Powder metallurgy is a material processing technique in which particulate materials are consolidated to intermediate and finished products. It concerns with the production of metal powders and converts them to useful shape. There are two main reasons to use powder metallurgy by industries. There are difficulties or impossibility to make by other methods to manufacture products like tungsten filament, tungsten carbide, porous self lubricating bearings and others. The second reason is that powder metallurgy process of manufacturing structural components minimizes or eliminates the machining, and scrap losses at the same time is suited to high volume production of components compared to other manufacturing methods such as casting, machining and forging. This is more economic, saving more energy and raw materials along with mass production of quality precision components. [1]
The traditional powder metallurgy process consists of blending the metal powders and other constituents followed by compaction to produce the desired size and shape. The green compact is then sintered by heating at elevated temperatures, preferably below the melting point of the major constituent to get a product of desired density, structure and properties. The two stages of compaction and sintering are combined into a single step in hot pressing. Powders can also be rolled continuously and sintered to produce strips and other flat products or can be forged to get high strength finished components. [1]
One of the important characteristics of powder metallurgy process is the shaping of metallic objects from powdered materials without melting. Iron objects were known at least from 3000 Before Christ (B.C.). Much before the iron age of about 1200 B.C., Ancient Egyptians knew the carburizing of iron in 1200 B.C., and the quenching technique in 900 to 700 B.C. Iron tempering was employed in Roman times. In the tomb of the Egyptian Pharaoh Tutankhamon, who lived sometime in the fourteenth century B.C., daggers ornamented with gold powder were found. These ornamental gold, silver, copper and other powders were made by grinding the particles or amalgams in a special mortar and pestle. The mercury from the amalgam was driven off by the heat and the resulting powders were processed to make the pigments. Powders were also prepared by mechanical disintegration in water and also by the reduction of oxides. In 800 to 600 B.C., the manufactures of iron components were widespread in Greece. In the earlier days, metallic iron was used by man as weapons and useful in the struggle for existence. In those days, the only way to produce useful objects of iron was by hammering together lumps of sponge iron to the desired shapes. The iron lumps or sponge were prepared by reduction of iron ore in Charcoal fires. Majority of the products made by this way were limited in size to pieces weighing a few kilograms. But the smiths of India produced the famous Delhi iron pillar, shown in Figure 1, weighing about 6.5 tons and other objects even larger as early as 300 A.D. Since the technology to obtain temperature high enough to melt pure iron was not available until about 1800, these are likely to be processed by hammering of hot pieces of reduced iron, produced by the direct reduction of iron oxide without melting. The natives of Matakam tribe of Central Africa have also formed their iron tools from sponge iron by hammering and heating. In the above discussion, the hammering process may be compared to the compaction and heating to the sintering of the powder metallurgy process. At about 1000 A.D, powder metallurgy principles were also used by the Arabs and Germans to make their high quality swords from iron powder. The iron powder was produced by filling the hammered steel lumps. After certain rusting, the powder were again hot forged and the treatment repeated until the carbon content is low enough and the impurities were finely dispersed in the matrix. In the seventeenth and eighteenth centuries, these principles of powder metallurgy processes for making iron and steel were forgotten and replaced by the metallurgy of molten iron with the development of high temperature furnaces for the reduction and treating of iron and steel. The same procedure of powder metallurgy principles were adopted in the history of Platinum metal because of its high melting point of the order of 2042˚K over a period of 1750 to 1850. Platinum compacts were made out of reduced platinum powder, which are subsequently sintered and hot worked. The credit to this work is to be given to Wollaston in England and Sobolevskiy in Russia who performed the experiments almost simultaneously. Sobolevskiy’s powder metallurgy was used to make platinum coins from powders. Nowadays, powder metallurgy is also used in Canada to make the 5 cent coins from nickel powder on account of its economic viability. Wollaston succeeded in producing some of the best platinum ware, particularly crucible, of his time. The powder metallurgy of platinum was replaced by melting first by using the oxy-hydrogen flame. By 1860, the fusion method ,which is quicker and cheaper, became very popular and almost replaced the powder metallurgy process for making platinum products. [1]
Figure 1: Delhi Iron Pillar. [1]
