Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C1/00—Making non-ferrous alloys
  • C22C1/04—Making non-ferrous alloys by powder metallurgy
  • C22C1/0433—Nickel- or cobalt-based alloys
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
  • B22F1/00—Metallic powder; Treatment of metallic powder, e.g. to facilitate working or to improve properties
  • B22F1/10—Metallic powder containing lubricating or binding agents; Metallic powder containing organic material
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
  • B22F1/00—Metallic powder; Treatment of metallic powder, e.g. to facilitate working or to improve properties
  • B22F1/14—Treatment of metallic powder
  • B22F1/148—Agglomerating
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
  • B22F3/00—Manufacture of workpieces or articles from metallic powder characterised by the manner of compacting or sintering; Apparatus specially adapted therefor ; Presses and furnaces
  • B22F3/12—Both compacting and sintering

Definitions

• a powder metallurgy process suitable for high performance alloys difficult to compact and/ or sinter involves coating the particles with a solid organic binder by dry blending the alloy powder with particles of the binder, mixing the blended particles with a solvent for the binder to produce a plastic mass, consolidating this mass to discrete bodies of an intermediate density, drying the bodies, crushing them and screening the resulting agglomerates, pressing them into green compacts, and sintering those compacts.
• Some or all of the starting powder may be ground to smaller particle size. As fine grinding is always accompanied by oxidation of the powder, which is detrimental to sinterability, the grinding is adjusted to produce particles of a limited average size range.
• This invention relates to the production by powder metallurgy of articles of hard metal alloys. It is more particularly concerned wth production by this method of articles of high performance metal alloys.
• the alloys with which this invention is concerned are high performance cobalt-base, nickel-base, and iron-base chromium-containing alloys resistant to wear, heat and corrosion. These alloys either are not workable or are worked with difficulty, and are commonly produced as castings, which may be ground or machined where necessary. Many small articles made from high performance alloys, such as thread guides for textile mills, valve seat inserts, and the like, are tedious and expensive to cast in the quantities that are required. Attempts have been made to produce such articles by powder metallurgical process, such as by slip casting or pressing the articles to shape from fine powders, and then sintering them. However, such processes, which have proved satisfactory and economical for many alloys, have turned out to be difiicult and expensive to adapt to alloys as hard as the high performance alloys here concerned.
• the powder particles should be of spherical configuration and of a random size distribution over a rather wide range of sizes to provide optimum packing density and so facilitate subsequent densification.
• US. Pat. 3,639,179 of Steven Reichman et al. of Feb. 1, 1972, Method of Making Large Grain Sized Superalloys the patentees recommended a size range of about 150 microns to about microns. We have found, however, that a number of high performance alloy powders when compacted in this way can be sintered only in a very narrow range of temperatures, or in some cases not at all.
• the density of the sintered article is generally required to be of cast density or better, all compacts shrink from about 25 to as much as 40% or more during sintering, Where the sintered compact must meet close dimensional tolerances the compacts are constrained during sintering.
• the green compacts are slipped over mandrels and sintered in that position. If the cohesion between the powder particles is insufficient the compacts will crack.
• the average particle size required for effective compacting in the worst case, was found to be less than about 5 microns, and the grinding time necessary for such powder was measured in days. This, of course, considerably increased its cost. Moreover, the greatly increased surface area of the very fine powder and the length of time required for its grinding facilitated oxidation of the powder so that, in spite of all precautions, its oxygen content was much greater than that of atomized powder. This high oxygen content is undesirable for several reasons, one overriding reason being that it narrows the sintering range of the powder. Thus, the sinterable powders were not compactible for many of the alloys, and the compactible powders were, effectively, not sinterable.
• Our process to be described is adapted to utilize the full size range of atomized melts of many high performance alloys if maximum density in the resulting article is not desired. It is also adapted to high performance alloys which by conventional processes are unsinterable or marginally sinterable.
