US20240051018A1 - Unidirectional solidification device, unidirectional solidification method, unidirectionally solidified casting, and unidirectionally solidified ingot - Google Patents

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

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US20240051018A1
US20240051018A1 US18/277,800 US202218277800A US2024051018A1 US 20240051018 A1 US20240051018 A1 US 20240051018A1 US 202218277800 A US202218277800 A US 202218277800A US 2024051018 A1 US2024051018 A1 US 2024051018A1
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mold
directional solidification
cooling
region
heating
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Yoshio Ebisu
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EBIS Corp
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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
    • B22D27/00—Treating the metal in the mould while it is molten or ductile ; Pressure or vacuum casting
    • B22D27/04—Influencing the temperature of the metal, e.g. by heating or cooling the mould
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
    • B22D27/00—Treating the metal in the mould while it is molten or ductile ; Pressure or vacuum casting
    • B22D27/04—Influencing the temperature of the metal, e.g. by heating or cooling the mould
    • B22D27/045—Directionally solidified castings
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22C—FOUNDRY MOULDING
    • B22C9/00—Moulds or cores; Moulding processes
    • B22C9/22—Moulds for peculiarly-shaped castings
    • B22C9/24—Moulds for peculiarly-shaped castings for hollow articles
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
    • B22D27/00—Treating the metal in the mould while it is molten or ductile ; Pressure or vacuum casting
    • B22D27/003—Treating the metal in the mould while it is molten or ductile ; Pressure or vacuum casting by using inert gases
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
    • B22D27/00—Treating the metal in the mould while it is molten or ductile ; Pressure or vacuum casting
    • B22D27/02—Use of electric or magnetic effects

Definitions

  • This invention is concerned with improved directional casting apparatus, directional casting method, and directional castings in the production of columnar dendrite structure consisting of polycrystalline grains (so called DS material) or dendrite structure consisting of a single crystalline grain (so called Monocrystal or SX material).
  • DS material polycrystalline grains
  • SX material Monocrystal
  • the apparatus of a typical Bridgman method (referred to herein as Standard Bridgman method) consists of a heating furnace, a cooling chamber, a mechanism for withdrawing a mold from the heating furnace to the cooling chamber, an adiabatic baffle that separates the heating furnace and the cooling chamber, and a cooling chill that initiates solidification (see, for example, Non-Patent Reference 1).
  • the mold is preheated above the melting temperature by a resistance heater, and after the pouring of the molten metal the mold is withdrawn into the cooling chamber at a prescribed speed.
  • the mold is set on the cooling chill to start solidification by heat conduction to the chill.
  • the effective cooling range by the chill is relatively short, and in the case of a large casting, its range is limited to the height of a grain selector (refer to FIG. 1 ).
  • Non-Patent Ref. 2 by Konter et al Non-Patent Ref. 2 by Konter et al.
  • the standard Bridgman method is of the one removing the cooling gas pump system 13 and the superconducting coil 14 from FIG. 1 ]
  • the casting is solidified by radiation cooling in the cooling chamber.
  • LMC Method Liquid Metal Cooling Method
  • LMC method Liquid Metal Cooling method
  • U.S. Pat. No. 6,276,433B1 uses an Al eutectic alloy as a medium for cooling metal bath (Patent Reference 1).
  • Patent Reference 3 Non-Patent Reference 3
  • Liu et al. Non-Patent References 4 and 5
  • the microstructure can be refined and the high temperature creep strength of directionally solidified Ni-based superalloy can be increased (For example, the creep rupture time at 1050° C. and 160 Mpa was approximately doubled from 84 hrs to 131 hrs. See Non-Patent Reference 5).
  • FIG. 2 An example of a typical apparatus for the LMC method is shown in FIG. 2 (Non-Patent Reference 3).
  • the heating area is equipped with upper and lower heaters (reference symbols 5 a and 5 b ).
  • the mold 1 is gradually immersed and cooled in the molten metal bath 18 by the mold withdrawal arm 21 .