The next significant development was with Tungsten. Tungsten is another metal with a higher melting point than platinum, and hence more difficult to melt and process. Tungsten metal has also undergone the same history of development in which principles of powder metallurgy process were used. The invention of electric lamp in October 1879 by Thomas Edison has contributed substantially to the progress of tungsten powder metallurgy for the manufacture of filaments. Although in 1878, Joseph Wilson Swan, an Englishman, has already produced some electric lamps with carbon filaments but they were unsuitable for mass production. Edison’s electric lamp consisted of an evacuated bulb with carbonized cotton thread as filament glowed for more than forty hours. In 1898, metal filaments for electric lamps were introduced with the osmium filament of Austria, followed by metallised carbon filament. In 1905, Tantalum filaments were used in Germany. But all of these filaments were brittle, although it was recognized that tungsten would be the best metal for filaments because of its high melting point of the order of 3653˚K along with good electrical properties. Around 1906, many attempts were made and several patents were filed regarding the manufacture of the filaments by powder metallurgy process. But the development of making tungsten ductile at room temperature is due to W. D. Coolidge, who took his patent in 1909. Coolidge realized that when a sufficiently high temperature was used for sintering and swaging, metal being subjected to mechanical work will increase its ductility, until finally it became so ductile that it could be drawn into wire at room temperature. Further, it is possible to shape filaments into coils of very small diameter which greatly improved the efficiency of electric lamp filaments. This development may be considered as the modern renaissance of powder metallurgy where the sintering process was used to make filaments for the incandescent electric lamp industry. [1] [2]
Figure 2: Powder Metallurgy Automotive Structural Parts. [1]
The subsequent development includes the production of contacts and electrode materials, sintered porous bearings, cemented carbides, a wide range of electrical and magnetic materials and finally the use of powder metallurgy process for producing certain components as a competitive process to the conventional methods of casting, working and machining. The Coolidge process consists of briquetting and subsequently swaged at high temperatures to reduce its cross section and to improve the ductility until a stage is reached where the metal is ductile at room temperature and can be drawn to wire. All the high temperature operations were carried out below the melting point of tungsten. Later other refractory metals such as molybdenum, tantalum, niobium and other metals were also processed through the powder route. Powder metallurgy process is also extremely useful to add thorium oxide in a finely divided form to tungsten during the processing so that this prevented the grain growth of tungsten crystal during the use of the finished tungsten filament in an incandescent lamp which greatly increased the resistance of the filament to sagging and embrittlement. [1]
Another advancement of the powder metallurgy took place with the manufacture of metallic objects deliberately made porous, so that it could be impregnated with lubricants. Several patents were taken including the one for production of bronze materials with graphite around 1910. Hence considerable developments took place in the field of powder metallurgy during the late 19th and early 20th century. In 1922, the production of cemented carbides started and revolutionized the machining and metal working industry. From this work, many other products developed such as heavy alloys, cemented multi carbides and contact materials using the principle of liquid phase sintering or infiltration techniques. From the sintered porous bronze bearings, other development of products such as porous filters and electrodes and metal-non-metal aggregates such as friction materials took place. The automobile industries and a variety of these products (sintermetallwerk-Krebsoge) are shown in Figure 2 are the major consumers of traditional powder metallurgy products. Modern applications of powder metallurgy techniques include the processing of ferrites, garnets and piezoelectric materials for electronic industry, beryllium, super alloys and titanium alloys for aircraft and aerospace and a variety of fuel elements for nuclear reactors. Recent developments in powder metallurgy techniques promise to produce a variety of new materials and alloy systems tailored to meet specific technological requirements. [1]
Although India had an ancient and rich heritage in the iron powder metallurgy, the production of modern powder metallurgy products such as bronze and iron brushes, filters and structural components started in India only in 1958. Subsequently, cemented carbide products such as cutting tools, mining bits and wire drawing dies were manufactured. Other products include metal-graphite brushes, electrical contact materials and tungsten filaments. Powder metallurgy is also used in fabricating nuclear fuel elements and for fabricating components for defence and aerospace applications. [1]
