US3656983A

US3656983A - Shell mold composition

Info

Publication number: US3656983A
Application number: US80793A
Authority: US
Inventors: Harry V Sulinski
Original assignee: US Department of Army
Current assignee: US Department of Army
Priority date: 1970-10-14
Filing date: 1970-10-14
Publication date: 1972-04-18
Anticipated expiration: 1989-04-18

Abstract

Composition for making shell molds for precision casting of metals containing silica, aqueous collodial silica and graphite in sufficient amounts to control certain properties of the finished mold.

Description

United States Patent Sulinski [4 1 Apr. 18, 1972 [54] SHELL MOLD COMPOSITION 72 lnventor: Harry v. Sulinski, Philadelphia, Pa. [56] Refmms CM [73] Assignee: The United States of America as UNITED STATES PATENTS represented by the Secretary of the y 2,948,032 8/1960 Reuter 1 06138.3 x [22] Filed: Oct 14, 1970 3,396,935 8/1968 Synder 106/389 X [21] App]. No.: 80,793 Primary Examiner-Lorenzo B. Hayes Attorney-Harry M. Saragovitz, Edward J. Kelly, Herbert Berl Related U.S. Application Data and Sheldon Kanars -[63] Continuation-in-part of Ser. No. 8,697, Feb. 4, 1970,

abandoned, which is a continuation-in-part of Ser. No. [57] ABSTRACT 780,605, 1399- 2, 1968 abandoned- Composition for makingshell molds for precision casting of I metals containing silica, aqueous collodial silica and graphite "106/383! 106/3835, 106/389 in sufficient amounts to control certain properties of the [51] Int. Cl ..B28b 7/34 finished mold [58] Field of Search ..l06/38.2-38.9,

10 Claims, No Drawings SHELL MOLD COMPOSITION This application is a continuation-in-part of my co-pending patent application, Ser. No. 8,697, filed Feb. 4, 1970, now abandoned, entitled Method and Means for Fabricating Complex Metal Powder Parts, which, in turn, is a continuation-in-part of application Ser. No. 780,605, filed Dec. 2, 1968, now abandoned, entitled A Process for Fabricating Complex Metal Powder Parts and relates to powder metallurgy and more particularly concerns the fabrication of porous metal powder parts of complex shape from improved ceramic shell molds.

The invention described herein may be manufactured, used, and licensed by or for the Government for governmental purposes without the payment to me of any royalty thereon.

Powder metallurgy is a versatile method for producing many materials for various requirements in the form of finished parts or semi-finished forms. In the area of porous materials, however, an improved manufacturing technique is needed for forming complex shaped porous parts which cannot be conventionally cold pressed, i.e., parts which contain undercuts, re-entrant angles and holes normal to the axis of compaction. The methods commonly used in powder metallurgy for producing such shapes are slip casting and the process of sintering metal powder in machined metal or graphite molds. Both of these methods, however, are cumbersome and expensive, have shape limitations and are not readily adaptable to mass production. Those parts which from a bridge in the mold or contain cores present difficulty in withdrawal and tend to tear as the part shrinks in the mold.

It is therefore an object of this invention to substantially overcome or at least minimize the disadvantages aforementioned.

Another object of the invention is to provide means for fabricating powder metallurgy parts using improved ceramic shell molds.

Still another object of the invention is to provide means for economically fabricating high quality complex metal powder parts having a broad range of shapes and sizes.

Other objects and advantages of the invention will in part be obvious and in part appear hereinafter in the following detailed description.

Very briefly, the invention involves filling an expandable ceramic shell mold with metal powder and sintering the powder in situ.

More specifically, and in accordance with the objects aforementioned, ceramic shell molds, heretofore employed in the foundry industry for making castings, have been improved so slurry with stuccoed grain are used. Metal powder shapes comprising bars, rods, discs, and the like can readily be formed using the abovedescribed conventional ceramic shell molds. More complex parts, however, having cores and recesses therein, exhibit distortion and develop cracks due to restraint offered by the mold during metal powder shrinkage. Difficulties are also encountered in removing the mold material from recessed portions of complex parts. The above infirmities are believed to be related to the strength of the mold. The strength of the mold could reduce by the simple expedient of reducing the number of coatings. It was determined that at least 3 coatings were necessary in order for a mold to withstand the dewaxing operation. When complex powder metal parts were fabricated from copper, bronze, or stainless steel powders, the pieces were distorted and cracked due to mold restraint of metal powder shrinkage.