• compositions of a number of alloys for which our process is suitable are listed in the accompanying Table.
• Noun-Balance includes incidental impurities.
• the alloy powder which we employ is preferably produced by the atomization of a melt of the desired composition.
• This melt is heated to a temperature of 200 F. or so above its fusion temperature in a crucible.
• this melting is carried out in vacuum or under a blanket of inert gas such as argon.
• the melt is then poured into a preheated refractory tundish which is formed with a small diameter nozzle in the bottom through which the stream of metal flows into an atomizing chamber.
• the stream emerging from the nozzle is broken up into fine particles by a high-pressure jet of inert gas, or of water, which makes contact with the molten stream just below the nozzle.
• the particles or droplets are almost instantaneously quenched by the atomizing fluid and fall into a reservoir in the bottom of the atomizing chamber. Only the fraction is used which passes through a mesh screen. These particles are approximately spherical in shape and about 25% to of the particles are 325 I mesh. A 325 mesh screen will pass particles the greatest dimension of which is 44 microns.
• polyvinyl alcohol as a binder for our powder, but other solid binders which are known to the art are employed. Examples are camphor, methyl alcohol, paradichlorobenzene, chloroacetic acid, naphthalene, benzoic acid, phthalic anhydride, glycerine, Acrowax C, which is a proprietary compound, the ethylene oxide polymers sold as Carbowax, synthetic gums such as acrylamide, and metal stearates.
• the solvent for the binder must be appropriately chosen. Water is satisfactory for water-soluble binders.
• the blending of the powder and binder particles is accomplished in any suitable mixing apparatus.
• the amount of binder is not critical, and a few percent by weight is sufficient. Extrusion of the plastic or putty-like blend of particles, binder and solvent is the most convenient way of consolidating the plastic mixture into agglomerates, but other methods, such as roll briquetting, may be employed.
• the extrusions are then dried, crushed in a roller crusher, hammer mill or the like, and screened.
• the l00 mesh fraction of crushed extruded binder powder is largely fines. From about 60 to 80% of the particles are 325 mesh with corresponding apparent densities of about 2.0 to 3.3 grams per cc. Both the percentage of fines and the compacting pressure can be as low as 20 tons per sq. inch or as high as 70 tons per sq. inch, the density of the green compacts being higher at higher compaction pressures. At a compaction pressure of 20 tons per sq. inch, compact density is about 56 to 58% of cast density, and at 70 tons per sq. inch it is 70 to 72% of cast density.
• the desired density of the finished article is obtained by sintering the compact in vacuum or reducing atmosphere at a temperature between the solidus temperature and liquidus temperature of the alloy. Sintering can be completed in about an hour, but if the time is extended to two or at most three hours, the temperature can be reduced somewhat without impairing the properties of the article. Compacts properly sintered have densities of 98% or better of cast density.
• Our process also contemplates grinding, when necessary, of part or all of the powder particles resulting from the atomization of a melt as above described.
• relatively coarse atomized powder such as -30 mesh by ball miling, impact milling, attriting, vibrating milling, or other known process so as to convert it to particles more than 98% of which are 325 mesh and process those particles in the way described above to produce sintered articles having improved properties.
• the milling vehicle which we prefer to use is methanol, the mill is preferably evacuated to minimize oxidation of the charge, and, in the case of ball milling, the balls charged are made of a wear-resistant alloy of a composition compatible with the product being produced.
• Milling time ranges from about 8 to 36 hours and the average particle size of the 325 mesh product ranges from about 30 microns to as low as 9 microns, depending on milling conditions.
• the charge is dumped from the mill and the powder allowed to settle.
• the alcohol is decanted and the sludge is vacuum filtered.
• the powder filter cake is allowed to dry under vacuum or in air, and is then crushed to 60 mesh agglomerates of bindered particles.
• the 30 mesh atomized powder milled to an average particle size of about microns can be sintered to 95% density or better within a temperature range of about to This range is broad enough for commercial operation.
