CIATION ACHERS
POWDER METALLURGY' ALDEN M. BURGHARDT Watertown Arsenal, Watertown, Massachusetts
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PRINCIPAL interest of the powder metallurgist is the forming of objects from metal powders. In general, metal powders are pressed in a mold to the desired shape and are simultaneously or subsequently heated to cause strong bonding between the particles of powder. Bonding may be accomplished in the absence of mdting but often the fusion of a minor phase may occur. Theoretically, two identical pieces of massive metal would strongly bond together on contact a t room temperature if the atoms on the contact surfaces were brought so close together that atomic forces of attraction could become operative. The contact surfaces would also have to be entirely free of contaminants such as oxides, or adsorbed gases. Under optimum conditions the massive metals would weld together upon contact and the plane of welding would be identical in strength and appearance to other portions of the metal. Experimentally, massive metals cannot be readily bonded together a t room temperature by simple contact. However, sections of mica may be bonded at room temperature. If a thin, flexible sheet, cleaved from a large crystal, is replaced immediately in its former position the sheet adheres strongly to the parent crystal. The force required to remove the adhered sheet is comparable with the force that was required to cause the initial cleavage. Resistance welding without fusion is an example of the bonding of massive metal in the absence of a molten phase, but pressure and heat are required. In this process metal sections are forced together under high pressure and a t the same time the area of contact is locally heated until the metal becomes plastic. In this manner, welds of high mechanical strength may be produced. Heating not only causes the metal to become plastic but provides greater mobility to the metal atoms. Pressure creates an intimate contact between the abutted sections and does not permit gases of the atmosphere to contaminate the weld surfaces during bonding.
In powder metallurgy, metal powders of high puritjr are bonded together in a sintering or heating operation. Metal powders are produced by various methods such as reduction of metallic compounds, electrolytic deposition, or chemical precipitation. Metal powders for compacting are finely divided and will generally pass through a 100-mesh sieve in which the wires are spaced approximately 150 microns apart. Loose metal powders may be poured into ceramic molds and sintered in vacuum or in a protective atmosphere such as purified hydrogen or cracked ammonia. Upon heating, the points of contact between the powder particles bond together. At high temperatures, but below the melting point of the metal, the porosity between particles is reduced, presumably as a result of viscous flow of the metal under the influence of surface tension forces. As the porosity of the compact is reduced, during sintering, shrinkage of the metal occurs. In the cold press sinter process, metal powders are consolidated in dies a t room temperature under pressures ranging as high as one hundred tons per square inch. Small parts have been compacted in automatic cold presses a t rates as high as 500 units per minute. Pressing powders in a die causes more intimate contact between particles than when the particles are loosely packed. The pressed compacts are then sintered in a protective atmosphere. I n forming parts to specified dimensional tolerances, allowance is made for the shrinkage that occurs during sintering. In order to maintain very accurate dimensional tolerances a repressing operation is usually performed after sintering. Figure 1 shows various products that were prepared by cold pressing and sintering metal powders. No machining was conducted on the products. In hot pressing, metal powders are simultaneously pressed as heat is applied, causing welding of the paru r e snot b e high in hot ' Presented before the Ninth Annual Summer Conference of ticles. ~ o m ~ a c t i n ~ ~ r e s sneed the New Ennlsnd Association of Chemistrv Teachera. Welleslv pressing since become plastic a t temperaCollege, AU& 23, 1947. tures. 517
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proper application a porous bearing with its self-contained reservoir of oil will function satisfactorily for long periods of time without attention. Porous clock hearings operate continuously for five years or more. In other applications porous bearings sealed within the motor are expected to function throughout the life of the equipment. Reservoirs of oil may be connected to porous bearings by means of porous metal wicks if necessary. POROUS FILTER MATERIALS
