Powder Metallurgy

modern technology is making important use of a relatively new development in industrial metal working the technique of powder metal while the principles underlying powder Metallurgy are modern in their industrial application to Metals actually they have long been known and they've been used for centuries in the field of Ceramics take bricks for example brick manufacturer starts with the raw material clay the clay is formed Under Pressure then it is subjected to heat the result is the finished brick an essentially similar process is used in powder Metallurgy here metal powder is the raw material pressure forms the powder in the desired shape heat sets up a strong bond among the separate powder particles and produces a piece which is already finished or adapted to further work this this process of powder Metallurgy has made possible thousands of different useful Industrial Products the major industrial applications of powder Metallurgy can be divided into five broad categories the first of these is the use of powder Metallurgy in combining essentially non-alloy Metals For example electrical circuits which carry heavy currents at high voltages require circuit breaking contact points which must be highly conductive and at the same time resistant both to wear and to Fusion at extreme arcing temperatures here are two metals which together have these desired properties one is silver a metal highly conductive but lacking good wear and heat resistance the other is nickel an excellent wear and heat resistant but a poor conductor a combination of these two metals is wanted but they cannot be alloyed by casting methods by reducing these metals to powder form however and mixing them a raw material is obtained which makes it possible to fabricate a product in which are combined the properties best suited for the job powder Metallurgy also provides a practical means of combining metals and non-metallic materials when the brakes on the landing wheels of large planes are applied temperatures as high as 1200° f are developed in the brake bands within a few seconds a friction material capable of withstanding such a high temperature can be fabricated from a mixture of copper and Tin powders for a binder and Lead iron graphite and silica for controlling friction from this combination of powders can be obtained a brake material of superior heat and wear resistant properties which stands up under extremely severe operating conditions a third major application of powder Metallurgy is in utilizing Metals with melting points too high for casting the filament of an incandescent light must be made from a ductal material with an extremely high melting point tungsten has the highest melting point of any known metal but this virtue in itself prevents the working of tungsten by ordinary melting and casting methods with tungsten powder however powder Metallurgy techniques can be employed to eliminate the need for melting and produce a ductal tungsten metal which can be drawn into to fine wire strong and remarkably heat resistant one of the most spectacular applications of powder Metallurgy is in fabricating metals to obtain unique structural properties for example adequate lubrication for sealed in electric motors and other enclosed Machinery is a frequently recurring problem the bearings in which the enclosed drive shaft of this dust proof motor revolves cannot be provided with an external source of lubrication the ideal answer to this problem seen in an identical motor which has been dismantled is a self-lubricating bearing which contains its own oil supply to be able to store its oil supply such a self-lubricating bearing is Thoroughly porous much like a sponge in structure its paracity can be demonstrated by plugging it at both ends and then forcing water into it the water blocked at either end escapes through the walls of the bearing such a porous bearing can be obtained by pressing powders usually copper tin and graphite and a volatile organic material such as steric acid the steric acid under heat is an effect itive pore generating agent oil impregnated in the bearing is automatically and uniformly distributed by capillary action to the working surface as required by the moving part in addition powder Metallurgy in many cases offers a means of replacing lengthy and expensive Machining operations in mass production substituting a simpler and more efficient molding process these and many other products are produced more cheaply and easily by powder Metallurgy process than by Machining take for instance this gear when gears have to be cut by Machine the process is slow and entails the loss of a considerable amount of scrap metal some gears however as well as many other parts can be pressed directly from powders the result mass production at considerably reduced cost these then are the principal ways in which industry employs this useful metalworking technique the basic powder Metallurgy process can be seen by following through the laboratory procedure which is used in the first of these applications combining essentially non-alloy Metals silver and nickel are two metals which cannot be alloyed when reduced to a molten State they will not mix this can be illustrated by The Familiar demonstration of putting water and oil together the oil quickly separates to the top just so does molten silver separate from molten nickel these two metals therefore cannot be combined by ordinary Fusion methods silver and nickel powders however can bring about such a desired combination here in a laboratory the powders are mixed according to a formula this formula calls for 60% of silver powder and 40% of nickel powder powder particles shown here greatly magnified come in various shapes the shape is determined by the method used in making the powder for instance powders produced by atomization are round teardrop or dumbbell like powders produced by reduction of oxides are irregular and porous powders produced by electrolytic deposition have a fern-like shape all metal powders are produced in graduated size groups ranging from very coarse to the finess of dust particle size is measured by the smallest mesh size through which the particle will pass usually a given powder