Process for producing metal powders

BACKGROUND OF THE INVENTION

This invention relates to the production of metallic alloy powders. More particularly it relates to a process for producing finely divided homogeneous metallic alloy particles.

In powder metallurgy metal powders are generally com- 40 pressed and then heated under controlled conditions to thereby yield bars of metals that can thereafter be worked by normal metal working techniques to metals in various forms such as sheets, wire, rods, and the like. When single component or unimetal materials such as iron, tungsten, 45 molybdenum and the like are produced there are no problems concerning composition of particles. The primary concern is with the physical shape of the particles. Multimetal alloys are known to have some properties that are superior to unimetal materials. When an alloy is produced, ⁵⁰the distribution of the various metals within each of the particles becomes of major significance because of the effects upon the properties of the alloy. One of the major problems that heretofore faced those who attempted to produce metal powders suitable for the production of ⁵⁵metal alloys via powder metallurgy was a homogeneous distribution of all the metals in the alloys throughout the material produced. Physical mixtures of solid metallic compounds or metals when processed by standard powder metallurgy techniques resulted in the subsequent formation ⁶⁰of alloys having a non-uniform or non-homogeneous distribution of the various components. Other techniques such as heating the two or more metals to form liquids, mixing the liquid metals and then cooling the mixture under conditions that produce discrete particles overcome 65 some of the homogeneity problems, however, this added significant costs to the manufacture of the resulting product.

It is believed, therefore, that a process that produces multimetal powders that contain finely divided particles having a homogeneous distribution of all of the metals and materially reduces processing costs would be an advancement in the art.

SUMMARY OF THE INVENTION

In accordance with one aspect of this invention there is provided a process for producing multimetal powders containing discrete particles having the metals homogeneously distributed therein. The process comprises forming an aqueous solution consisting essentially of at least two thermally reducible metallic compounds and water, atomizing the solution into droplets having a maximum size of 150 microns into a chamber containing an atmosphere at a temperature sufficient to form discrete solid particles from said droplets and heating said particles in a reducing atmosphere at temperatures of from the reduction temperature of the metal compounds to below the melting point of any of the metals in the alloy.

For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the following disclosure and appended claims.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the practice of this invention an aqueous solution consisting essentially of at least two thermally reducible metallic compounds and water is formed. As used herein “thermally reducible” means that upon heating to a temperature of below about 1250° C. in a reducing atmosphere, such as hydrogen, the metallic compounds will decompose or disassociate to form metallic mixtures of metals without the formation of a liquid phase. It is also necessary that in addition to being thermally reducible the metallic compounds should be water soluble to at least the extent of 0.1 gram of the metallic compound in 100 grams of water at 25° C. Use of slurries where an appreciable amount of at least one of the metallic compounds is present in the solution as a solid is to be avoided since the solid particle can be larger than the desired droplet size and would tend to plug the atomizing means that is used to form the droplets. Although the particular metallic compound can vary depending upon the metal that is desired, the metallic salts of inorganic acids such as nitric and hydrochloric are suitable in most instances.

After the solution of the two or more metal containing compounds is formed, it is atomized in a chamber containing a gas that is heated to a temperature sufficient to evaporate a sufficient amount of water to convert the droplets into discrete solid particles that will not agglomerate to an appreciable extent. The temperature and flow of gas will be dependent upon the concentration of the metals in the solution, the particular metals and their tendency to form free-flowing hydrates. The particles will be less than about 150 microns since the maximum permissible droplet size is about 150 microns. In most instances, there is at least some size reduction during the drying in the chamber. Heated air or an inert gas such as flue gas is suitable to form the particles. In most instances the gas will flow upward through the chamber while the solution is being atomized at the top and the droplets will be dried as they fall through the gas. Inlet gas temperatures of

3,663,667 from about 400° C. to about 500° C. will normally be used with about 450° C. being preferred. The exit temperature of the gas will generally be in excess of about 100° C. to evaporate at least some water from the droplets at the point of entering. It is generally preferred to have the exit gas temperature above about 150° C.

