Preparation Method of Metal Powder

Powder metallurgy is a process of preparing metal powder and using metal (or metal and non-metal mixture) powder as raw material to obtain parts and products through molding and sintering. As the main raw material of industry, metal powder is widely used in the fields of machinery, metallurgy, chemical industry, and aerospace materials. Metal powder is the basic raw material of powder metallurgy industry, and its output and quality determine the development of powder metallurgy industry.

Metal powder is usually a collection of metal particles less than 1mm. There is no uniform regulation for the division of particle size range. The commonly used division method is: particles in 1000~50µm are conventional powders; 50~10µm are called fine powders; 10~0.5µm are called ultrafine powders; <0.5µm are called ultrafine powders; 0.1~100nm are called nano-scale powders. Each powder particle may be a crystal or composed of many crystals, depending on the particle size and preparation method.

Preparation methods of metal powders At present, there are dozens of methods for industrial production of powders, but in terms of the actual analysis of the production process, they are mainly divided into two categories: mechanical and physical methods and physical and chemical methods. They can be obtained by direct refinement of solid, liquid and gaseous metals, and can also be obtained from metal compounds in different states through reduction, pyrolysis and electrolysis. Carbides, nitrides, borides and silicides of refractory metals can generally be directly prepared by chemical combination or reduction-combination methods. Due to different preparation methods, the shape, structure and particle size of the same powder often vary greatly.

Mechanical and physical method

Ball milling method

Mechanism: Ball milling method is mainly divided into rolling ball method and vibration ball milling method. This method utilizes the mechanism that metal particles produce strain and break and refine at different strain rates.

Application: This method is mainly suitable for the preparation of powders such as Sb, Cr, Mn, Fe-Cr alloys, etc.

Advantages and disadvantages: The advantages are continuous operation, high production efficiency, suitable for dry grinding and wet grinding, and can be used for the preparation of powders of various metals and alloys. The disadvantage is that the material selectivity is not strong, and it is difficult to classify during the powder preparation process.

Grinding method

Mechanism: The grinding method is to spray compressed gas into the grinding area after passing through a special nozzle, thereby driving the materials in the grinding area to collide with each other and rub into powder; the airflow expands and rises with the material into the classification area, and the turbine classifier selects the material that reaches the particle size, and the remaining coarse powder returns to the grinding area for further grinding until the required particle size is separated.

Application: It is widely used in ultra-fine grinding in non-metallic, chemical raw materials, pigments, abrasives, health care drugs and other industries.

Advantages and disadvantages: Since the grinding method adopts dry production, the dehydration and drying of materials are omitted; its products have high purity, high activity, good dispersibility, fine particle size and narrow distribution, and smooth particle surface. However, the grinding method also has the disadvantages of high equipment manufacturing cost. In the production process of metal powder, continuous inert gas or nitrogen must be used as the compressed gas source, the gas consumption is large, and it is only suitable for the crushing and powdering of brittle metals and alloys.

Atomization method

Mechanism: Atomization method generally uses high-pressure gas, high-pressure liquid or high-speed rotating blades to break the metal or alloy melted by high temperature and high pressure into fine droplets, and then condense in the collector to obtain ultrafine metal powder. This process does not cause chemical changes. Atomization method is one of the main methods for producing metal and alloy powders. There are many methods of atomization, such as double-flow atomization, centrifugal atomization, multi-stage atomization, ultrasonic atomization technology, tightly coupled atomization technology, high-pressure gas atomization, laminar atomization, ultrasonic tightly coupled atomization and hot gas atomization.

Application: Atomization method is usually used in the production of metal powders such as Fe, Sn, Zn, Pb, Cu, and can also be used to produce alloy powders such as bronze, brass, carbon steel, and alloy steel. The atomization method meets the special requirements of metal powders for 3D printing consumables 

Advantages and Disadvantages: Atomized powder has the advantages of high sphericity, controllable powder particle size, low oxygen content, low production cost and adaptability to the production of a variety of metal powders. It has become the main development direction of high-performance and special alloy powder preparation technology, but the atomization method has the disadvantages of low production efficiency, low yield of ultrafine powder, and relatively large energy consumption.

Physical and chemical method

Reduction method

Mechanism: The reduction method is a method of reducing metal oxides or metal salts under certain conditions to produce metal or alloy powders. It is one of the most widely used powder making methods in production. Commonly used reducing agents include gas reducing agents (such as hydrogen, decomposed ammonia, converted natural gas, etc.), solid carbon reducing agents (such as charcoal, coke, anthracite, etc.) and metal reducing agents (such as calcium, magnesium, sodium, etc.). The most representative preparation method is the hydrogenation-dehydrogenation method with hydrogen as the reaction medium. It uses the easy hydrogenation property of the raw metal to make the metal react with hydrogen at a certain temperature to generate metal hydride, and then mechanically crush the obtained metal hydride into powder of the desired particle size, and then remove the hydrogen in the crushed metal hydride powder under vacuum conditions to obtain metal powder.

Application: Mainly used in the preparation of metal (alloy) powders such as Ti, Fe, W, Mo, Nb, W-Re, etc. For example, metal titanium (powder) begins to react violently with hydrogen at a certain temperature. When the hydrogen content is greater than 2.3%, the hydride is loose and easy to crush into fine particles of hydride titanium powder. The hydride titanium powder is decomposed at a temperature of about 700°C and most of the hydrogen dissolved in the titanium powder is removed to obtain titanium powder.

Advantages and Disadvantages: The advantages are simple operation, easy control of process parameters, high production efficiency, low cost, and suitable for industrial production; the disadvantage is that it is only suitable for metal materials that are easy to react with hydrogen and become brittle and easy to break after absorbing hydrogen.

Electrolysis method

Mechanism: The electrolysis method is a method of depositing metal powders at the cathode by electrolyzing molten salt or salt aqueous solution.

Application: Electrolysis of aqueous solution can produce metal (alloy) powders such as Cu, Ni, Fe, Ag, Sn, Fe-Ni, etc., and electrolysis of molten salt can produce metal powders such as Zr, Ta, Ti, Nb, etc.

Advantages and Disadvantages: Its advantage is that the purity of the prepared metal powder is high, and the purity of the general single substance powder can reach more than 99.7%; in addition, the electrolysis method can well control the particle size of the powder and can produce ultra-fine powder. However, the electrolysis method consumes a lot of electricity and has a high cost of powder production.

Hydroxyl method

Mechanism: Certain metals (iron, nickel, etc.) are synthesized with carbon monoxide to form metal carbonyl compounds, which are then thermally decomposed into metal powders and carbon monoxide.

Application: 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.

Advantages and Disadvantages: The powder obtained in this way is very fine and of high purity, but the cost is high.

Chemical Replacement Method

Mechanism: The chemical replacement method is to replace the less active metal from the metal salt solution with a more active metal according to the activity of the metal, and further refine the metal (metal powder) obtained by the replacement by other methods.

Application: This method is mainly used for the preparation of inactive metal powders such as Cu, Ag, and Au.