A metal powder is a group of metal particles smaller than 1 mm in size. Including single metal powder, alloy powder and some refractory compound powder with metal properties, it is the main raw material of powder metallurgy.
The preparation and application of metal powder have a long history. In ancient times, gold, silver, copper, bronze and some oxide powder were used as coatings for the coloring and decoration of pottery, jewelry and other utensils.
At the beginning of the 20th century, American Cu Li Ji (W.D.Coolidge) produced tungsten powder by hydrogen reduction tungsten oxide to produce tungsten wire, which was the beginning of modern metal powder production. After that, copper, cobalt, nickel, iron, tungsten carbide and other powders were obtained by chemical reduction method, which promoted the development of early powder metallurgy products (oil porous bearings, porous filters, cemented carbides, etc.). At this time, the carbonyl method was also invented to produce iron powder and nickel powder.
In the 1930s, iron powder was prepared by eddy current grinding and then by solid carbon reduction. In the early 1930s, molten metal atomization also appeared. This method was first used to produce low melting point metals such as tin, lead and aluminum, and developed into high pressure air atomization to produce iron powder in the early 1940s. Alloy steel and various alloy powders were prepared by high pressure water atomization in the 1950s. In the 1960s, a variety of atomization methods were developed to produce high alloy powder, which promoted the development of high performance powder metallurgy products. Since the 1970s, many kinds of gas-phase and liquid-phase physical and chemical reaction methods have appeared to produce coated powder and ultrafine powder with important applications.
The basic properties of the powder can be determined by a specific standard detection method. There are many methods to determine the particle size and distribution of powder, such as sieve analysis (>44μm), sedimentation analysis (0.5~100μm), gas transmission method, microscope method and so on. Ultrafine powders (<0.5μm) were determined by electron microscopy and X ray small-angle scattering. Metal powder is divided into five grades: coarse powder, medium powder, fine powder, fine powder and ultrafine powder.
In the solution of metal brine, the metal ions are discharged and precipitated on the cathode to form a deposition layer which is easy to break into powder. Metal ions generally come from the dissolution of the same metal anode and migrate from the anode to the cathode under the action of current. The main factors affecting powder particle size are electrolyte composition and electrolysis conditions (see aqueous electrolysis). In general, the electrolytic powder is dendritic and has high purity, but this method has high power consumption and high cost.Electrolysis is also widely used to produce copper, nickel, iron, silver, tin, lead, chromium, manganese and other metal powders. For rare refractory metals such as tantalum, niobium, titanium, zirconium, beryllium, thorium and uranium, composite molten salts are often used as electrolytes (see molten salt electrolysis) to prepare powders.
Some metals (iron, nickel, etc.) were synthesized into metal carbonyl compounds with carbon monoxide, which were then thermally decomposed into metal powders and carbon monoxide. The resulting powder is fine (several hundred angstroms to several microns in size), high in purity, but also expensive. The industry is mainly used to produce nickel and iron fine powder and ultrafine powder, as well as Fe-Ni、Fe-Co、Ni-Co alloy powder.
Atomizing the molten metal into tiny droplets, solidified into powder in a cooling medium (Fig .3). Fig .4 The widely used two-stream (melt flow and high-speed fluid medium) atomization method is to use high pressure air, nitrogen, argon, etc. (gas atomization) and high pressure water (water atomization) as injection medium to crush metal liquid flow. Centrifugal atomization using rotary disc crushing and melt self-rotation (self-consuming electrode and crucible), and other atomization methods such as hydrogen dissolved vacuum atomization, ultrasonic atomization, etc. Because of the fine droplets and good heat exchange conditions, The condensation velocity of droplets can reach 100~10000 K/s, in general, several orders of magnitude higher than the ingot. So the composition of the alloy is uniform, Small tissue, The alloy material made of it has no macroscopic segregation, Excellent performance. Aerosolized powders are generally nearly spherical, Water atomization can make irregular shapes. the characteristics of the powder, such as particle size, shape and crystalline structure, mainly depend on the properties of the melt (viscosity, surface tension, superheat) and atomization process parameters (e.g., melt flow diameter, nozzle structure, the pressure of the injection medium, flow rate, etc). Almost all molten metals can be produced by atomizing, Especially suitable for the production of alloy powder. This method is efficient, And easy to expand the scale of the industry. Not only for the mass production of industrial iron, copper, aluminum powder and various alloy powders, They are also used to produce superalloy, high speed steel, stainless steel and titanium alloy powders with high purity (O2<100 ppm. In addition, Fast condensing powder (condensing speed >100, 000K/s) increasing attention. It can be used to produce high performance microcrystalline materials (see fast cold microcrystalline alloys).
The oxygen in the metal oxide powder is captured by a reducing agent, and the metal is reduced to powder. Gas reductants include hydrogen, ammonia, gas, converted natural gas and so on. Solid reductants have carbon and sodium, calcium, magnesium and other metals. Hydrogen or ammonia reduction is often used to produce tungsten, molybdenum, iron, copper, nickel, cobalt and other metal powders. Carbon reduction is often used to produce iron powder. metal powders such as tantalum, niobium, titanium, zirconium, vanadium, beryllium, thorium, uranium can be produced with metal strong reducing agents such as sodium, magnesium, calcium (see metal thermal reduction). nickel, copper, cobalt and their alloys or coated powders can be obtained by reducing metal salt aqueous solution with high pressure hydrogen (see wet metallurgy). Most of the powder particles formed by reduction are irregular shapes of sponge structure. The particle size of powder mainly depends on the factors such as reduction temperature, time and particle size of raw material. The reduction method can produce most metal powders, which is a widely used method
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