US3888657A

US3888657A - Process for production of metal powders having high green strength

Info

Publication number: US3888657A
Application number: US353659A
Authority: US
Inventors: Erhard Klar; Elbert K Weaver
Original assignee: SCM Corp
Current assignee: SCM Metal Products Inc
Priority date: 1970-12-30
Filing date: 1973-04-23
Publication date: 1975-06-10
Anticipated expiration: 1992-06-10

Abstract

Metal particles of high green strength are made by the process of thermally agglomerating a mass of finely atomized particulates of said metal into a self-supporting cake, then milling said cake into a body of agglomerate particles having average particle size substantially greater than said atomized particles. Water or gas atomization can be used, and the metals can be ferrous or nonferrous. The product is useful in powder metallurgy.

Description

United States Patent Klar et all. June 10, 1975 1 PROCESS FOR PRODUCTION OF METAL 3,479,180 11/1969 Lambert 75/213 POWDERS HAVING HIGH GREEN 3,482,963 12/1969 Osborn r 75/0.5 B 3,539,334 11/1970 Goedde1.... 75/0.5 B STRENGTH 3,647,417 3/1972 Wetzel 75/5 Ilnventors: Erhard Klar, Baltimore, Md.; Elbert K. Weaver, Westborough, Mass.

Assignee: SCM Corporation, Cleveland, Ohio Filed: Apr. 23, 1973 Appl. No.: 353,659

Related US. Application Data Continuation of Ser. No. 102,682, Dec. 30, 1970, abandoned.

US. Cl 75/0.5 B; 29/420; 75/200 Int. Cl B22f 9/00 Field of Search 75/05 B, 5, 200, 211,

References Cited UNITED STATES PATENTS 10/1969 Patrick 264/118 Primary ExaminerL. Dewayne Rutledge Assistant Examiner-Peter D. Rosenberg Attorney, Agent, or FirmRichard H. Thomas [5 7] ABSTRACT Metal particles of high green strength are made by the process of thermally agglomerating a mass of finely atomized particulates of said metal into a selfsupporting cake, then milling said cake into a body of agglomerate particles having average particle size substantially greater than said atomized particles. Water or gas atomization can be used, and the metals can be ferrous or non-ferrous. The product is useful in powder metallurgy.

16 Claims, 2 Drawing Figures PATENTEUJUK'IQ 1975 3,888,857

awe/MM ERHARD KLAR EL BERT K. W EAVER W wW W PROCESS FUR PRODUCTION OF METAL POWDERS HAVING HIGH GREEN STRENGTH This is a continuation of application Ser. No. 102,682, tiled Dec. 30, 1970, and now abandoned.

This invention relates to a process for producing metal particles having desirably high green strength and more particularly to such process wherein the product metal particles are generated from conventionally finely-atomized particulates of metal.

Heretofore, it has been proposed to obtain steel particles of high green strength by a process of water atomization wherein irregular chain-like or clump-like atomized particles were produced, which interlocked well at the pressures used for powder metallurgy and yielded very good green strength (U.S. Pat. No. 3,325 .277 While such process is an excellent and economic one, it is restricted as to formation of chain-like or clump-like agglomerates of steel under a narrow range of processing conditions and is oflimited versatility.

It has also been proposed to prepare various metal powders optionally with additives such as selenium, tellurium, and other additives to make irregular particles which compact well under powder metallurgy pressures give articles of desirably high green strength (U.S. Pat. No. 3,383,198). However, for metals other than steel by the process of U.S. Pat. No. 3,325,277, and even for steel using more conventional gas or water atomization processes, it has been generally recognized in the powder metallurgy art that one conveniently cannot make desirably high green strength metal particles by atomization.

Advantages of our process over prior proposals include versatility with respect to metals and particle size control thereof, high purity of metal where necessary or desirable because additives are not required, control of flowability of the resulting particles and control of purity, e.g. oxygen content or insolubles such as refractory oxides.

The instant process comprises thermally agglomerating a loose mass of discrete finely-atomized particulates of the metal in process into a self-supporting cake, and milling said cake into a body of agglomerated particles having average particle size substantially greater than said original atomized particulates.

The drawings are reproductions of scanning electron micrograph pictures (S.E.M.) showing the unique topology of agglomerate metal particles made in accordance with the principles of this invention.

