CASTING FURNACE FOR SOLIDIFICATION RESTRUCTURING (FSR)

Abstract

A casting furnace for manufacture of metal and/or ceramic components at high production rate is disclosed. The casting furnace comprises a melting chamber, a dual zone mold heating chamber and a dual zone mold cooling chamber. The melting chamber provides a source of molten alloy or ceramics with adequate superheat. The dual zone mold heating chamber includes an independently controlled primary heating zone and the secondary heating zone. The primary heating zone raises the mold temperature adequately to impart high gradient solidification conditions. The secondary heating zone assists the primary heating zone to minimize overheating of the majority of the mold. The dual zone mold cooling chamber comprises a primary cooling chamber and a secondary cooling chamber. The primary cooling chamber speeds up solidification in order to prevent defect formation and refine microstructure. The secondary cooling chamber slows down the cooling of castings to reduce residual stresses build up and minimize elemental segregation through augmenting solid-state diffusion of lower melting elements.

Claims

1. A casting furnace for solidification restructuring (FSR), comprising: a melting chamber to provide a source of molten alloy or ceramics with adequate superheat; a dual zone mold heating chamber, wherein the dual zone mold heating chamber comprises a primary heating zone and a secondary heating zone, wherein the primary heating zone and the secondary heating zone are independently controlled, wherein the primary heating zone raises the temperature of the mold adequately to impart high gradient solidification conditions, and wherein the secondary heating zone assists the primary heating zone to minimize overheating of the mold; and a dual zone mold cooling chamber, wherein the dual zone mold cooling chamber comprises a primary cooling chamber and a secondary cooling chamber, wherein the primary cooling chamber and the secondary cooling chamber are independently controlled, wherein the primary cooling chamber speeds up the solidification to prevent defect formation and refine microstructure, and wherein the secondary cooling chamber slows down the cooling of castings to reduce residual stresses build up and minimize elemental segregation through augmenting solid-state diffusion of lower melting elements.

2. The casting furnace of claim 1, wherein the primary heating zone is narrow and connects to a power supply, heating and insulation to increase the temperature of the metal in the mold and provides thermal stability and prevention of magnetic flux leakage to the metal, and wherein the secondary heating zone is wider than the primary heating zone.

3. The casting furnace of claim 2, wherein the dual zone mold heating chamber prevents overheating of the mold and reduces mold bulging and mold-metal reaction and associated casting defects.

4. The casting furnace of claim 1, wherein the primary cooling chamber is equipped with cooling fluid flow and pressure control linked to the withdrawal rate to regulate the heat extraction from the mold, and wherein the secondary cooling chamber comprises an insulator or heat reflector to reduce the heat loss from hot mold entering the secondary cooling chamber to slow down the cooling of the metal with a heat resistant metal baffle separating them.

5. The casting furnace of claim 4, wherein the dual zone mold cooling chamber prevents rapid cooling of the casting to reduce residual stresses build up, casting cracking and to improve homogenization of casting through facilitating solid state diffusion of low melting elements.

6. The casting furnace of claim 1, wherein the dual zone mold heating chamber and the dual zone mold cooling chamber provide conditions for ultra-high thermal gradient solidification in excess of 3000? F./inch resulting in solidified articles with finer structure and reduced segregation and porosity.

7. The casting furnace of claim 6, wherein the solidification of the articles allows the manufacture fine equiaxed castings with improved porosity and gating efficiency.

8. The casting furnace of claim 6, wherein the reduced segregation of low melting elements resulting from ultra-high thermal gradient of solidification enhances full heat treatability of castings and results in reduced heat treat related scrap.

9. The casting furnace of claim 6, wherein the increased heat extraction facilitates higher speed of withdrawal resulting in either productivity enhancements or production of larger molds to reduce manufacturing costs.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Further features and advantages of the present subject matter will become apparent from the following detailed description, taken in combination with the appended drawings, in which:

[0019] FIG. 1 is a schematic illustration of a casting furnace with a standard melt chamber and unique dual zone mold heating and unique dual zone cooling chambers, in accordance with one embodiment of the present invention;

[0020] FIG. 2 is an illustration of a mold heating chamber, in accordance with one embodiment of the present invention;

[0021] FIG. 3 is an illustration of a mold cooling chamber, in accordance with one embodiment of the present invention;

[0022] FIG. 4 is a schematic illustration of a mold used for casting directionally solidified alloy for documenting the effect of thermal gradient on structure, segregation and soundness, in accordance with one exemplary embodiment of the present invention; and

[0023] FIG. 5 illustrates the temperature profile comparison between conventional Bridgman and the casting furnace of the present invention.

