Patent Application
20240316626
This application claims priority to and benefit of Indian patent application Ser. No. 20/2341019538, filed on Mar. 21, 2023, the contents of which are incorporated by reference in its entirety.
The present invention relates to a casting furnace for restructuring of metals and/or ceramics and more particularly resulting from improved thermal processing conditions for control of microstructure and elemental segregation so that the resulting components are relatively free of defects and microstructural inhomogeneity normally associated with conventional cast parts. The invention encompasses refining of all types of cast structures including random and directional oriented metals such as gas turbine blades and/or ceramics and magnetic materials with specific directional properties.
The increased demand for gas turbines with greater power and efficiency have significantly expanded the need for more complex, lighter in weight and larger in size cast components with improved soundness, reduced segregation and optimum microstructure. To achieve increased power and efficiency, the turbine needs to be not only larger but also have to operate at much higher temperature to enhance fuel combustion and thrust. Cast components with single crystal or columnar structure that can operate at much higher temperature than conventional random crystal structure because of reduced grain boundaries and favorable crystal orientation to the stress axis are increasingly used in these turbines. Improved soundness and homogeneity along with preferred crystal orientation is required to improve reliability of these components.
In the past, the conventional practice of manufacturing of castings was to pour metal into a mold cavity and let the entire casting transform from the molten to solid state. Solidification of this metal was mostly dictated by casting cross sections and has been relatively uncontrollable because different sections froze at different times. The solidification time and the reservoir of the metal above determined the casting soundness and structure and ultimately the mechanical properties. Therefore, the properties of casting solidified in the conventional manner, will vary in proportion to the solidification pattern/structure achieved. The economy and product performance dictates development of new processes with enhanced filling and feeding characteristics that promote progressive solidification of metal to impart castings with uniform structure and improved soundness. Progressive solidification under high gradient conditions provides the ideal situation for control of solidification microstructure while enhancing the feeding of molten alloy from the molten alloy reservoir above to improve casting soundness.
A directional solidification casting method, which is commonly known as Bridgman method, is typically used to produce single crystal or columnar cast structure. In this method, the molten metal is poured in to a mold maintained considerably above the melting point of the alloy in a mold heating chamber and then the mold is withdrawn gradually from hot mold chamber to a cooler lower chamber. The metal solidifies in the lower chamber due to radiation heat loss or the use of a water-cooled chill plate. The resultant crystal structure is primarily influenced by the rate of mold withdrawal, temperature gradient at the solidification boundary between the heating and cooling chambers. The temperature gradient in itself is influenced by the type of the alloy, amount of heat contained in the mold and rate of heat loss and therefore, a small mold with less metal will solidify under higher thermal gradient due to reduced heat contained in the mold and faster heat loss. A higher temperature or thermal gradient invariably results in finer microstructure through formation of many crystals.
It is a well-known fact that the refinement of microstructure requires solidification of alloys under very high gradient and controlled solidification rate. Literature survey shows several studies have been completed to document the effect of gradient, solidification and cooling rate of castings on their microstructure, carbide and gamma precipitate sizes. It is also well documented with increase in gradient and solidification rate, primary dendrite and secondary dendrite spacings decrease gradually, the morphology of gamma prime precipitates changes from cuboidal to spherical and distribute more uniformly in both dendritic and inter-dendritic regions. The MC carbide morphology changes from coarse block to fine strip to finally Chinese script mainly consisting of Ta, W and Hf elements. The refinement of microstructure and precipitates significantly improve both creep and low cycle fatigue properties which are critical to long term functioning of the turbine blades which rotate under very high speed and are subjected very high temperature and pressure.
In casting of large molds with many cast components and/or casting of large last stage blades, thermal gradient for solidification becomes inadequate leading to formation of coarser cast structures with increased solidification related defects, segregation and porosity. Also, the limited heat extraction possible at lower thermal gradient invariably results in excessively long casting time leading to lower productivity. Therefore, casting of large final stage blades, which are increasingly in demand because of higher power output required from the turbines, require an ultra-high gradient furnace capable of faster rate of directional heat extraction. In addition to the increased production rate, a faster solidification rate results in castings with significantly finer crystal structure, reduced defects, improved soundness and homogeneity. Refined microstructure significantly enhances fatigue properties of turbine blades while the reduced segregation assists in post cast processing of castings to homogenize microstructure which improves creep life of turbine blades. Reduced segregation helps optimization of heat treat parameters to avoid recrystallization or incipient melting scrap which are prevalent in conventional castings with high localized segregation. Ultra-high gradient furnace, because of its ability to extract a directionally large amount of heat, provides the ideal solidification conditions needed for the manufacture of large complex turbine blades with highest performance and reliability at a high production rate.
