Steps in powder metallurgy

Steps in powder metallurgy

The major steps of powder metallurgy method for
the fabrication of MMCs are as follows: 

(i) powder production,
(ii) powder
mixing or blending,
(iii) compaction,
(iii) sintering, 
(iv) other secondary
finishing operations, and 
(v) characterization. 

Flow chart of powder metallurgy
route for MMC fabrication is shown in Figure

Flow chart of powder metallurgy process for the fabrication of MMCs
Flow
chart of powder metallurgy process for the fabrication of MMCs

Production of metal powder

Production of metal
powder is the important stage of powder metallurgy process. There are few
important powder characteristics such as shape, size and surface area which
plays the vital role on the final properties of the finished products prepared
by using powder metallurgy process, due to this reasonsingle powder production method
for powder metallurgy cannot be used for all the applications. Methods for
powder production are broadly classified as mechanical, chemical or physical
methods based on the nature of energy used for the metal production.

Annealing of powder

Before mixing of powder step, annealing
is essential to heat treat the powder. Annealing the powder helps to soften the particles surface. This process is
carried out to remove the oxygen content which may be present after the powder
production. Another advantage of annealing to relive
stress and also to remove thepresence of
oxide layer if present. However, most importantly annealing of the metal powder
enhances the compressibility during compaction and green strength of compact
due to asoftened surface layer of metal
particles.

Mixing of selected
powder

Ball milling is one of the important
steps in powder metallurgy process. It is carried out to disperse the
reinforced particles uniformly throughout the matrix, and also to alter the
particle size. The various parameters associated with ball milling are ball to
powder ratio (BPR), rotations per minute (RPM), milling time duration and type
of ball milling machine (Planetary or Horizontal ball mill). In ball milling
process, the materials are fractured due to impact by balls. Finally,the particle size is increased due to cold
welding. All these stages resulted in the dispersion of reinforcement phase
homogeneously in the matrix phase. Increasing in milling time leads to very
less chance of formation of amorphous structure.



  Powder compaction

In
powder compaction process the composite powder is first transferred to die and
compacted by applying uni-axial force which is required to provide the desired dimension.
In this process, the particles are arranged in random order with point contact
to neighboring particle as the plastic flow occurs in thevicinity of contact as the force is applied
with punch. This increases the contact zone, thus thecenter of particles comes close to each other hence densify the
green compact. After densification, the
compact obtained are called ‘green compact’. For compaction of Al-graphite, various methods are available such
as cold pressing, hot pressing and spark plasma sintering (SPS) etc. In SPS
process both compaction and sintering carried out together under vacuum. In hot
pressing and SPS, it is necessary to
maintain avacuum to avoid oxidation of
metal materials.

 Sintering of green compact


Sintering is the process in which green compact
is heated in an inert atmosphere at a temperature up to 70-90% of matrix
melting point temperature. During this
process,
the surface of the particles
is melted
and gets bonded with surrounding particles resulting in a compact structure.
Volatile impurities are also eliminated during this process. Sintering is generally
carried out under nitrogen or argon atmosphere to avoid oxidation problems.
When sintering is carried out at higher temperature i.e. close to melting
temperature of matrix metal, the composite grain growth would occurred.
 

2 thoughts on “Steps in powder metallurgy”

  1. To avoid potential flammability issues, a burn off oven is used to remove combustible materials from shelves, hooks, components, and parts. It is completed by heating the substrate in an oxygen-starved environment to a certain temperature level. Fine, light ash, and gases make up the combustible product.

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