  • a molten metal vessel 20 containing and holding the molten metal bath 18 adjusts the temperature of the metal bath by circulating hot oil therein.
  • a heat insulating layer 22 made of floating alumina beads is provided on the surface of the molten metal bath 18 to block radiant heat from the heating area.
  • the molten metal bath 18 is agitated by an agitator 23 to make the temperature uniform.
  • GCC Method Gas Cooling Casting Method
  • the GCC method employs a forced gas cooling method in which inert gas (argon, helium, etc.) is used to intensely cool the mold pulled out into the cooling area (refer to Non-Patent Reference 2 and Patent Reference 2).
  • inert gas argon, helium, etc.
  • a cooling gas injection nozzle 11 for blowing a cooling gas is arranged immediately below the heat insulation baffle 9 provided to thermally separate the heating region and the cooling region, and during the directional solidification, the mold is cooled by blowing the inert gas onto it.
  • the cooling gas nozzle an appropriate number of ejection ports are provided to eject swirling flow obliquely downward.
  • the cooling gas blown out into the furnace is circulated by the gas pump system 13 along a path of suction/filtering/cooling/supply/suction to cool the mold in the cooling region. According to the above reference, it is possible to obtain a cooling intensity equal to or higher than that of the LMC method.
  • Non-Patent Reference 1 ASM Handbook, Vol. 15, Casting (1988), p. 320, FIG. 3 or p. 321, FIG. 4
  • Non-Patent Reference 2 M. Konter, et al: “A Novel Casting Process for Single Crystal Gas Turbine Components”, Superalloy 2000, TMS 2000, p. 189
  • Non-Patent Reference 3 A. J. Elliot et al: “Directional Solidification of Large Superalloy Castings with Radiation and Liquid-Metal Cooling”, Metallurgical and Materials Transactions A, Vol. 35A, October, 2004, pp 3221-3231
  • Non-Patent Reference 6 ‘A Numerical Method of Macrosegregation Using a Dendritic Solidification Model, and Its Applications to Directional Solidification via the use of Magnetic Fields’, Metallurgical and Materials Transactions B, vol. 42b (2011), pp. 341-369
  • Non-Patent Reference 10 X. Li, et al: ‘Influence of thermoelectric effects on the solid-liquid interface shape and cellular morphology in the mushy zone during the directional solidification of Al—Cu alloys under a magnetic field’, Acta Materialia, Vol. 55 (2007), pp. 3803-3813
  • Non-Patent Reference 12 J. Yu, et al: ‘Influence of Axial Magnetic Field on Microstructures and Alignment in Directionally Solidified Ni-based Superalloy’, ISIJ International, Vol. 57 (2017), No. 2, pp. 337-342
  • Non-Patent Reference 14 Y. Lian, et al: ‘Static Solid Cooling: A new directional solidification technique’, J. Alloys and Compounds, Vol. 687 (2016), pp. 674-682
  • the method of applying an axial static magnetic field to the standard Bridgman method is called M method (Magnetic process)
  • M method Magnetic process
  • MV1 method Magnetic process Version 1
  • forced gas cooling by conventional GCC method or molten metal bath cooling by LMC method may be used as a strong cooling means.
  • the apparatus of the invention comprises a mold 1 filled with molten metal 5 , a cooling chill 7 placed at the bottom of the mold, and a resistance heater 25 serving as the main heater for heating the mold.
  • the apparatus also includes a sub-heater 30 for heating the mold which is movable and targets a relatively small area, and a movable cooling gas nozzle 35 for blowing cooling gas onto the mold 1 .
  • the sub-heater 30 and the movable cooling gas nozzle 35 are ring-shaped and configured to be coaxially and integrally movable against the mold 1 from the cooling chill 7 side to the upper end side.
  • the movable cooling gas nozzle 35 is configured to blow the cooling gas obliquely downward onto the outer surface of the mold.
  • a heat insulating baffle 33 is arranged between the sub-heater 30 and the cooling gas nozzle 35 .