Highly porous rnetal made by powder nletallurg~ techniques is being fabricated into filtration equipment. Bronze, nickel and stainless steel filters are commercially available and many other metals can be produced in porous form. One method of forming highly porous metal consists of sintering loose powder in ceramic molds. In a second method, a low melting point component, such as wax, is uniformly mixed with the metal courtesy of The Chrysler Corporalion powder. The powder containing the admixed wax is F i s u n 1. Typical Roduct. M.nuf.ctu..d Without Machining next pressed to shape in dies. During sintering, the Op-afions horn Metal Powders wax volatilizes from the specimen leaving a porous With the appropriate choice of powder characteristics metal mass. Porous metal filters are generally used for removing and manufacturing conditions metal powder products may he made highly porous or dense. Sintered metals from gases or liquids small foreign particles that would of nearly theoretical density have mechanical properties pass through the finest conventional screens. Metallic that are comparable to those of cast material of the filters are now being used for straining locomotive Diesel fuels. Porous elements confining a desiccant permit the same composition. The powder metallurgy process is frequently consid- passage of gas through the drying agent without the ered to be a very recent development for the manufac- entrainment of the desiccant in the gas stream. The ture of small items. However, in the 15th century the familiar Gooch crucible is now being made of porous Incas of Ecuador prepared small objects of platinum by stainless steel. Millions of cup-shaped filters of porous nickel perpowder metallurgy methods. In the year 1800 Wollaston was producing platinum ware from metal powders. formed an important function in the mercury safety 'At the present time most powder metallurgy parts are switch associated with the variable time-fuze of high exrelatively small. However, some carbide dies used in plosive ammunition. The cup retained the mercury producing steel cylinders weigh as much as two hundred during normal handling of the shell. Upon firing, however, the spin of the projectile forced the mercury pounds. through the pores of the cup and closed an electrical cirPOROUS OR SELF-LUBRICATING BEARINGS cuit for operating radio equipment within the projectile. Up to the present time, porous metal bearings have The radio equipment caused the shell to detonate in the been a principal product of many powder metallurgy vicinity of the target. Too rapid flow of mercury concerns. Considering the cylindrical bearing, its func- through the cup would have resulted in detonation of tion is to provide support for a rotating shaft. In oper- the shell in the gun or in proximity to the gun crew. ating a motor, wear of the driving shaft and bearing Needless to say, close inspection insured that the cups must be held to a minimum in order to maintain proper were manufactured to an exact degree of porosity. Another porous metal item developed by the British shaft alignment and free rotation. Porous bronze bearings are widely used in small fractional horse-power during the past war was an airplane de-icing strip. The motors for household equipment such as sewing ma- strip was: made of corrosion resistant cupro-nickel (70 cymes, electric clocks, and refrigerators. The porosity per cent Cu and 30 per cent Ni) and was built into the of such bearings may vary between 15 and 35 per cent. front edges of the wings. A fluid was pumped through The bearings are impregnated with lubricating oil in the strip and prevented the formation of ice on the much the same manner that a sponge may be saturated wings. with water. In operation, frictional heat between the REFRACTORY METALS porous bearing and the rotating shaft causes the lubricating oil to expand and flow to the surface of the shaft. The refractory metals, tungsten, molybdenum, tanThe uniform oil film produced then lubricates the sur- talum, and columbium, are successfully processed by faces that bear on each other. Upon stopping the motor powder metallurgy methods. These metals have very the bearing cools and most of the oil on the shaft passes high melting points ranging betwen 1950' and 3370' C. back into the porous bearing by capillary action. In a In processing them, powder is compacted into a bar with
SEPTEMBER, 1948
equipment like that shown in Figure 2. The pressed bar is sintered by the passage of a current of several thousand amperes through it. With tungsten or molybdenum an atmosphere of hydrogen is maintained during sintering, but tantalum and columbium are sintered in vacuum because of their great affinityfor gases, including hydrogen. After sintering, the bar is swaged, rolled, or drawn. I n producing fine tungsten wire, initial drawing of swaged metal is carried out in tungsten carbide dies and diamond dies are used in the final stages of drawing. Tungsten is well known in the form of electric light filaments. Columbium or tantalum are used in electronic tubes because of their ability to absorb gases. Molybdenum heating elements in hydrogen atmosphere sintering furnaces afford temperatures up to 2000°C. Tantalum with high strength and ductility is an excellent material for chemical processing equipment since tantalum is totally inert t o hydrochloric acid, nitric acid, and boiling aqm-regia. Tantalum metal may also, be substituted for broken bones in the human body. CEMENTED CARBIDE TOOLS AND DIES
Cemented carbides are fabricated by bonding the carbides of tungsten, tantalum, or titanium by means of metals such as cobalt or nickel. Aft,er being sintered, cemented tungsten carbide has a specific gravity in the vicinity of 1.5, is extremely hard, and readily scratches glass. The characteristic hardness of carbides is utilized to advant,a~ein drawing dies t,hat are subject to heavy abrasion. The ability of the carbides to retain their hardness at high temperatures has resulted in their wide applicat,ion for machine tools. Most metals can he machined with carhide t,ools and carhide drills readily pierce ceramics.
