Classification and basic principle of preparation methods of MIM metal powder

Classification and basic principle of preparation methods of MIM metal powder

 

Preparation method of metal powder:

1.1 Physicochemical method:

1.1.1 Reduction method:

The reduction of metal oxides and salts is a widely used method for powder preparation. Iron powder and tungsten powder can be reduced by solid carbon, and tungsten, molybdenum, iron, copper, cobalt, nickel and other powders can be prepared by hydrogen or ammonia decomposition;

Iron powder can be prepared from converted natural gas and coal gas, and rare metal powder such as tantalum, niobium, titanium, zirconium, thorium and uranium can be prepared from sodium, calcium and magnesium as reducing agents. The basic principle of the reduction method of metal oxides and salts is that the affinity of the reducing agent used for oxygen is greater than that of the corresponding metal in the oxides and salts used for oxygen, so that the oxygen in the metal oxides or salts can be captured and the metal can be reduced. Because different metal elements have different effects on oxygen, the stability of oxide formation is not the same. The stability of the oxide can be characterized by the size of △ G in the oxidation reaction process. If the △ G value in the reaction process is smaller, it means that the stability of its oxide is higher, that is, its affinity for oxygen is greater.

Its 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 applicable to metal materials that are easy to react with hydrogen and become brittle and fragile after hydrogen absorption.

1.1.2 Metal thermal reduction and reduction method:

Metal thermal reduction is that the reduced raw material can be solid, gaseous or molten salt. The latter two have the characteristics of gas phase reduction and liquid phase precipitation. The metal thermal reducing agent method is commonly used in industry: reducing TiO2, ThO2, UO2, etc. with calcium; Reduce TiCl4, ZrCl4, TaCl5, etc. with magnesium; Reduce TiCl4, ZrCl4, K2ZrF6, K2TaF7, etc. with sodium; Nickel chromium stainless steel powder was prepared by co-reduction of chromium oxide and nickel oxide with calcium hydride (CaH2).

Reduction method refers to the reaction of carbon, boron carbide, silicon, nitrogen and refractory metal oxides to obtain carbides and borides. Nitride method.

1.1.3 Electrolysis method:

Electrolysis is a method of making metal powder precipitate at the cathode by electrolytic molten salt or salt aqueous solution. Almost all metal powders can be produced by electrolysis, especially copper powder, silver powder and tin powder. Electrolytic pulverization can also be divided into aqueous solution electrolysis, organic electrolyte electrolysis, molten salt electrolysis and liquid metal cathode electrolysis.

The advantages are that the purity of the metal powder prepared is high, and the purity of the general elemental powder can reach more than 99.7%; In addition, the electrolytic method can well control the particle size of the powder and can produce fine powder. However, the power consumption of electrolytic powder production is large, and the cost of powder production is high. Electrolytic aqueous solution can produce metal (alloy) powders such as Cu, Ni, Fe, Ag, Sn, Fe-Ni, and electrolytic molten salt can produce metal powders such as Zr, Ta, Ti, Nb.

1.1.4 Hydroxyl method:

Some metals (iron, nickel, etc.) and carbon monoxide are synthesized into metal carbonyl compounds, which are decomposed into metal powder and carbon monoxide by reheating. The powder thus prepared is very fine, with high purity, but high cost. In industry, it is mainly used to produce fine and ultra-fine powders of nickel and iron, as well as Fe-Ni, Fe-Co, Ni-Co and other alloy powders.

1.1.5 Chemical replacement method:

The chemical replacement method is based on the activity of the metal. The metal with strong activity is used to replace the metal with less activity from the metal salt solution, and the metal (metal powder) obtained from the replacement is further refined by other methods. This method is mainly applied to the preparation of inactive metal powders such as Cu, Ag, Au, etc.

1.2 Mechanical method:

1.2.1 Atomization method:

The atomization method belongs to the mechanical pulverization method. It is a method of directly crushing liquid metal or alloy and worth powder. It is widely used, and its scale is only second to the reduction method. The atomization method, also known as spray method, can be used to produce metal powders such as lead, tin, aluminum, copper, nickel and iron, and can also be used to produce alloy powders such as bronze, brass, carbon steel and alloy steel.

The atomization method generally uses high-pressure gas, high-pressure liquid or high-speed rotating blades to break the metal or alloy melted at high temperature and pressure into small droplets, and then condense in the collector to obtain ultra-fine metal powder. This process does not have chemical changes. Atomization is one of the main methods of producing metal and alloy powder. There are many methods of atomization, such as dual-flow atomization, centrifugal atomization, multistage atomization, ultrasonic atomization technology, tight coupling atomization technology, high-pressure gas atomization, laminar flow atomization, ultrasonic tight coupling atomization and hot gas atomization.

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

1.2.2 Mechanical crushing method:

The mechanical pulverization of solid metal is an independent pulverizing method, and its mechanism development is closely related to the state of solid strain and the formation and expansion of cracks in pulverization. At the same time, it is also an indispensable supplementary process for some milling methods. For example, hard and brittle cathode deposits obtained by grinding electrolysis, spongy metal blocks obtained by grinding reduction, etc. Therefore, mechanical crushing method plays an important role in powder production.

Due to the different properties of materials and the required grinding fineness, the grinding methods are also different. According to the different ways of applying external force, material crushing is generally carried out by extrusion, impact, grinding and splitting, and the working principles of various crushing equipment are mainly based on these principles.

Among them, ball milling method is mainly divided into rolling ball method and vibration ball milling method. This method utilizes the mechanism that metal particles are broken and refined due to deformation at different strain rates. It has the advantages of low selectivity for materials, continuous operation, high production efficiency, suitable for dry and wet grinding, and can be used for powder preparation of various metals and alloys. The disadvantage is that it is difficult to grade in the process of powder preparation.

1.2.3 Grinding method:

The grinding method is to spray compressed gas into the grinding area after passing through a special nozzle, thus driving the materials in the grinding area to collide with each other and rub into powder; After the air expansion, the material will rise into the classification area, and the material with the required particle size will be separated by the turbine classifier. The remaining coarse powder will return to the grinding area and continue grinding until the required particle size is separated. Because the grinding method is produced by dry method, the dehydration, drying and other processes of materials are omitted; Its product has high purity, high activity, good dispersibility, fine particle size and narrow distribution, and smooth particle surface. It is widely used in the ultra-fine grinding of nonmetals, chemical raw materials, pigments, abrasives, health care drugs and other industries. 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 crushing and pulverizing brittle metals and alloys.