US3155502A - Powder metallurgy - Google Patents

Description

Nov. 3, 1964 H. J. BROWN 3,155,502

PQWDER METALLURGY Filed Aug. 12, 1960 BINDER DISPERSANT METAL POWDER-4 FWATER DRY PAN MULLER LUBR|CANT l TWIN SHELL BLENDER WARM EXTRUDER LOW-PRESSURE COLD ROLL PUNCH SHAPING PRESS PRESS SINTERING SINTERING SINTERING FURNACE FURNACE FURNACE STAMPINGS SHEET CONTOURED STOCK SHAPES INVENTOR.

HARRY J. BROWN BY mklbwh A TTORNEV United States Patent 3,155,502 PGWDER METALLURGY Harry J. Brown, North Tonawanda, N.Y., assignor to Union Carbide Corporation, a corporation of New York Filed Aug. 12, 1960, Ser. No. 49,255 6 Claims. (Cl. 75-214) This invention relates in general to methods of forming finished products from powdered metals and metal compounds. The invention more specifically relates to an improved method of forming thin sheets, flat and shaped, from powders of normally brittle, hard-.to-form materials such as tungsten metal, chromium carbide and the like.

Thin sheet articles of refractory metals are useful in many high-temperature applications such as in furnace parts, and in jet engines as turbine blades, chamber linings, etc.

Heretofore, the fabrication of certain refractory metals and metal compounds into finished thin sheet articles has involved very extensive and costly procedures largely because of the brittleness and general lack of ductility exhibited by such materials. A typical example of these refractory materials is tungsten metal, for which the known methods of fabrication will be discussed hereinafter. Other materials, for example, chromium, chromium carbide, molybdenum silicide, etc., are similarly fabricated only with great diificulty and are therefore also to be considered as falling within the scope of my invention.

Prior art methods of fabricating thin sheets of metal lic tungsten consist of involved processes comprising numerous repeated rollings of either hammered or swaged rods. In the preliminary steps of the process, thigh temperatures must be maintained while the metal is between the rolls. The bar or sheet is heated in a furnace during the interim between each pass and is fed rapidly into heavy rolls which are normally heated by a gas flame. Similar to swaged and drawn wire, the tungsten sheet so produced tends to split in the direction of rolling if severely stressed and moreover becomes brittle when heated above the recrystallization temperature for tungsten. In producing tungsten sheet from ingots, the first step consists of hammering at high temperatures, usually 1500 to 1600 C., until the ingot thickness is reduced by about 20 percent. Hot-rolling at an intermediate temperature of about 1300 to 1400 C. follows, and the rolling is finished at a relatively lower temperature of about 700 to 800 C. Between passes the metal bar or sheet must be subjected to further heat-treatment and, in order to prevent excessive oxidation, a hydrogen atmosphere is usually employed. Cold-rolling is continued at 200 to 300 C. down to a final sheet thickness of about 8 mils (.008 inch). This process is obviously costly and unduly involved; and furthermore requires large amounts of manpower before a reasonably thin finished sheet is produced.

Numerous procedures have been devised in an attempt to improve upon the sheet-forming methods heretofore discussed; the most promising of which comprises the rolling of sheet directly from powdered refractory materials. In this process, metallic tungsten powder is compressed into sheet by introducing the powder directly into a roll gap formed by laterally spaced, oppositely disposed pressure rolls. Strips made in this fashion are generally thick and porous and, after an initial sintering, must be subjected to further rolling to achieve thin, dense sheet. The rerolling steps following this initial compacting and sintering are generally just as lengthy and involved as in the above-described sequence. Other direct rolling methods require that the metallic powder 3,155,502 Patented Nov. 3, 1964 and/or the rolls be heated to sintering temperature to effect densification. Needless to say, these modified processes also require elaborate and expensive processing equipment.

It is :an object of my invention to overcome these prior art difficulties by providing a new and improved method for fabricating thin sheets and shaped articles from normally brittle, hard-to-work refractory materials.

It is a further object of my invention to provide a method of fabricating a variety of thin articles of manu facture having complex curvature and being composed of normally brittle, hard-to-work materials.

Other objects and advantages of this invention will become apparent by referring to the following specification and accompanying drawing in which:

The drawing shows a flow sheet of the steps involved in the practice of the invention.