After the particles are formed they are heated in a reducing atmosphere at temperature from above the reducing temperature of the compounds and below the melting point of any of the metals in the particles. The temperature is below the melting point of any of the metals therein but sufficiently high to cause the metal compounds to reduce and leave only the metallic cation portion or the original molecule. In most instances, a temperature of at least about 500° C. is required to reduce the compounds. Temperatures below 500° C. can cause insufficient reduction while temperatures above the melting point causes the particles to agglomerate. The metals in the resulting multimetal alloy particle can either be combined as intermetallic compounds or can be solid solutions of the various metal components. In any event there is a homogeneous distribution throughout each particle of each of the metals. The powders of this invention when processed by normal powder metallurgy techniques yield materials having a higher percentage of the theoretical density than the materials heretofore produced by other processing techniques that have the same ratio of metals therein.

To more fully illustrate the subject invention the following detailed examples are presented. All parts, proportions and percentages are by weight unless otherwise indicated.

Example I

About 1000 parts of a ferric chloride solution containing a 10% by weight iron content is mixed with sufficient nickel and cobalt oxides to yield an iron to nickel to cobalt weight ratio of 57:29:17. The solution pH is adjusted with hydrochloric acid and the solution fed to a spray drier and atomized to a droplet size of less than about 10 microns. The temperature of the entering air is about 450° C. and the flow is adjusted to maintain the exit gas temperature of about 150° C. The resulting particles are heated by raising the temperature gradually from about 390° C. to about 690° C. over about 21 hours. The resulting powder is essentially completely reduced as evidenced by the analysis of the oxygen content of less than 1 % on a sample of the powder. Analysis of samples by X-ray diffraction and micromerographs indicate a homogeneous distribution of iron, nickel and cobalt throughout each of the particles.

The powder is isostatically pressed into bar using about 40,000 pounds per square inch pressure and the resulting bars are sintered at about 1150° C. for about 3 hours. The bars are cold rolled and strips are formed having a thickness of about 10 mils. These strips exhibit improved properties as evidenced by a 15% to 20% greater elongation on the strips produced from the materials of this invention than on similar strips produced in substantially the same manner except that the powder is produced from iron, nickel and cobalt salts that are dry mixed and then reduced.

Example II

An aqueous solution containing an iron to nickel ratio of about 49:51 is prepared from iron and nickel nitrates. The solution is spray-dried as in Example I and thereafter reduced in a furnace having a hydrogen atmosphere at about 500° C. for about 3 hours. The particle size of the resulting powder is about 3 microns (Fisher Sub-Sieve Sizer). The density of the powder after pressing and sintering for about 4 hours at about 1100° C. yields a material having a density of about 93% of theoretical for a Ni-49 Fe alloy. A material prepared by blending iron and nickel nitrates to achieve a relatively uniform mixture of the two components when reduced, pressed and sintered in the same manner yields a material having a theoretical density of about 81% of theoretical for a Ni-49 Fe alloy.

Example III

An aqueous solution is prepared by dissolving nickel chloride and chromium chloride in about 1000 parts of water to yield a nickel and chromium content of about 10% by weight and in an 80:20 weight ratio respectively. The solution is atomized to droplets of less than about 150 microns in a chamber having an inlet gas temperature of about 500° C. The particles are dried at about 150° C. for about 2 hours and then placed in a reduction furnace having a slight positive pressure of hydrogen. The temperature in the furnace is raised from 150° C. to about 900° C. over a period of about 12 hours. X-ray analysis of samples of the powder thus produced indicate a homogenerous distribution of nickel and chromium throughout each particle. X-ray diffraction analysis indicates a solid solution of nickel and chromium. Analysis of samples of materials prepared by dry mixing of the same raw materials and reducing the mixture under similar conditions indicate non-homogeneous distribution of nickel and chromium. The material is suitable as a component for producing nickel-chromium alloys by standard powder metallury techniques.

Example IV

An aqueous solution is prepared in essentially the same manner as in Example III except that a solution containing a weight ratio of iron to copper to indium of about 85:10:5 is prepared and atomized to produce particles having an average size of about 75 microns. After drying at about 150° C., the particles are reduced in a hydrogen atmosphere by raising the temperature at a relatively uniform rate to about 900° C. over a period of about 12 hours. Analysis of samples of the resulting powder indicate a homogeneous distribution of iron, copper and indium. The material can be mixed with iron powder to produce a ferrous alloy by standard powder metallury techniques.

While there have been shown and described what are at present considered the preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claims.