Method for preparing powder by impinging or otherwise breaking metal or alloy liquids into tiny droplets by a fast-moving fluid (atomizing medium) and then condensing them into solid powder. Atomization is the best method to produce fully alloyed powder, which is called pre-alloyed powder. Each particle of the powder not only has the same uniform chemical composition as that of a given molten alloy, but also refines the crystalline structure due to rapid solidification, and eliminates the macro-segregation of the second phase.
The atomization method can be divided into two kinds: the "Two-flow method"(crushing alloy liquid flow by atomizing medium flow) and the "Single-flow method"(crushing alloy liquid flow by other ways) . 846 the former is divided into gas (helium, fog, nitrogen, air) and liquid (water, oil) atomization medium, the latter such as centrifugal atomization and dissolved gas vacuum atomization.
The most widely used methods are gas atomization and water atomization. In the atomization process, the raw metal is melted into a qualified alloy liquid (overheated 100 ~ 150 ° C) in an electric or induction furnace, and then injected into a tundish located above the atomization nozzle. The alloy liquid flows out from the bottom hole of Tundish, and is atomized into small droplets when it meets with high-speed air or water flow through the nozzle. In general, inert gas atomized powder particles are round in shape with the lowest oxygen content (below L00 × 10) and can be made into densified products directly by thermoforming techniques such as hot isostatic pressing. Most of the water atomized powder particles have irregular shape, high oxygen content (above 600 × 10) and need to be annealed, but they have good compressibility and can be formed by cold pressing and then sintered into mechanical parts.
The above-mentioned atomization method is easy to be industrialized in large quantities, but because the alloy liquid is in contact with the slag and Refractory crucible, it is inevitable that non-metallic inclusions are introduced into the resulting powder. Therefore, according to the ESR principle, Soderfors Powder Company of Sweden first changed the Tundish with a capacity of 7 T into an ESR (electroslag heating) device, the content of non-metallic inclusions in the powder of high speed steel by nitrogen atomization was reduced to 1/10 of the original content, and the bending strength of ASP powder high speed steel was increased from 3500MPa to over 4000MPa.
The measure to avoid oxide contamination completely and effectively is to adopt“Single-flow” atomization method, for example, rotating electrode atomization method (see rotating electrode method) . In addition, there is a vacuum solution atomization method can also produce high-purity spherical powder. The principle is: when the gas supersaturated alloy liquid under pressure suddenly exposed to the vacuum, the dissolved gas will escape and expansion, causing alloy liquid atomization, and then condensed into powder. For the nickel, copper, cobalt, iron and aluminum matrix alloys, the method of dissolving hydrogen can be used to achieve the vacuum dissolved gas atomization powder