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substances such as graphite, silicon carbide, and alumina. The mixed powdered materials are pressed into sheet form which may be several hundred square inches in area. After sintering, the sheets are cut up into pieces as required. For additional strength, the friction facings are usually bonded during sinteringto st,eel hacking plates. Friction materials produced from powders have been used for several years in trucks, tractors, agricultural machinery, and airplanes. Figure 3 illustrates the type of sintered metal clutch facings that are used in automotive and earth-moving equipment. ALNICO MAGNETS
Permanent magnets of the Alnico type, with high magnetic strength, are familiar to everyone. Alnico is an iron base alloy containing, as the name implies, aluminum, nickel, and eohalt. When made by casting methods, expensive grinding operations are required t o maintain close dimensional tolerances, since the Alnico materials are difficult to forge or machine. Small magnets of controlled composition can he sintered to close dimensional tolerances with little or no grinding. The powder metallurgy process successfully competes economically with casting methods in the manufacture of magnets weighing less than two ounces. ORDNANCE APPLICATIONS
The well-established powder metallurgy products such as porous bearings, sintered friction materials for brake linings, and cemented carbide tools and dies performed important functions in the past war. High density parts of sintered iron and copper weresubstituted to some extent for parts that are normally machined from bar stock. Components for automatic rifles, iron pole pieces in Signal Corp field telephones, and bronze bomb sight parts were manufartured from metal powdem.
ELECTRICAL CONTACTS
Electrical contact materials, fabricated from metal powders, are incorporated into circuit breakers and electrical relays. A contact is comprised of a thin metal facing hrazed to a copper backing. In an electrical circuit, two contacts hearing on each other permit the passage of current hut upon being separated the current is interrupted. The contacts are expected to interrupt current flow without welding to each other. In addition, contacts must have good elect,rical conductivity and resist wear. I n telegraph relays, which may make as many as six million breaks per day, sintered contacts perform satisfactorily for long periods of time. Since copper and tungsten are not soluble in each other, these metals cannot be alloyed by conventional melting methods. Powder metallurgy, however, permits the fabrication of electrical contacts from these two metals and the sintered products have excellent conductivity and refractory characterist,ics. FRICTION MATERIALS Courtesy of Fansled Melallurgiol Corporation
Friction materials for brake and clutch linings are produced from mixtures of metal powders and powdered
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BWO-Ton Prrss Used for Compacting Rehsctorsr pordar. into B- suitable for sinterin.
