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.
The preparation and application of metal powder has a long history. In ancient times, gold, silver, copper, bronze and some oxide powders were used as paints for the coloring and decoration of pottery, jewelry and other utensils. At the beginning of the 20th century, American W.D. Coolidge used hydrogen to reduce tungsten oxide to produce tungsten powder to make tungsten wire, which was the beginning of modern metal powder production. Since then, a variety of powders such as copper, cobalt, nickel, iron, tungsten carbide have been prepared by chemical reduction, which promoted the development of early powder metallurgy products (oil-containing porous bearings, porous filters, cemented carbide, etc.); at this time, they also invented Carbonyl method to prepare iron powder and nickel powder. In the 1930s, iron powder was first produced by the eddy current grinding method, and then the iron powder was produced by the solid carbon reduction method, at a very low cost. The molten metal atomization method also appeared in the early 1930s. This method was originally used to prepare low-melting-point metals such as tin, lead, aluminum and other powders, but in the early 1940s it developed into the use of high-pressure air atomization to prepare iron powder. In the 1950s, high-pressure water atomization was used to prepare alloy steel and various alloy powders. 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, a variety of gas-phase and liquid-phase physical and chemical reaction methods have emerged to prepare coated powders and ultra-fine powders with important uses.
Metal powder is a loose substance, and its performance comprehensively reflects the nature of the metal itself, the properties of individual particles and the characteristics of particle groups. Generally, the properties of metal powders are divided into chemical properties, physical properties and process properties. Chemical properties refer to metal content and impurity content. The physical properties include the average particle size and particle size distribution of the powder, the specific surface and true density of the powder, the shape, surface morphology and internal microstructure of the particles. Process performance is a comprehensive performance, including powder fluidity, loose packing density, tap density, compressibility, formability and sintering dimensional changes. In addition, for some special applications, powders are required to have other chemical and physical properties, such as catalytic properties, electrochemical activity, corrosion resistance, electromagnetic properties, internal friction coefficient, etc. The performance of the metal powder depends to a large extent on the production method of the powder and its preparation process. The basic properties of the powder can be determined by specific standard testing methods. There are many methods for determining the particle size and distribution of powder, generally using sieve analysis (>44μm), sedimentation analysis (0.5-100μm), gas permeation method, microscopy, etc. Ultrafine powder (<0.5μm) was measured by electron microscope and X-ray small-angle scattering method. Metal powder is conventionally divided into five grades: coarse powder, medium powder, fine powder, fine powder and superfine powder.
