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Knowledge of powder metallurgy (PM) process and sintering(1)

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 Knowledge of powder metallurgy (PM) process and sintering(1)

4.1 Powder metallurgy process

Powder metallurgy is a method of making various kinds of parts and products by using metal powder (or a mixture of metal and non-metal powder) as raw materials and sintering after pressing.

Features of powder metallurgy:

1) To be able to produce products that would otherwise be impossible or difficult to manufacture. Can make refractory, extremely hard and special properties of materials,

2) High utilization rate of materials, close to 100%.

3) Powder metallurgy is more economical.

4) Powder metallurgy is suitable for mass production.

The production process of powder metallurgy is as follows:

4.1.Preparation and properties of metal powder

(1) preparation of metal powder

There are many ways to produce metal powder, including mineral reduction, atomization, mechanical pulverization, etc.

1) Powder mineral reduction method is to obtain metal materials of certain shape and size after metal ores are reduced under certain metallurgical conditions, and then metal materials are crushed to obtain powder.

Mineral reduction method is mainly applied to production of iron powder. The purity of iron powder is directly related to purity of iron ore. In addition to iron powder, cobalt, molybdenum and calcium can be produced by mineral reduction. For example, refractory powders of metal compounds such as carbides, borides, and silicides are obtained by combination or chemical replacement of metal oxide powders with carbon, boron, or silicon powders.

2) Electrolysis is the use of aqueous solution of metal salt electrolysis precipitation or molten salt electrolysis precipitation of metal particles or spongy metal blocks, and then mechanical method to crush.

3) Atomization method is to atomize molten metal liquid through jet air (air or inert gas), mechanical force of water vapor or water and quench effect, so as to obtain metal powder.

Due to high purity and alloying of atomized powder, powder has its characteristics, high yield and low cost, so its application has developed rapidly. It can be used to produce powders of iron, steel, lead, aluminum, zinc, copper and its alloys.

4) Most commonly used method of mechanical crushing is ball grinding of steel or cemented carbide blocks or granular raw materials, which is suitable for preparing some brittle metal powder or metal powder treated by brittleness (such as titanium powder brittle by hydrogenation).

(2) Characteristics of metal powders

The characteristics of metal powder have great influence on pressing, sintering process, pre-sintering strength and performance of final product. Basic properties of metal powders include chemical composition, particle size distribution, particle shape and size, and technical characteristics.

1)Chemical composition of a powder usually refers to the content of major metals or components, impurities and gases. The content of main metals in metal powder is not less than 98% - 99%, which can meet the requirements of sintering machinery parts. However, when making high performance powder metallurgy materials, a higher purity powder is needed.

The most common inclusions in metal powders are oxides. Oxide makes compressibility of metal powder bad and increases wear of die. Sometimes, a small amount of reducible metal oxides is beneficial to sintering metal powders. As metal powders are large in surface and small in volume, a large amount of gases are adsorbed on metal powders surface. During preparation of metal powder, a lot of gases will be dissolved in it. Main gases in metal powder are oxygen, hydrogen, carbon monoxide and nitrogen, which increase brittleness of metal powder and make compactness worse, especially plasticity of some refractory metals and compounds (such as Ti, Zr, Cr, carbides, borides and silicides) worse. During heating, gas precipitates strongly, which may affect normal shrinkage of green part during sintering. Therefore, some metal powders are often degassed by vacuum.

2) Particle shape and size are one of factors that affect technical characteristics of powder (such as loose packing density and fluidity, etc.).Usually, powder grains are spherical or granular. Industrial powder size is usually in 0.1-500 micron, 150 micron or more as coarse powder, 40-150 micron as medium powder, 10-40 micron as fine powder, 0.5-10 micron as very fine powder, 0.5 micron or less called ultrafine powder.

Powder size directly affects performance of powder metallurgy products, especially for cemented carbide and ceramic materials, the finer powder the better. But making fine powder is difficult and economical.

3) Particle size distribution refers to relative content of different particle sizes, also known as particle size composition. Wide range composition of powder size, product density is high, with good performance, especially benefit for product edge strength

4) Forming technical features of powder mainly include: 

(1) Loose packing density is also called loose packing ratio, refers to unit volume of free loose packing powder quality. It is determined by powder size, particle shape, particle size composition and intergranular pore size. Loose-load ratio affects properties of pressing and sintering, and is a very important parameter for die design.

(2) Fluidity refers to time for 50g powder drop freely in powder flow meter until flow finished. The shorter the time, the better the liquidity. Powder with good fluidity is conducive to rapid and continuous powder loading and uniform powder loading of complex parts.

(3) Powder compressibility and formability. Compressibility determines strength and density of green part. It is usually expressed by powder’s compression ratio before and after pressing. Compressibility of powder is mainly determined by hardness, plastic deformation and work hardening. Powder after annealing has good compressibility. In order to ensure green part’s quality, make it have certain strength, and facilitate transportation in production process, powder should have good formability. The formability is related to physical properties of powder, and is also affected by particle size, particle shape and particle size composition. In order to improve formability, a small amount of lubricant such as zinc stearate, paraffin, rubber and so on is often added into powder. Usually bending strength or compressive strength of green part is used as formability test index.

 


Keywords in the article: Powder metallurgy (PM)
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