FIG. 1 shows such particles thermally agglomerated from discrete gas-atomized copper particulates and milled, said particles being under 175 power magnification.

FIG. 2 shows inventive agglomerate metal particles thermally agglomerated from discrete water-atomized copper particulates said particles being under about 1,500 power magnification. The preparation of these unique particles is described more fully in the examples which follow.

In general, the atomized particulates can be formed by conventional gas atomization or liquid atomization processes. Advantageously, for efficiency and economy, water atomization is employed. A leading patent in liquid atomization is the Batten U.S. Pat. No. 2,956,304, for making such atomized particulates.

Other pertinent U.S. Pat. Nos. in the gas atomization art include 2,968,062 and 3,253,783.

The starting atomized particulates for our thermal agglomeration treatment form a loose mass, e.g. a bed, pile, or similar assembly. For practical operation, the major portion of such mass should be finer than 100 mesh, and advantageously finer than 325 mesh with only a trace exceeding 100 mesh size, (Tyler Standard Sieve). If the major part of the starting particulates is substantially larger than 100 mesh, the green strength of the resulting product agglomerate particles is substantially reduced. Preferably, the starting atomized particulates are finer than about 400 mesh, with only a trace thereof exceeding 100 mesh in size. Generally, the proportion of product particles finer than 325 mesh does not substantially exceed 30 percent, and product particles coarser than about mesh are present in trace amounts only.

While not intending to be bound by any theory, it is our belief that the porosity and peculiar topology of the particles resulting from the inventive process makes for their good green strength. Additionally, this process affords excellent independent control over the atomizin g and the agglomerating rather than forcing compromise of any aspects of one operation for the sake of the other.

Thus, one can visualize a single ideally smooth, spherical, metal particulate of, say 0.1 inch diameter, having no pore volume whatsoever, accordingly, a pore volume-to-weight ratio of zero. However, when an infinite number of these are joined together at points of contact (agglomerated) in a gross mass, the pore volume of the interstices between such spheres reaches a constant limiting value. Unfortunately, such mass is too large to handle well and to pour into molds.

Now, we have discovered that by selecting a mass of small enough atomized particulates, thermally agglomerating them into a self-supporting cake, then braking the cake into small agglomerate particles which are substantially larger in average mesh size than that of the original particulates, e.g. up to seven times larger, advantageously about 1.5 to 7, and preferably 3 to 7 times larger, the maximum ratio of the pore volume to weight of an individual agglomerate particle can be approached in a practical way to yield a product of useful, pourable size for powder metallurgy.

We have further discovered that the green strength of the product takes a steep rise from a low value to a desirably high one when such maximum ratio is approached in accordance with our processing. The most dramatic green strength obtained is for milled cakes having original particulate makeup of 400 mesh (with only a trace on mesh) milled to a resulting agglomerate particle size of to 100 mesh (with only a trace on 100 mesh). Lesser, but valuable, green strength can be obtained for milled cakes having original particulate makeup of 100 to 140 mesh milled to 80 to 100 mesh resulting agglomerate particle size. Table III, below, illustrates this observation.

The green strength of conventional compacting grade metal powders decreases sharply if the powder is densified by grinding and milling to greater apparent density than its original density. In contrast, powders made by the technique described herein are remarkably resistant towards such deterioration from grinding or milling. This behavior is of great practical significanoe, since it permits an independent control over the apparent density, which in turn is useful to control the flow and die-filling depth of the powder.

To preserve purity ofthe particles, the thermal agglomeration is donepreferentially under conventional non-oxidizing conditions, e.g. in the presence of-hydrogen, carbon 'monoxide, nitrogen, mixtures thereof, using. gas, electric resistance or induction heat, and

generally in an atmosphere containing no reactable ox- .ygen. However, if oxidation can be tolerated, such nonoxidizing conditionswould not necessarily. be used in thethermal agglomeration or in the optional annealing Preparatory or during the thermal agglomeration, it

is generally'undesirable to physically compact the atomized particulates too greatly. Advantageously, one

' piles them into a loose mass of'about 2-6 inch'thickness without vibration, extraneous pressure or other induced compaction. The bed depth, e.g. in a boat, on a belt, or in a sagger, often will be'dictated by the structural strength of equipment. The agglomeration is done at temperatures of approximately one-half the absolute melting point temperature of the metal in process up to a temperature approaching the melting pointof the atomized particulates for times up to about 30 minutes, e.g. a few minutes to about 30 minutes, usually at least about 5 minutes. A form of sintering takes place in the thermal agglomeration step of the atomized particulates. The agglomeration can be done continuously or batchwise.