By following the teachings of my invention hereinafter disclosed, sheet stock as thin as 6 mils (.006 inch) composed of normally brittle, hard-to-work refractory mate rials, such as, for example, tungsten metal, may be readily cold-formed. My novel method comprises rolling at ambient temperatures and under relatively low pressures precompacted powders made sufiiciently plastic by the addition of selected binders and lubricants.

In order to accomplish the objects of my invention, finely divided powder of the refractory :material to be fabricated into thin sheets is blended with binder, lubricant, and a small amount of Water; precompacted to convert the loose powder into a coherent plastic body, then cold formed into a thin sheet of desired size and shape, and finally sintered to produce a hard, strong, dense, distortion-free and crack-free article of manufacture.

Suitable binders for the purposes of this invention are preferably water soluble, polymerized, synthetic gums, e.g., acrylamide. Lubricants should be waterinsoluble stearates. The lubricant and the'binder chosen are preferably fugitive, i.e., they should volatilize or burn-out during sintering so as to leave no residual impurities in the final article of manufacture. The amounts of binder and lubricant used in my invention are considerably less than those amounts employed in the wellknown paste process, with the result that the mixture to be compacted and processed has the consistency of a nearly free-flowing damp powder. The specific amount of binder employed will vary according to the composition and total surface area of the powdered refractory material to be processed, varying from about 2 percent to about 8 percent by weight of the total mix and preferably being about 3 percent to 4 percent by weight of the total. Lubricant is incorporated in amounts up to about 2 percent by weight. In a preferred embodiment the amount of lubricant is from about 0.1 percent to about 2 percent by weight. One combination of binder and lubricant that has been found to produce especially desirable results is a combination of polyacrylamide binder and diglycol stearate lubricant.

In the practice of the present invention the binder, in dry powder form, is thoroughly blended with the powdered, refractory base material, e.g., tungsten powder. A preferred method of blending is by mulling since the mashing action of the mailer tends to smear the binder over the surface of the refractory powder and thus achieve an even distribution of binder. The lubricant and suficient water (i.e., about 2.5 percent to 7 percent by weight of the total mass) to uniformly moisten the mixture are then added. While the order of adding the several ingredients may vary, the preferred order of addition is that shown in the flow sheet of the drawing. In this preferred embodiment the dry binder and refractory {c.g. metallic tungsten) powder are first thoroughly blended. The water is .then added in small increments with continuous blending, until all the water has been added; and the lubricant is then stirred in. The addition of lubricant is preferably performed in a twin shell blender. The resulting mixture has the consistency of a nearly free-flowing damp powder.

If desired, a small amount, e.g., 0.1 percent to 0.2 percent by weight, of a suitable dispersing agent may be added with the binder to aid the achievement of a homogeneous distribution of the ingredients. One type of dispersant found to be especially useful for this purpose is pine wood lignin sulfonates, such as polyfon O. Other v in the mix to be compacted usually varies from about 1.5

microns to 10 microns. The particle size of the binder and lubricant is generally not critical, and each may be used in the form as received from the manufacturer.

Prior to rolling into thin sheet, the mixture, prepared .as described above, is first compacted, preferably by a 'warm extrusion. .suitab1e apparatus, for example, in a piston-type ex- 'truder.

. exerted upon the charge to compact it to a green (i.e.

Extrusion may be carried out in any During extrusion, sufficient pressure must be unbaked) density of from about 30 percent to 65 percent of theoretical density, preferably as close to 65 percent as is possible. This is considerably in excess of the green density obtained in slip-casting methods. In order to allow coldprocessing of the compact the extruded column must also possess a green strength (tensile strength in p.s.i. of the unbaked compact) of from 5 to 500 p.s.i. and preferably 50 to 200 p.s.i.

During extrusion, the damp, powdery mixture is heated to a temperature sufficient to melt the lubricant so as to enable the lubricant to function as such. Heat may be maintained in the charge through the use of any suitable means, such as, for example, a gas flame applied to the chamber or die body of the extruder. Electrical heating means, such as those provided by a heating tape or mantle may alsobe used if desired. The temperature of the charge varies depending on the melting point of the lubricant, which in the case of diglycol stearate is about 50 C. Excess heat must be avoided so as to prevent overly rapid evaporation of water. Extrusion temperatures are generally on the order of 50 to 60 C.

The extruded column is then rolled at ambient temperatures to the desired thickness. Low rolling pressures are suflicient in this operation since the extruded compact is easily deformed, even at the low temperatures employed. In situations where precision rolling is not required, the compacted column can be reduced in thickness as much as 90 percent by hand pressure alone, using a device such as a rolling pin.

