Raw materials in the crucible, to take into account the induced magnetic field and the temperature in the melting process of space distribution, usually induction coil around the crucible outside the side, the crucible inside the side of the magnetic field is the strongest, gradually weakened to the center, but the crucible side, the bottom and the opening is the main way of heat leakage, so the crucible lower side temperature in the middle, the upper and the bottom of the middle temperature is low, The hottest part is in the middle. Therefore, when loading the low melting point of small pieces of material more dense on the bottom of the pot; High melting point material, bulk material in the lower middle; The bulk material with low melting point is placed on top and loosened to prevent bridging. At present, continuous melting and casting technology has been widely used, in which raw materials are successively added into the crucible at high temperature through the feeding chamber. In order to control the volatilization of rare earth materials, pure iron is usually added first to melt it, and then high melting point metals or alloys are added successively, and finally rare earth is added.
2.Casting
In order to achieve the desired quench effect, the traditional ingot casting technology has been striving to reduce the thickness of alloy ingot. The advantages of ingot casting are low equipment cost, simple operation, and can meet the requirements of general magnet production, but the disadvantages are uneven grain size and α-Co or α-Fe phase precipitation. Long time heat treatment of alloy ingot at temperature below the melting point of alloy is helpful to eliminate α-Co or α-Fe phase, but will cause the accumulation of Nd-rich phase, which is not conducive to the optimization of grain boundary phase distribution of sintered magnets.
In order to further reduce the thickness of the alloy ingot, a "disc-scraper" structure similar to pancake was developed, so that the alloy thickness reached about 1cm, but the increase of the alloy area brought a lot of trouble to the receiving of the large capacity melting furnace. Another effective technology development path is to go in the opposite direction, starting from the extremely high cooling rate of fast quench Nd-Fe-B alloys, and try to reduce the cooling rate to produce fast cooling crystalline alloys. A technology called strip casting or SC was developed. It is to pour molten alloy on a rapidly rotating water-cooled metal wheel through a diversion channel to obtain alloy thin slices with ideal phase composition and texture and thickness of 0.2-0.6mm. The uniform distribution of Nd-rich phase and the inhibition of α-Fe reduce the total rare earth content in the alloy structure of strip casting, which is beneficial to obtain high performance magnets and reduce the cost of magnets. The disadvantage is that, due to the reduction of the volume fraction of the Nd-rich phase, the magnet is brittle and difficult to finish compared with the magnet produced by ingot casting.