The slurry abovedescribed comprises fused silica, liquid colloidal silica, and a wetting agent. By modifying this standard slurry in accordance with one aspect of this invention, excellent controlled collapsibility characteristics of the mold are achieved. Controlled collapsibility of the mold means that it will predeterminedly break down to thus ofier no restraint to the shrinkage of the metal powder during sintering, and permitting crack-free complex parts to be produced therefrom. This controlled collapsibility characteristic of the mold is achieved by the addition of graphite and water to the slurry mixture of fused and colloidal silica, and wetting agent, if necessary. The strength of the mold decreases with increasing amounts of graphite and attendant water. The geometry of the parts to be fabricated dictates the amount of graphite needed. Those parts which form a bridge in the mold or contain cores and the like require about 6 to 9 percent graphite or even up to about 10 percent graphite. Simple shapes require less than about 6 percent graphite or, if very simple, such as rods, bars, and the like, little or no graphite will be needed. Thus, my inventive concept comprises the addition of about 1.13 to 10 weight percent of graphite to the slurry, and water, if necessary, to control the collapsibility of the mold.

The following examples illustrate typical amounts of graphite needed for fabricating various powder metallurgy parts. Included in each example are typical respective refractory slurry compositions:

EXAMPLE I Metal Powder Part: Rod, Bar, Plate or Disc Amount of Graphite: 1.13 wt. percent Refractory Slurry Compositions:

Stucco grain mesh fused silica 20 mesh fused silica I Slurry No. 1 contains -200 mesh fused silica. 2 Slurry No. 2 contains mesh fused silica. Triethanolamirie salt of dodecyl benzene sulfonic acid.

4 Drops.

as to be most useful in making complex powder metal parts, when my processes are followed, to be described hereinafter.

The procedure for making the prior art ceramic shell molds is not new. Ordinarily, four to seven coatings of refractory 75 As aforediscussed, for very simple shapes, such as rods, bars, and the like, it may be advantageous to use no graphite at all, or in amounts even less than about 1.13 weight percent, depending upon the nature of the materials being used.

EXAMPLE ll Metal Powder Part: Pulley Wheel Stucco Grain: 80 mesh Fused Silica No. 2 Slurr Amount of Graphite: 4 wt. y Refractory Compositions: Fused Silica, l mesh 1 pound Slurry No. 1 Slurry No 2 Material Amount Wt. percent Amount Wt. pcrccm l nsv1lsilirn'- 2 151141118 130,157 151 gllls 51.52 (olloitlal .silicn 17! cc Jill. .25 261) cu. 35.41] \Vvttingugollt. iltlrops... 3 (imphilc, l25 mosh. 4.01) 35.2. 4.00 \Vnlcl' .02 0b.... 5. 08 80.02 cc fl. ()8 Slurry viscosity. 2,100 to 2,500 cps. at. 75 to 85 F. 350 to 500 cps. at. 70 to 75 F. Stucco grain 80 mesh fused silica -.2t)1nush fused Silica l Slurry N0. 1 contains 200 mesh fused silica.

2 Slurry No. .2 contains 100 mesh fused silica.

3 '1riot.hanolannnc salt of dodccyl licnezene sullonic acid.

l Drops.

M LE I Colloidal Silica, 260 cc 20 Graphite, -325 mesh M t lPowde Part Hollow c linder e a r y Same as in No. l Slurry above Amount Of Graphite: 9 Wi As required to control mold strength Refractory-S y COmPOSIIIOHSI Wt. percent water= (3.27 X wt. percent graphite) 4 Slurry No. 1 Slurry No. 2

Material Amount Wt. percent Amount Wt. percent Fused silica 1 454 gms 51.43 38. 86 Colloidal silica. 172 cc. 25. 64 26. 71 Wetting agent 3 5 drops... 4 5 Graphite, 3.!5n1csli 70.45 grits 9.00 \Vatcr. 122..l8cc. 13.93 25.43 Slurry viscosity. 2,100 to 2,500 cps. at 75 to 85 F. F. Stucco grain 80 mesh fused silica mesh fused silica Slurry No. 1 contains 2l)tl mosh fusod silica.

2 Slurry No. 2 contains 1lill mosh fused silica.

-' 'lrii-tlinnolalnino salt. of dodocyl lmnzono sulfonic acid. Drops.

