Unidirectional solidification device, unidirectional solidification method, unidirectionally solidified casting, and unidirectionally solidified ingot

Abstract

This invention is concerned with the production of directionally solidified castings or ingots to eliminate casting defects such as macrosegregation and misoriented grain defects that occur in the blades for jet engines and industrial gas turbines. The mechanism of the occurrence of the above casting defects was clarified by the computer simulation system developed by this inventor, and it was found that, by strongly cooling the solid phase region and applying an axial static magnetic field, the heat pulses at the solidification interface due to convection of the liquid phase can be suppressed, and harmful lateral liquid flow in the solid-liquid coexisting phase can be suppressed by the synergistic effect of these two measures. This eliminates casting defects such as macrosegregation and misoriented grain defects, also refines the microstructure to produce high-quality products with excellent mechanical properties (creep strength). Regarding the strength of the static magnetic field, it was found that there is a range where the macrosegregation becomes minimum in a relatively low magnetic field range. This makes it possible to keep the required magnetic strength low, which significantly reduces the price of expensive superconducting coils. In addition, productivity can be improved by increasing the withdrawal speed.

1 . A directional solidification apparatus for making directionally solidified castings or ingots having a grain structure consisting of a single crystal structure, or a polycrystalline columnar dendrite structure or a mixture of said single crystal structure and said polycrystalline columnar dendrite structure, wherein
 (1) a first means of said directional solidification apparatus includes a mold for casting molten metal,
 an adiabatic baffle for dividing said mold into a heating region to heat said mold and a cooling region to cool said mold during directional solidification process, 
 a means for moving said mold from said heating region to said cooling region, 
 a heating means for heating and keeping said molten metal in said mold at a prescribed temperature, 
 and a fierce cooling means for enhancing the heat removal capability from a side surface of said mold so as to rectify the liquid flow within a solid-liquid coexisting phase in a directional solidification direction, and by moving said mold at a prescribed speed to accomplish said directional solidification process, and 
   (2) during said directional solidification process, a second means is provided which applies a static magnetic field onto at least the entire solid-liquid coexisting phase in a direction substantially parallel to the directional solidification direction to suppress convection in the liquid phase, thereby eliminates heat pulses at the solidification interface and suppresses turbulent liquid flow within said solid-liquid coexisting phase so as to rectify in the directional solidification direction, and,
 said directional solidification apparatus is characterized by synergistic effects based on the respective rectifying effects of said first means of (1) and said second means of (2) so that said synergistic effects suppress the formation of macrosegregation or misoriented grain defects and refine the microstructure. 
   
     
     
         2 . The directional solidification apparatus described in  claim 1 , wherein said heating means is equipped with a main heater by resistance heating and at least one resistance sub-heater located directly above said insulating baffle. 
     
     
         3 . The directional solidification apparatus described in  claim 1 , wherein said fierce cooling means is configured to cool said mold in that a nozzle for an inert gas is placed immediately under the insulating baffle to blow said inert gas against the side surface of said mold. 
     
     
         4 . The directional solidification apparatus described in  claim 1 , wherein said fierce cooling means is configured to cool said mold by immersing said mold in a molten metal bath made of a low-melting-point material. 
     
     
         5 . The directional solidification apparatus described in  claim 1 , wherein said mold is formed by alternating layers of graphite having high thermal conductivity and thermally insulating material. 
     
     
         6 . A directional solidification method for making directionally solidified castings or ingots having a grain structure consisting of a single crystal structure, or a polycrystalline columnar dendrite structure or a mixture of said single crystal structure and said polycrystalline columnar dendrite structure, wherein
 (1) a first step of a directional solidification process, wherein molten metal is cast into a mold and cooled to produce said directionally solidified castings or ingots, consists of
 a heating region to heat said mold, 
 a cooling region to cool said mold, and 
 a thermally insulating region between said heating region and said cooling region, and 
 moving said mold from said heating region to said cooling region to proceed directional solidification, 
 wherein in said heating region the molten metal in said mold is heated and kept at a prescribed temperature, and in said cooling region the solid region is strongly cooled so as to rectify the liquid flow within the solid-liquid coexisting phase in directional solidification direction and said mold is moved at a prescribed speed to accomplish said directional solidification, and 
   (2) during said directional solidification process, a second step is provided which applies a static magnetic field in a direction substantially parallel to said directional solidification direction onto at least the entire solid-liquid coexisting phase in order to suppress convection in the liquid region, thereby eliminates heat pulses at the solidification interface and suppresses turbulent liquid flow within said solid-liquid coexisting phase, and,
 said directional solidification method is characterized by synergistic effects based on the respective rectifying effects of said first step of (1) and said second step of (2) so as to suppress the formation of macrosegregation or misoriented grain defects and refine the microstructure. 
   
