US3498782A - Compactible fused and atomized metal powder - Google Patents

Description

United States Patent 3,498,782 COMPACTIBLE FUSED AND ATOMIZED METAL POWDER Donald R. Spink, East Amherst, and Allen C. Goodrich, East Aurora, N.Y., assignors to Amax Specialty Metals, Inc., a corporation of Delaware No Drawing. Filed Feb. 18, 1966, Ser. No. 528,390 Int. Cl. B22f 1/00 U.S. Cl. 75-.5 4 Claims ABSTRACT OF THE DISCLOSURE Metal powders produced by fusing and a-tomizing metals selected from Groups 4, 5 and 6, the rare earths, and the actinium series of the periodic table and their alloys are made highly compactible by reducing their relatively high energy state in the as-produced condition to a lower energy state accompanied by the discharge of energy in the form of heat. This reduction in energy state is accomplished by controlled annealing or by a plurality of compacting and recrushing steps, or a combination thereof.

This invention relates to compactible metal powders and a process for making powders compactible. More particularly this invention rel ates'xto plasma-arc produced powders of high melting metals such as those selected from Groups 4, 5, and 6, the rare earths and the actinium series of the periodic table and a process for making these metal powders more compactible.

The necessity for metal parts capable of operating under severe conditions, such as in nuclear reactors where such parts must be highly corrosion resistant and must possess high temperature resistance and mechanical strength, has increased the demand for parts formed from alloys of such high melting metals as zirconium, titanium, tungsten, tantalum, uranium, and niobium. Fabrication of these metals and their alloys has been restricted due to ease of contamination of these metals by elements such as oxygen, nitrogen, and iron, and the difiiculty in working the metals by conventional metal working techniques. For example, parts formed from zirconium based alloys frequently exhibit undesirable anisotropic properties as well as low corrosion resistance when formed by conventional metal working techniques. The economical optimization of corrosion properties, physical properties and fabrication of the aforementioned metals and their alloys have been shown to lie in the use of the powder metallurgical techniques, and these are an object of this invention.

Alloys of the aforementioned group of metals, pan ticularly zirconium alloys, are frequently used in the alpha stabilized form. Alpha stabilized alloys are those in which the alpha phase is stabilized, by alloy additions, to temperatures above that which it normally survives. -In the alpha stabilized form the effect of conventional metal working and heat treating operations on the mechanical and chemical properties of the alloy is readily apparent. An alpha stabilized alloy comprising 1.2 to 1.7 weight percent tin, 0.07 to .20 weight percent iron, 0.05 to 0.15 weight percent chromium, and 0.03 to 0.08 weight percent nickel, and referred to in the industry as zircaloy 2, is particularly suitable in regard to neutron absorption, corrosion resistance and mechanical strength. However, when prepared by conventional metal working techniques this alloy may exhibit mechanical weakness and poor corrosion resistance which could cause premature failure in stressed applications. Using conventional techniques zircaloy is frequently forged starting at 1700 to 1800 F. for 12 to 16 dia. ingots. Working is completed well above the 1450 F. so that a condition of slow cooling from 3,498,782 Patented Mar. 3, 1970 the apha-beta phase is present during the production of forged bars.

The mechanical weakness may be associated with a relatively high degree of anisotropy of properties (i.e. the finished articles have different chemical and physical properties in one direction than in other direction). This slow cooling tends to form a microstructu-re of large alpha zirconium grains of relatively low alloy content surrounded by a network of coarse grains of intermetallic compounds and this microstruct-ure can exhibit anisotropic properties, especially in objects having thin cross sections.

Moreover such a microstructure has been observed to yield poor resistance to corrosion apparently due to the intermetallic compounds. For example, the precipitation of iron and chromium intermetallic compounds from slowly cooled alloys was found to be the cause of intergranular corrosion in the heat affected zone of welds made in commercial zirconium of high impurity content.

Heat treatment is required to affect a resolution of the offending intermetallic compounds in order to avoid the undesirable side eifects which may occur when such metals as zirconium, hafnium, titanium and the like are worked by conventional techniques. It appears to be necessary to change at least a major portion, if not all of the structure, to the beta phase since the beta phase has a greater solubility for the alloying elements. Rapid quenching from the beta phase temperature region is performed to prevent the segregation of intermetallics which may be subsequently worked into stringers which extend through the grains for considerable distances (as much as inch or more) causing anisotropic properties and areas of reduced corrosion resistance.

