Powder investment casting binder and molds derived therefrom

Abstract

A powder binder product for use in making a slurry for investment casting molds comprising Nano-sized powders; and an organic polymer powder, wherein it does not require aqueous colloidal silica to produce slurries used to build investment casting molds. The Nano-sized powders comprise fumed alumina, boehmite, fumed silica, or fumed titanium oxide or combinations thereof. The coarse refractory powder, combined with the powder binder for mold manufacture, comprises milled zircon, tabular alumina or fused alumina, fused silica, alumino-silicate, zirconia, and yttria or combinations thereof. The organic polymer in a powder binder comprises a cellulose-based material. A powder investment casting binder, that once fired, consists of up to 96 weight percent aluminum oxide.

Claims

1. A powder binder product for use in making a slurry for investment casting molds comprising: Nano-sized powders; and an organic polymer powder; wherein the composition of the Nano-sized powder comprises: boehmite or pseudo boehmite, 71% to 94% by weight; aluminum oxide; silicon oxide, 2.5% to 23% by weight; titanium oxide, 0.4% to 1.0% by weight; or combinations thereof; and wherein the powder binder product does not comprise aqueous colloidal silica to produce slurries used to build investment casting molds.

2. The powder binder product of claim 1, wherein the composition of the Nano-sized powder comprises: pseudo boehmite, 71% to 94% by weight; aluminum oxide; titanium oxide, 0.4% to 1.0% by weight; or combinations thereof.

3. The powder binder product of claim 1, wherein said Nano-sized powder is between 80 nm and 1.2 μm.

4. The powder binder product of claim 1, wherein the organic polymer is between 2.0 and 6.0% by weight of the total powder binder mass.

5. The powder binder product of claim 1, wherein the organic polymer comprises: at least one of a cellulose-based material or acrylic combined with polyethylene glycol.

6. The powder binder product of claim 1, wherein the organic polymer comprises: a cellulose-based material or acrylic combined with polyethylene glycol; and a methyl cellulose binder.

7. The powder binder product of claim 1, which, when fired, comprises: up to 96 weight percent crystalline aluminum oxide and not less than 70 weight percent.

8. The powder binder of claim 1 for a mold manufacture wherein sizes of particles of a coarse refractory powder are: -325 mesh; -200 mesh; -120 mesh; or combinations thereof.

9. The powder binder product of claim 1 wherein said Nano-sized powder component comprises: particles less than about 1.2 μm.

10. The powder binder product of claim 1 wherein, when dispersed in deionized water, and buffered to between 3.0 and 5.0 pH, produces an aqueous sol to produce investment casting molds.

11. The powder binder product of claim 1, wherein, once used to produce molds, yields molds subsequently dewaxed by flash-fire.

12. A method for producing an investment casting comprising: obtaining a dry powder (605) comprising: fumed alumina, boehmite, 71% by weight to 94% by weight; fumed silica, or fumed titanium oxide, 0.4% by weight to 1.0% by weight, or combinations thereof; aluminum oxide, zircon, mullite, alumino-silicate, zirconium oxide, yttrium oxide, silicon oxide, 2.5% by weight to 23% by weight, or combinations thereof; wherein said aluminum oxide, zircon, mullite, alumino-silicate, zirconium oxide, yttrium oxide, silicon oxide, or combinations thereof are added as −325 and −120 flours to make the slurry; and a cellulose-based material; obtaining water (610) and buffering said water; combining said dry powder and said buffered water to form a slurry or sol only (615); adjusting the pH of said slurry (620); providing a pattern (630); applying said slurry with a stucco to said pattern to create a mold (635); allowing said mold to harden (640); removing said pattern from said mold (645); filling said mold with molten casting material (650); allowing said casting material to solidify (655); and removing said mold from a cast article (660).

13. The method of claim 12 wherein said dry powder comprises: Nano-sized powders; and an organic polymer powder; wherein the composition of the Nano-sized powder comprises: boehmite or pseudo boehmite, 71% by weight to 94% by weight; aluminum oxide; silicon oxide, 2.5% by weight to 23% by weight; titanium oxide, 0.4% by weight to 1.0% by weight; or combinations thereof.

14. The method for producing an investment casting of claim 12, wherein said step of obtaining water (610) and buffering said water comprises: adding nitric acid to a pH between about 3.0 and about 5.0.