No. l Slurry: wt. percent water (1.77 X wt. percent Graphite) 2 No. 2 Slurry: wt. percent Water (3.27 X wt. percent Graphite) 4 TABLE 1 REFRACTORY SLURRY DATA No. l Slurry Amount 1 pound 172 cc Material Fused Silica, 200 mesh Colloidal Silica Wetting Agent Graphite, 325 mesh Water Colloidal silica, aqueous based, having a minimum specific gravity of 1.200 at oilF. and containing approximately 30% by weight OI'SiO Wetting agent, an alkyl aryl sulfonate, anionic surface-active, suitably a triethanolamine salt ofdodecyl benzene sulfonic acid.

Afew drops(2to5) As required to control mold strength Wt. percent water t 1.77 x wt. percent graphite) 2 Viscosity ofthe Slurry: 2.l00 to 2,500 cps at 7 F.

Viscosity of the Slurry: 350 to 500 cps at 70 75F. Stucco Grain: 20 Mesh Fused Silica To lend more precision to the quantity of wetting agent required, if about pounds of fused silica is present with about 60 pounds colloidal silica, then about 1 to 2 teaspoons of wetting agent will suffice ordinarily. The rule is not to use more wetting agent than necessary to make the slurry wet the pattern surfaces.

No wetting agent is needed with No. 2 slurry but the addition of only a teaspoon to about 250 pounds of total silica will make a smoother slurry.

In calculating the weight of water needed for the amount of graphite present, it is apparent that if only about 1% graphite is present, then from the formulas presented above, no water need be added. In the case of No. 1 slurry, any amount of graphite present which is below about 1.13 percent will require no water. Similarly, in slurry No. 2, if the total amount of graphite present is below about 1.22 percent of the total weight, no water need be added. The upper limit of graphite for very complex parts will be about 10 percent of the total weight of the slurry.

The ratios of fused silica to liquid colloidal silica may also vary somewhat from those presented in the above table in order that the viscosity requirements above listed will be met.

Ceramic shell molds may be suitably dewaxed by flashdewaxing the molds in a hot furnace. Alternatively, the molds may be dewaxed using an autoclave, a molten bath of fusible alloy, hot refractory powder, hot metal shot or a solvent vapor such as trichlorethylene vapor.

A typical procedure for making my dual strength ceramic shell mold, Le, a mold having high green strength and a low fired strength, is a follows:

1. A wax pattern assembly is washed in acetone to remove any residual die lubricant therefrom.

2. The pattern assembly in dipped into No. l slurry, drained, and while still wet, stuccoed with coarse grain using a fluidized bed of 80 mesh fused silica powder to form a first coating of the mold.

3. The mold is dried.

4. The mold is dipped into water, and while still wet, dipped into No. l slurry, drained and stuccoed with silica powder as described in Step 2.

5. The mold is dried.

6. The mold is dipped into water, and while still wet, dipped into No. 2 slurry, drained and stuccoed with minus mesh fused silica powder.

7. The mold is then thoroughly dried.

8. Steps 6 and 7 may be repeated, if desirable.

9. The mold is dewaxed.

10. The mold is fired at l,400-1,600 F in an oxidizing atmosphere to burn off the graphite and any wax or carbon residue remaining from the dewaxing operation. The molds thus prepared may be stored indefinitely and used as needed.

Stainless steel, copper, bronze, and nickel-coated tungsten 4 powders, among others, have been used successfully with my invention. These powders were spherically-shaped of high tap density. Their tap density was at least 55 percent of theoretical.

Complex porous metal powder parts were successfully fabricated using the following sintering schedules:

TABLE II.-COMPLEX METAL POWDER PART FABRICAT- ING INFORMATION in nitrogen and for minutes at 1,400 F. in a dissociated ammonia atmosphere. Nickel-coated tungsten (0.25% Sintered 16 hours at 2,500 F. in a dis- N i). sociate-d ammonia atmosphere.

The sequence of operations for fabricating complex metal powder parts as practiced in this invention is as follows:

1. The ceramic shell mold is manufactured to the desired strength level in order to obtain the desired collapsibility.

2. The mold and metal powder are conditioned by drying in an oven at about 250 F in order to drive-off absorbed moisture.

3. The mold is filled with metal powder using a vibrator to insure complete filling and dense packing.

4. The metal powder in the mold is presintered to a coherent metal powder part with a minimum amount of mold restraint of metal powder shrinkage. This treatment alone, in many cases, may be sufficient to impart the final desired properties, e.g., fabrication of filters.