contains a specified mixture of particle sizes this is called called size distribution shape and size distribution influence the behavior of the powder throughout the process and bear importantly on the ultimate structural properties of the finished product powders selected and measured according to formula are placed in a mixing jar the powders must be thoroughly mixed to obtain uniform distribution of metals and of particle sizes the mixing period here will be for 1 hour with some mixtures it may be as long as 3 days after mixing the powders are taken to the Press they are poured into the D cavity to assure a complete fill of the cavity the D Block is tapped around the powder and the excess powder is removed the Dy is positioned in the press the upper plunger of the Dy is set on top of the filled cavity the Press is turned on now the powders are compacted as the upper plunger is forced down into the cavity under heavy pressure in this case 40,000 lb which with this size briquette is nearly 20 tons per square in with some powder mixtures pressures as high as 50 tons per square in may be used the pressure is released the upper plunger is removed to eject the compacted powders two spacers are placed on top of the d block and again the Press is turned on now the lower plunger is forced upward ejecting the brickette into which the powders have been formed Under Pressure this is the briquette what has happened to the metal powders here in schematic treatment we see the loose powder in the cavity of the spring floated D this is the movable upper plunger which will apply downward pressure this is the stationary lower plunger which will provide opposing pressure and will also eject the Press piece as pressure is applied the Loose Powder is packed into less and less space in this enlarged cross-section view we see the powder before pressure is applied the light gray particles are silver the dark gray are nickle the black spaces are voids between the particles after compression the gray silver particles being relatively soft p have lost their original shape and tended to become irregular masses of metal this has also happened to a lesser degree to the harder nickel powders as they were compacted the particles filled in the greater part of the voids and they mechanically interlocked in addition a bonding action took place we can see this bonding action here metal particles tend to acquire a surface film of oxides and other substances as the particles slide against each other Under Pressure they break through this surface film thus clean metal surfaces come into contact in some spots and a minute but actual welding takes place with the pressure stroke completed the Loose Powder has been compacted into a greatly reduced face usually 1/2 to 1/3 the original powder volume the interlocking and Welding of the particles have set up an initial bond which assures that the powder compact will retain the shape and form it has received under pressure when the pressure is removed now the lower plunger ejects from the die cavity a brickette which has enough strength to be handled it must however be handled with care as at this St AG it can be broken quite easily this brickette is ready for the third phase of the powder Metallurgy process the Heat or centering phase heat acting in an atmosphere of protective gases will complete the transformation of the metal powders this phase takes place in a heating unit the centering furnace briquettes set in a powder to prevent their sticking to the tray are pushed into the furnace where the brittle briquettes will be transformed into a strong durable material of desired properties here is an overall view of the centering furnace this is the entrance chamber this section is the heat chamber where the centering takes place the entrance chamber serves merely as a kind of vestibule the briquettes are pushed immediately from the entrance chamber into the heat chamber centering is a partially unexplained phenomenon its essential principle is that it takes place below the melting point of at least the major constituent metal in the briquette in this case the temperature while very hot 1600° f is still below the melting point of both the nickel and the silver at this temperature the briquettes in the heat chamber are red hot they are surrounded by an atmosphere of hydrogen containing gas this atmosphere reduces whatever oxides are present in the compacted powders and prevents further oxidation here again is the cross-section view of a part of the briquette as we saw it after pressing but before centering here is the same cross-section as it looks after centering the heat has developed a closer contact among the powders there has been in fact some migration of atoms from particle to particle centering continued the welding of the silver particles which was well started by the pressing action the similar coalescing of the nickel particles which was only slight under pressure is now pronounced centering also caused shrinkage of the briquette further reducing the voids many of the original particle boundaries have disappeared and a new more intimately bonded crystal structure formed the centering time in this heat chamber varies for different types of work from a few minutes to several hours in this case it is 30 minutes after centering the briquettes are moving into this water jacketed cooling chamber here the transfer into the cooling chamber is made controlled cooling still in the protective gas atmosphere preserves the structure of the briquettes which the centering has achieved after cooling the briquettes are removed these briquettes brittle when they went into the furnace are now so strong that placed in this testing machine for a transverse rupture test they are able to resist a mounting pressure of hundreds of pounds and in finally yielding reveal a strength more than adequate for the uses to which this nickel Sil compound may be put where two metals would not alloy This Modern metal working process has wrought the best of each into a unique new metal powder pressure heat these are the principles on which powder Metallurgy is based a field yet in its infancy the future of which holds both promise and challenge for the engineer e