US3672882A - Slip casting - Google Patents

Slip casting Download PDF

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Publication number
US3672882A
US3672882A US827846A US3672882DA US3672882A US 3672882 A US3672882 A US 3672882A US 827846 A US827846 A US 827846A US 3672882D A US3672882D A US 3672882DA US 3672882 A US3672882 A US 3672882A
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United States
Prior art keywords
slip
metal
particles
cast
casting
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Expired - Lifetime
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US827846A
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English (en)
Inventor
Joseph R Sagmuller
Richard I Hunter
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National Standard Co
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Battelle Development Corp
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    • 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
    • B22F9/00—Making metallic powder or suspensions thereof
    • B22F9/16—Making metallic powder or suspensions thereof using chemical processes
    • B22F9/18—Making metallic powder or suspensions thereof using chemical processes with reduction of metal compounds
    • 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
    • B22F3/22—Manufacture of workpieces or articles from metallic powder characterised by the manner of compacting or sintering; Apparatus specially adapted therefor ; Presses and furnaces for producing castings from a slip

Definitions

  • This invention relates to improvements in slip casting and relates in particular to a new and novel method for making slip cast and sintered metal articles.
  • Slip casting is a method for making articles by introducing a slurry or dispersion of a particulate material in a carrier liquid into a mold that is constructed of a substance that is disposed to absorb the carrier liquid.
  • the mold draws olf the carrier liquid or vehicle leaving the particulate material deposited on the inner walls of the mold in the desired shape. Partial or complete drying causes the slip-cast deposited particulate material to shrink a sufficient amount for removal from the mold.
  • the slipcast article consisting of compacted particles is generally further consolidated into a solid body usually by heat treatment and fusion of the particles.
  • slipcast clay articles are fired to effect ceramic bodies
  • slipcast metals and metal oxides are sintered to effect dense metal or ceramic objects.
  • the art of slip casting is very complex.
  • the slip must be of a viscosity that lends itself to easy pouring since if the slip is too thick it will not fill in the details of the mold. If the dispersed material settles out too rapidly when making hollow castings the wall thickness of the castings will vary.
  • the flow properties or viscosity of a slip must be reasonably constant over a range of solid-liquid ratios. There must be some shrinkage to permit the casting to be removed from the mold but excessive shrinkage may cause strains in the walls of the greenware and increases the chances for cracking both before and during firing or sintering. Additional factors to consider in formulating a slip include greenware strength, the release rate from the molds and the tendency of slips to undergo chemical change during storage.
  • the parameters involved in creating a slip that will meet the above-enumerated varying properties include the selection of the carrier liquid or vehicle, additives to the slip such as dispersing agents and the particle size of the oxide or metal.
  • the carrier liquid or vehicle be water and the preferred dispersing agent, if required at all, will vary in accordance with the casting being made and exact slip-casting practice involved. In any event, regardless of these considerations, the single most important parameter in effecting proper rheological control over the slip is particle size and particle size distribution.
  • slip casting of metals is carried out utilizing particles below 10 microns in diameter.
  • particles or powders are not, however, classified so that they actually consist of powders or particles having a particle size distribution ranging from about 20 to 1 microns. If the particles are finer the exercise of proper rheological control over the' slip becomes difficult. Fine particles tend to interact with the surrounding atmosphere so that minor changes in the ionic nature of the atmosphere can effect undesirable viscosity changes.
  • iron powder may be manufactured by the direct reduction of iron oxide powder.
  • the iron powder produced has a particle size distribution wherein a major portion is composed of particles of less than about 2.5 microns diameter the powder reverts to iron oxide within a few minutes of being exposed to air.
  • the ultimate density of sintered metal compacts is related to the size of the metal particles. Generally, the finer the particle size the greater the density. As shown above, however, the pyrophoric nature of metal powders such as iron powders has a limiting effect on the usable particle sizes requiring pretreatment or controlled atmosphere processing. However, where powdered metals are slip cast an added limitation on the size of particles which may be utilized relates to the rheological control over the slip.
  • a recent development in creating metal objects through slip casting techniques involves slip casting of metal oxides, reducing the cast metal oxide object by heat treatment in a reducing atmosphere and sintering (M. Zadrovitch and A. Mitav Mohanty, Powder Metallurgy, 1965, volume 8, No. 15, pages 152-161) to obtain a metal object.