  • the resistance heater 25 is formed by winding a belt-like resistance heating element about one turn in the circumferential direction, raising it, and then winding it in the opposite direction about one turn, there by repeating this procedure.
  • a slit-like gap is formed, through which the cooling gas introduction pipe 34 , the heat insulating baffle 33 , and the sub-heater 30 can move up and down.
  • the main heater 25 is made of a resistance heating element such as carbon graphite, and is attached inside a cylindrical heat insulating sleeve 26 . Further, the main heater 25 is connected to sliding contact terminals 27 arranged at the outside of the heat insulating sleeve 26 . Then, a sliding brush 28 is attached onto the contact terminals 27 . Electric power can be supplied to the main heater 25 through the contact terminal 27 at the uppermost end and the sliding brush 28 at the current position as shown in the figure.
  • a resistance heating element such as carbon graphite
  • Variable Resistance Heating method is referred to as Variable Resistance Heating method.
  • the brush 28 is positioned at the lower end so that the entire region of the mold is heated and held at a prescribed temperature higher than the melting point of the alloy.
  • the molten metal is poured and solidification commences at the chill.
  • the brush 28 is slid upward at a prescribed speed together with the cooling gas nozzle 35 and the sub-heater 30 so that solidification proceeds upward.
  • the heating zone shrinks and the cooling zone expands.
  • the heating zone disappears, and the entire zone becomes a cooling zone to end the operation, and to finish the solidification.
  • FIG. 4 illustrates a schematic diagram incorporating the above devices.
  • Reference symbol 29 is a power supply of the main heater, which supplies electric power through the upper end contact terminal 27 and the sliding brush 28 .
  • the sub-heater copper cable 31 connects the sub-heater power supply 32 and the sub-heater 30 to supply power.
  • 38 is a vacuum pump, and 40 is a superconducting coil.
  • the cooling gas pump system 37 supplies the cooling gas to the cooling gas nozzle 35 through the cooling gas inlet pipe 34 , and the suction port 36 is an intake port for circulating cooling gas blown into heat insulating sleeve 26 .
  • the cooling gas is circulated along a path of suction/filtering/cooling/supply/suction.
  • the melting chamber 3 containing the induction melting furnace 4 and the mold chamber containing the mold 1 can be separated, and after solidification the mold is taken out.
  • This DS method is called MV2 method (Magnetic process Version 2: S+sliding brush +GCC+Bz, S means single chamber).
  • the synergistic effects are obtained based on the MV1 method or MV2 method as follows.
  • FIG. 1 is a schematic drawing showing an embodiment of a directional solidification apparatus by the present invention (referred to as MV1 method).
  • FIG. 2 shows an example of a directional solidification apparatus by the LMC method.
  • FIG. 3 ( a ) is a schematic diagram showing the outline of the directional solidification apparatus using a sliding electrode system by the present invention, and ( b ) shows the main resistance heater of (a).
  • FIG. 4 is a schematic diagram showing an example of the sliding electrode system (referred to as MV2 method). Shown are vacuum chamber 3 , induction melting furnace 4 , cooling gas circulation system 37 and magnetic coils 40 for applying an axial static magnetic field onto the mushy zone.
  • FIG. 8 shows a schematic diagram showing the morphology of Al macrosegregation of Ni-10 wt % Al blade by the standard Bridgman method (longitudinal section at the center of thickness direction).
  • Broken lines with arrows denote stream lines. Two broken lines in horizontal direction denotes mushy zone.
  • ZB is the height from the bottom of the blade to the baffle,
  • FIG. ( b ) shows the flow pattern within the mushy zone denoted by the two broken lines in horizontal direction of Figure (a), No. I-1, and
  • FIG. 11 shows the effects of the axial static magnetic fields on normalized segregation standard deviations of IN718 blades by the MV2 method (GCC, sliding brush 40 cm/h).
  • the standard deviations of each elements (Table 7) are normalized by initial alloy contents. S in the figure denotes single chamber.
