Custom Search

Powder Metallurgy

The technology of powdered metals can be used in the production and utilization of metallic powders for fabricating massive materials and shaped objects.

This process in which articles and components are produced by agglomeration of fine metallic powder, is employed in cases where other methods of shaping such as casting, forging and machining are impracticable or where special material properties have to be achieved.

The materials used in powder metallurgy,metallic posers or, for some purposes, mixtures of metallic and nonmetallic powders are shaped by cold pressing at room temperature between steel dies, which produces initial adhesion of the particles. This is followed by heating of the compacts in a nonoxidizing atmosphere (sintering) to obtain final cohesion. The dies usually consist of two parts thrusting against each other, and each part may be subdivided to produce the required shape (Fig 4). Another technique is isostatic pressing: the powder is pressed in a closed flexible container rubber plastic under liquid pressure.

The function of the sintering treatment is to bond the powder particles of the compact into a coherent mass. As a rule, the sintering temperature is somewhat below the melting point of the powder, or the temperature may be so controlled that fusion of certain constituents of the powder mixture is achieved. Sintering as a subsequent separate treatment may be dispensed with by pressing of the powder at elevated temperature or by subjection of cold-pressed compacts to hot shaping e.g., by drop forging, rolling or extrusion. In certain cases, it is advantageous to process the powder in a protective metal envelope which provides mechanical strength and/or protection against oxidation (Fig.2c). To prevent oxidation, hot pressing or sintering is usually carried out under the protection of a shielding gas or in a reducing atmosphere.

Shaping of the powder is generally done by the application of pressure. However, in slip casting process, a technique adopted from the ceramics industry (Fig.1), the powder is mixed with a suitable liquid suspension medium to form a slip (a thick suspension), which is put into a mold (a, Fig.1). The liquid is absorbed by the walls of the mold, usually consisting of gypsum plaster (b). Then the shaped component is removed from the mold, dried and sintered (c). The powders used in powder metallurgy are produced by comminution of solid materials, by atomizing of molten materials in a stream of gas or water (Fig.3), or by chemical processing. It is essential to obtain particles that are suitably graded in size and are of regular shape and surface condition, so that they interlock and adhere properly when compressed.

The technique has numerous applications. It is used in the production of high melting point metals such as tungsten and molybdenum. For instance, pure tungstic oxide is prepared from the ore and then reduced to tungsten powder, which is cold-pressed and sintered. Another important application of powder metallurgy is the manufacture of hard-metal cutting and working tools in which cemented carbides –e.g., tungsten carbide- are incorporated: cobalt and carbide powders are mixed together, pressed and sintered, so that the cobalt fuses.

Metals produced by powder metallurgy techniques are characterized by their fine porosity, a fact that is utilized for making filters and bearings, more particularly porous bronze bearings that can soak up oil like a sponge and require no subsequent lubrication. Copper-tungsten and similar combinations produced by powder metallurgy are used as electrical-contact materials. Permanent-magnet alloys are also produced by such techniques. There are many other applications, including the combination of metallic with nonmetallic materials to produce high-temperature-resisting materials called cermets.

In conclusion, powder metallurgy is the metallurgy of powdered metals; how to produce solid metal objects from powdered metal by compaction and sintering.