[0024] It will be noted that throughout the appended drawings, like features are identified by like-reference numerals.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0025] The detailed description set forth below in connection with the appended drawings is intended as a description of exemplary embodiments in which the presently disclosed subject matter may be practiced. The term exemplary used throughout this description means serving as an example, instance, or illustration, and should not necessarily be construed as preferred or advantageous over other embodiments. The detailed description includes specific details for providing a thorough understanding of the presently disclosed casting furnace. However, it will be apparent to those skilled in the art that the presently disclosed subject matter may be practiced without these specific details. In some instances, well-known structures and devices are shown in functional or conceptual diagram form in order to avoid obscuring the concepts of the presently disclosed casting furnace.

[0026] In the present specification, an embodiment showing a singular component should not be considered limiting. Rather, the subject matter preferably encompasses other embodiments including a plurality of the same component, and vice-versa, unless explicitly stated otherwise herein. Moreover, the applicant does not intend for any term in the specification to be ascribed an uncommon or special meaning unless explicitly set forth as such. Further, the present subject matter encompasses present and future known equivalents to the known components referred to herein by way of illustration.

[0027] Although the present subject matter describes a casting furnace, it is to be further understood that numerous changes may arise in the details of the embodiments of the casting furnace. It is contemplated that all such changes and additional embodiments are within the true scope of this subject matter.

[0028] The following detailed description is merely exemplary in nature and is not intended to limit the described embodiments or the application and uses of the described embodiments. As used herein, the word exemplary or illustrative means serving as an example, instance, or illustration. Any implementation described herein as exemplary or illustrative is not necessarily to be construed as preferred or advantageous over other implementations. All of the implementations described below are exemplary implementations provided to enable persons skilled in the art to make or use the embodiments of the subject matter and are not intended to limit the scope of the subject matter.

[0029] It should be understood that the present invention describes a novel casting furnace for manufacture of metal and/or ceramic components at high production rate. The furnace includes dual heating and dual cooling zones to regulate both the heat content and heat extraction rate from the ceramic mold to grow crystals of desired density, orientation and microstructure. The furnace with upper mold heating chamber containing dual heating zones and a lower mold cooling chamber containing dual cooling zones enables to regulate the heat extraction rate necessary to affect the restructuring of crystals during primary solidification in the molten state; and a reduction of secondary segregation in the solid state. This restructuring which encompasses refinement of microstructure, improvement in soundness and segregation significantly results in improvement properties those are of critical importance for high temperature performance of turbine blades and quality of ceramics. The aforementioned unique mold heating and cooling chambers along with a standard induction melting chamber to supply the molten metal or ceramic constitute the essential components of this advanced casting furnace.

[0030] Various features and embodiments of a casting furnace are explained in conjunction with the description of FIGURES (FIGS.) 1-5.

[0031] Referring to FIG. 1, one form of a casting furnace 10 for precise control of temperature profiles at the solidification front based on optimization of heat extraction rate specifically tuned to the heat content of the mold is shown, in accordance with one embodiment of the present invention. The casting furnace 10 consists of three chambers, i.e., a melting chamber M within a furnace F, a mold heating chamber H and a mold cooling chamber C. The melting chamber M preferably contains an induction furnace 11 to melt the alloy in a crucible for providing the source of a molten alloy 12 at the right temperature necessary for pouring into a mold through pour inlet 13. The melting chamber M is isolated from the mold heating chamber H by a thick insulating top cover 14 usually made up of graphite. The melting chamber M includes a mold cavity 15. The mold cavity 15 forms a tortuous path through which molten alloy will travel as it fills the mold. Preferably, the mold heating chamber H is characterized as being composed of thin-walled structures made by the standard investment casting process with higher thermal transfer properties. A mold heating chamber H which is thin and very conductive is preferred in this process to reduce both heat content of the mold as well as facilitate rapid heat extraction.

[0032] Referring now to FIG. 2, another embodiment of the present invention is shown in which the mold heating chamber H consists of two independent heating chambers i.e., a primary heating zone PH and a secondary heating zone SH. The primary heating zone PH and the secondary heating zone SH are separated by a baffle 16. The secondary heating zone SH helps to maintain the mold sufficiently above the melting point of the alloy but at much lower temperature than in the conventional process. For example, in a conventional process with single heating, a furnace controlled at 2800? F., the middle zone can easily overheat by 150? F. to 250? F. whilst the mold in the present invention can be about 100? F. cooler because of independent secondary heating control. Most conventional ceramics creep/distort when exposed to temperatures in excess of 2800? F. for prolonged time resulting in increased dimensional scrap and leakers. The considerably reduced temperature to which the mold is exposed in the present invention, will permit faster rate of heat extraction leading to increased rate of production. The lower mold temperature also helps to reduce extent of mold-metal reaction especially with reactive alloys, elemental segregation, bulging of molds and leakers.

