Science: Solids out of Powders

The most radical innovation in metallurgy since the Bronze Age is hidden inside the shiny 1942 models rolling out of U.S. automakers’ plants this week. For perhaps 5,500 years man has started off with molten metal, which he has then cast, forged, rolled, extruded, hammered, machined, hobbed, drilled, milled and ground. But in the new General Motors cars there are some 25 parts, and in the new Chryslers 30, which started as fine metallic powders and were simply pressed into solid shape.

The new powder metallurgy’s products are often much cheaper than those of fusion metal particularly for small complicated shapes like gears. They look just like ordinary metal to the naked eye; they can ultimately be made equally strong. But they have distinct advantages, chiefly 1) light weight, 2) porosity which enables them to absorb large quantities of oil, giving them semi-permanent built-in lubrication.

The powdered metal parts in the 1942 cars add up to only three pounds, but automotive engineers predict that before long there will be 100 pounds in almost every model. Meantime, subsidiaries of both Chrysler and General Motors are busy supplying thousands of other manufacturers with powdered metal gears, bearings, parts for airplane engines, guns, ships, household equipment—everything in which wheels and levers turn.

All told, some 5,000 tons of metallic powder will be used in metallurgy this year—up 25% over 1940, 100% over 1936. Students returning this fall to several engineering colleges—Michigan, Minnesota, Ohio State, others—found a new department had been added to teach them powder tricks which many an oldtime engineer wished he knew. So fast has powder metallurgy expanded in industry that shop practice has sometimes outstripped basic theory. Leading U.S. academic metal powder laboratory is directed by Gregory Jamieson Comstock at Stevens Institute of Technology.

From Pills to Gears. Reason most powdered metal parts are small is that pressures of from five to 100 tons per square inch are needed to force the minute particles close enough together to become locked into a genuine solid. Thus a solid gear with a 4-in. diameter would have a top surface of twelve square inches and might require a pressure of as high as 400 to 1,200 tons. The largest presses now in use are of 80-ton capacity, although 400-ton, 600-ton, 800-ton machines are on order. The pressure needed can be lowered as much as 90% by simultaneous application of heat, but this method is still under development.

The individual dustlike particles in a cubic centimeter of metal powder may have a total surface of approximately 384,000 square centimeters, making a tremendous amount of surface energy available ; but even so the cold-pressed briquets are not very strong and can be easily broken by hand unless they are strengthened by sintering—consolidating heat treatment by baking at temperatures well below their melting points. This heat also shrinks the pressed part, in some cases very little, in others 20%. Yet in each metal the shrinkage is controllable enough so that parts can be made precise to within a few thousandths of an inch.

Platinum & Diamond Dust. A pioneer of powder metallurgy was an Englishman, William Hyde Wollaston, who in 1829 described a process for working platinum, whose melting point (3224° F.) was too high for the crude furnaces then in use. As better furnaces were developed, his technique was little used until about 1910 when U.S. scientists, notably General Electric’s William David Coolidge, revived it as the only practical way of making ductile tungsten (melting point 6100° F.) from which thin wires for light bulb filaments could then be drawn through holes in diamonds.

Germany’s contribution to powder metallurgy came about 1916 when the great Krupp Works learned from the electrical industry to press and sinter mixtures of tungsten carbide with cobalt into the hardest cutting compound known, began producing it commercially. These hard-cemented carbides have a hardness between diamond and sapphire. They are often shaped into cutting tools by another product of powder metallurgy: a solidified mixture of diamond dust and bronze powder. They work without softening at high, cherry-red heats while cutting ordinary armament steels two to ten times faster than cutting tools made of the toughest high-speed steel; and they gave Germany some advantage in arms-making efficiency throughout World War I, when they were unknown to the Allies. In 1926 Krupp began to assist the American development of hard-cemented carbides, and these materials are now playing a major part in the U.S. rearmament program, trimming rough castings into smooth-cheeked gun barrels and shell cases.

Up from the Door Latch. About 1922 the U.S. electrical industry created a byproduct of its work with tungsten: bearings pressed from copper and iron alloys. Their sponginess was their advantage: the fine continuous pores (up to 40% by volume) can absorb oil, exude it by capillary action as needed. Often they require no further oiling after impregnation; they can be sealed into machinery (e.g., household refrigerators) and forgotten. By 1932 “oil-less” bearings were used for many purposes in automobiles and were in time found to outlive the rest of the machine. Billions of such bearings are now in use in the U.S.

About five years ago Chrysler turned to powder metallurgy to make a door-latch part which would be 1) self-oiling, like a bearing, 2) quieter than clangy solid metal. Besides offering these advantages, this part surprised engineers by being easier and cheaper to make from powder than by former methods. From this and similar pressed parts a wave of interest in powder metallurgy at once swept U.S. industry. First powdered-metal automotive gear appeared in the oil pump of the 1940 Oldsmobile, and this year more new parts have been made from powders.

Meantime, powder metallurgy has also produced:

> Welding electrodes which combine the high electrical conductivity of silver or copper with the heat resistance of tungsten, molybdenum or nickel.

> Current collector brushes for generators which combine the conductivity of copper with the lubricating properties of graphite. > Metal filters, with perhaps 50% porosity, which can be used, for example, in cleaning diesel fuels.

> Fine bronze screens stamped from powders, eliminating the need for making wire and weaving it.

Most intriguing new experimental use for powder metallurgy is in typewriters and other clerical machines, where powder-metal parts will not only be quieter and cheaper but may well banish ink ribbons, since porous typefaces can soak up ink and stamp it on paper.