1. Field of the Invention
The invention relates to a method for the manufacture of powders from molten materials by using fluid media to atomize the melt to particles and cooling the particles during their fall, having a melting system, an atomizing system, and an atomizing tower associated with the atomizing system and attached to the melting chamber.
2. Discussion of Related Art
It is known that powders, such as powdered metals, can be produced by atomizing a melt (DOS 30 34 677). In this method a jet of the melt is disrupted by a gas or by a liquid such as water or oil, at a high relative velocity of the atomizing medium to the molten jet, to fine droplets which solidify in a cooling section that follows.
It has, however, been found to be disadvantageous and limiting that gas-atomized powders have only a coarse average particle size of typically 4 to 150 microns. Even in the case of powders produced by ultrasound only a gradual reduction of the average particle size has been achieved. To this is added the poor cooling action of the gas in the cooling section, especially below about 600.degree. C., which in the case of alloys of metals, such as aluminum-base metals, for example, can lead to undesirable segregations during the manufacture of the powder.
In the case of powders atomized with water or oil smaller average particle sizes can be established than is possible by atomizing with gases. Powders are obtained, however, which have undesired reaction products at least on the surface of the powder particles, such as oxides, hydrides, carbides etc. (In the case of gas atomizing this can be prevented by the use of inert gases.)
Additional difficulties can result in the case of powders atomized with water or oil, due to the necessity of separating the powders from the atomizing medium.
Furthermore, a method is also known for the manufacture of fine metal powders, in which a stream of molten metal is introduced into an opening in a tank by the action of a gas flowing at ultrasonic velocity. The ratio of the gas pressure in the vicinity of the opening outside of the tank to the gas pressure inside of the tank is made greater than 5, and the gas flowing into the tank a temperature ranging between 0.7 and 1.5 times the solidification temperature of the metal (in .degree. K) before injection. The molten stream is first divided into fibers, these fibers being converted to droplets in the area of the pressure drop in the hot gas, and these droplets hardening into spherical metal powders (DPS 33 11 343). The molten metal is brought into contact with the gas at a point in the tank opening at which the gas pressure has dropped to less than 60 % of the pressure ahead of the opening. This previously known process makes it possible with relatively low energy consumption, to produce very fine powders with particle diameters of less than 40 microns, with a relatively narrow particle diameter distribution.