US1873223A - Porous metal and method of forming the same - Google Patents

Description

Patented Aug. 23, 1932 um'reo s'i CHARLES F. SHERWOOD, OF HIGHLAND PARK, MICHIGAN, ASSIGNOR TO SHERWOOD PATENTS, LTD., A CORPORATION OF CALIFORNIA POROUS METAL ANT) METHOD OF FORMING THE SAME No Drawing.

The present invention relates to bearings, bushings,or other articles and to the method of manufacturing the same.

More particularly it relates to bearings 01' other articles composed of sintered metal.

that'it may be made in large as well as in 1 small sizes, the particlesof the powdered metals being united by coalescence.

In carrying out the process, the powdered metal is formed under pressure into an agglomerate or briquette including a mixture go of the powdered metal and a material of a lubricating character which lubricant has a xvolatilization point below the temperature at which sintering of the powdered metal takes place. ,The effect of the volatilization .25 of the lubricant below the sintering temperature is that there will be no residue or extraneous material between the particles of metal, but on the contrary, at the time the sintering temperature is reached the lubricating so binding material will have been volatilized and driven off leaving the metal particles in contact metal to metal at the juxtaposed or contacting portions of their surfaces, so that coherence and coalescence will take place 85 throughout these surfaces at the sintering temperature.

Another object of the invention is the posi tive reduction of any existent metallic oxides, the prevention of oxidation, and the production of a metallic body improved in strength, ductility and machineability.

Other objects will appear from the following specification.

The improved process is applicable to the manufacture of bearings, bushings, brushes, brake linings and other articles from various powdered metals and mixtures thereof.

The art of manufacturing articles from 5o powdered metals is extremely old, originating Application filed. November 18, 1929. Serial 1T0. 407,022.

ilrsifagllannheim, Germany, in approximately The old method of procedure was, briefly, making use of finely divided metallic powders and by finely divided, I refer to metal particles that will pass 150 mesh screendry, and under pressure forming them into a coherent briquette, placing the same, after formation, in heat for varying periods of t1me, and alloying the metal by the heat at a temperature lower than that of the melting point of't-he main constituent metal. For example, a mixture of 84% copper, 10% tin, and 6% graphite, will alloy and sinter ata temperature of 1425 F. The melting point of copper is 1820 F. and that of tin approximately 400 F so that it will be seen that the sintering temperature is a mean of the two temperatures of the constituent metals. This method has been in use commercially in the making of small bushings. The briquettes are formed on pressure machines of the pill type.

After forming, the briquettes are packed in oil coke in cast iron boxes, with covers, and of rectangular shape, approximately 6 by 12 inches in size. They are then placed in a cold furnace, electrically or gas heated, and are brought to a temperature of 1425 and maintained at that temperature for from five to six hours, when they are allowed to cool in the boxes and are then removed and scratch brushed prior to machining.

One objection that has been encountered is that the product can not be successfully machined, owing to the large amount of copper oxide which occurs in the bushings, due to the method of sintering. Copper oxide being extremely errosive, rapidly Wears and dulls the tools, so that it is impossible to machine the product of the prior process. The presence of copper oxide also tends to destroy the mechanical strength of the bushing, and it is very liable to break on the mandrel while machining is attempted.

The method of sizing the bearings in prior use is that of plain broaching or sizing under pressure with smooth tools. This has the objectional effect of closing the pores of the bearing to a large degree, and reduces capacity as a carrier or holder of lubricant Which is supplied to the bearing.

I have found that the reduction and pre-' from 30 seconds to 2 minutes, at the predetermined temperature," which temperature is governed by the metallic constituents of the composition to be sintered.

By so sintering a bushing the time that it is exposed to high temperature is decreased and by the use of the reducing gas, the formation of copper or other oxides is prevented. Also any small amount of oxide that may be present in the powder prior to briquetting is reduced.

The prior methods of briquetting call for a substantial amount ofgraphite in the mixture. This the present manufacturers perforce must use to prevent extreme die wear in forming the briquettes. The use of this graphite destroys the mechanical strehgth of the briquette after sintering, as graphite, being lamellar in structure, prevents the perfect coalescing of the metallic particles during sintering, graphite being light and 6% by weight being about 25% by volume.