A variety of articles may be manufactured from the thus-fabricated cold-rolled strip. If no further shaping is to be performed, the thin cold rolled sheets are sintered (after an optional air-drying step at low temperatures, if desired) at relatively high temperatures and preferably in an inert atmosphere, to produce the final dense, strong, crack-free article. If very thin sheet is desired, the strip, after initial reduction in the plastic state, may be subjected to a sintering step followed by a sequence of rerolling and heat-treating steps. Since in my invention the strip is already reduced to nearly the final desired thickness (i.e., down to a thickness of 20 mils or less), considerable manpower is saved in the re-rolling steps as compared to the conventional prior art methods hereinbefore described.

If a contoured shape is desired, the strip, cut to proper dimensions, .can beformed overa mandrel, mold or die by light pressure prior to the optional air-drying and sintering steps. Flat articles can be produced by simply stamping or cutting the cold-rolled strip prior to sintering.

The following specific examples illustrate in greater detail the practice of my invention. Proportions as expressed in the examples and elsewhere in the specification and claims are parts by weight.

Example I Ninety-three and one-half parts by weight (about 92.7 percent) of 99.72 percent pure tungsten powder having an average particle size of 2 microns were thoroughly blended in a mortar and pestle with 2.9 parts (about 2.9 percent) of dry, powdered polyacrylamide and 0.1 part of dry, powdered polyfon O (a sulfonate of pine wood lignin). To this mixture was added in small increments, a total of 5.0 milliliters of water, representing approximately 2.5 parts (about 2.5 percent). The water was also thoroughly distributed throughout the mix by the mashing action of the pestle until a uniformly moistened mixture was obtained. About 1.9 parts (about 1.9 percent) of dry, powdered diglycol stearate were then stirred into the moistened mix using a stirring rod rather than a pestle so as not to disturb the binder coating on the metal particles.

After the above mix was prepared, it was intorduced into the chamber of a piston type extruder and compacted by extrusion through the die thereof. Prior to extrusion, the die was heated by means of a heating tape, the heat being allowed to penetrate through the die and into the charge until the charge reached a temperature of approximately 54 C. as indicated by a thermometer embedded therein. The thermometer was withdrawn at this point, and the piston advanced so as to extrude the charge. Heat was continuously applied during the extrusion operation. The column of compacted mix emerged from the die under a pressure of approximately 10,000 p.s.i. The resulting compact was a rod inch in diameter and approximately 5 feet long. It was strong and dense, having a green density of about 12.5 gm./cc., which is approximately 65 percent of the theoretical density. A 4- inch segment was cut off of this rod, laid upon a plate glass surface and manually cold-rolled by means of a rolling pin into a strip about 2 inches wide with an average thickness of about 0.50 inch (i.e. 50 mils). A small rectangular piece was cut from the cold-rolled strip with a knife, laid on a plaster of Paris. surface shaped like a turbine blade and lightly pressed to conform to the contoured surface. After an air drying, the shaped piece was sintered in a hydrogen atmosphere at about 1700 C. The resulting piece retained the complex aerodynamic shape without distortion or cracking and was both hard and strong.

Example II The procedure of Example I was followed except that a variety of flat shapes were cut from the same coldrolled strip with a manually operated cookie cutter-like device using ordinary hand pressure.

The resulting air-dried and sintered articles were strong,

dense, and free from distortion and cracking.

Example III Example IV Eighty-six and one-half parts of chromium powder havlng an average particle size of 7 microns; 7.4 parts of dry,

powdered polyacrylamide; and 0.2 part of dry, powdered polyfon O were blended in a mortar and pestle device. To this mixture .a total of 4.0 parts of water were added in small increments, and thoroughly blended into the mix. 2.0 parts of dry, powdered diglycol stearate were then stirred (as in Example I) into the moistened mix. The mix thus prepared was then extruded in the manner described in Example I above. Portons of the extruded rod were then easily cold-rolled at room temperature into thin sheets approximately mils thick, using a hand operated roll. After an air drying and sintering at 1500 C. for minutes in a 100 percent hydrogen atmosphere, a strong, crack-free, flat sheet was produced.