An example of such heat treatment is disclosed in U.S. Patent 2,894,866 to Picklesimer wherein a procedure is set forth for preventing the formation of intermetallic stringers and anisotropic properties during the fabrication of alpha stabilized zirconia based alloys. This process includes performing major size reduction at :a malleabilizing temperature excluding the alpha plus beta range, heating the workpiece to a temperature above 970 C. {beta range) for at least approximately 30 minutes, cooling the workpiece rapidly, working at a temperature below approximately 500 C. to reduce the cross sectional area of the workpiece by at least 20 percent, annealing at a temperature from approximately 700 C. to 810 C. for at least approximately 15 minutes and finally cooling the article thus fabricated. This involved procedure involves many possibilities for the introduction of undesirable interstitial elements and other contamination. Moreover this procedure does not readily lend itself to the production of intricate final shapes normally required for nuclear reactor or other applications.

Powder metallurgy offers an attractive alternate approach to the intricate shaped hardware of many industries. In the fabrication of zircaloy 2 and other metals, by powder metallurgy techniques, the constituents may be blended to the desired homogeneity, pressed into an intermediate high density green compact which requires a minimum of hot or cold working and annealing to provide the desired finished shape. The powder metallurgical technique provides a simpler method which may require less costly fabricating equipment. The application of this powder metallurgical technique and using fused and atomized particles which have been rapidly quenched from the beta phase excludes the formation of intermetallic stringers and coarse preferentially oriented crystals by introducing only fine crystals which are randomly oriented. A final shape more resistant to corrosion, of high purity, having improved mechanical properties and a simplified fabricating technique are advantages which result from the availability of the compactible microspherical powder which is a subject of this invention. The application of plasma produced microspherical powders which had a desirable shape and chemical purity was, prior to this invention, impractical because of the poor compactibility (low compact green strength and low compact density) of this material.

Accordingly, it is an object of this invention to provide metal powder produced from high melting metals such as those of Groups 4, 5 and 6, the rare earths, and actinium series of the Periodic Table and their alloys, which is readily and easily compacted into a dense, homogeneous, isotropic shape.

Another object is to provide a fused and atomized metal powder which can be formed into a compact which has high green strength.

A further object is to provide a process for improving compaction of metal powders at moderate pressures.

A still further object of this invention is to provide a process for making compactible metal powder from fused and atomized metal powders.

Various other objects and advantages will appear from the following description of the embodiments of this invention, and the novel features will be particularly pointed out in connection with the appended claims.

We have found that metal powders formed by fusing and atomizin-g metals selected from Groups 4, 5, and 6, the rare earths, and the actinium series of the Periodic Table can be treated in accordance with this invention to form highly compactible metal powders which are capable of being formed by powder metallurgical techniques into articles possessing excellent chemical and mechanical properties.

The metal powders treated in accordance with this invention are produced from Groups 4, 5, and 6, the rare earths, and the actinium series of the Periodic Table and include such metals as zirconium, hafnium, titanium, tungsten, tantalum, niobium, lanthanum, thorium, uranium, and cerium. The metal powders are produced by fusion and atomization, preferably by plasma jet techniques. In producing metal powder by plasma jet techniques, the metal or alloy to be treated may be introduced as a shape, such as a rod or bar, or as a powder, into a stream of inert gas that has been heated to such a temperature that it is dissociated or ionized. The ionized gas sweeps over the metal shape melting the surface metal and carrying off the molten metal asdroplets in the gas stream. The metal droplets are subsequently atomized and extremely rapidly quenched and the resultant product is in the form of uniformly shaped microspheres which may range in size from about +60 mesh to about 325 mesh.

There are several advantages to using metal powder produced by plasma techniques. First, metal alloys, as well as pure metals, may be converted into powder efiiciently and economically. Other methods of producing alloy powders are rather cumbersome, and usually require long diffusion treatments to insure complete alloying. Secondly, the metal powder is of high purity and uniform shape and structure due to the method of forming the metal droplets and their rapid quenching. The resulting particles, known as microspheres, are less susceptible to surface take-up of impurities because the uniform spherical shape of the microspheres provides low surface to mass ratio. The spherical shape of the particles and the method of forming the microspheres avoids crystal orientation that could contribute to anisotropy. Prior to this invention the use of fused and atomized metal powders of metals from Groups 4, 5, and 6, the rare earths, and the actinium series of the Periodic Table has been restricted since such powders are only moderately compactible and compacts made from these metal powders are extremely weak and may require pre-sintering before they are capable of being handled. On the other hand compacts made from fused and atomized metal powders which have been processed according to this invention are highly compactible and the compacts, before sintering, may achieve more than 97 percent of the theoretical density of the component metal.