Ordinarily the sintered and milled product from our process is characterized by a low apparent density, e.g. below 50 percent of the real density of the metal from which it is made.

The cooled cake can be milled by conventional means, e.g. hammermill, disc mill, fluid energy mill or the like, and if necessary, classification procedures such as screen classification can be used on the resulting product to obtain especially desirable fractions. The following examples show how this invention has been practiced, but should not be construed as limiting the invention. Green strength of atomized particulates and product particles is measured in accordance with ASTM B3l2-64 except that a constant compacting pressure of 12 tsi is used. Particle size distribution recited throughout the specification, examples and claims are US. Sieves Equivalent (Fine Sieves). It is understood that Tyler Standard Screen Scale Sieves and US. Sieves can be used interchangeably.

EXAMPLE 1 Seventeen pounds of copper were melted in a kw Ajax induction furnace. The melting was done in a graphite crucible. Molten copper was water-atomized in accordance with the principles of the Batten US. Pat. No. 2,956,304 at a rate of 40 pounds of metal per minute under a water pressure of 2000 psi and a water velocity of 400 feet per second. The orifice of the tundish feeding zone measured 3/16 inch. The yield was 92 percent finer than 100 mesh and 55 percent finer than v 325 mesh. Thermal agglomeration was done in a laboratory tube furnace with these atomized copper partic ulates loosely piled in a boat to a bed depth of l /inches. Temperature of thermal agglomeration was 1'500F under H gas atmosphere, and total timethereof was about 10 minutes. The resulting selfsupporting porous cake was hammermilled into'product agglomerate particles virtually all but a trace passing mesh,'thesize distribution being'about 14 percent finer than 325'mesh, '20 percent between 200 and 325 mesh, 12 percent between 80 and mesh, and

the balance between 100 and 200 mesh. This powder had green strength of 1380 psi. In contrast, startingat omized particulates, has a green strength of psi 5 when tested the same way.

EXAMPLES 2-5 Examples 25'are shown in Table'l. The same steps were followed as inExample 1, except that the proportion of finer than 325 mesh particulates in the mass thereof subjected to thermal agglomeration ranged from 37 percent to 72 percent, with all but a trace passing l00. mesh. The porous cake formed was milled to v virtually all but a tracev passing 100 mesh, the size distribution being about :20 percent finer than 325, 30 per% cent between'200 and 325 mesh, and the rest-between 100 and 200 mesh.

7 TABLE I Example No. '2 3 4 5 --325 mesh particulates in feed, 63% 69% 72% Thermal agglomeration temperature, "F Time, minutes Atmosphere Apparent Density g/cc. according to ASTM 8212-48 Flow (sec/50g) according to ASTM B213-48 H Loss according to ASTM El59-63T HNO lnsolubles according to ASTM El94-62T Density of compacted samples according to ASTM B33l63T Green strength of atomized starting particulates, (psi) Green strength of agglomerate product particles, (psi) Compacts for green strength testing were made with the addition of 0.5 percent stearic acid in Examples 3, 4, and 5, and with no additive in Example 2.

EXAMPLES 69 Samples of the product agglomerate particles of these Examples 6-9 were compacted with 0.5 percent stearic acid, then sintered for minutes at 1500F, then compared to compacts made in like manner from a conventional compacting grade of copper powder made by reducing copper oxide.

The foregoing illustrates some of the desirable properties of the instant process, for example: adaptability of the metal for powder metallurgy; good green strength, high purity, high sintered strength both with and without tin, and excellent compressibility.

EXAMPLE 10 To ascertain the relationship between starting particulate size and product agglomerate particle size with respect to green strength, the following tests were run with nitrogen gas-atomized copper particulates of three selected screen fractions as individual batches. Each selected fraction was subjected to thermal agglomeration under conditions identical to those set forth in Example One. Each resulting cake was milled to make a mass of product agglomerate particles. Each mass was classified into fractions of the size ranges indicated in Table III, Sections A, B, and C. These classified product fractions were compacted to density of 7.00 grams per cc by the use of the pressure indicated. Compactions for green strength weremade without the use of admixed stearic acid.

