The physical properties of metal powder are mainly reflected in the following aspects:
- 1.Theparticle size and composition
The particle size and composition of the metal powder depends on the preparation conditions of the metal powder, and it has a great influence on the behavior of the metal powder during pressing and sintering as well as the properties of the product.
Using particle diameters to characterize metal powder particle sizes is only accurate for ideal spherical metal powders, while metal shapes for other shapes can only be approximated. The metal powder used in the metal powder metallurgy porous material has a particle size mainly between several micrometers and 500 micrometers.
The shape of the metal powder particles is an important indicator of the properties of the metal powder, which has a great influence on the process performance of the metal powder. The strength, permeability, and uniformity of properties (isotropy) of the article are related to the shape of the metal powder particles.
Both spherical metal powder and non-spherical metal powder can be used to produce a porous material, but it is desirable that the metal powder be spherical in order to improve the pore uniformity and permeability of the article. For the convenience of describing the spherical metal powder, we cite a characteristic coefficient such as the ratio of the major axis to the minor axis of the spherical metal powder, and consider the metal powder having a coefficient of less than 1.2 as a spherical metal powder.
For complex shaped metal powders, this can be represented by deviation from the equivalent volume of spheres or by the length of the particles: the ratio of width to thickness. In the production of a highly permeable porous material, the sphericity of the metal powder (i.e., the percentage of the number of spherical metal powder particles to the total number of metal powders) is required to be 60% or more.
- 3.Metal powder specific surface
Most reactions start on the surface of the particles, so the ratio of the surface area of the metal powder particles to their volume or weight - specific surface is one of the important parameters in the metal powder metallurgy process, which directly affects the pressing properties and sintering properties of the metal powder.
For most metal powders, the specific surface value can range from 0.01 square meters per gram to tens of square meters per gram. The specific surface of the metal powder depends not only on the particle size and shape of the metal powder, but also on the surface state of the particles (or the degree of development of the surface).
The finer the particle size of the metal powder, the more complicated the shape, and the rougher the surface, the larger the specific surface of the metal powder; on the contrary, the coarser the particle size of the metal powder, the more regular the shape (such as a sphere), the smoother the surface (no unevenness) of metal powder, the smaller the specific surface. The particle size, shape and surface state of the metal powder are determined by the conditions and methods for preparing the metal powder.
- 4.True density and microhardness of metal powder
The density of the metal powder particles is usually smaller than the theoretical density of the raw materials from which it is produced because many of the metal powders produced by the method have considerable internal pores and a large number of lattice vacancies. The true density of the so-called metal powder refers to the density of the metal powder, including only the closed pores inside the particles. The true density of the metal powder varies with the method of milling, and the true density of the metal powder is also strongly dependent on the oxide content.
The microhardness of metal powder particles is an indicator of the plasticity of metal powder. The microhardness value depends to a large extent on the content of various impurities and alloying elements in the matrix metal, and is related to the degree of lattice twist. When preparing a porous material, the metal powder has a certain hardness, which is beneficial for ensuring high permeability of the product. Therefore, for a plastically good metal (such as sodium titanium powder), in order to achieve a certain hardness value, it is often performed before pressing.
- 5.Lattice state of metal powder
The metal powder particles are usually composed of crystal grains of various sizes, and the size and orientation of the crystal grains also depend on the manufacturing method of the metal powder. In many cases, there is a correlation between metal powder particle size and grain size. In the process of atomizing the metal powder, the droplets are cooled from the molten liquid phase, and the smaller particles are cooled rapidly, so that the crystal grains of the atomized metal powder fine particles are usually smaller than the coarse particles.
In general, metal powders are produced under non-equilibrium conditions. Therefore, the metal powders obtained by various methods have crystal defects to varying degrees. For example, in the reduction of oxides, the crystal structure of the oxide is converted into a metal crystal structure, but in practice this transformation is incomplete; the atomized metal powder is crystallized from the liquid state quickly, and oxides are present, of course, there may be lattice defects.