The relatively uncompactible nature of the fused and atomized powders of the metals of Groups 4, 5, and 6, the rare earths, and the actinium series of the Periodic Table is attributed to the high energy state in which the powder particles exist in the as-produced condition. This high energy state is presumably due to the extremely rapid quenching of the molten droplets of metal that occurs after fusing and atomizing the metal during the process for producing fused and atomized metal powder. We have found that lowering the energy state of the particles by annealing or cold working the powder in accordance with this invention the particles become highly compactible. This changing of the particles from the high energy state to a low energy state is marked by a release of the energy in the form of heat and is accomplished without deleteriously affecting the chemical and physical properties of the metal or alloy powder so treated.

In the practice of one embodiment of this invention, fused and atomized metal powders are annealed under an inert condition at a temperature above 300 C. but below the temperature at which the crystals change to a less corrosion resistant state. Inert conditions, as used throughout this specification, mean conditions wherein the environment surrounding the particular metal or alloy being annealed is substantially unreactive with that metal or alloy. Such an inert condition exists for example when annealing in a vacuum or in an atmosphere of argon. However this term includes annealing a metal or alloy in any atmosphere which is unreactive with the material being treated and may include annealing in air in some cases. The temperature varies with the metal or alloy being annealed. For example when using zirconium and zirconium alloys the temperature at which the crystals change to a state of poor corrosion resistance is approximately 870 C. Therefore when annealing zirconium or its alloys, such as zircaloy 2, according to this invention, the annealing temperature is maintained between 300 and 870 C. The upper annealing temperatures are determined from experience and by a study of the phase diagrams of the particular metal or alloy being treated.

The exothermic nature of the change that is coincident with the change to greater compactibility (higher green strength and density of compacts) has led to the following theory. The rapidly quenched, fused and atomized particles are in a high energy state. On heating and holding at temperatures above 300 C. (more particularly at a temperature of approximately 780 C. for zirconium alloys such as zircaloy 2) an exothermic reaction accompanies the change to a lower energy state which is more amenable to compacting. This lower energy state can also be achieved by cold working.

Thus, although it is the preferred embodiment of this invention to anneal the microspheres in order to aifect the change of crystalline energy state from the uncompactible high energy state to the compactible low energy state, it is within the scope of this invention to affect the change by a multiplicity of compacting and crushing steps. I. is also within the scope of this invention to lower the particle energy level by a combination of annealing and compacting and crushing steps.

The compacts are usually formed at a pressure of between 50,000 p.s.i. and 200,000 p.s.i. using conventional powder metallurgical techniques and equipment.

Compactibility was determined by measuring the volume and weight of a green compact using ASTM procedure (D70-52). Compact green strength was determined by the following procedure. Five gram samples of the metal powder being tested were pressed in a cylindrical mold having a diameter of /2 inch. These cylinders (not less than 5 per test) were dropped from a height of 36 inches into a 2000 ml. stainless steel beaker having a firmly supported flat bottom. The contents of the beaker were carefully poured onto an 8 mesh screen. The pieces retained on the 8 mesh screen were weighed. This weight when divided by the total sample weight and multiplied by 100, resulted in the compact green strength for the article being tested.

The objects and advantages of this invention are shown in greater detail in the following examples and by reference to the appended claims.

The following example illustrates the increase in powder compactibility obtained when fused and atomized metal powders are treated according to this invention.

EXAMPLE 1 A fused and atomized metal powder, formed by conventional plasma jet technique, such as described above, was prepared from a zirconium alloy comprising, in addition to zirconium, 1.21.7 weight percent tin, .05-.l weight percent chromium, .07.20 weight percent iron, and .03-.08 weight percent nickel.

The zirconium alloy metal powder had a mesh size ranging from +60 to --325 US. Standard Mesh.

Five gram samples of the fused and atomized zirconium alloy powder were pressed at various pressures to test the compactibility of the powder in the as-produced condition. Tests were conducted on the as-produced powder both with and without a die lubricant The die lubricant was a powdered fat manufactured by the Capital City Products Company, Columbus, Ohio and sold under the mark Sterotex. The results are shown in Table I.

TABLE I.GREEN" STRENGTH OF COMPACIS FORMED FROM AS-PRODUCED ZIRCONIUM ALLOY FUSED AND ATOMIZED POWDER Percent compacted The results of Table I show that even at pressures of 200,000 p.s.i.g., using die lubricant, the compactibility of the as-produced powder is relatively poor.

To improve compactibility, samples which had been pressed at 150,000 p.s.i.g. without die lubricant were recrushed and recompacted an additional four times. The compacts were tested for green strength after each recompaction. Recrushing may be carried out by any conventional means and although the compact may be reduced to a coarser powder than the original metal powder, it is preferred that crushing be continued until the compact is reduced to powder of substantially the same mesh size as the original powder. Recompacting was carried out at 150,000 p.s.i.g. Table II shows the improvement in powder compactibility that was obtained by mechanically working the fused and atomized metal powder of Example 1.