Example V Using the procedure of Example IV, a mix was prepared by thoroughly blending 884 parts of chromium carbide powder, 4.4 parts of dry, powdered polyacrylamide, 0.2 part polyfon 0 and 7.0 parts of water. The thus prepared mix was squeezed by hand into a lump and then cold-rolled at room temperature by means of hand rolls. It was not necessary in this case to add lubricant to the mixture because the compaction step was performed by hand and not in an extruder. A thin sheet approximately 20 mils thick was produced. After an air drying and sintering at 1500 C. for 30 minutes in a 100 percent hydrogen atmosphere, a strong, crack-free, flat sheet was produced. This sheet may, if desired, be subsequently rerolled according to conventional procedures to produce even thinner sheets. However, because of the preliminary reduction in thickness down to 20 mils, a considerable saving in time and manpower as compared to conventional processes is obtained.

Example VI A thin sheet was fabricated by thoroughly blending, extruding and cold-rolling as described in Examples I and IV a mixture of 91.2 parts (about 91 percent) of molybdenum disilicide, 4.0 parts (about 4 percent) of dry, powdered polyacrylamide, 0.1 part (about 1 percent) of dry, powdered diglycol stearate, and 4.0 parts (about 4 percent) of water. After sintering in a vacuum at 1650 C. for 1 hour, a strong, flat, crack-free sheet was produced.

The fore-going examples show that by following the procedures of my invention a decided advance in the art of fabricating wrought articles of normally brittle, hardto-work refractory materials has been achieved. A particular advantage of my invention, especially insofar as fabrication of tungsten sheet is concerned, is the ability to accomplish in essentially one step approximately a 90 percent reduction in size of the stock by simple coldrolling techniques. The many intermediate re-rolling and heat-treatings steps heretofore found necessary, and which involve tremendous expense, are thus eliminated. The fact that a variety of fiat and contoured shapes can be made using the same process is an additional benefit obtainable through the practice of my invention. It should furthermore be noted that large raw material cost savings are achieved in my process as compared to the known paste process" since the mix to be compacted (e.g., by extrusion) is a nearly free-flowing powder rather than a viscous, plastic mass; thus resulting also in greater ease in control of the size, shape and continuity of the preformed compacted mass.

While I have emphasized several specific embodiments of my invention, various modifications and changes will readily suggest themselves to those skilled in the art. It is understood that such modifications and changes are to be considered as being within the scope of my invention as defined in the appended claims.

What is claimed is:

1. A method of making dense, crack-free and distortion-free articles of sinterable normally brittle and hardto-work refractory materials comprising preparing a blended damp, powdery mixture of nearly free-flowing consistency consisting essentially of from about 83 percent by weight to about 95.6 percent by weight of the powdered sinterable refractory material having a particle size of from 1.5 to 10 microns, from about 2 percent to about 8 percent by weight of a water-soluble, polymerized, synthetic gum, from about 2.5 percent to about 7 percent by weight water, and from about .1 percent to about 2 percent by weight of a water-insoluble stearate, compacting said powdery mixture under pressure to produce a compact thereof having a green density of from 30 percent to 65 percent of the theoretical density and a green strength of from 5 p.s.i. to 500 p.s.i., cold-rolling said compact at ambient temperatures and low pressures to reduce the thickness thereof at least 80 percent, thereby producing a thin sheet from said compact, and sintering said sheet in an inert atmosphere.

2. A method of making dense, crack-free and distortion-free articles of refractory materials selected from the group consisting of tungsten, chromium, chromium carbide, and molybdenum disilicide, comprising preparing a damp, powdery mixture of nearly free-flowing consistency by first thoroughly blending about 83 percent to about 94.5 percent by weight of a selected powdered refractory material having a particle size of from 1.5 to 10 microns with from about 2 percent to about 8 percent by weight polyacrylamide and from about 2.5 percent to about 7 percent by weight water, and then blending in from about 1 percent to about 2 percent by weight diglycol stearate, extruding said powdery mixture at a temperature from about C. to C. toproduce a compact having a green density about percent of the theoretical density and a green strength of from about 50 p.s.i. to 200 p.s.i., cold-rolling said compact at ambient temperatures and low pressures to reduce the thickness thereof at least percent, thereby producing a thin sheet from said compact and sintering said sheet in an inert atmosphere.