Some of the conventional Bridgman casting units containing a single heating zone and radiation cooling have been disclosed in the past. For example, United States granted U.S. Pat. Nos. 10,717,617, 10,082,032, 6,311,760 and 5,921,310 disclose the use of additional gas or liquid cooling to increase heat extraction. The cooling suggested in the above disclosures constitute active cooling with high pressure gas directed on the mold with nozzles to impart turbulent or supersonic cooling.
Therefore, there is a need in the art to provide an improved casting furnace for restructuring of metals and/or ceramics.
It is an object of the present invention to provide an improved casting furnace that avoids the drawback of known furnaces.
It is another object of the present invention to overcome the above-mentioned problems and difficulties encountered in conventional casting of large molds with many components and/or large blades that contain large amounts of heat through improvement of solidification thermal gradient and cooling.
It is another object of the invention to provide a superior casting furnace with high heat extraction capability for manufacture of large sized molds or increased number of parts of desired soundness and microstructure including directional and non-directional in a shorter time thus enhancing productivity.
It is yet another object of the invention is to control nucleation and growth of crystals through optimization of thermal gradient and withdrawal rate at the solidification boundary to refine primary and secondary dendrite arms spacing of crystals. The reduced primary spacing should help in minimizing elemental segregation and porosity while decreasing the propensity for formation of solidification defects. The finer microstructure and reduced segregation should considerably aid in improvement of low cycle fatigue properties.
It is yet another object of this invention is to provide appropriate thermal conditions to promote diffusion of interstitial elements such as carbon and boron, and harmful trace elements such as zirconium, silicon, and substitutional element such as aluminum and titanium to improve heat treatability and final properties of the casting including creep.
The casting furnace of the present invention contains a unique mold heating chamber and mold cooling chamber and a standard melt chamber. The purpose of the melt chamber is to provide the necessary source of molten alloy or ceramics for production of the component and hence is not discussed in detail. The mold heating chamber comprises two independently controlled heating units or zones i.e., a primary heating zone and a secondary heating zone. The secondary heating zone operates above the melting point of the alloy but a lot colder than the conventional directional solidification casting furnace. This helps not to excessively superheat the metal and ceramic mold/core thereby reducing the extent of the mold-metal reaction, distortion of ceramic mold/core which negatively impacts casting yield. By raising the mold sufficiently near the casting temperature, the secondary heating zone helps to reduce the heat needed in the primary heating zone thereby improving flexibility and control for the primary heating zone. The primary heating zone provides an additional source of heat to bring the melt and mold temperature above those used in conventional Bridgman casting furnaces. The higher mold temperature of the primary heating zone combined with much lower mold temperature of primary cooling zone attained in the present invention, imparts the increased thermal gradient for solidification needed for growth of directional crystals. The purpose of the secondary mold cooling unit or the secondary cooling chamber is to affect the cooling rate of casting thereby promoting solid state diffusion of lower melting and interstitial elements. The principle of the present invention is based on control of temperature and heat extraction rate needed at critical locations in each zone to provide the optimum solidification conditions for control of crystal nucleation and growth and hence restructuring.
In one advantageous feature of the present invention, the cooling achieved is passive, and does not need the use of nozzles to generate turbulent cooling and the gas pressure is low enough to facilitate recirculation without the need for expensive compression units. The present invention uses double heating and dual cooling systems to impart precise control of temperature and temperature gradient across the solidification front for restructuring of metals with fine structure, reduced segregation and improved soundness.
Features and advantages of the subject matter hereof will become more apparent in light of the following detailed description of selected embodiments, as illustrated in the accompanying FIGURES. As will be realised, the subject matter disclosed is capable of modifications in various respects, all without departing from the scope of the subject matter. Accordingly, the drawings and the description are to be regarded as illustrative in nature.
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:
It will be noted that throughout the appended drawings, like features are identified by like-reference numerals.
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.
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.
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.
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.
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.
Various features and embodiments of a casting furnace are explained in conjunction with the description of FIGURES (FIGS.) 1-5.
Referring to
Referring now to
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.
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.
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.
Referring now to
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.
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.
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.
Referring to
| Number | Date | Country | Kind |
|---|---|---|---|
| 202341019538 | Mar 2023 | IN | national |