     
     
         7 . The directional solidification method described in  claim 6 , wherein a method to heat said mold is characterized by heating around the lower edge of said heating region and directly above said insulating region so as to heat and keep said molten metal in said mold at a prescribed temperature. 
     
     
         8 . The directional solidification method described in  claim 6 , wherein a method to cool said mold is characterized by blowing an inert gas onto the side surface of said mold. 
     
     
         9 . The directional solidification method described in  claim 6 , wherein a method to cool said mold is characterized by immersing said mold in a molten metal bath made of a low-melting-point material. 
     
     
         10 . The directional solidification method described in  claim 6 , wherein said mold is formed by alternating layers of graphite having high thermal conductivity and thermally insulating materials. 
     
     
         11 . In a directional solidification apparatus for producing castings or ingots having a grain structure consisting of a single crystal structure or a polycrystalline columnar dendrite structure or a mixed structure of said single crystal structure and said polycrystalline columnar dendrite structure, wherein a heating region for heating a mold, a strong cooling region for cooling the mold, and an insulating region for thermally separating and blocking these two regions are contained in a single chamber which is equipped with:
 said mold for casting the castings or ingots,   a cooling chill at the bottom of said mold to initiate solidification,   a sliding heat resistant main heater for heating said mold,   an insulating sleeve to support said main heater and block heat radiation to the outside,   a mold cooling gas nozzle to cool said mold, and   an insulating baffle located on the upper part of said mold cooling gas nozzle,   said insulating baffle and said mold cooling gas nozzle being able to move up and down synchronously and integrally,   said main heater providing with a passage to allow said insulating baffle and said mold cooling gas nozzle to move up and down together,   said main heater connecting to sliding contact terminals set up on the outside of said insulation sleeve, and said sliding contact terminals contacting with a sliding brush,   power supplied range between the upper end of said main heater and said sliding brush being variable by sliding said sliding brush up and down synchronously and integrally with said insulating baffle and said mold cooling gas nozzle,   said single chamber is also provided with means for applying a static magnetic field in a direction substantially parallel to said directional solidification direction onto an entire solid-liquid coexisting phase in said mold,   and at the start of operation, said sliding brush, said insulating baffle and said mold cooling gas nozzle are positioned at the lower end of said mold,   power is supplied to the energized region to heat and keep said mold at a prescribed temperature above the melting point of the metal material after melting and casting of said metal material,   the performance of directional solidification of said casting or ingot is characterized by supplying cooling gas to said mold cooling gas nozzle while reducing said heating region by moving said energized region upward at a prescribed speed and at the same time applying said static magnetic field onto said entire solid-liquid coexisting phase in a direction substantially parallel to the directional solidification direction.   
     
     
         12 . The directional solidification apparatus described in  claim 11 , wherein a heating means is equipped with at least one resistance sub-heater located directly above said insulating baffle to heat around the lower edge of said heating region 
     
     
         13 . The directional solidification apparatus described in  claim 11 , wherein said mold is formed by alternating layers of graphite having high thermal conductivity and thermally insulating materials. 
     
     
         14 . A directional solidification method for producing castings or ingots having a grain structure consisting of a single crystal structure or a polycrystalline columnar dendrite structure or a mixed structure of said single crystal structure and said polycrystalline columnar dendrite structure, wherein a heating region for heating a mold, a strong cooling region for cooling the mold, and an insulating region for thermally separating and blocking these two regions are contained in a single chamber,
 wherein a heating method for heating said mold is done by resistance heating the energized range between a fixed position at an upper end of said heating region and a lower end of said heating region whose said energized range reduces and varies and at the same time said mold is strongly cooled by blowing inert gas against a side surface of said mold,   and at the start of operation, said energized region surrounds said entire mold to heat and keep said mold at a prescribed temperature above the melting point of the metal material, and after melting and casting said metal material, said energized range is reduced at a prescribed speed from the lower end of said mold to a position fixed at said upper end, while strongly cooling the lower region under said insulating region to solidify and at the same time a static magnetic field is exerted onto at least an entire solid-liquid coexisting phase of said casting or ingot in a direction substantially parallel to the directional solidification direction.   
     
     
         15 . The directional solidification method described in  claim 14 , wherein said heating method is equipped with at least one resistance sub-heater located directly above said insulating region to heat around the lower end of said heating region. 
     
     
         16 . The directional solidification method described in  claim 14 , wherein said mold is formed by alternating layers of graphite having high thermal conductivity and thermally insulating materials. 
     
     
         17 - 22 . (canceled)