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Write a note on following:- i) Compacting ii) Sintering
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I. Compacting

Definition: It is a process of obtaining the desired shape of definite dimension from the loose powders. The blended & mixed metal powders are then fed into suitable dies to give them desired shape. This process requires specific pressure. This is an important step in powder metallurgy, because proper shape of finished product governs many properties.

It is important because, It reduces void space between metal particles, It increases density of compact, It produces adhesion & cold welding of powders & It facilitates the sintering operation, By deforming the powder plastically & By increasing the contact areas between the powder particles.

The compacting is carried out in either hydraulically or mechanically operated press. The dies are usually made of high grade hardened steel with fine finish. The solid mass is called Green Compact.

It is divided into following types.

  1. Cold compacting: The bulk powder with or with out lubricant or binder is compacted in rigid dies by axially loaded punch. Pressure is applied from both upper & lower & upper sides. It is very economical for mass production.

  2. Powder injection molding: The metal powder is converted into moldable state by mixing with binder. Binder also provide green strength & and great stability. Usually heating is done in order to force it through spruce & running channel. Uses: It is used for preparing parts with complicated shapes from materials like cemented carbide, tungsten, heavy alloy & ceramics.

  3. Hot compaction: Compaction is done at higher temperature & pressure. The powder is placed in a rigid die & compacted using direct current or induction currents. The material used for tooling is graphite, tungsten & cobalt alloys fiber reinforced fiber, heat resistant alloy.

II. Sintering

Definition: It is process of heating presintered material just below the melting point but not up to melting point in a furnace in hydrogen atmosphere. Sintering is required for increasing cohesiveness and strength.

The sintering operations consist of following steps: Heating the compacted components in furnaces provided with controlled atmosphere, Soaking & Cooling.

The heating is done keeping temperature below the melting point of all principal constituents of compact, or in certain cases at least any one constituent of compact.

The different types of furnaces’ used for sintering operation are; Belt type, Walking beam type, Sealed high temperature batch type, Pusher type, Roller hearth type & Vacuum type.

In most commonly employed commercial sintering operations, controlled atmosphere applied/maintained, is with respect to endothermic gas, nitrogen-hydrogen mixture, exothermic gas, cracked ammonia, etc.

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