15. The method for producing an investment casting of claim 12, wherein said step of adjusting pH of said slurry (620) comprises: a slurry pH range of about 3.5 to about 5.0 (620).

16. The method for producing an investment casting of claim 12, wherein said step of removing said pattern from said mold comprises: flash-fire (645).

17. The method for producing an investment casting of claim 12, comprising: a Nano-sized powder comprising boehmite or pseudo boehmite 71% by weight to 94% by weight, aluminum oxide, silicon oxide, 2.5% by weight to 23% by weight, or titanium oxide, 0.4% by weight to 1.0% by weight, or combinations thereof; an organic polymer powder; and a coarse refractory powder comprising aluminum oxide, zircon, mullite, alumino-silicate, zirconium oxide, yttrium oxide, fused silicon oxide, or combinations thereof.

18. A method for producing an investment casting comprising: obtaining a dry powder binder (605) comprising fumed alumina, boehmite, fumed silica, or fumed titanium oxide or combinations thereof, and aluminum oxide, zircon, mullite, alumino-silicate, zirconium oxide, yttrium oxide, silicon oxide, or combinations thereof, and a methylcellulose cellulose-based material; obtaining (deionized) water (610) and buffering said deionized water with nitric acid to a pH between about 3.0 and about 5.0; combining said dry powders and said buffered water to form a slurry (615); adjusting pH of said slurry as-needed to an about 3.5 to about 5.0 range (620); providing a pattern (630); applying said slurry with a stucco to said pattern to create a mold (635); allowing said mold to harden (640); removing said pattern from said mold by flash-fire (645); firing said mold to between 1,000 and 1,200 deg C; filling said mold with molten casting material (650); allowing said casting material to solidify (655); and removing said mold from a cast article (660).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The foregoing and other objects, features, and advantages of the invention will be apparent from the following description of particular embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. For purposes of clarity, not every component may be labeled in every drawing.

(2) FIG. 1 illustrates the weight and volume difference between aqueous colloidal silica and a powder equivalent in accordance with an embodiment.

(3) FIG. 2 shows ceramic investment casting molds produced by an embodiment, and by state-of-the-art technology.

(4) FIG. 3 shows castings manufactured by an embodiment, equivalent in appearance and surface quality to the present state-of-the-art.

(5) FIG. 4 shows Flexural Creep data of fired shell material by an embodiment, and state-of-the-art colloidal silica.

(6) FIG. 5 shows thermal expansion data for firing shell material in accordance with an embodiment.

(7) FIG. 6 is a flow chart of a method in accordance with an embodiment.

(8) These and other features of the present embodiments will be understood better by reading the following detailed description, taken together with the figures herein described.

DETAILED DESCRIPTION

(9) The features and advantages described herein are not all-inclusive and, in particular, many additional features and advantages will be apparent to one of ordinary skill in the art in view of the drawings, specification, and claims. Moreover, it should be noted that the language used in the specification has been selected principally for readability and instructional purposes, and not to limit in any way the scope of the inventive subject matter. The invention is susceptible of many embodiments. What follows is illustrative, but not exhaustive, of the scope of the invention.

(10) Advantages of embodiments: 1) Cost savings for shipping; not shipping water, and no need for temperature control during shipping and storage as required for colloidal silica. 2) Simplicity of use by the end user, just add water, fewer materials to source and inventory. 3) Less reaction between the metal and mold surface, easing shell removal, and improving the surface finish of the casting. 4) The ability to employ both Nano-sized silica and aluminum oxides in “backup” slurries to improve high temperature dimensional stability of the mold and casting. 5) Powder binder can be used in both prime and backup slurry. 6) Higher firing temperatures can be employed since the composition of the self-bonded refractory is more stable than colloidal silica at high temperatures.

(11) FIG. 1 illustrates the weight and volume difference 100 between aqueous colloidal silica and a powder equivalent.

(12) Table 1 illustrates the essential constituents and percentages within the embodiment.