  • the products obtained by this procedure exhibit a considerable degree of porosity. Additionally the original particles are visible in the microstructure and the density of these products are approximately only percent of a theoretically fully dense product. Such porosity and low density properties renders the process commercially unattractive.
  • metal compound powders may be slip cast, reduced, and sintered to obtain metal objects having relatively smooth pore free surfaces and density properties exceeding percent of theoretically percent dense cast metal if at least 35 percent, by weight, of the oxide particles have a particle size of 10 microns or less as determined by Coulter Counter Analysis.
  • the particle size distribution will be considerably below the maximum of 35 percent, by weight under 10 microns and will have a mean particle size no greater than about 6 microns and at least 25 percent, by weight, of the particles will be below 2.5 microns in diameter.
  • the minimum particle size is that which can be tolerated in creating a slip with acceptable viscosity for slip casting.
  • Optimum densities are obtained with powders that have a particle size distribution wherein substantially all of the particles are below 10 microns in size and at least 50 percent, by weight, of the particles are under 1 micron in diameter.
  • objects slip cast, reduced, and sintered in accordance with the method of the present invention exhibit superior reproducibility of the' mold surface when compared to prior practices. It is possible to take advantage of the shrinking characteristics of reduced and sintered slip cast metal compounds to provide particularly fine detail on the surface of the cast object.
  • FIG. 1 is an illustrative cross-sectional view of a composite slip cast shape that constitutes an embodiment of the present invention.
  • FIGS. 2a and 2b are illustrative fragmented enlarged cross-sectional views of a mold surface and corresponding slip cast surface showing the slip cast surface before reduction and sintering and after reduction and sintering respectively.
  • the method of the present invention is applicable to any reducible metal compound particularly those susceptible to reduction with hydrogen which have standard free energies of reaction with hydrogen that are less than about kilocalories per gram atom of hydrogen at the reduction temperature.
  • the metal compounds of particular interest are the metal oxides such as the oxides of Fe, Co, Ni, Cu, Mo, and W.
  • Any metal compound powders having particles of any general shape i.e., spherical, oblong, needles, or rods, etc.
  • any source i.e., ore deposits, ore concentrates, precipitates, etc.
  • the sintered article derived will possess a substantially pore free structure, a smooth surface, and will exhibit densities in excess of 90 percent of theoretically completely dense material.
  • metal oxide powders obtained by the process of spray drying provide superior slips that reduce and sinter in a manner to provide objects of greater density and better surface and structural integrity than slips made from other sources of metal oxide.
  • Spray drying of solutions containing soluble metal compounds to effect metal oxide powders is a well known prior art procedure.
  • this method is utilized to regenerate hydrochloric acid pickling solutions that have been used in the iron and steel industry to remove mill scale and other forms of iron oxide from iron and steel products.
  • the used aqueous pickling solution containing up to about 11 percent, by weight, free hydrochloric acid, and up to about 35 percent ferrous chloride is sprayed through a nozzle into a heated chamber (about 1000 F.) where the ferrous chloride is converted into iron oxide and hydrochloric acid, as follows:
  • spray-dried metal oxides, and particularly spraydried iron oxides are believed to consist of minute hollow spheroids.
  • the spheriods themselves cannot be used to make satisfactory slips for slip casting, reducing, and sintering in accordance with the method of the present invention and it is our theory that when fragmented the resultant powders produce a slip of superior characteristics for use in conjunction with the method of the present invention.
  • the slip cast metal compounds of the present invention may, of course, consist of blends or mixtures of two or more compounds of varying metals so as to effect a metal alloy product.
  • the metal compound particles or a portion thereof may consist of bi or multi metallic compounds containing more than one metal.
  • elemental metals or metal alloy powders may be blended with the metal compound slip. In the latter practice the advantages of the present invention are largely lost where more than about 50 percent, by volume, of the mixture consists of metal particles.
  • the alloying compounds When practicing the preferred embodiment of the present invention wherein spray-dried and fragmented metal oxides are utilized to produce the slip, it will be preferred that the alloying compounds also be of the spray-dried-fragmented variety. Some advantage will be experienced in utilizing any amount of spray-dried and fragmented metal oxides in the slip regardless of how small the proportion of these metal compound fragments are in relation to the metal compound particles; however, such advantages (green and sintered densities and sintered structure) are not readily discernble where such fragments do not constitute at least about 10 percent, by volume, of the particles present.