[0033] The secondary heating zone SH comprises a special induction coil 17 that is widely spaced to provide dispersed heating. Further, the secondary heating zone SH comprises a susceptor or graphite heating material 18 and an insulation material 19 and source of the power supply are similar to the one used in single zone heating. Here, a dual thermocouple 20 is used to control and monitor the temperature of the secondary heating zone SH.

[0034] The purpose of the primary heating zone PH is to raise the temperature of the molten alloy in the mold at least 50? F. above those used in the conventional process for a short time to enhance thermal gradient of solidification. The primary heating zone PH is hence designed to be short in length with a high heat source and is typically about 20% of the total height of the entire heating zone. The primary heating zone PH contains a very closely spaced coil 21 to provide an intense heat in a very narrow zone. The primary heating zone PH includes a heating material 22 and the insulation 23 are designed to be thicker to retain heat as well prevent leakage of flux. The primary heating zone PH comprises a dual thermocouple 24. The dual thermocouple 24 controls and monitors the temperature of the primary heating zone PH. Unlike the entire furnace to be raised to the very high temperature needed near the solidification boundary in the conventional process, the present invention allows the primary heating zone PH to be at the highest temperature at the solidification zone needed to grow directional crystals, without superheating the rest of the mold.

[0035] The dual heating zones i.e., the primary heating zone PH and the secondary heating zone SH with independent controls, provide significant flexibility for maintaining the optimum temperature where needed. The increased temperature of the primary heating zone PH along with the cooler temperature of a primary cooling zone PC, in the present invention, leads to conditions desired for higher thermal gradient of solidification. The primary heating zone PH is isolated from the primary cooling zone PC below by a composite baffle 25. This baffle system is supported on a water-cooled copper shelf 26 which also helps to shield leakage of magnetic field. In order to achieve the higher gradient of solidification in the present invention, the higher primary heating zone PH temperature is coupled with lower temperature of the primary cooling zone PC. The temperature of the primary heating zone PH and the primary cooling zone PC, in addition with controlled withdrawal rate to regulate the amount of metal entering the solidification boundary between the primary heating zone PH and the primary cooling zone PC provides the ultra-high solidification conditions needed.

[0036] Referring now to FIG. 3, another embodiment of the present invention is shown in which the mold cooling chamber C consists of a primary cooling chamber PC and a secondary cooling chamber SC separated by a water-cooled metal baffle 27. The mold cooling chamber C is connected to a suitable vacuum source 28 via a pump 29 incorporating a vacuum valve 30. The mold cooling chamber C can also be connected to sources such as auxiliary gas source AS and a primary gas source PS via a blower B for an auxiliary fluid cooling 31. In the majority of the cases, a cooling gas is introduced in the secondary cooling chamber SC through a manifold 32 and pulled out of a manifold 33 in the primary cooling chamber PC. The intake and exhaust manifolds 32; 33 contain several dozen holes' precision drilled in preselected direction, the diameter and number of these holes are based on the volume flow of gas, residual pressure of gas in the chamber and the pressure of gas at inlet. The exhaust gas can be reused, if and when necessary, through a combination of reversible recirculation pump and heat exchangers/chillers to lower the gas temperature. A dual thermocouple 34 is used to monitor and control the temperature of the primary cooling zone PC.

[0037] The gas cooling system used in the present invention is passive and does not use high pressure to induce supersonic velocities as stated in several earlier inventions. A lower pressure of the gas used in this invention also facilitates recirculation of gas, if required without the need for expensive compression units. The gas cooling system employed varies based on the materials being cast and may compose of evaporative, endothermic and reactive cooling. The design of the intake and exhaust manifolds with multiple pre-inclined holes is critical to improve the robustness of the primary cooling.

[0038] The narrow and unique designs of the primary heating and primary cooling zones in the present invention along with optimization of mold withdrawal rate to allow small amounts of molten alloy to pass through the baffle, permits maximum directional heat extraction needed for solidification under ultra-high thermal gradient. The furnace of present invention is capable of achieving a thermal gradient in excess of 450? F./inch with optimization of complex composite baffle when primary heating zone PH and the primary cooling zone PC temperatures compared to about 150? F./inch typically achieved in conventional Bridgman furnaces. Solidification under higher thermal gradient facilitates refinement of structure resulting in improved soundness and homogeneity.

[0039] The purpose of the secondary cooling chamber SC is to minimize the heat loss from the mold to slow down the cooling of the metal solidified in the primary cooling zone PC. A slower cooling provides adequate time for diffusion of trace, interstitial and lighter elements such as aluminium to homogenize microstructure which further helps to improve heat treat solution treatment. The slow cooling also helps to minimize build-up of residual stress in castings which can lead to increased tendency for distortion or cracking of casting containing variable cross sections. An insulated metal shield 35 preferably through a resistance or induction heating coil 36 can be used to slow down casting heat loss. A dual thermocouple 37 is used to monitor and control temperature of the secondary cooling chamber SC.

