From ordinary machinery manufacturing to precision instruments, from hardware tools to large machinery, from the electronics industry to motor manufacturing, from civil industry to military industry, from general technology to cutting-edge high technology, powder metallurgy technology can be seen. In the field of civil industry, powder metallurgy products have become indispensable supporting basic parts for industries such as automobiles, motorcycles, home appliances, power tools, agricultural machinery, and office appliances.
The huge potential of the market is also driving technological progress. With the increasing application of powder metallurgy products, the requirements for the size, shape and performance of metal powder particles are getting higher and higher, and the performance and size and shape of metal powders depend to a large extent on the production method and preparation process of the powders. Therefore, the preparation technology of powders is also constantly developing and innovating. At present, many methods have been developed for the preparation of metal powders, which are mainly divided into physical and chemical methods and mechanical methods according to the production principle. The main methods in the mechanical method are atomization and mechanical crushing. The main methods in the physical and chemical method are reduction, electrolysis and hydroxyl method.
Reduction method
The metal is reduced to powder by using a reducing agent to capture oxygen in the metal oxide powder. Gaseous reducing agents include hydrogen, ammonia, coal gas, converted natural gas, etc. Solid reducing agents include carbon and metals such as sodium, calcium, and magnesium. Hydrogen or ammonia reduction is often used to produce metal powders such as tungsten, molybdenum, iron, copper, nickel, and cobalt. Carbon reduction is often used to produce iron powder. Using strong metal reducing agents such as sodium, magnesium, and calcium, tantalum, niobium, titanium, zirconium, vanadium, beryllium, thorium, and uranium can be produced. Metal powders (see metal thermal reduction) can be produced by reducing metal salt aqueous solutions with high-pressure hydrogen (see hydrometallurgy).
The powder particles produced by the reduction method are mostly irregular shapes with sponge structures. The powder particle size mainly depends on factors such as reduction temperature, time and particle size of raw materials. The reduction method can produce powders of most metals and is a widely used method.
Atomization method
The molten metal is atomized into fine droplets and solidified into powder in a cooling medium. The widely used two-stream (melt flow and high-speed fluid medium) atomization method uses high-pressure air, nitrogen, argon, etc. (gas atomization) and high-pressure water (water atomization) as injection media to break up the metal liquid flow. There are also centrifugal atomization methods that use rotating disk crushing and the rotation of the melt itself (consumable electrode and crucible), as well as other atomization methods such as hydrogen vacuum atomization and ultrasonic atomization. Due to the small droplets and good heat exchange conditions, the condensation rate of the droplets can generally reach 100-10000K/s, which is several orders of magnitude higher than that of ingot casting. Therefore, the alloy has uniform composition and fine structure. The alloy material made from it has no macroscopic segregation and excellent performance.
Gas atomized powder is generally nearly spherical, and water atomization can produce irregular shapes. The characteristics of powders, such as particle size, shape and crystal structure, mainly depend on the properties of the melt (viscosity, surface tension, superheat) and atomization process parameters (such as melt flow diameter, nozzle structure, pressure of the spray medium, flow rate, etc.). Almost all meltable metals can be produced by atomization, especially suitable for producing alloy powders. This method has high production efficiency and is easy to expand industrial scale. It is not only used for mass production of industrial iron, copper, aluminum powder and various alloy powders, but also for the production of high-purity (O2 <100ppm) high-temperature alloys, high-speed steel, stainless steel and titanium alloy powders. In addition, the use of quenching technology to produce fast condensing powders (condensation rate >100,000K/s) is increasingly valued. It can be used to produce high-performance microcrystalline materials (see fast cooling microcrystalline alloys).
Electrolysis method
When direct current is passed through a metal salt aqueous solution, metal ions are discharged and precipitated on the cathode to form a deposition layer that 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 factors that affect the powder particle size are mainly the composition of the electrolyte and the electrolysis conditions (see aqueous solution electrolysis).
Generally, electrolytic powders are mostly dendritic and have high purity, but this method consumes a lot of electricity and is costly. The application of electrolysis is also very wide, and it is often used to produce various metal powders such as copper, nickel, iron, silver, tin, lead, chromium, and manganese; alloy powders can also be produced under certain conditions. For rare and 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 produce powders.
Mechanical pulverization method
Mainly, solid metals are crushed into powders through crushing, crushing, and grinding. Equipment is divided into coarse crushing and fine crushing. The main crushing equipment includes crushers, roller mills, jaw crushers, and other coarse crushing equipment. The main crushing and grinding equipment includes hammer crushers, rod mills, ball mills, vibrating ball mills, stirring ball mills, and other fine crushing equipment.
Mechanical crushing is mainly suitable for crushing brittle and easily hardened metals and alloys, such as tin, manganese, chromium, high carbon iron, iron alloys, etc. It is also used to crush sponge metals obtained by reduction method and cathode deposits obtained by electrolysis; it is also used to crush titanium that becomes brittle after hydrogenation, and then dehydrogenate to produce fine titanium powder. Mechanical crushing is inefficient and energy-intensive, and is often used as a supplement to other powder making methods, or for mixing powders of different properties. In addition, mechanical crushing also includes vortex grinders, which rely on two impellers to create vortices, so that the particles entrained by the airflow collide with each other at high speed and crush, which can be used to crush plastic metals.
Cold flow crushing method
Use high-speed and high-pressure inert gas flow to carry coarse powder and spray it onto a metal target. Due to the adiabatic expansion of the airflow at the nozzle outlet, the temperature drops sharply to below 0°C, so that the coarse powder of metals and alloys with low temperature brittleness is crushed into fine powder. The mechanical alloying method uses a high-energy ball mill to grind different metals and high-melting point compounds into a solid solution or finely dispersed alloy state.
Carbonyl method
Certain metals (iron, nickel, etc.) are synthesized with carbon monoxide to form metal carbonyl compounds, which are then thermally decomposed into metal powder and carbon monoxide. The powder obtained in this way is very fine (particle size ranges from several hundred angstroms to several microns) and has high purity, but the cost is also high. It is mainly used in industry to produce fine and ultrafine powders of nickel and iron, as well as alloy powders such as Fe-Ni, Fe-Co, and Ni-Co.
Direct compounding method
Carbon, nitrogen, boron, and silicon are directly compounded with refractory metals at high temperatures. The reduction-combination method uses carbon, nitrogen, boron carbide, and silicon to react with refractory metal oxides. Both methods are commonly used methods for producing carbide, nitride, boride, and silicide powders.
Fine powders and ultrafine powders less than 10μm have a special position in the manufacture of materials (such as dispersion-strengthened alloys, ultra-microporous metals, and metal magnetic tapes) and direct applications (such as rocket solid fuels and magnetic fluid seals, magnetic inks, etc.) due to their uniform composition, fine grains, and high activity. In addition to the carbonyl method and electrolysis method, vacuum evaporation condensation method, arc spray, coprecipitation complex salt decomposition, gas phase reduction and other methods are also used to manufacture this type of powder. Coated powders are increasingly showing their excellence in special applications such as thermal spraying and atomic energy engineering materials. Various coated powders mixed with metals and metals, metals and non-metals can be produced by using two types of chemical powder making methods, gas phase and liquid phase deposition, such as hydrogen reduction thermal dissociation, high-pressure hydrogen reduction, replacement, and electrodeposition.