• the oxygen content of the milled powder is about 0.44%. It is interesting to find that the addition of a relatively minor amount of a fine fraction of the atomized particles to milled powder appreciably impairs its sinterability.
• a charge of 30 +270 mesh atomized powder of No. 7 alloy was ground in a ball mill for 25 hours to an average particle size of about 10 microns. This material was mixed with 270 mesh atomized powder in amount representing 30% by weight of the aggregate. The average particle size of this aggregate was 23.5 microns.
• Compacts of the aggregate did not sinter as well as compacts of -30 +270 mesh atomized powder milled in a ball mill for 18 hours to an average particle size of 15 microns.
• the first mentioned powder had to be sintered at a temperature of 2300 F. for better than an hour to achieve 95 density. Sintering at 2310 F. for an hour resulted in an article density of 98.25%.
• the second mentioned powder achieved a compact density of 95 after one hour of sintering at 2280 F. and 98% after one hour at 2290 F.
• EXAMPLE I The 325 mesh fraction of atomized powder of Alloy No. 3 of the Table was dry blended in a mixer with particles of a binder, preferably 100 mesh polyvinyl alcohol, in amounts of 2% to 3% by weight.
• the powder particles used had an average particle size of about 30 microns.
• enough warm water was added to form a plastic mixture of the powder and binder.
• This mixture was then extruded into cylinders or roundels of about two inches long and one-half inch in diameter under pressure sufficient to consolidate the mixture to a density of about 60% of cast density.
• the roundels were dried, then crushed in a roller crusher, hammer mill, or the like, and the crushed material was screened to -100 mesh.
• the -l00 mesh agglomerates of blended alloy powder particles were formed under pressure of about tons per sq. inch into green compacts of the desired shape, which had sufficient strength to withstand further processing.
• the green compacts were then sintered for 1 to 3 hours at a temperature of between 2260 F. and 2325 F.
• the binder volatilized during sintering and the sintered articles had a density of 97% to 99% of cast density.
• EXAMPLE II Inert gas atomized powder of Alloy No. 7, a nickelbase alloy, was screened through a 30 mesh screen. One hundred pounds of the screened powder were charged into a 28" long ball mill along with 13 gallons of methanol and about 800 pounds of Haynes Stellite Alloy No. 3 balls. The mill was evacuated and run at approximately 80% of critical speed (54 r.p.m.) for 10 hours. The average particle size of the resulting powder was about 17.5 microns. About 98% of the powder was 325 mesh.
• the powder was removed from the mill, filtered, dried, and dry blended with 2% by weight of 100 mesh polyvinyl alcohol particles, and 1% by weight of Acrowax C, mixed with water to form a putty-like mass, extruded into roundels, dried, crushed, charged into a die, pressed and removed from the die.
• the coherent green compacts were placed in a sintering furnace and sintered at a temperature between 2210 F. and 2230 F. for a period of time of 1 to 3 hours.
• the articles resulting had a density of 98% to 99% of cast density and Rockwell C scale hardness of 41 to 44.
• EXAMPLE IV Inert gas atomized particles of Alloy No. 8, which is an iron-base alloy, were screened through a 325 mesh screen. The particles passing through the screen were then mixed with a binder as described in Example I, except that the binder was 3% polyvinyl alcohol, and further processed as there described into green compacts. These compacts held their shape, and were transferred to a sintering furnace and sintered at a temperature between 2150" F. and 2170 F. to articles having a density of 97% of cast density.
• EXAMPLE V Inert gas atomized particles of Alloy No. 8 of 30 mesh size were ground in a ball mill for 24 hours to particles of an average particle size of about 9 microns. These particles were then blended with 3% by weight of polyvinyl alcohol particles and 1% by weight of particles of Acrowax C and further processed as is described in Example I into coherent green compacts. Those compacts were sintered at a temperature between 2140 F. and 2170 F. to articles having a density of 97% of cast density.