Example No. 15 7 8 9 Conventional Powder Sintered Properties (Plain Copper) Green Density g/cc 6.32 15.38 6.24 6.33 6.30 Compacting Pressure, (tsi) 13.3 114.0 13.3 13.9 16.5 Sint. Density, g/cc 6.1 1 615 6.03 6.09 6.33 Sint. Modulus of Rupture, lpsi) 17.500 18,600 16,200 14,900 9,400 Dimensional Change 1%) +1.06 +1.29 +1.30 +1.52 +0.47

. TABLE III-A Section B For comparison, the Same conventional compacting Starting Particulates Fraction 10 microns to 400 mesh er owder was closed with tin owder and Compacting grade of copp p p Product Fraction Apparent Den- Green Pressure treated in like manner shown. Samples of the product h sity, gms/cc Strength, psi Used, tsi

agglomerate particles of these Examples 6-9 were com- Starting culates without pacted with 0.5 percent stearlc acld and 10 percent tin further treatment 485 2% 1 5 owder then sintered for 15 minutes in h dro en at 325 to 400 mesh 3.32 13 p i y g 200 to 325 mesh 2.70 1860 15.8 1,500F. The tin powder used in all instances had the 140 to 200 mesh 3%: 5828 {2.2

t 100 to 140 mesh following characteristics; apparent denslty 3.90 g/cc 80 to 00 mesh 5E8 and virtually all but a trace finer than 100 mesh. 50 mesh Example No. b 7 8 9 Conventional Powder Sintered Properties(Copperl 0% Tin) t g2? Densl y 6.32 6.30 6.31 6.29 6.32 Com actin PFCSSEH'B, (t si) 111.8 11.5 12.0 11.9 14.2 D 't ens, y 600 5.98 5.97 5.72 5.85 Sint. Modulus of Rupture, lpsi) 29,000 128,900 28,600 24,500 22,600

' 1 Ch i izii ange +2.21 +2.23 +2.05 +2.65 +2.59

Based on 12500" die size.

TABLE III-B Starting Particulates Fraction 325 to 200 mesh Starting Particulates Fraction 140 to 100 mesh Compacting Product Fraction Apparent Den- Green Pressure Mesh sity, gms/cc Strength, psi Used tsi Starting particulates without further treatment 5.03 85 10.7 80 to 100 mesh 340 11.0 50 to 80 mesh 3.78 450 11.4 35 to 50 mesh 3.26 630 12.0 to mesh 2.94 760 12.2

Table 111. A, B, and C show that green strength initially increases with the increasing size of the product agglomerate particles made from each, then becomes substantually constant and levels off as a critical size of the product agglomerate particles is reached. This critical size decreases with decreasing size of the starting particulates. The greatest gain in green strength results from the agglomeration of very fine particles.

EXAMPLE 1 l The same steps were followed as in Example 1, except that the molten metal was A.I.S.I. grade 304-L stainless steel. The original atomized stainless steel particulates, when screened, contained 49 percent finer than 325 mesh powder. A portion (67 percent) of the -325 mesh fraction was thermally agglomerated for 8 minutes at 2100F in an H atmosphere to form a porous cake. The porous cake was milled to virtually all but a trace passing 80 mesh and added back the remainder of the unagglomerated particulates from which the 325 mesh was screened. Particle size distribution after agglomeration, milling and blending was as follows: 6 percent between 80 and 100 mesh, 6 percent between lOO and 140 mesh, 18 percent between 140 and 200, 28 percent finer than 325 mesh, and the rest between 200 and 325 mesh. The green strength of the resulting particulate mixture measured 1,020 psi when pressed at tsi with 1 percent lithium stearate admixed as a lubricant. The green strength of the starting particulates was 650 psi, when pressed the same way with the same additive.

What is claimed is:

1. A process for the production of metal particles from finely divided atomized metal particulates for use in powder metallurgy comprising the steps of:

a. disposing said particulates in a loose mass and heating said loose mass at a temperature and for a time to produce a porous cake of miniumum density, said cake being particulates sintered together; and

b. breaking up said cake into particles each having multiple particulates and having a particle size substantially greater than said particulates, said cake being broken up under conditions which minimize densification.

2. The process of claim 1 wherein said particulates are heated under nonoxidizing conditions.

3. The process of claim 1 wherein said particles have an average particle size 1.5 to 7 times greater than the particle size of said atomized metal particulates.