  • green slip cast metal oxide objects cast in accordance with the present invention may be joined by positioning such objects in abutting relationship to one another prior to reducing and sintering. After reducing and sintering, the weld or juction between the abutting objects is not discernible by microscopic examination of polished and etched segments. This discovery is significant since it increases the versatility of the process immeasurably. By following this practice it is possible to join solid and hollow slip cast objects that cannot be slip cast in a single mold. Additionally, it is possible to slip cast around an already green slip cast part to effect unique cast structures.
  • the advantages of slip casting in accordance with the method of the present invention as compared to conventional metal casting techniques are considerable.
  • the greatest advantage relates to the reproducibility of the mold surface, particularly in terms of detail in the final product.
  • metal compounds such as metal oxides are slip cast, reduced, and sintered
  • shrinkage from the as-cast to the as-sintered object is considerable.
  • iron objects derived from reducing and sintering slip cast iron oxide are about two-third of the size of the original slip cast objects.
  • metal particles are slip cast and sintered shrinkage is generally less than percent and in ordinary molten metal casting shrinkage is less than 2 percent.
  • the sintered objects of the present invention exhibit relatively smooth, bright, and continuous surfaces and additionally exhibit remarkably accurate reproductions of the mold surface though reduced in size to about two-thirds of the original slip casting. This is significant in that it enables one to provide tolerances in surface detail not previously regarded to be feasible.
  • a groove of about 10.1 mils must be provided on the surface of the mold pattern where one is casting molten metal and about 10.5 mils where one is slip casting and sintering metal particles (the fraction .1 and .5 accounting for shrinkage).
  • Such a small groove is difiicult to produce by any conventional means such as machining with any degree of accuracy.
  • conventional molten metal casting and slip casting techniques will not consistently provide accurate detailed surface reproductions of depressions as small as 10 mil wide grooves.
  • the mold pattern groove and mold projection 22 will have a width (W-l) of about mils since after reducing and sintering the casting including groove 16 will shrink to two-thirds of its orginal size to provide an accurate reproduction of the pattern groove but having a width (W) of the desired 10 mil dimension. It is obviously easier to provide a 15 mil groove in the mold surface than a 10.1 or 10.5 mil groove; consequently, the method of the present invention constitutes a considerable advance in the art of casting metal objects where surface details are concerned.
  • the slip was cast into plaster of Paris molds having mold cavities in the shape of a horsehead bookend and an ornamental cup (both about 2 inches in diameter and 4 inches deep).
  • the wall thickness of these objects was about 0.25 inch.
  • the cast slips were permitted to dry sufiiciently to shrink away from the mold cavity and were then removed from the molds and dried in a drying oven at 140 F. They were then heated in the presence of a hydrogen atmosphere at approximately 1200 F. for a sufiicient time to reduce the iron oxide to elemental iron and were then sintered at about 2100" F. in a reducing atmosphere.
  • the resultant objects exhibited crack-free smooth finishes and densities in excess of percent of theoretical. Although they had shrunk to approximately one-third of their original size they retained the original ornamental shape.
  • Accurate particle size determinations of fine grained powders are diflicult to obtain, particularly where the particle size distribution of such powders includes a fraction that is less than 10 microns in diameter. Such determinations are most difficult where the particles are of nonuniform shape. For example, if the particles consist of crushed or ground spheroids as is speculated in regard to ball milled spray dried HCl pickle liquor oxides many of the particles are likely to be of a relatively elongated or semicircular shape (sections of a hollow spheroid) so that it is diflicult to determine actual diameter. Elongated particles will not pass through a screen having a mesh that is designed to accommodate a relatively symmetrically shaped particle of equivalent mass.
  • particle size determinations and limitations are in terms of Coulter Counter measurements and shall include metal compound particles meeting such determinations irrespective of particle size determinations by other means.
  • the method of making high density sintered metal objects comprising:
  • particulate metal compounds wherein said particulate metal compounds are selected from the group consisting of the oxides of iron, cobalt, nickel, copper, molybdenum, and tungsten mixed with a carrier liquid, at least 35 percent, by weight, of the metal compound particles being less than 10 microns in diameter;
  • metal compound particles have a mean particle size no greater than 6 microns and at least 25 percent, by weight, of which do not exceed 2.5 microns.
  • metal compound particles consist essentially of particles derived from spray drying solutions of soluble compounds of a metal of the group consisting of iron, cobalt, nickel, molybdenum and tungsten.
  • a method for making high density sintered metal objects comprising:
  • particulate metal compounds wherein said particulate metal compounds are selected from the group consisting of the oxides of iron, cobalt, nickel, copper, molybdenum, and tungsten mixed with carrier liquids, at least 35 percent, by weight, of the metal compound particles being less than 10 microns in diameter;
  • a method for making high density sintered metal objects comprising:

