Preparation of dense molybdenum

The purpose of press forming is to produce compacts of a certain shape and size with a certain density and strength. When the compact size of the compact is pressed, the large-sized molybdenum compact is pressed by press molding, and formed by a cold isostatic press. The pressure of molding on a mechanical press is generally controlled at 200 to 300 MPa, and the pressing pressure on a cold isostatic press is 2000 MPa. The pressed compact has a relative density of 60% to 65%.
In order to increase the strength and electrical conductivity of the molybdenum compact, low temperature pre-sintering is performed first, followed by high-temperature sintering. The low temperature pre-sintering is to place the compact in a hydrogen furnace and sintering at 1100 to 1200 ° C for 30 to 60 minutes. In order to obtain a dense metal article molybdenum, calcined briquettes to high temperature sintering. High temperature sintering is divided into direct high temperature sintering (dipfusion) and indirect high temperature sintering. Direct high-temperature sintering (dip-fusion) is to directly pass an electric current through a pre-fired molybdenum slab and heat it to 2200 to 2400 ° C for sintering. The maximum current intensity for sag melting is about 90% to 92% of the melting current. The Mo strip is determined by its size to determine the sag specification as shown in Table 1. In recent years, the program of the sag current has been developed automatically, thus eliminating artificial current and time errors, saving labor and stabilizing product quality. Indirect sintering is mainly used for the sintering of large-sized molybdenum billets. The high-temperature furnace generally used has a resistance heating high temperature furnace and an induction heating high temperature furnace. Sintering is carried out in a protective atmosphere or in a vacuum. It must be noted that if a graphite rod is used as the heating element or a graphite boat is used as the container, the container must be coated with a molybdenum powder or a phenolic resin mixture to prevent the molybdenum from being carbonized. At high temperature sintering, the temperature is 1700 ~ 1850 ° C, and the sintering time is 3 ~ 15h to obtain dense metal molybdenum products.
Vacuum melting
The metal molybdenum powder, molybdenum grains or sintered strips are subjected to vacuum consumable arc melting or electron beam melting into a metal molybdenum ingot in a vacuum melting furnace. The single-weight molybdenum ingot can be obtained by vacuum melting method. However, the metal molybdenum ingot obtained by this method has coarse crystal grains and cannot be directly processed. It must be extruded to make the ingot grain refined before processing. Plate, foil, tape and other materials.
Compared with the powder metallurgy method, the vacuum melting method produces the processed material by the vacuum melting method, and the yield is low, and the electric energy consumption is large, which greatly increases the production cost. The molybdenum billet produced by the powder metallurgy method has fine crystal grains and uniform structure, and can be directly processed deep. The metal loss is small, the processing yield is high, the cost is greatly reduced compared with the vacuum melting method, and the flexibility is large. In the early stage, only small-sized products could be produced by powder metallurgy. After the 1970s, with the advent of large-scale pressing equipment such as isostatic pressing and large-scale high-temperature sintering furnaces, large-size and large-weight molybdenum can also be obtained by powder metallurgy. Products, so the application of powder metallurgy is gradually expanding. [next]

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