• the vehicle chosen for the ball milling has some effect on the sintering process. While we would prefer to use water, we find that its use results in a measurable increase in the oxygen content of the sintered article and a narrowing of the temperature range for sintering. Where the oxygen content of the alloy is critical or where the sintering range is restricted we use a solvent other than water. In the case of No. 7 alloy, for example, made from powder of about 18 microns average size, the increase in oxygen content of the alloy arising from the use of water as a vehicle is about 0.43%. We prefer to use methanol as a vehicle, which brings about an increase in oxygen content of only about 0.12%. Other organic solvents that may be used as vehicles are ketones, aromatic hydrocarbons and methane series compounds.
• the decomposition of organic binders increases the carbon content of the sintered article in amounts between about 0.1% and 0.2%.
• this increase can be significant, and in such cases we added to the powder small amounts of an oxide of a metal which is reduced by carbon at the sintering temperature.
• Cobalt oxide is suitable for Alloy No. 3 and is preferred by us.
• nickel oxide or oxides of other metals compatible with the alloy composition may be used.
• alloys consisting of a matrix of Alloy No. 2 having particles of tungsten carbide dispersed therein in amounts from about 25% to about 60% by weight.
• the tungsten carbide powder is added to the alloy powder and mechanically mixed therewith.
• the powder mix is then blended with a suitable binder and processed from that point on in the same way as is described in the examples above set out.
• the screen sizes are ASTM screen sizes. Average particle sizes were determined by Sharples Micromerograph.

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• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• Chemical & Material Sciences (AREA)
• Manufacturing & Machinery (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Powder Metallurgy (AREA)
• Manufacture Of Metal Powder And Suspensions Thereof (AREA)

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Cited By (31)

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• 1973
  • 1973-01-15 US US00323502A patent/US3846126A/en not_active Expired - Lifetime
  • 1973-12-09 ZA ZA739585A patent/ZA739585B/xx unknown
  • 1973-12-28 DE DE2365046A patent/DE2365046C2/de not_active Expired
• 1974
  • 1974-01-02 IN IN09/CAL/74A patent/IN140276B/en unknown
  • 1974-01-08 AR AR251846A patent/AR199602A1/es active
  • 1974-01-11 BR BR74159A patent/BR7400159D0/pt unknown
  • 1974-01-12 PL PL1974168106A patent/PL88512B1/pl unknown
  • 1974-01-14 ES ES422274A patent/ES422274A1/es not_active Expired
  • 1974-01-14 BE BE139794A patent/BE809726A/xx not_active IP Right Cessation
  • 1974-01-14 HU HUCA360A patent/HU168185B/hu unknown
  • 1974-01-14 SE SE7400430A patent/SE401110B/xx not_active IP Right Cessation
  • 1974-01-14 IT IT67094/74A patent/IT1009104B/it active
  • 1974-01-14 CH CH45874A patent/CH587091A5/xx not_active IP Right Cessation
  • 1974-01-14 FR FR7401195A patent/FR2213826B1/fr not_active Expired
  • 1974-01-14 JP JP49007494A patent/JPS5756523B2/ja not_active Expired
  • 1974-01-14 AT AT74274A patent/ATA27474A/de not_active IP Right Cessation
  • 1974-01-14 CA CA190,070A patent/CA1031603A/en not_active Expired
  • 1974-01-14 NL NL7400498A patent/NL7400498A/xx not_active Application Discontinuation
  • 1974-01-14 LU LU69150A patent/LU69150A1/xx unknown
  • 1974-01-15 DD DD176014A patent/DD112725A5/xx unknown
  • 1974-01-15 CS CS74250A patent/CS216185B2/cs unknown
  • 1974-01-15 GB GB180174A patent/GB1457661A/en not_active Expired
• 1981
  • 1981-04-02 JP JP56050035A patent/JPS5933654B2/ja not_active Expired

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