In my process, however, it is not essential to employ graphite. I use a solid lubricant in molding the briquettes. Such a lubricant is also volatile at temperatures very much below that of sintering. This solid lubricant is introduced into the metal powders in a dissolved state. For example, of 1% to 5% stearic acid is dissolved in one of its many solvents, such as ether. The dissolved stearic acid is added to the metallic powder,.and

thoroughly mixed. After mixing, the ether being extremely volatile, is easily driven ofi, leavlng each finely divided metallic particle entirely coated with a lubricating film. The mix may then be briquetted under high pressures. The pressure may vary considerably but willusually be between 5,000 and 55,000 pounds per square inch. Under. the briquetting pressure, the finely divided metallic particles being covered with the solid volatilizable lubricant, will slidereadily over each other in arranging themselves relative to each other in the formation of the briquette. The coating of lubricant on the particles also serves to prevent oxidationfnBy reason of the lubricant covering the metal particles, the briquette will not in anyway abradeor wear tools or dies used in the operation. At the same time the push-out pressures may be reduced to approximately one-tenth of the formation pressure. For instance, in the use of graphite as a lubricant in the prior practice, bearings are formed at a pressure of 55,000 ounds, and it takes 40,000 pounds to eject the formed briquette from the die. With the use of my lubricant, a bushing formed at 55,000 pounds maybe ejected from the mold or die with a push-out pressure of 5,500 pounds. This is of commercial importance, as it is necessary in the first case to water-jacket dies, owing to excessive heat due to push-out pressures, but it is not necessary in the second instance.

89% of copper with 10% of tin and 1% of stearic acid dissolved in ether is-a specific example of proportions producing a bushing of great strength, ductility and machineability;

Furthermore, by the use of stearic acid,

or other solid volatilizable lubricant, I am able to volatilize completely the stearic, leaving no residual particles to interpose themselves between the metallic particles. Consequently, I get perfect cohesion between the individual metallic particles on sintering, thereby increasing the strength over five times that of the sintered bushing using graphite as a lubricant.

I have also used hydrocarbon oils for this purpose, but find that on sintering residual carbon is left, which materially reduces the strength of the finished bushing;

My invention therefore involves the use of a solid lubricant, dissolved and in a liquid state at the time of mixing with the metallic constituents, the solvent being volatile, and being allowed to evaporate prior to formation of the briquettes, and the solid volatilizable lubricant being volatilized and driven off after the briquettes are formed at a temperature substantially below that of the sintering operation. The particles are then in condition to coalesce and cohere perfectly at sintering temperature, producing a very ductile product, readily machined,

substantially free from oxides, and having 1great strength on radial and compressive cad.

Among other fatty acids that may be used in this connection, as well as stearic, are palmitic and oleic, all of which have lubricating properties.

In connection with the use of stearic as a lubricant,it serves not only as a lubricant, but by increasing the amount over and above thatv necessary as a lubricant, one can control the ultimate porosity of the finished bushing or otherarticle within certain.

ranges. For instance, by using 5% stearic acid, one would, increase the porosity approximately 20%. In this instance of 1% or 1% of stearic will be required to lubricate while the 4% additional will serve to increase the porosity. The lubricating 1% of stearic in volatilizing-will also leave voids and cause porosity, but additional stearic will materially increase the porosity.

I propose to accomplish the sintering of the metallic briquette y the application of electrical heat obtained, through the resistance of the molded metallic article itself, or by eddy currents induced within the molded article by the use of an induction furnace While the article enveloped in a reducing atmosphere or an atmosphere of a reducing gas, though I do not desire to limit myself in this regard.

According to this method a high voltage electric current is passed through or into the metallic briquette while in a suitably vented chamber containing an atmosphere of the reducing gas, the heat. being produced by the resistance of or by eddy currents set up within the briquette itself.

Suitable reducing gases which may be used are hydrogen, helium, or ordinary coal gas commonly known as illuminating gas.