TABLE II.EFFECT OF CRUST-TING AND RE COMPACT- ING 0N FUSED AND ATOMIZED METAL POWDER Number of recrushing and recompacting cycles:

Green strength (percent compacted) As can be seen from the results of Table II, fused and atomized zirconium alloy powder, which is only moderately compactible in the as-produced condition, forms compacts of high green strength which are at least 99 percent compacted after four recrushing and recompacting cycles.

The following example illustrates the increase in powder compactibility when fused and atomized metal powder is annealed in accordance with this invetion.

EXAMPLE 2 To illustrate the effect of annealing on the compactibility of the microspherical powder, samples of zirconium microspherical powder, as produced in Example 1, were annealed in the following manner. The microspherical powder was placed in a cool annealing furnace C.) and the furnace was purged with argon to exclude oxygen. The temperature of the furnace was gradually increased to 775 C. :25" C. The temperature was increased gradually because at approximately 600 C. the power in the furnace began to give off energy in the form of heat and consequently less heat input was needed in order to raise the temperature of the powder to the desired annealing temperature. It was observed that if the furnace temperature increase was too rapid the exothermic nature of the fused and atomized powder caused the furnace temperature to rise above 810 C. thereby forming undesirable corrosion susceptible crystals in the microspheres and the mechanical properties of articles containing this material may be very poor.

The furnace was held at 775 C.:25 C. for sufiicient time to allow substantially all of the energy to be discharged in the form of heat (approximately three hours) and then cooled slowly to room temperature. The annealing time may vary depending on the size of the charge and the temperature at which the energy discharge is carried out. For example the energy discharge which marks the energy level change of the particles will occur more rapidly at 775 C. than at 300 C. Five gram samples of the annealed powder were pressed in a /2 inch diameter cylindrical mold at pressures of 50,000 psi. and 150,000 p.s.i. Compact green strength was measured in the same manner as in Example 1. The results are set forth in Table III.

TABLE IIL-EFFECT OF ANNEALING ON COMPACTI- I SILITY OF FUSED AND ATOMIZED ZIRCONIUM ALLOY POWDER Compacting pressure, p.s.i.: Percent compacted 50,000 99+ 150,000 99+ Although the examples of this specification are directed toward fused and atomized powders of zirconium and its alloys it is within the scope of this invention to include the high melting metals of Groups 4, 5, and 6, the rare earths, the actinium series and their alloys, which are only moderately compactible when formed into a powder by fusion and atomization.

While the invention has been described in connection with specific embodiments thereof, it will be understood that it is capable of further modification, and this application is intended to cover any varations, uses, or adaptations of the invention. It will, therefore, be recognized that the invention is not to be considered as limited to the precise embodiments shown and described but is to be interpreted as broadly as permitted by the appended claims.

We claim:

1. A method for producing highly compactible metal powder from fused and atomized particles of metals selected from a group consisting of Groups 4, 5, 6, the rare earths, and the actinium series of the Periodic Table and their alloys which comprises reducing said metal particles from a high energy state to a lower energy state as evidenced by the release of energy in the form of heat whereby the particles are readily compactible into compacts having high green strength, wherein said particles are reduced from said high energy state to said lower energy state by a plurality of compacting and recrushing steps.

2. A method for producing highly compactible metal powder from fused and atomized particles of metals selected from a group consisting of Groups 4, 5, 6, the rare earths, and the actinium series of the Periodic Table and their alloys which comprises reducing said metal particles from a high energy state to a lower energy state as evidenced by the release of energy in the form of heat whereby the particles are readily compactible into compacts having high green strength, wherein said particles are reduced from a high energy state to a lower energy state by a combination of steps comprising heating said powder under inert conditions to a temperature of at least 300 C. and compacting and comminuting said powder at least once whereby said powder is made compactible.

3. The method of claim 1 comprising the steps of compacting said powder at a pressure of at least 50,000 psi. and comminuting said compacted powder, repeating said compacting and comminuting steps at least once thereby reducing said particles of said powder from said high energy state to said lower energy state.

4. A highly compactible metal powder comprising fused and atomized particles of metal selected from a group consisting of Groups 4, 5, 6, the rare earths, and the actinium series and their alloys, said powder particles having been produced by the method of claim 1.

References Cited UNITED STATES PATENTS OTHER REFERENCES Modern Developments in Powder Metallurgy, vol. 2, p. 269, John Googin et al.

HYLAND BIZOT, Primary Examiner W. W. STALLARD, Assistant Examiner U.S. Cl. X.R.