Application of powder metallurgy
The Powder metallurgy refers to a branch of metallurgy that deals with the production of metal powders and their further processing. The manufacturing processes of powder metallurgy are characterized by the mechanical compression of metal powders in molds or presses and simultaneous or subsequent sintering of the "green body" at high temperatures or hot rolling to form a block.
The result is solid, fine-grained semi-finished or finished parts. Sintering is particularly suitable for the mass production of small, lightweight molded parts and rollers for large sintered metal blocks.
The history of powder metallurgy and the sintering of metals and ceramics are closely related. Sintering is the term used to describe the heat treatment of pre-pressed molded parts made of powder, whereby solid metal or ceramic parts with precisely defined dimensions and properties can be produced.
It has been proven that as early as the 12th century BC Pre-pressed objects were produced with this method. At that time, the powder was extracted manually or mechanically from sponge iron using a reduction process and then sintered or melted.
Three manufacturing steps
The manufacture of powder metallurgical products essentially comprises three sub-areas:
Metal powders of pure metals or alloys with grain sizes below 0.6 mm are required. The type of powder production has a strong influence on the properties of the powder, which is why many different processes have been developed. Mechanical processes, chemical reduction processes or electrolytic processes, as well as the carbonyl processes, centrifugal, atomizing and other processes are used.
The powder is compacted into green compacts in pressing tools under high pressure (between 1 and 10 t / cm² (tons per square centimeter). Other methods include compacting by vibration, the slip casting method, bulk method and methods with the addition of binders.
During the heat treatment (the actual sintering), the powder grains are brought into a firm bond at their contact surfaces by diffusion of the metal atoms. The sintering temperature is for single-phase (powders made from uniform Material) between 65 and 80% of the solidus temperature. In the case of multiphase powders, on the other hand, sintering is generally carried out in the vicinity or above the solidus temperature of the phase with the lowest melting point. After sintering, the spaces between open-pored workpieces can also be filled by dipping into a molten metal.
All sintered metals are porous, whereby tightnesses of up to 99% can be achieved. Complete densification is achieved in some cases by hot rolling. After calibration, the workpieces show high dimensional accuracy and surface quality.
The processes are used for the production of hard metal parts and powder injection molding. Because of the expensive pressing tools, powder metallurgy is only used for large series and only for small, light parts.
The most important area of application is the automotive industry. Typical products are e.g. (oil-soaked) bearing shells and bearings, engine and transmission molded parts, sieves, filters and permanent magnets.
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