[0040] Referring to FIG. 4, the schematics of the mold and furnace setup (single zone heating and cooling furnace setup SZ and dual zone heating and cooling furnace setup DZ) is shown, in accordance with one exemplary embodiment of the present invention. Here, a special mold 38 with slot for insertion of thermocouple 39 and a reservoir was fabricated for this evaluation. The reason for choice of in-situ melting stemmed from the need for an additional power supply needed for dual zone heating. The melt power supply was used as the auxiliary source to power the primary heating zone while the mold heating power supply was used for secondary heating. A laboratory 16 cubic zirconia single crystal furnace with 12 heating zone was modified to cast one mold with conventional single zone heating and cooling and two molds with dual zone heating and cooling per the present invention. A charge 40 was inserted into mold and cast separately under single and dual zone heating and cooling conditions. Thermocouple 41 was used to control heating of single zone furnace, while thermocouples 42 and 43 were used to control dual zone heating. The mold was loaded onto a chill plate 44 attached to a ram 45 in the mold cooling chamber C and the chamber was evacuated. The mold was then raised into preheated mold heating chamber H and soaked for one hour during which the metal melted and filled the mold. Both molds were withdrawn at 10/hour for the first 30 minutes and 6/hour for one hour with 1 transition in speed.

[0041] FIG. 5, depicts the temperature recorded by the thermocouple 39 during the withdrawal of molds with single and dual heating and cooling. The alloy used in this evaluation was MarM247 with a melting range of 2350? F. to 2500? F. An analysis of thermocouple data in a 2 zone around the baffle separating PH and PC zones, where the solidification was occurring, show thermal gradients were 190? F./inch per single zone heating and cooling versus 425? F./inch for dual zone heating and cooling. The results clearly demonstrate that the dual zone heating and cooling with differentiated control of temperatures per the present invention, can significantly increase thermal gradient than possible in the conventional Bridgman furnace. To corroborate the benefits of ultra-high gradient solidification with dual heating and cooling, primary dendrite arms spacing was measured 3 and 9 from the bottom of the slabs. The primary dendrite spacings were 0.016 and 0.023 for single zone heated and cooled slab versus 0.012 and 0.016 for dual zone heated and cooled slab depicting increased refinement of microstructure with dual heating and cooling. The dual zone heated and cooled slab exhibited no bulging and significantly reduced porosity. Electron dispersive analysis of aluminum ratio between the dendritic and inter-dendritic regions showed dual zone heating and cooling improved the aluminum ratio from 0.65 for single zone heating and cooling to 0.81 for dual zone heating and cooling. As aluminum is a strong gamma prime former, a more uniformly dispersed aluminum permits full heat treatment which should help to improve strength and creep properties of turbine blades. Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the applied claims, the invention may be practiced otherwise than specifically described herein.

REFERENCE NUMERALS

[0042] Casting furnace 10 [0043] Melting chamber M [0044] Furnace F [0045] Mold heating chamber H [0046] Mold cooling chamber C [0047] Induction furnace 11 [0048] Molten alloy 12 [0049] Inlet 13 [0050] Insulating top cover 14 [0051] Mold cavity 15 [0052] Primary heating zone PH [0053] Secondary heating zone SH [0054] Baffle 16 [0055] Induction coil 17 [0056] Susceptor or graphite heating material 18 [0057] Insulation material 19 [0058] Dual thermocouple 20 [0059] Coil 21 [0060] Heating material 22 [0061] Insulation 23 [0062] Dual thermocouple 24 [0063] Baffle 25 [0064] Water-cooled copper shelf 26 [0065] Primary cooling chamber PC [0066] Secondary cooling chamber SC [0067] Water-cooled metal baffle 27 [0068] Vacuum source 28 [0069] Pump 29 [0070] Vacuum valve 30 [0071] Auxiliary gas source AS [0072] Primary gas source PS [0073] Blower B [0074] Auxiliary fluid cooling 31 [0075] Manifold 32 [0076] Manifold 33 [0077] Dual thermocouple 34 [0078] Insulated metal shield 35 [0079] Resistance or induction heating coil 36 [0080] Dual thermocouple 37 [0081] Mold 38 [0082] Thermocouple 39 [0083] Charge 40 [0084] Thermocouple 41 [0085] Chill plate 42 [0086] Ram 43 [0087] Single zone heating and cooling furnace setup SZ [0088] Dual zone heating and cooling furnace setup DZ