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means of cold or hot working procedures with vacuum anneals a t various stages of working, sintered titanium is successfully shaped into ductile sheet and wire. PRESENT LIMITATIONS OF THE POWDER METALLURGY PROCESS
Powder metallurgy practices are becoming less of an art since educational institutions and commercial laboratories are becoming interested in the scientific principles that are involved in forming metal powders into sintered products. With further research some of the present limitations of the sintering process will be overcome. At present the cost of metal powders is high. On the other hand, production facilities for the manufacture of metal powders increased greatly during the war. In due time low-cost metal powder will become available when additional metal powder manufacturing facilities are established. During compacting of dry powders pressure is not uniformly transmitted throughout the powder mass. Dry powders also do not readily flow around corners. Large parts and very complex shaped parts are, thereCourtesy of S. K. Wellman Company fore, difficult to manufacture from dry powders. ReFig".. 3. Clutch Fecinge Consisting of Sintered M e t d pressing or forging treatments are generally required to Bonded t o Steel or Copper Supporting Plate produce parts of theoretical density from metal powders. Hot pressing offers a means of producing dense, I h ~ e s t i v~~rojrrtilrs equip11td with sintrred ~wppt~r IN complex-shaped parts, but mold materials tthat have brans nItntins hnllck fllllctiolled s:ltisfl~crul~ily in limited high strength at high temperatures have yet to be defiring tests. The sintered bands could not compete on a veloped. cost basis, however, with bands cut from wrought copSince the porosity of sintered metal may be varied, a per tubing. Shell rotating bands of sintered iron were wide range of mechanical properties may be achieved. extensively substituted for copper bands by the Ger- Highly porous sintered metal and sintered metal-cemans. The Germans also used tungsten carbide cores in ramic compositions are unique products of the powder ammunition for antitank weapons. The high density of metallurgy process. In addition, high production rates cemented tungsten carbide is a contributing factor to and the elimination of machining procedures allows the effective penetration of thick armour plate. powder metallurgy process to compete with other standard methods of manufacture when the size and design DUCTILE TITANIUM of the product permits the use of compacting techniques. Ductile titanium has recently been developed on a pilot plant scale by the U. S. Bureau of Mines. Prior to REFERENCES this development, ductile titanium had been produced only on a laboratory scale and by methods that were not BAEZA,W. J., "A Course in Powder Metallurgy," Reinhold Pubsuitable for large-scale commercial production. Titalishing Corporation, New York, 1947. nium sheet, rod, and wire products are now being fabri- HAUSNER,H. H., "Powder Metallurgy," Chemical Publishing Company, Brooklyn, New York, 1947. cated by powder metallurgy methods. Undoubtedly, R. E., AND R. E. POLLARD, United Slates Patents on P o w JAGER. titanium will assume an important role in metal compoder Metallurgy, National Bureau of Standards Miscellaneous nents of the future. Titanium ranks ninth in abundance Publication No. MlM. United States Government Printine of all the elements in the earth's crust. It has a specific Office. Wmhindon. - , 1947. gravity of 4.5 as compared to 7.86 for iron. Pure tita- JONES, W. D., "Powder Metallurgy," Longmans, Green and Company, New York, 1937. nium can be processed to have properties comparable E. E.,I N D A. G. SOUDEN, The Bell System Techniwith medium strength steels. In addition, pure tita- SCHWMACHER, cal Journal, 23,422-57 (1944). nium has corrosion-resistant properties similar to those S C ~ A R ~ K OP.,P E "Powder , Metallurm,'' The Mamillan Comof 18-8 stainless steel. pany, New York, 1947. A. J., AND J. WULFF,2nd. Eng. Chem., 40,83&43 (1948). When heated, titanium reacts chemically with hydro- SHALER, . "Powder Metallurw." -", American Societv for Metals. gen, oxygen, and nitrogen and becomes brittle. The W u ~ mJ.. ~lev&add,Ohio, 1942. brittle character of impure titanium does not permit U. S. Office of Education. Film No. 346, Powder Metallurgy, Part shaping. To avoid the absorption of gases titanium , I , Prinezples and Uses; Film No. 347, Part II, Manufacture of Porous ~ r o n z eBearings (distributed by Castle Films). powder is pressed and then sintered in vacuum. By
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