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

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. 

• Tweet
• Share
• Share
• Share
• Share

Related Post