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  • Chemical & Material Sciences (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • General Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Manufacturing & Machinery (AREA)
  • Mechanical Engineering (AREA)
  • Powder Metallurgy (AREA)
US827846A 1969-05-26 1969-05-26 Slip casting Expired - Lifetime US3672882A (en)

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US82784669A 1969-05-26 1969-05-26

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US3672882A true US3672882A (en) 1972-06-27

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US (1) US3672882A (fr)
AT (1) AT304089B (fr)
BE (1) BE751245A (fr)
CA (1) CA928482A (fr)
DE (1) DE2025793C3 (fr)
ES (1) ES380870A1 (fr)
FR (1) FR2048854A5 (fr)
GB (1) GB1300433A (fr)
LU (1) LU60995A1 (fr)
NL (1) NL7007610A (fr)
TR (1) TR17159A (fr)
ZA (1) ZA703567B (fr)

Cited By (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE3023605A1 (de) * 1979-06-25 1981-01-15 Nat Standard Co Viskoelastische zusammensetzung zur herstellung von formkoerpern
US4491559A (en) * 1979-12-31 1985-01-01 Kennametal Inc. Flowable composition adapted for sintering and method of making
US4503009A (en) * 1982-05-08 1985-03-05 Hitachi Powdered Metals Co., Ltd. Process for making composite mechanical parts by sintering
US6551551B1 (en) 2001-11-16 2003-04-22 Caterpillar Inc Sinter bonding using a bonding agent
US8734715B2 (en) 2011-01-13 2014-05-27 Ut-Battelle, Llc Method for the preparation of ferrous low carbon porous material
US20140356216A1 (en) * 2013-06-04 2014-12-04 Michael T. Stawovy Slip and pressure casting of refractory metal bodies

Families Citing this family (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE102009039604A1 (de) * 2009-09-01 2011-03-10 Henri Dr.-Ing. Cohrt Verfahren zur Herstellung eines gasdichten Körpers und zur mechanischen Bearbeitung zu einem Zahnersatz vorgesehener Grundkörper

Cited By (9)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE3023605A1 (de) * 1979-06-25 1981-01-15 Nat Standard Co Viskoelastische zusammensetzung zur herstellung von formkoerpern
US4298383A (en) * 1979-06-25 1981-11-03 National-Standard Company Low viscosity composition for forming shaped bodies
DE3051089C2 (fr) * 1979-06-25 1989-03-09 National-Standard Co., Niles, Mich., Us
US4491559A (en) * 1979-12-31 1985-01-01 Kennametal Inc. Flowable composition adapted for sintering and method of making
US4503009A (en) * 1982-05-08 1985-03-05 Hitachi Powdered Metals Co., Ltd. Process for making composite mechanical parts by sintering
US6551551B1 (en) 2001-11-16 2003-04-22 Caterpillar Inc Sinter bonding using a bonding agent
US8734715B2 (en) 2011-01-13 2014-05-27 Ut-Battelle, Llc Method for the preparation of ferrous low carbon porous material
US20140356216A1 (en) * 2013-06-04 2014-12-04 Michael T. Stawovy Slip and pressure casting of refractory metal bodies
CN105263655A (zh) * 2013-06-04 2016-01-20 H·C·施塔克公司 难熔金属体的粉浆和压力铸造

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Publication number Publication date
ES380870A1 (es) 1972-09-16
BE751245A (fr) 1970-11-16
CA928482A (en) 1973-06-19
ZA703567B (en) 1971-01-27
TR17159A (tr) 1974-04-25
DE2025793A1 (de) 1970-12-03
GB1300433A (en) 1972-12-20
NL7007610A (fr) 1970-11-30
DE2025793C3 (de) 1979-10-11
FR2048854A5 (fr) 1971-03-19
LU60995A1 (fr) 1970-08-20
AT304089B (de) 1972-12-27
DE2025793B2 (de) 1979-02-22

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AS Assignment

Owner name: NATIONAL-STANDARD COMPANY, NILES, MI

Free format text: ASSIGNMENT OF ASSIGNORS INTEREST.;ASSIGNOR:BATTELLE DEVELOPMENT CORPORATION;REEL/FRAME:004040/0221

Effective date: 19820715