(13) TABLE-US-00001 TABLE 1 Powder Binder Formulation Raw material Minimum Maximum Powder Boehmite 71.0% 94.0% Fumed silica 2.5% 23.0% Fumed Titania 0.4% 1.0% Cellulose binder 2.7% 4.9%

(14) The powder characteristics of the embodiment were characterized by industry standard techniques; BET Surface Area Analysis, and Dynamic Laser Scattering (DSL). For the formulations in Table 1 the Single Point Specific Surface Area was 40.31 and 40.36 m.sup.2/g and the particle-size range, 80 nm to 1.2 μm covered both the Minimum and Maximum formulations. Data was measured by a certified testing laboratory, Particle Technology Laboratory, in Downers Grove, Ill.

(15) FIG. 2 shows ceramic investment casting molds 200 produced by the embodiment (A), and by known state-of-the-art technology (C). Each flash-fired at 815 deg. C.

(16) Table 2 lists the distinctions between the embodiment and state-of-the-art technology.

(17) TABLE-US-00002 TABLE 2 Summary of Mold Manufacture in FIG. 2 State-of-the-art Powder Binder Colloidal Silica Weight Percent Binder 8% .sup.  38% (as aqueous in Slurry, 66% total colloidal silica) solids in each. Colloidal Binder pH    4.5   10.8 Weight Percent 19.8% .sup.   24.2% colloidal solids, % Mold fire temperature, 2 1,200 deg C. 1,000 deg. C. hours at temperature Oxide composition in 96% Al2O3, 3% SiO2, 100% SiO2, binder and 1% TiO2 trace Na2O Percent Polymer in 2.7%  .sup.   1.3% binder Fired Mold Strength at 654 300 Room Temperature, 3-pt. MOR, psi

(18) FIG. 3 shows A356 aluminum castings 300 manufactured by the embodiment (A), equivalent in appearance and surface quality to the present known state-of-the-art (C).

(19) FIG. 4 Flexural Creep data 400 shows that the embodiment (A) is 10× more resistant to dimensional distortion at high temperature compared to known state-of-the-art colloidal silica (C).

(20) Table 3 shows, after firing at high temperature, the embodiment can consist of up to 96% by weight crystalline aluminum oxide. Colloidal silica under the same conditions consists of 19% non-crystalline glassy phase.

(21) TABLE-US-00003 TABLE 3 Quantitative X-Ray Diffraction Analysis, Calcined Powder Binder, and Colloidal Silica Atomic Powder SP-30 Phase Formula Binder CS corundum Al.sub.2O.sub.3 51.5 alumina Al.sub.2O.sub.3 27.9 theta alumina Al.sub.2O.sub.3 19 kappa alumina Al.sub.2.427O.sub.3.64 eta alumina Al.sub.2O.sub.3 1.2 sigma cristobalite SiO.sub.2 0.4 11.6 tridymite SiO.sub.2 66.6 quartz SiO.sub.2 0.2 silica SiO.sub.2 2.1 amorphous Non- 19.5 crystalline

(22) In embodiments, the concentration of the organic polymer comprises between 2.0% and 5.0% of the total dry mass. In embodiments, the organic polymer provides the required mechanical strength associated with dipping, drying, and the mold dewax operation.

(23) In embodiments, the powder binder contains titanium oxide, silicon oxide, and aluminum oxide. FIG. 5 shows how, when silicon oxide and titanium oxide are added, the firing temperature for the material decreases 500. In addition, note in Table 4, with the addition of titanium oxide, the mechanical strength of the fired mold material increased two-fold from 242 psi and 267 psi to 660 psi. Furthermore, the 2,000F−2 hrs and 401 psi MOR mechanical strength results, when Silica and Titania are added together, would provide mold properties expected by industry today. This shows the special value of these powder formulations and the special role that titanium oxide plays. Gas permeability, critical in commercial air-melt investment casting, is also shown to increase 25% from 14 and 15 cDarcy to 20 cDarcy with the addition of titanium oxide. This shows that high-alumina powder binder, formulated and fired correctly, yield results useful by industry today but which are not presently commercially available.

(24) TABLE-US-00004 TABLE 4 Mold Properties and Oxide Additives, materials in FIG. 5 Colloidal Silica Colloidal Colloidal Silica and Titania Alumina only Added added Fired MOR 242 267 660 Strength, psi, fired at 2,200 F.- 2 hrs Fired MOR 70 78 401 Strength, psi, fired at 2,000 F.- 2 hrs firing Gas 14 15 20 Permeability, cDarcy

(25) In embodiments, a small amount of wetting agent and anti-foam emulsion is used. A phosphate based wetting agent, Victawet 12, and Dow Corning antifoam 1430 and 1400 are used. Both (initially added to the water) were an asset to disperse the powders and reduce entrapped air. Dilute nitric acid was used to buffer the deionized water, between pH of 3.0 and 4.0, before preparing the slurry.

