metal mold sand box. The made graphite sand mold is dried and solidified at low temperature, and then high-temperature roasted under the cover of dry loose graphite powder or under the protection of a non-oxidizing atmosphere. After the casting combination is made, it can be loaded into the furnace for casting. For aviation castings with strict quality requirements, both the processed mold and the rammed mold need to be vacuum degassed before casting . Investment casting
Investment casting is the main process for producing titanium alloy castings for aviation with high precision, complex shape, smooth surface and dense interior. The titanium investment casting method is basically the same as the steel investment casting process, except for the shell material and some processes.
There are three processes used to produce titanium precision castings today:
(1) Graphite shell process.
(2) Metal surface ceramic shell process.
(3) Oxide ceramic shell process. The first one is inexpensive and can be used for the production of small and medium-sized castings, and the latter two can be used for the production of large thin-walled precision castings.
Casting titanium alloy and casting quality Most deformed titanium alloys can be used for casting, and the most widely used is Ti-6A1-4V alloy. It has good casting properties and stable microstructure. Compared with deformed alloys of the same composition, the strength of cast titanium alloys is basically the same, but its plasticity and fatigue properties are about 40% to 50% lower, and its fracture toughness is slightly better.
Titanium castings are generally treated by stabilization annealing. The solution treatment and hydrogenation treatment in the research can refine the grains, improve the structure, and improve the fatigue performance of the alloy to the level of forgings. Hot isostatic pressing is a common treatment method for high-quality titanium castings. After high temperature and high pressure treatment, the size of the casting does not change, while the internal structure becomes denser, and the stability of the mechanical properties will be greatly improved. The quality of the casting should be inspected according to the national standard GB6614, the national military standard GJB2896 or the aviation standard HB5448. The technical characteristics of titanium alloy castings are listed in the table.
Casting Application
The primary user of cast titanium is the aerospace industry. Important parts include: engine compressor casing, intermediate casing, blades, hollow guide, inner ring, supercharger impeller, bearing housing and support, aircraft bracket, parachute compartment, ear piece, short beam, flap slide rail, brake housing; missile control cabin, tail wing, rocket rear head, common bottom, etc.; satellite support, scanner frame, lens barrel, etc. In the civil industry, it is widely used to manufacture corrosion-resistant pump bodies, valves, impellers; screw propellers for ships; housings, brackets, and cylinders for precision machinery; artificial joints and prosthetic components for medical use; golf heads, horse harnesses, and bicycle parts for sports equipment, etc.
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