4 The process of claim 3 wherein a major portion of the atomized metal particulates is finer than mesh.

5. The process of claim 3 wherein a major portion of the atomized metal particulates is finer than 325 mesh and only a trace exceeds 100 mesh size.

6. The process of claim 11 further including the step of preparing a compacted metal part from said particles, said compacted metal part having substantially higher green strength than if prepared from said atomized metal particulates.

7. The process of claim 1 wherein the particles are annealed.

8. The process of claim 1 wherein the metal of the metal particulates is non-ferrous.

9. The process of claim 8 wherein the metal particulates contain copper and heating is carried out at a temperature in the range of about 1,000F. to about 1,800F.

10. The process of claim 1 wherein the metal of the metal particulates is ferrous.

i l. The process of claim 1 wherein the heating is carried out at a temperature of approximately 42 the absolute melting point temperature of the metal of the metal particulates up to a temperature approaching the melting point of the particulates.

12. The process of claim 1 characterized in that the apparent density of the metal particles is less than about 50% of the real density of the metal of said metal particulates.

13. A process for the production of metal particles from finely divided atomized metal particulates for use in powder metallurgy comprising the steps of:

a. disposing said particulates in a loose mass and heating said loose mass at a temperature and for a time to produce a porous cake, said heating being such as to minimize densification of the mass but being sufficient whereby said cake is selfsupporting; and

b. breaking up said cake into particles having a particle size substantially greater than said particulates under conditions which minimize densification.

14. The process of claim 13 including the steps of preparing a compacted metal part from said particles, said compacted metal part having substantially higher green strength than if prepared from said atomized metal particulates.

15. Metal particles thermally agglomerated from dis crete gas-atomized metal particulates according to the process of claim 11, said particules having essentially the topology shown in FIG. 1 under about powder magnification.

116. Metal particles thermally agglomerated from discrete water-atomized metal particulates according to the process of claim 11, said particles having essentially the topology shown in FIG. 2 under about 1,500 powder magnification.

Claims

1. A PROCESS FOR THE PRODUCTION OF METAL PARTICLES FROM FINELY DIVIDED ATMIZED METAL PARTICULATES FOR USE IN POWDER METALLURGY COMPRISING THE STEPS OF: A. DISPOSING SAID PARTICULATES IN A LOOSE MASS AND HEATING SAID LOOSE MASS AT A TEMPERATURE AND FOR A TIME TO PRODUCE A POROUS CAKE OF MINIUMUM DENSITY, SAID CAKE BEING PARTICULATES SINTERED TOGETHER; ABD B. BREAKING UP SAID CAKE INTO PRTICLES EACH HAVING MULTIPLE PARTICULATES AND HAVING A PARTICLE SIZE SUBSTANTIALLY GREATER THAN SAID PARTICULATES, SAID CAKE BEING BROKEN UP UNDER CONDITIONS WHICH MINIMIZE DENSIFICATION.

2. The process of claim 1 wherein said particulates are heated under non-oxidizing conditions.

4. The process of claim 3 wherein a major portion of the atomized metal particulates is finer than 100 mesh.

6. The process of claim 1 further including the step of preparing a compacted metal part from said particles, said compacted metal part having substantially higher green strength than if prepared from said atomized metal particulates.

7. The process of claim 1 wherein the particles are annealed.

9. The process of claim 8 wherein the metal particulates contain copper and heating is carried out at a temperature in the range of about 1,000*F. to about 1,800*F.

11. The process of claim 1 wherein the heating is carried out at a temperature of approximately 1/2 THE absolute melting point temperature of the metal of the metal particulates up to a temperature approaching the melting point of the particulates.

13. A process for the production of metal particles from finely divided atomized metal particulates for use in powder metallurgy comprising the steps of: a. disposing said particulates in a loose mass and heating said loose mass at a temperature and for a time to produce a porous cake, said heating being such as to minimize densification of the mass but being sufficient whereby said cake is self-supporting; and b. breaking up said cake into particles having a particle size substantially greater than said particulates under conditions which minimize densification.

15. Metal particles thermally agglomerated from discrete gas-atomized metal particulates according to the process of claim 1, said particules having essentially the topology shown in FIG. 1 under about 175 powder magnification.

16. Metal particles thermally agglomerated from discrete water-atomized metal particulates according to the process of claim 1, said particles having essentially the topology shown in FIG. 2 under about 1,500 powder magnification.