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Introduction to Powder Metallurgy-the basic procedure in the manufacture of PM parts

2022-01-05

 Powder Metallurgy (PM) is a continually and rapidly evolving technology embracing most metallic and alloy materials and a wide variety of shapes. Powder Metallurgy is a highly developed method of manufacturing reliable ferrous and non-ferrous parts. The European Market alone has a turnover of over Six Billion Euros per year, with annual worldwide metal powder production exceeding one million tonnes.

PM Components are created by mixing elemental or alloy powders and compacting the mixture in a die, with the resultant shapes heated or “sintered” in a controlled atmosphere furnace to bond the particles metallurgically. The high precision forming capability of PM generates components with near net shape, intricate features and good dimensional precision pieces. Pieces are often finished without the need of machining. Whether it is by controlling porosity to develop the unique capability of self-lubrication or by optimising the chemistry to provide improved longevity in wear parts, PM has a solution.

The basic procedure in the manufacture of PM parts is:

 

(1) Mix the metal powder or powders with a suitable lubricant.

(2) Load the mixture into a die or mould and apply pressure. This gives what is called a compact, which requires only to have sufficient cohesion to enable it to be handled safely and transferred to the next stage. Such compacts are referred to as green, meaning unsintered. Hence the terms green density and green strength.  

(3) Heat the compact, usually in a protective atmosphere, at a temperature below the melting point of the main constituent so that the powder particles weld together and confer sufficient strength to the object for the intended use. This process is called sintering, hence the term sintered parts. In certain cases, a minor constituent becomes molten at the sintering temperature in which case the process is referred to as liquid phase sintering. The amount of liquid phase must be limited so that the part retains its shape. In certain special cases stages 2 and 3 are combined i.e. compaction is done at an elevated temperature such that sintering occurs during the process. This is termed hot pressing, or pressure sintering. One specialised variant on this process - the production of Soft Magnetic Composite (SMC) components - involves a heating step aimed at curing a resin binder, added to insulate the individual iron powder particles, rather than at sintering of the metal particles. In many cases the sintered part is subjected to additional processing - repressing, plating etc and these will be dealt with in later sections. In certain special cases, e.g. in the manufacture of filter elements from spherical bronze powder, no pressure is used, the powder being placed in a suitably shaped mould in which it is sintered. This process is known as loose powder sintering.