Historical development of powder metallurgy
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The powder metallurgy has long time history in human being development. As we know, Powder metallurgy is a forming and fabrication technique consisting of three major processing stages. The first stage is the formation of the primary material which is physically powdered, and then divided into many small individual particles. Next, the powder is injected into a mold or passed through a die to produce a weakly cohesive structure (via cold welding) which is very near the dimensions of the object ultimately to be manufactured. This method is very useful when we come across those products which need high accuracy dimension and tight tolerance in dimension.
Two main techniques used to form and consolidate the powder are sintering and metal injection molding. However, recent developments have made it possible to use rapid manufacturing techniques which use the metal powder for the products. Because of this technique the powder is melted and not sintered. Thus, better mechanical strength can be accomplished.
The history of powder metallurgy and the art of metals and ceramics sintering are intimately related. Sintering involves the production of a hard solid metal or ceramic piece from a starting powder. There is evidence that iron powders were fused into hard objects as early as 1200 B.C. In these early manufacturing operations, iron was extracted by hand from metal sponge following reduction and was then reintroduced as a powder for final melting or sintering.
Powder metallurgy has been called a lost art. Unlike clay and other ceramic materials, the art of molding and firing practical or decorative metallic objects was only occasionally applied during the early stages of recorded history. Sintering of metals was entirely forgotten during the succeeding centuries, only to be revived in Europe at the end of the 18th century, when various methods of platinum powder production were recorded. Metal powders such as gold, copper, and bronze, and many powdered oxides were used for decorative purposes in ceramics, as bases for paints and inks, and in cosmetics since the beginnings of recorded history. This was because most of the decorative apparatus like necklaces, ear rings at that time are mostly small in size especially for cosmetic purpose. Usually those iron oxide are use as pigment for the decorative equipment to make the looking more attractive and creative.
Powdered gold was used to illustrate some of the earliest manuscripts. It is not known how these powders were produced, but it is possible that some of the powders were obtained by granulation after the metal was melted. Low melting points and resistance to oxidation (tarnishing) favored such procedures, especially in the case of gold powder. The use of this method for pigments and decorative purpose cannot truly consider as true powder metallurgy. This is because the true powder metallurgy is the production of powder and the consolidation of it into solid state using pressure or heat at the temperature below the melting point of the major constituent. However, early man learned by chance that particles of metal could be joined together by hammering, resulting in a solid metallic structure. In time, man learned how to build furnaces and develop temperatures high enough to melt and cast metals and to form lower melting alloys, such as copper and tin to make bronze.
As the introduction of new material coming in, metal like platinum which brought by the conquistadores from South America, this metal could not be melted, but the early part of the 19th century workers in England, Spain, and Russia developed similar process for making wrought platinum.
Another important product is tungsten wire filaments, which is pioneered in USA. Unlike the earlier products that were made from powder because the metal concerned could not readily or at all be processed by melting. This are made by powder metallurgy is because of the special properties that result. With this method, it can be arranged that a considerable volume of interconnected porosity remain. Besides, if the gas is extracted from the pores and the parts are immersed in lubricating oil, the pores are filled with oil. Such parts are used as bearing in most small rotating or reciprocating machinery which no further lubrication during the life of the equipment.
After the First World War, another powder metallurgy product came out. In 1925, a German company F Krupp, was granted a patent for a process and product consisting of tungsten carbide particles held together by a "cement" consisting of metallic cobalt. This material is used originally in the form of wire drawing dies, for tungsten as a replacement for diamond dies.
Since powder metallurgy come to human being technology, it bring a lot of consequence and change in metal industry.
Advantages of Powder Metallurgy
There are advantages and disadvantages in powder metallurgy. The advantages are main from technical and commercial aspects.
The technical and commercial advantages of producing parts from powder can be summarized as below:
- production to near net shape. This means that the product can have very tight tolerance of dimension. The accuracy of the dimension of product can be reach higher degree using powder metallurgy.
- few or no secondary operations. Usually powder metallurgy didn't required secondary operation such as cutting.
- high material utilization from low levels of 'in process scrap' For example, those scrap are being recycle used by crushing it into powder and reform new product using powder metallurgy.
- homogeneous powder, and hence part, chemical composition due to absence of gross solidification segregation and uniform pre-alloyed powder particle composition
- unique compositions and structures possible as there is no melting e.g. Introduction of specific particles to give special properties such as silica and graphite in brake pads, and porosity in bearings for oil retention
- non-equilibrium compositions possible e.g. Copper-chromium alloys
- metallurgical structures are usually fine and isotropic e.g. Carbide distribution in atomized high speed steel powder parts
Disadvantage of Powder Metallurgy
Inevitability there is some limitations including:
- costs of powder production.
- limitations on the shapes and features which can be generated e.g. The process cannot produce re-entrant angles by fixed die pressing or radial holes in vertically pressed cylinders
- the size will always change on sintering. This can usually be predicted as it depends on a number of factors including 'as-pressed' density which can be controlled
- potential workforce health problems from atmospheric contamination of the workplace. Powder particles size can be very small up to micrometer measurement scale. So it is very hard to see using eyes and prevent it from taking inside of lungs.
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