Powder Metallurgy

Powder making method

Making powder is the first step in powder metallurgy.

Powder metallurgy materials and products continue to increase, and its quality continues to increase, requiring more and more types of powders.

For example, from the viewpoint of the material range, not only metal powder but also alloy powder, metal compound powder, etc.;

From the viewpoint of powder shape, it is required to use powders of various shapes, such as when a filter is produced, it is required to form a powder;

From the viewpoint of powder particle size, powders of various particle sizes are required, and the coarse powder has a particle size of 500 to 1000 μm, an ultrafine powder particle size of less than 0.5 μm, etc.

In order to meet the various requirements for powder, there are also various methods for producing powder.

These methods are nothing more than converting metal, alloy or metal compound into a powder state in solid, liquid or gaseous state.

Various methods for making powder

A method of converting metal and alloy or metal compound into powder in a solid state

Include:

(1) Mechanical pulverization and galvanic corrosion methods for making metal and alloy powders from solid metals and alloys:

(2) Reduction method for making metal and alloy powders from solid metal oxides and salts

Reduction-chemical treatment of metal compound powders from metal and alloy powders, metal oxides and non-metal powders.

The method of converting a metal to an alloy or a metal compound into the powder in a liquid state

Include:

(1) An atomization method for making alloy powder from liquid metal and alloy.

(2) A replacement method and a solution hydrogen reduction method for making and extracting metal alloy from metal salt solution;

A molten salt determination method for making metal powder by precipitation from molten metal salt;

A metal bath method for precipitating metal compound powder from an auxiliary metal bath.

(3) An aqueous solution electrolysis method for making metal and alloy powder by electrolysis from metal salt solution;

A molten salt electrolysis method for making metal and metal compound powders from metal molten salt electrolysis.

A method of converting metal or metal compound into powder in a gaseous state:

(1) A steam condensation method for making metal powder from metal vapor condensation;

(2) Carbon-based thermal dissociation method for dissociating gaseous metal carbon substrates from metals, alloys and coated powders

(3) A gas phase hydrogen reduction method for obtaining metal, alloy powder, and metal or alloy coating from gas phase reduction of gaseous metal halide;

A chemical vapor deposition method is used to make metal compound powder and coating from a gaseous metal halide deposition.

However, from the essence of the process, the existing milling methods can be broadly classified into two major categories, mechanical and physical chemical methods.

The mechanical method is a process in which the raw material is mechanically pulverized, and the chemical composition is substantially unchanged;

Physical chemical method is a process of obtaining powder by changing the chemical composition or aggregation state of raw materials by chemical or physical action.

The production method of powder is mostly from the industrial scale, the most widely used Hans reduction method, atomization method and electrolysis method.

Some methods such as vapor deposition and liquid deposition are also important in special applications.

The basic processes of the powder metallurgy technique

  1. Making of raw material powder.

Existing milling methods can be broadly classified into two categories: mechanical methods and physical chemical methods.

The mechanical method can be divided into: mechanical pulverization and atomization;

The physical and chemical law is further divided into:

  • Electrochemical corrosion
  • Reduction method
  • Chemical combination method
  • Restore-chemical method
  • Vapor deposition method
  • Liquid deposition method
  • Electrolysis method

Among them, the most widely used methods are reduction, atomization and electrolysis.

  1. The powder is formed into a compact of the desired shape.

The purpose of the forming is to produce a compact of a certain shape and size and to have a certain density and strength.

The molding method is basically divided into press molding and pressureless molding.

The most widely used in press molding is compression molding.

In addition, 3D printing technology can also be used for the production of embryo fragments.

  1. Sintering of the compact.

Sintering is a key process in powder metallurgy processes.

The formed green compact is sintered to obtain the desired final physical and mechanical properties.

Sintering is further divided into unit system sintering and multi-component sintering.

For solid phase sintering of unit and multicomponent systems, the sintering temperature is lower than the melting point of the metals and alloys used;

For multi-phase liquid phase sintering, the sintering temperature is generally lower than the melting point of the refractory component and higher than the melting point of the fusible component.

In addition to ordinary sintering, there are special sintering processes such as loose sintering, melt immersion, and hot pressing.

  1. The subsequent processing of the product.

The post-sintering treatment can be carried out in various ways depending on the requirements of the product.

Such as finishing, oil immersion, machining, heat treatment and plating.

In addition, in recent years, some new processes such as rolling and forging have also been applied to the processing of powder metallurgy materials after sintering, and have achieved satisfactory results.

Property of powder

A general term for all properties of powder.

It includes:

Geometric properties of the powder

  • Granularity
  • Specific surface
  • Aperture
  • Shape

Chemical properties of the powder

  • Chemical composition
  • Purity
  • Oxygen content
  • Acid insoluble matter

Mechanical properties of powder

  • Bulk Density
  • Fluidity
  • Formability
  • Compressibility
  • Stacking angle
  • Shear angle

Physical properties and surface properties of the powder

  • True density
  • Luster
  • Absorbing
  • Surface activity
  • Magnetic

Powder properties often determine the performance of powder metallurgy products to a large extent.

Geometric properties of the powder

The most basic geometric performance is the particle size and shape of the powder.

(1) Particle size.

It affects the processing of the powder, the shrinkage during sintering, and the final properties of the product.

The performance of certain powder metallurgical articles is almost directly related to the particle size.

For example, the filtration accuracy of the filter material can be empirically determined by dividing the average particle size of the original powder particles by 10.

The properties of cemented carbide products are strongly related to the grain size of the wc phase.

To obtain a finer grain size carbide, it is only possible to use a finer particle size wc material.

Powders used in production practice range in size from a few hundred nanometers to hundreds of microns.

The smaller the particle size, the greater the activity and the easier the surface is to oxidize and absorb water.

When it is as small as a few hundred nanometers, the storage and transport of the powder is not easy.

Moreover, when the quantum effect begins to function to a certain extent, the physical properties will change greatly.

For example, the ferromagnetic powder will become superparamagnetic powder, and the melting point will decrease as the particle size decreases.

(2) Particle shape

It depends on the milling method, such as the powder obtained by electrolysis, the particles are dendritic;

The iron powder particles obtained by the reduction method are in the form of sponge sheets;

The gas atomization method is basically a spherical powder.

In addition, some powders are ovoid, disc-shaped, needle-shaped, onion-like, etc.

The shape of the powder particles affects the fluidity and bulk density of the powder.

Due to the mechanical meshing between the particles, the strength of the green compact of the irregular powder is also large, and in particular, the dendritic powder has the highest strength of the pressed compact.

However, for porous materials, it is best to use spherical powder.

Mechanical properties

The mechanical properties of the powder are the process properties of the powder.

It is an important process parameter in the powder metallurgy forming process.

The bulk density of the powder is the basis for weighing by volume method during pressing;

The fluidity of the powder determines the filling speed of the powder to the stamper and the production capacity of the press;

The compressibility of the powder determines the difficulty of the pressing process and the level of pressure applied;

The formability of the powder determines the strength of the blank.

The chemical properties mainly depend on the chemical purity of the raw materials and the milling method.

Higher oxygen levels reduce the compression properties, green strength and mechanical properties of the sintered product.

Therefore, most of the technical conditions of powder metallurgy have certain provisions for this.

For example, the powder has an allowable oxygen content of 0.2% to 1.5%. This corresponds to an oxide content of 1% to 10%.