3. A method of making dense, crack-free and distortion-free articles of tungsten comprising preparing a damp, powdery mixture of nearly free-flowing consistency by first thoroughly blending about 83 percent to 94.5 percent by weight powdered tungsten metal having a particle size of from 1.5 to 10 microns with from about 2 percent to about 8 percent by weight polyacrylamide and from about 2.5 percent to about 7 percent by weight water, and then blending in from about 1 percent to about 2 percent by Weight diglycol stearate, extruding said powdery mixture at a temperature from about 50 C. to 60 C. to produce a compact having a green density about 65 percent of the theoretical density and a green strength from about 50 p.s.i., cold-rolling said compact at ambient temperatures and low pressures to reduce the thickness thereof at least 80 percent, thereby producing a thin sheet from said compact, and sintering said sheet in an inert atmosphere.

4. A method of making dense, crack-free and distortion-free articles of tungsten comprising preparing a damp, powdery mixture of nearly free-flowing consistency by first thoroughly blending about 92.7 percent by weight powdered tungsten metal having a particle size of from 1.5 to 10 microns with about 2.9 percent by weight polyacrylamide and about 2.5 percent by weight water, and then blending in about 1.9 percent by Weight diglycol stearate, extruding said powdery mixture at a temperature from about 50 C. to 60 C. and at a pressure of about 10,000 p.s.i., to produce a compact having a green density of about 12.5 grn./cc., cold-rolling said compact at ambient temperatures and low pressures to reduce the thickness thereof at least 80 percent, thereby producing a thin sheet from said compact, and sintering said sheet in an inert atmosphere.

5. A method of making dense, crack-free and distortion-free articles of chromium comprising preparing a free-flowing, damp, powdery mixture by first thoroughly blending about 86.5 percent by weight powdered chromium metal having a particle size of from 1.5 to 10 microns with about 7.4 percent by weight polyacrylamide and about 4 percent by weight water, and then blending in about 2 percent by weight diglycol stearate, extruding said powdery mixture at a temperature from about 50 7 S to 60 C. and at a pressure of about 10,000 p.s.i. to C. to 60 C. and at a pressure of about 10,000 p.s.i. to cent of the theoretical density, cold-rolling said compact at ambient temperatures and low pressures to reduce the thickness thereof at least 80 percent, thereby producing a thin sheet of said compact, and sintering said arnide and about 4 percent by weight water, and then blending in about 1 percent by weight diglycol stear'atef extruding said powdery mixture at a temperature from about 50 C. to 60 C. and at a pressure of about 10,000 p.s.i. to produce a compact having a green density about percent of the theoretical density, cold-rolling said compact at ambient temperatures and low pressures to reduce the thickness thereof at least percent, thereby producing a thin sheet from said compact, and sintering said sheet in an inert atmosphere.

References Cited in the file of this-patent UNITED STATES PATENTS 1,757,846 Schroter May 6, 1930 2,659,132 Leontis et al Nov. 17, 1953 2,659,136 Leontis et a1. Nov. 17, 1953 2,792,302 Mott May 14, 1957 OTHER REFERENCES 7 Schwarzkopf: Cemented Carbides, MacMillan Co., New York 1960, pp. 42-49.

Wehrmann Jan. 19, 1960

Claims (1)

1. A METHOD OF MAKING DENSE, CRACK-FREE AND DISTORTION-FREE ARTICLES OF SINTERABLE NORMALLY BRITTLE AND HARDTO-WORK REFRACTORY MATERIALS COMPRISING PREPARING A BLENDED DAMP, POWDERY MIXTURE OF NEARLY FREE-FLOWING CONSISTENCY CONSISTING ESSENTIALLY OF FROM ABOUT 83 PERCENT BY WEIGHT TO ABOUT 95.6 PERCENT BY WEIGHT OF THE POWDERED SINTERABLE REFRACTORY MATERIAL HAVING A PARTICLE SIZE OF FROM 1.5 TO 10 MICRONS, FROM ABOUT 2 PERCENT TO ABOUT 8 PERCENT BY WEIGHT OF A WATER-SOLUBLE STEARATE, COMPACTNG SAID POWDERY MIXTURE UNDER PRESSURE TO PRODUCE A COMPACT THEREOF HAVING A GREEN DENSITY OF FROM 30 PERCENT TO 65 PERCENT OF THE THEORETICAL DENSITY AND A GREEN STRENGTH OF FROM 5 P.S.I. TO 500 P.S.I., COLD-ROLLING SAID COMPACT AT AMBIENT TEMPERATURES AND LOW PRESSURES TO REDUCE THE THICKNESS THEREOF AT LEAST 80 PERCENT, THEREBY PRODUCING A THIN SHEET FROM SAID CMPACT, AND SINTERING SAID SHEET IN AN INERT ATMOSPHERE.

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