US2216769A - Metal powder - Google Patents

Description

Patented a. 8, 1940 PATENT OFFICE 2,216,769 METAL POWDER Joseph E. Drapeau, Jr., Calumet City, 11]., and Louis G. Klinker, Hammond, Ind., assignors to The Glidden Company,

poration 01' Ohio Cleveland, Ohio, a cor- No Drawing. Application February 11, 1937, Serial No. 125,303

5 Claims.

The invention relates to metallic powders and has particular reference to a new type of powder.

Powdered metal has been used fora good many years in the decorative industries, to simulate 5 metallic leaf. This type of powder is made by milling or stamping thin sheets of metal, with a lubricant, and the resultant flake product, with its lubricant coating, has then been suspended in a menstruum for application.

More recently a demand has sprung up for metallic powders of nodule like shape, free of such libricating coatings, for manufacture into 'moldedmetal objects. The metallic powder is mixed with other metal powders to make a desired alloy, and the entire mass molded under high pressure to the desired shape. The molded mass is then sintered by heating, in order to sinter the mass and obtain the desirable mechanical strength.

This field of powdered metals has but recently been opened up. 'Because of the diiference in action between metal in the powder form, and in large homogeneous masses, an entirely new technique has had to be developed. Any treatment of powdered metal must be of such type as not to interfere with the powdery nature of the material; and this factor has led to considerable difiiculties in many instances.

Particularly in the manufacture of bearings and other molded objects made from powdered metals and non-metallic powders, extreme care in manufacture of the metal powder must be exercised, if a desirable finished molded object is to be obtained. One particularly essential characteristic is the manner in which the powder acts on molding. If, on the application of pressure, a molded object is obtained which is then to be sintered, it is highly desirable that the molded object should not shrink on sintering, as such shrinkage is always uneven, and the resultant object will be out of shape. The characteristic desired is a fixed or constant slight expansion, in order to permit of finishing to shape by machining. 45 It has been discovered that the desirable expansion on sintering can be obtained by using a highly compressible metal powder. With copper, the principal metal powder in use, a simple standard test has been adopted by some users, which comprisesmixing 20 grams of powder and /g%- graphite and placing the powder in a mold of 0.20 square inch cross sectional area. The mixture is subjected to a hydraulic pressure of 20,000 pounds per square inch. The height of the molded cylinder should not be over 1.070

inches, as experience has indicated that shrinkage is obtained on sintering a less compressed mass. Lengths from 1.000 to 1.070 inches are considered good.

Copper powder with sufficient compressibility 5 has been produced by electrolytic deposition of spongy copper, using cathodic current densities of 50 to 150 amperes per square inch, in the presence of certain colloidal agents. The spongy mass is washed as free as possible of electrolyte, 10 and dried under non-oxidizing conditions. The outstanding disadvantages of this type of powder are high cost, and a marked tendencyto reoxidize when exposed to atmospheric conditions. The traces of electrolyte salts apparently serve as 15 catalytic agents promoting oxidation of the copper powder.

Another method of making copper powder has been to stamp it out of thin sheets of copp r alloy metal, using a lubricant. The resultant powder, when examined under the microscope, is in the form of flat sheets, coated with lubricant. While excellent color and leafing are obtained in paint vehicles, the libricant results in poor mechanical strength and electrical conductivity. The excellent color is due to the alloying of small percentages of zinc or aluminum withcopper. This type of powder, because of its lubricant coating, is useless in the mechanical industries, and is largely confined to the paint industry. I

It has also been proposed to obtain copper powder by reducing cupric or cuprous oxide powder, in a furnace, by the use of various reducing gases. The process may be carried out on the furnace hearth, or in trays. The mass that is removed from the furnace in the formof sintered lumps or cakes, is then broken up by milling. The resultant powder has an apparent gravity ranging from just above 2.5 up to 3.0 and 40 higher; under the miscroscope, it gives the appearance of solid nodules. It has fair electrical conductivity, and may be used in bearings with fair results, but the compressibility is poor, the cylinders obtained on the standard compression test being 'often as high as 1.150.

. Another method suggested, and outlined in the co-pending Drapeau application, Serial Number 3,499, filed January 25, 1935, comprises the reduction of copper oxide powder at low temperatures, of the order of 350 to 750 F., with reducing gas, while subjecting the inass to constant agitation, followed by cooling of the reduced powder in the same atmosphere. The raw material should be a copper oxide (preferably cuprous oxide) powder prepared by oxidizing substantially pure copper by roasting; this is essential if the presence of occluded salts is to be avoided, as these cause rapid atmospheric corrosion. By the use of low temperatures, a porous structure is obtained, together with low apparent density and fairly good mechanical properties. The compressibility, however, is not good, the standard molding varying from 1.090 to 1.120 inches.

In our co-pending application, Serial Number 125,304, filed February 11, 1937, which has matured into Patent 2,181,123, Nov. 28, 1939, we have disclosed that these and other powders may be improved in compressibility and other properties by subjecting them to mechanical deformation, followed by heating in a non-oxidizing atmosphere below the sintering point of the metal to remove the internal stresses set up.

