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Introduction to Powder Metallurgy
1. Powder composition and performance
The aggregate of scattered particles with size less than 1mm is often referred to as powder and it is normally measured in units of μm or nm.
(1) Chemical composition and performance of powder
Common metal powders include: iron, aluminum, copper and their alloys. The content of impurity and gas must not exceed 1%~2%; otherwise the quality of product will be affected.

2. Physical performance of powder
(1) Granular shape
The shape of powder is granular and such shape varies according to different methods of powder production. These include: irregular, sliced, multi-dimensional, spherical, water drop and fiber, shapes. The granular shape has a great influence on density, fluidity and forming ability, which is one of the major performances of the powder.
(2) Particle and its distribution
The smallest substance that can be separated and exist independently in powder is the single particle. The actual powder is often the aggregated particles, i.e. the secondary particle. In actual powder particles, the ratio for different sizes of particles within overall mass is referred to as the particle distribution.

3. Craftsman performance of powder
The craftsman performance of powder includes: fluidity, filling, compression and forming ability.
(1) Filling ability
This refers to the tightness for powders piling freely without any influence of external conditions, and which is often expressed as loose and piled density. The filling ability of powder relates directly to the size, shape and surface property of the particles.
(2) Fluidity
This refers to the flowing ability of powder and it is often expressed as time required for 50g powder to flow through a standard funnel; the shorter the time, the better the fluidity. According to the regulation, allow the powder gently flowing into a container with given volume and scrape the container flat after it is full to derive the powder’s mass; the loose density of powder will be derived by dividing mass with volume of container. An arch bridge will form between the powders due to friction and it will collapse with further vibration. The fine powders will then fill up space between coarse particles and reduce volume occupied by the overall powders. The density tested in such manner is referred to as the tap density.
The loose density of powder is directly related to the die design; particularly, the variation of loose density during automatic press directly affects the weight of the compressed piece.
The powder’s category, particle and particle distribution, shape, loose density, and moving method of particles, are extensively related to fluidity; furthermore, fluidity is also affected by the reaction of particle attachment.
(3) Compression ability
This refers to the density of compact specimen achieved by the compacting ability of powder during compression with unit pressure regulated, which is tested in the standard die under regulated lubrication condition. The factors affecting powder compression include: plasticity of particle, micron hardness and better compression ability of plastic metal powder compared to hard and brittle material. The shape and structure of particles will also affect the powder’s compression ability.
(4) Forming ability
This means the compact specimen’s ability to maintain its given shape after power compression.

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Overview of powder metallurgy process

  • Powder
  • In addition to the powder's properties, such as: particle, particle distribution, particle shape and chemical property, the factors related to production include: density, fluidity, compression ability and raw specimen strength.
  • Mixing
  • This is the aggregate made by mixing even proportions of raw or alloy particles with lubrication agent or other additives.
  • Forming
  • Fill mixed raw material into the die and compress it into a formation of compact specimen. The current forming machine ranges from 0.5 to 1000 tons.
  • Inside of the specimen after forming (bronze material)





  • Sintering
  • Place the specimen in the sintering furnace and allow bonding of particles to reach the expected mechanical strength, which is the core technology within processes of powder metallurgy.
  • Inside diameter of specimen after sintering

 

  • Coining
  • After sintering, the specimen induces contraction and expansion. In order to reach coarseness, precision or density and strength, the specimen must be placed in the compression die for further coining or precision.
  • Inside diameter of specimen after coining
 

  • Wash
  • All specimens must be washed after coining for removal of attached powder and oil stains.
  • Oil Impregnation
  • The bearing must be provided with vacuum oil impregnation at the end to effect self lubrication.
  • Heat Treatment
  • Parts of powder metallurgy can also be annealed, quenched, tempered and surface hardened; for example, hardness of both carburizing process and high-frequency induction process are tested with HV and HRA.
 
 
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