5. The mold material is removed from the metal part.

6. The metal powder part is sintered to the final desired property level.

It is apparent from the foregoing description that complex parts containing undercuts, re-entrant angles and cores may readily be fabricated from powdered metals by using ceramic shell molds having a most desirable characteristic of controlled collapsibility. Further, the limitations on shape and size which existed has been substantially overcome.

lclaim:

1. ln a slurry for a ceramic shell mold, said slurry including fused silica and aqueous colloidal silica;

the improvement which comprises a quantity of graphite in said slurry, said graphite being present in an amount of from about 1.13 to about 10 percent by weight of said slurry, said amount being sufficient to provide dual strength and controlled collapsibility characteristics to said mold.

2. The slurry as described in claim 1 wherein said fused silica is about 1OO mesh in grain size and is present in said slurry with colloidal silica in a ratio of about 1 pound to about 260 cc respectively, wherein said graphite is about -325 mesh in size, and wherein water is present in said slurry and the percent graphite exceeds about 1.22 percent of the weight of the said slurry.

3. The slurry as described in claim 1 wherein the graphite exceeds about 1.22 percent of the weight of the said slurry, and wherein water is present in an amount of about (3.27 X wt. percent graphite) minus 4.

4. The slurry as described in claim 3 wherein the viscosity of said slurry is about 350 to 500 centipoises at 70 to F.

5. The slurry as described in claim 1 further characterized by minor additions of an anionic surface-active wetting agent thereto, said wetting agent being an alkyl aryl sulfonate.

6. The slurry of claim 5 wherein said wetting agent is triethanolamine salt of dodecyl benzene sulfonic acid.

7. The slurry as described in claim 5 wherein said fused silica is about 200 mesh in grain size and is present in said slurry with colloidal silica in a ratio of about 1 pound to about 172 cc respectively, wherein said graphite is about 325 mesh in size, wherein water is present in said slurry and the percent graphite exceeds about 1.13 percent of the weight of said slurry, and wherein said wetting agent is present in a quantity of two to five drops.

8. The slurry as described in claim 7 wherein said wetting agent is a triethanolamine salt of dodecyl benzene sulfonic acid.

9. The slurry as described in claim 1 wherein the graphite exceeds about 1.13 percent of the weight of said slurry, and wherein water is present in an amount of about (1.77 X wt. percent graphite) minus 2.

10. The slurry as described in claim 9 wherein the viscosity of said slurry is about 2,100 to 2,500 centipoises at 75 to F.

Claims

2. The slurry as described in claim 1 wherein said fused silica is about -100 mesh in grain size and is present in said slurry with colloidal silica in a ratio of about 1 pound to about 260 cc respectively, wherein said graphite is about -325 mesh in size, and wherein water is present in said slurry and the percent graphite exceeds about 1.22 percent of the weight of the said slurry.
3. The slurry as described in claim 1 wherein the graphite exceeds about 1.22 percent of the weight of the said slurry, and wherein water is present in an amount of about (3.27 X wt. percent graphite) minus 4.
4. The slurry as described in claim 3 wherein the viscosity of said slurry is about 350 to 500 centipoises at 70* to 75* F.
5. The slurry as described in claim 1 further characterized by minor additions of an anionic surface-active wetting agent thereto, said wetting agent being an alkyl aryl sulfonate.
6. The slurry of claim 5 wherein said wetting agent is triethanolamine salt of dodecyl benzene sulfonic acid.
7. The slurry as described in claim 5 wherein said fused silica is about -200 mesh in grain size and is present in said slurry with colloidal silica in a ratio of about 1 pound to about 172 cc respectively, wherein said graphite is about -325 mesh in size, wherein water is present in said slurry and the percent graphite exceeds about 1.13 percent of the weight of said slurry, and wherein said wetting agent is present in a quantity of two to five drops.
8. The slurry as described in claim 7 wherein said wetting agent is a triethanolamine salt of dodecyl benzene sulfonic acid.
9. The slurry as described in claim 1 wherein the graphite exceeds about 1.13 percent of the weight of said slurry, and wherein water is present in an amount of about (1.77 X wt. perceNt graphite) minus 2.
10. The slurry as described in claim 9 wherein the viscosity of said slurry is about 2,100 to 2,500 centipoises at 75* to 85* F.