In this heating operation the upper temperature limit, is, of course, the point where the metal powder begins to sinter. Likewise, where copper oxide and other oxides are being reduced, even far below the sintering temperature, a certain amount of sintering occurs due to local overheating, particularly between small numbers of individual particles, thereby producing oversized material.

We have discovered that copper and other metal powders may be exposed to heat treatment, with considerably less tendency toward sintering, by the introduction into the powder of a small percentage of some carbonizable liquid or plastic solid which will form a thin film over the metal surfaces and be carbonized in place to produce a metal powder coated with a carbon film.

For example, we prepared a batch of substantially pure cuprous oxide, all passing through a 325 mesh sieve (particles less than 44 micron diameter). This was placed in a revolving drum, and coal gas was passed through, being burnt under the drum to maintain a temperature of 600 F. The reduction was continued for 36 hours, and the charge was then allowed to cool in the reducing atmosphere; it was a somewhat clinkered mass containing less than 0.20% oxygen. The product was given one pass through an impact hammer mill; the powder resulting gave only a 55% yield of material going through a 325 mesh sieve.

We then added 0.10% by weight of 20 Baum lubricating oil to the same amount of the same cuprous oxide, and made a run which exactly duplicated the original. The resultant product yielded of powder passing through 325 mesh, indicating considerably less sintering. The copper powder obtained, when viewed under a high powered microscope, showed the presence. of carbon on the surface of the particles.

In another example of our invention, we heated pounds of copper powder in a tray, in an atmosphere of city artificial gas consisting largely of hydrogen and carbon monoxide, to improve its compressibility. It was necessary to hold the heat down to 1,500 F., andafter a heating period of 60 minutes, sufiicient action had taken place to get the desired change in compressibility. Attempts to shorten the time by increasing the temperature were found useless, as the mass sintered to such an extent that the after milling required destroyed the effect of the treatment, or else produced excessive oversize powder which must be rejected in the classification system.

We then blended 0. of 20 Baum lubricating oil with the powder. It was then possible to heat the batch to 1,700 F., at which temperature surfiut'es.

cient compressibility change occurred in 20 min- The batch had not sintered, and was in just as good shape as the untreated batch which had been heated to 1.500 F. A microscopic examination disclosed the presence of carbon on the surface of the powder.

The tendency of iron powder to sinter into solid, or semi-solid masses during a reduction of iron oxides to pure iron powder is substantially overcome through the additions of 0.1% of 26 Baum lubricating oil. This not only substantially eliminates losses of iron powder as coarse oversize product but permits the production of a powder carrying decidedly more lower micron particle size product.

Similarly, we find that presence of these carbonizable liquids to the extent of 0.10% substantially overcomes sintering during heating treatment to improve the compressibility of iron powders. Certain other favorable characteristics were noted in this use of lubricating oil. The metal powder, when heated in steel containers without the oil, had tended to diffuse into the steel and alloy with it at the elevated temperatures employed. This resulted in a loss of powder yield, and a weakening of the container. The use of oil in the powder prevented this diffusion, as first the oil, and later the carbon, prevented the diffusion.

The type and amount of carbonizable liquid to be added varies with the time and temperature of heat treatment. Obviously, a product volatile at the heating temperature, without carbonizetion, is valueless. Similarly, the product must have the ability to spread over the individual particles of metal powder in a thin film, before carbonization, or it fails of its purpose. We have found that lubricating oils and greases are particularly suitable addition products. Other products which may be used are vegetable oils such as corn oil, olive oil, cottonseed oil, cocoanut oil, etc.

The amount of product added should be such as to carbonize to substantial completion during the treatment, but there should be sufficient present to insure the presence of carbon on the metal particle surfaces at the end of the reaction. The carbon formed tends to diffuse into the metal particles, and if heating is carried on for a sufficient period to carbonize the liquid and diffuse the carbon formed, the effect of the addition disappears. The presence of substantial quantities of uncarbonized lubricant is, of course, detrimental to the product. We find that amounts of the order of 0.1% of a percent of the metallic powder to be sufficient for the typical reduction on heating process. Much higher amounts should be avoided; if more than 0.25% is allowed to carbonize in the product, a visible darkening of color occurs.

The carbonizable compound need not be thoroughly admixed with the powder before heating. The type of product used becomes very mobile as the heating progresses, and will effectively penetrate the powder mass merely by being smeared about the container.

The carbon coated metal powders obtained show somewhat less tendency to dust than the untreated powders; and the thin film of carbon does not materially affect the color of the product.

While we have shown the preferred uses 01' our invention, it is obvious that it is applicable to any heating of powdered metal in which the desired end product is discrete particles of metal powder.

The method of producing the powder is claimed in our application Ser. No. 267,186, filed Apr. 10, 1939. K

We claim:

1. A metal powder comprising discrete particles of metalpowder carrying coatings of sub-.

stantially undiffused amorphous carbon on their surfaces.

2. A copper powder comprising discrete particles of copper powder carrying coatings of sub- Y stantially undifiused armorphous carbon on their surfaces.

3. A copper powder comprising discrete parof 0.1% by weight of the copper.

JOSEPH E. DRAPEAU, JR. LOUIS G. KLINKER.