Preparation and processing of molybdenum and molybdenum alloy products
At present, the main manufacturing methods of molybdenum and molybdenum alloy products are smelting, powder metallurgy, chemical vapor deposition, rolling and forging.
Plastic working is not only the forming method of molybdenum and molybdenum alloy, but also can improve the strength and low temperature plasticity of molybdenum alloy. Molybdenum and its alloy can be produced by conventional plastic processing method to produce plate, strip, foil, tube, bar, wire and profile. The characteristic of molybdenum alloy processing is that each hot deformation process has a significant impact on the final performance of the product. Molybdenum begins to oxidize above 400 ℃ and increases with the increase of temperature. Therefore, hydrogen or other reducing atmosphere is usually used to protect molybdenum and its alloys. Because the contamination layer of molybdenum is very thin and easy to be washed away by melting alkali, hot working can be carried out under atmospheric conditions, but it is better to do it quickly. The cold working of molybdenum and its alloys should be carried out above the plastic brittle transition temperature.
1. Smelting
Mo has high melting temperature and strong chemical activity. Interstitial elements (N, O, H) have obvious effects on its properties. When casting in the atmosphere, there will be rapid oxidation and nitriding, forming a large number of inclusions, which can not get satisfactory ingot. Therefore, vacuum melting method must be used to produce ingots or castings, mainly using arc melting, electron beam melting and regional melting. In the smelting process, tungsten and molybdenum powders are usually made into electrode rods by powder metallurgy, and then melted, and then processed into the required materials or products.
2. Powder metallurgy
Powder metallurgy is an effective method to prepare refractory metals, which is widely used in manufacturing molybdenum and molybdenum alloy products. Mold pressing, extrusion, cold isostatic pressing and injection molding can be used for forming molybdenum products, and then sintered at high temperature in protective atmosphere. The sintering process of Mo was carried out in hydrogen atmosphere. The molybdenum billets used for processing are mainly formed by isostatic pressing, and a small amount are still formed by die pressing, then sintered at high temperature, and the required products are obtained through deformation processes such as drawing, forging and rolling.
3. Metal organic chemical vapor deposition of carbonyl molybdenum
Due to its high melting point, hard and brittle nature, molybdenum is difficult to produce large-scale, complex shaped and thin-walled dense metal and its compound products (such as curved surface and internal thread products) by ordinary powder metallurgy and mechanical methods, which greatly limits its application in aviation, aerospace and nuclear industries. Therefore, it is an important task to explore the technology of high-purity and high-density metal forming.
Metal organic chemical vapor deposition (MOCVD) is one of the effective methods to prepare refractory metal complex parts and high temperature refractory metal coatings. The complexes of transition metals with carbon monoxide are highly volatile (sublimated) and dissociated into metals and - carbon oxides at lower temperatures. With carbon based metal as precursor, thin-walled thin-walled parts with complex shapes can be successfully prepared by adjusting MOCVD process and atmosphere. ）Base molybdenum is a kind of powder material, which sublimates at 150 ℃ to form carbon based molybdenum vapor. MOCVD deposition of Mo (based on) as precursor has the advantages of low deposition temperature, high deposition speed, high purity, compact film, good surface finish, short process, no pollution, no corrosion and so on. By adjusting the process and atmosphere, molybdenum films of pure metals, compounds and alloys and thin-walled parts with complex shapes can be formed.
4. Deformation processing of molybdenum
Molybdenum rolling is mainly used for molybdenum plate, molybdenum strip, molybdenum foil, molybdenum bar and molybdenum rod production. The blooming temperature of billets provided by rolling melting extrusion is generally between 1200 ℃ and 1250 ℃, while that of billets provided by powder metallurgy is generally around 1400 ℃. In order to reduce the non-uniform deformation, the pass deformation during Blooming should be between 20% and 40%, and the total rolling deformation after each heating is about 75%. When the total deformation is more than 85% (plate thickness is about 6 mm), the rolling temperature can be reduced to 700 ~ 900 ℃; when the plate thickness is 1 ~ 2 mm, the rolling temperature can be reduced to 200 ~ 400 ℃. According to the plastic brittle transition temperature of the material, the thickness of the cold rolled sheet is 0.5-1 mm. Cross rolling can be used to improve the anisotropy of products. In order to obtain enough work hardening and improve low temperature plasticity, the appropriate cold work rate of the final product should be about 70%.
The forging of molybdenum includes rotary forging and ordinary forging. Rotary forging is mainly used to produce 2.5 mm diameter thin bar and drawing wire blank, the blank used is 10 ~ 30 mm square sintered bar. The open forging temperature of pure molybdenum is usually about 1400 ℃, and the pass deformation is generally 10-20%, which can also reach about 30%. With the decrease of the diameter, the forging temperature decreases gradually, and it can be reduced to about 800 ℃ when the diameter is 3 mm. For ordinary forging, hammer forging is more suitable than press forging. Large size blank and heavy forging can be obtained by ordinary forging. The open forging temperature of pure molybdenum is about 1400 ℃, but the open forging temperature of extruded blank can be lower.
Molybdenum drawing is mainly used for manufacturing molybdenum and molybdenum alloy wire.
