Preparation method of Powder Metal

Metal powder refers to a group of metal particles with a size of less than 1 mm. Including single metal powder, alloy powder and some refractory compound powder with metallic properties, it is the main raw material of powder metallurgy.
The simple substance of metal is generally silver-white. When the metal is under certain conditions, it is a black powder. Most metal powders are black.

It is usually divided into mechanical method and physical chemical method according to the principle of transformation. It can be obtained directly from solid, liquid, and gaseous metals, and can be transformed from metal compounds in different states through reduction, pyrolysis, and electrolysis. Preparation. Carbides, nitrides, borides, and silicides of refractory metals can generally be prepared directly by compounding or reduction-compounding methods. Due to different preparation methods, the shape, structure and particle size of the same powder often differ greatly (Figure 2). The powder preparation methods are listed as follows, among which the most widely used are reduction method, atomization method, and electrolysis method.
Reduction method
The reducing agent is used to deprive the oxygen in the metal oxide powder, so that the metal is reduced to powder. Gas reducing agents include hydrogen, ammonia, coal gas, and converted natural gas. Solid reducing agents include carbon and metals such as sodium, calcium and magnesium. Hydrogen or ammonia reduction is commonly used to produce metal powders such as tungsten, molybdenum, iron, copper, nickel, and cobalt. Carbon reduction is often used to produce iron powder. With strong metal reducing agents sodium, magnesium, calcium, etc., metal powders such as tantalum, niobium, titanium, zirconium, vanadium, beryllium, thorium, and uranium can be produced (see metal thermal reduction). Using high-pressure hydrogen to reduce metal salt aqueous solutions, nickel, copper, cobalt and their alloys or coated powders can be obtained (see hydrometallurgy). Most of the powder particles produced by the reduction method are irregular shapes with a sponge structure. The particle size of the powder mainly depends on factors such as the reduction temperature, time and the particle size of the raw materials. The reduction method can prepare most metal powders and is a widely used method.
Atomization
Figure 3 Atomization method
Figure 3 Atomization method
Figure 4 Two-stream atomization method
Figure 4 Two-stream atomization method
The molten metal is atomized into fine droplets, which solidify into powder in a cooling medium (Figure 3). Figure 4 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 the spray medium to break the metal liquid stream . There are also centrifugal atomization methods that use rotating disk pulverization and the rotation of the melt itself (consumable electrode and crucible), as well as other atomization methods such as hydrogen-dissolved vacuum atomization, ultrasonic atomization, etc. Due to the small droplets and good heat exchange conditions, the condensation speed of the droplets can generally reach 100～10000K/s, which is several orders of magnitude higher than that during ingot casting. Therefore, the composition of the alloy is uniform, the structure is fine, and the alloy material made with it has no macro-segregation and excellent performance. Gas atomized powder is generally nearly spherical, and water atomization can produce 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 (such as melt flow diameter, nozzle structure, spray medium pressure, flow rate, etc.) . Almost all metals that can be melted can be produced by the atomization method, which is especially suitable for the production of alloy powders. This method has high production efficiency and is easy to expand the industrial scale. It is not only used for mass production of industrial iron, copper, aluminum powder and various alloy powders, but also used to produce high-purity (O2<100ppm) high-temperature alloys, high-speed steel, stainless steel and titanium alloy powders. In addition, the use of chilling technology to produce fast-condensing powder (condensation rate>100,000K/s) has received increasing attention. It can be used to produce high-performance microcrystalline materials (see fast cooling microcrystalline alloys).
Electrolysis
When direct current is applied to the metal salt aqueous solution, the metal ions are discharged on the cathode to precipitate, forming a deposited layer that is easy to be broken 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 electric current. The main factors affecting the particle size of the powder are the composition of the electrolyte and the electrolysis conditions (see aqueous electrolysis). Generally, electrolytic powders are mostly dendritic and have higher purity, but this method consumes a lot of electricity and costs. The electrolysis method is also widely used, commonly used to produce copper, nickel, iron, silver, tin, lead, chromium, manganese and other metal powders; alloy powders can also be prepared under certain conditions. For rare refractory metals such as tantalum, niobium, titanium, zirconium, beryllium, thorium, uranium, composite molten salts are often used as electrolytes (see molten salt electrolysis) to prepare powders.
Mechanical pulverization
Mainly through crushing, crushing and grinding, the solid metal is broken into powder. There are two types of equipment: coarse crushing and fine crushing. The main crushing equipment is crushers, roller mills, jaw crushers and other coarse crushing equipment. The main functions of crushing and grinding are hammer crushers, rod mills, ball mills, vibrating ball mills, agitating ball mills and other fine crushing equipment. The mechanical crushing method is mainly suitable for crushing brittle and easy to work hardening metals and alloys, such as tin, manganese, chromium, high-carbon iron, ferroalloys, etc. It is also used for crushing sponge-like metals made by reduction and cathode deposits made by electrolysis. It is also used to break the brittle titanium after hydrogenation, and then dehydrogenate to prepare fine titanium powder. The mechanical pulverization method has low efficiency and high energy consumption. It is mostly used as a supplement to other powder milling methods or used to mix powders of different properties. In addition, the mechanical pulverization method also includes a vortex grinder, which relies on two impellers to create a vortex, so that the particles enclosed by the airflow collide with each other at a high speed and pulverize, which can be used for the crushing of plastic metals. The cold flow crushing method uses a high-speed and high-pressure inert gas stream 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 below 0°C, making the coarse metal and alloy powders with low temperature brittleness pulverized into fine powders. 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
Some metals (iron, nickel, etc.) and carbon monoxide are synthesized into metal carbonyl compounds, which are then thermally decomposed into metal powder and carbon monoxide. The powder prepared in this way is very fine (a few hundred angstroms to several micrometers in size), and has a high purity, but the cost is also high. Industrially, it is mainly used to produce fine and ultra-fine powders of nickel and iron, as well as alloy powders such as Fe-Ni, Fe-Co, and Ni-Co.
Direct legalization
Directly combine carbon, nitrogen, boron, and silicon with refractory metals at high temperatures. The reduction-chemical method uses carbon, nitrogen, boron carbide, silicon and refractory metal oxides. Both of these methods are commonly used to produce carbide, nitride, boride and silicide powders.
Other methods
Fine powders and ultrafine powders smaller than 10μm are used in manufacturing materials (such as dispersion-strengthened alloys, ultra-microporous metals, metal tapes) and direct applications (such as solid fuels for rockets and magnetic fluid seals) due to their uniform composition, fine grains, and high activity. , Magnetic ink, etc.) has a special status. In addition to the carbonyl method and electrolysis method, the production of this type of powder also uses methods such as vacuum evaporation condensation method, arc spray, co-precipitation double salt decomposition, and gas phase reduction.

Coated powders are increasingly showing superiority in special applications such as thermal spraying and nuclear engineering materials. Two types of chemical powder preparation methods, gas phase and liquid phase deposition, such as hydrogen reduction thermal dissociation, high pressure hydrogen reduction, replacement, electrodeposition, etc., can be used to prepare various coated powders mixed with metals and metals, and metals and non-metals.