(26) Mechanical strength is critical at two points in the process; 1) before dewax in the so-called green state, and 2) after firing before casting to hold liquid metal during casting. During dewax the ceramic and wax assembly is heated rapidly to remove the wax. Before the wax melts it expands and puts a strain and a corresponding stress on the ceramic material it is encased in. If the stress exceeds the mechanical strength of the ceramic in the ‘green’ state it will crack. Therefore, during investment casting manufacture cracks that form in dewax produce positive metal defects and ‘fins’ that need to be removed by grinding. If the cracks are excessive the mold may leak and fail completely resulting in scrap and a safety risk to manufacture workers. In this context, the absence of positive metal and ‘fins’ after casting is evidence of sufficient strength in the green state. As a result, the absence of cracks and ‘fins’ in FIGS. 2 and 3, is evidence that powder binder in a prime or backup slurry provides sufficient mechanical strength for investment casting.

(27) Mechanical strength in the fired state is also critical to counter the hydrostatic pressure of the liquid metal during casting. A higher mold strength in the fired state is an advantage because the thickness of the mold can be reduced. The increase from 300 to 650 psi with powder binder could be a significant advantage. Money can be saved because less material is needed to produce the mold. Money is also saved by reducing the work space and labor associated with the manufacturing of the ceramic molds. Fewer ceramic raw materials are shipped and the carbon-footprint of manufacturing associated with products is further reduced. A thinner mold can also increase the casting rate of the metal which is known to reduce the grainsize of the metal which in turn increases the mechanical strength and reliability of cast components like turbine blades in aircraft engines.

(28) Furthermore, the dimensional integrity of the mold is also critical both during firing and casting. FIG. 4 shows evidence that, under high temperature and load, the colloidal silica-bonded mold material distorted 0.055 inches and under identical conditions with powder binder, only 0.005 inches with the same aluminosilicate refractory compositions. Significant savings would also be realized from reduced machining, and or straightening due to mold distortion during casting.

(29) Table 3 shows evidence that a greater advantage in dimensional stability could be realized with powder binder from the crystalline aluminum oxide formed and no amorphous phase detected. Casting manufacturers would be able to better meet the dimensional tolerances set by their customers. The X-ray Diffraction Analysis of greater than 95 weight percent is a huge benefit and stands in stark contrast with the 19% amorphous glassy phase with state-of-the-art colloidal silica binder.

(30) FIG. 6 is a flow chart of a method 600 for producing an investment casting. Steps of the method comprise: obtaining a dry powder binder (605) comprising fumed alumina, boehmite, fumed silica, or fumed titanium oxide or combinations thereof, and aluminum oxide, zircon, mullite, alumino-silicate, zirconium oxide, yttrium oxide, silicon oxide, and combinations thereof, and a methylcellulose cellulose-based material; obtaining (deionized) water (610) and buffering the deionized water with nitric acid to a pH between about 3.0 and about 5.0; combining the dry powders and the buffered water to form a slurry (615); adjusting pH of the slurry as-needed to an about 3.5 to about 5.0 range (620); providing a pattern (630); applying the slurry with a stucco to the pattern to create a mold (635); allowing the mold to harden (640); removing the pattern from the mold by flash-fire or steam autoclave (645); firing the mold to between 1,000 and 1,200 deg. C; filling the mold with molten casting material (650); allowing the casting material to solidify (655); and removing the mold from a cast article (660).

(31) The foregoing description of the embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of this disclosure. It is intended that the scope of the present disclosure be limited not by this detailed description, but rather by the claims appended hereto.

(32) A number of implementations have been described. Nevertheless, it will be understood that various modifications may be made without departing from the scope of the disclosure. Although operations are depicted in the drawings in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results.

(33) Each and every page of this submission, and all contents thereon, however characterized, identified, or numbered, is considered a substantive part of this application for all purposes, irrespective of form or placement within the application. This specification is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of this disclosure. Other and various embodiments will be readily apparent to those skilled in the art, from this description, figures, and the claims that follow. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.