The current when applied by contact with the briquette is passed through the briquette from end to end while the same is disposed in the reducing gaseous atmosphere, similarly to the manner in which a welding current is passed through the pieces to be welded as they are held in the. clamps of an electric welder.

Induction furnaces are well known in the art and can be purchased on the open market. In these furnaces, the applied primary voltage in a separate winding causes a secondary current to be set up in the secondary winding. The briquette may be placed within the secondary winding and in the path of the duced current passing creates the sintering quette itself.

The briquette formedof the metallic particles coated with the solid volatilizable lubricant is protected against oxidation by the said lubricant coating or covering and induced current. Thus, the inthrough the briquette heat within the briupon heating the briquette in the reducing gaseous atmosphere, the lubricant is volatilized and any contained or existent oxides are effectively reduced and further oxidation is substantially prevented.

I claim:

1. The hereindescribed method of form-,

ing porousbodies from powdered metal comprising dissolving a solid volatilizable lubricant in a volatile solvent, mixing the solution' with the powdered metal, volatilizing the volatile solvent until it is removed leaving each particle of metal powder coated with a lubricating and oxidization preventing film of the solid volatile lubricant, forming the mix into a briquette under pressure, and subjecting the briquette to sintering heat in a reducing medium or atmosphere below sintering temperature, and to sinter'and coalesce the metal particles metal to metal with voids distributed throughout the mass. 2. The hereindescribed method of forming porous bodiesfrom powdered metal comprising incorporating stearic acid with the lubricant or other foreign material and with voids distributed throughout the metallic 3. The hcreindescribed method of forming porous bodies from powdered metals comprising dissolving stearic acid in a volatile solvent, mixing the solution with the powdered metal and allowing'the volatile solvent to volatilize until it is removed leaving each particle of metal powder coated with a lubricating and oxidization preventing film of the stearic acid, forming the mix under high pres sure into a briquette, and heating the briquette in a reducing medium or atmosphere to volatilize and drive ofi the lubricant stearic acid at a temperature below that required to sinter the metal, and at sintering temperature to sinter and coalesce the metal particles metal to metal with voids distributed throughout the mass.

. 4. The hereindescribed method of forming porous bodies from powdered metal comprising dissolving a fatty acid in a volatile solvent mixing the solution with the powdered metal and allowing the volatile solvent to evaporate, leaving the particles of metal powder coated with a lubricating and oxidization preventing film of the fatty acid, forming the mix into a briquette under pressure, heating the briquette in a reducing medium or atmosphere to volatilize and drive off the fatty acid at a temperature below that required to sinter the metal particles, and on reaching sintering temperature to sinter and coalesce the metal particles metal to metal with voids distributed throughout the mass.

5. The method of claim 3 in which is used an amount of fatty acid in excess of 1% of the mix, the 1% of fatty acid being suflicient to lubricate and produce porosity on volatilizing, and the excess being used to materially increase the porosity of the sintered article, the porosity being controlled by the amount of the excess.

6. The hereindescribed method of forming porous bodies from powdered metals comprising dissolved stearic acid in a volatile solvent, mixing the solution with. the powdered metal and allowing the volatile solvent tovolatilize until it is removed leaving each particle of metal powder coated with a lubricating and oxidation preventing film of the stearic acid, forming the mix under high pressure into a briquette, and heating the briquette to a sintering temperature.

7. The hereindescribed method of forming porous bodies from powdered metal comprising dissolving a fatty acid in a .volatile solvent, mixing the solution with the powdered metal and. allowing the volatile solvent to evaporate, leaving the particles of metal powder coated with a lubricating and oxidation preventing film of the fatty acid, forming the mix into a briquette under pressure, and

heating the briquette to a sintering temperature.

8. The method of forming an article of manufacture which consists in mixing powdered metals together, thoroughly distributin g s'tearic acid throughout said mixture, compressing the mixture to a briquette form, and sintering the briquette to 'form a hard porous structure and to simultaneously volatilize the stearic acid.

In testimony whereof, I aflix my signature.

CHARLES F. SHERWOOD;