  • FIG. 12 shows the effects of the axial static magnetic fields on Nb segregation standard deviations of IN718 blades by the simple M method (standard Bridgman, withdrawal speed 15 cm/h) and MV2 method (GCC, sliding brush 40 cm/h). S in the figure denotes single chamber.
  • the calculations of the upper parts of the blades were omitted because of the formation of shrinkage.
  • FIG. 18 shows a schematic diagram of the Static Solid Cooling method (see Non-Patent Reference 14).
  • FIG. 19 shows a schematic diagram of a mold design by the Static Solid Cooling method adopted in the directional solidification apparatus by the MV2 method of the present invention (however, the heating and cooling means depend on those of the present invention.
  • the overall design including magnetic coils is omitted for brevity).
  • FIG. 20 shows an overview of a directional solidification monitoring system by the MV1 method (and MV2 method) of the present invention.
  • the cooling rate during solidification is increased and the solidified structure is refined as compared with the simple M method. Furthermore, casting defects such as macrosegregation or misoriented crystals have been eliminated, and at the same time, the required static magnetic field strength has been reduced, thereby making it possible to reduce the cost of superconducting coils.
  • ⁇ L ⁇ L ( C 1 L , C 2 L , . . . , T ) (1)
  • An alloy in which ⁇ L decreases as solidification proceeds is called upward type of buoyancy, on the other hand, an alloy in which ⁇ L increases is called downward type of buoyancy. It depends on the alloy compositions whether it is an upward type, a downward type, or a mixed type (i.e., ⁇ L decreases first and increases again with the progress of solidification, or vice versa).
  • Ni-10 wt % Al is an upward type alloy
  • IN718 is a downward type alloy (see FIG. 13 of Non-Patent Reference 6).
  • the temperature is lower at the root of the dendrites than at the tip, and therefore denser at the root so that convection does not occur. That is, it is ‘thermally stable’.
  • solute instability is greater than the thermal stability, a density inversion layer is formed, and the liquid phase in the mushy zone tends to generate ascending convection so that so-called chimney type freckles are likely to occur. Macrosegregation with such morphology is likely to occur in upward-type alloys. However, regardless of upward, downward or mixed type of buoyancy, it exhibits various forms depending on the casting conditions.
  • thermoelectric current is generated in the direction of the temperature gradient (so-called Seebeck effect).
  • Seebeck effect the current field is expressed as follows.
  • ⁇ is obtained by solving Eq. (6), J is obtained by Eq. (3), and then the Lorentz force f can be calculated from Eq. (5).
  • V must be calculated by the numerical analysis which includes momentum equation where the flow field and the electromagnetic field have a highly coupled relationship.
  • f is included in the body force term of the momentum equation. The electrical boundary condition at the blade-mold boundary (including the blade-cooling chill boundary) were assumed insulated.
  • Non-Patent literature will be reviewed which take into account the thermoelectromagnetic force.
  • Fautrelle, et al (Non-Patent Reference 9) have applied a static magnetic field of 0.08T in the thickness direction (horizontal direction) to an Al—Cu alloy of a width 5 mm ⁇ height 5 mm ⁇ thickness 200 ⁇ m and performed X-ray in-situ observation during solidification. Then, it has experimentally been shown that the liquid phase or the solid phase moves due to the Lorentz force generated even by as low a magnetic field of 0.08T for the temperature gradient in the height direction.
  • Non-Patent Reference 10 has applied a static magnetic field during the DS cellular growth process of an Al—Cu alloy (3 mm in diameter ⁇ 200 mm in length) and shown that convection due to the thermoelectromagnetic force affects the cellular morphology. That is, a ring-shaped cellular structure was formed by a weak magnetic field of 0.5T or less (see FIG. 6 of the reference).
  • Non-Patent Reference 11 investigated the effects on dendrite morphology by applying an axial static magnetic field in the DS dendrite growth process of Al-4.5 wt % Cu alloy using ⁇ 001> oriented 4 mm diameter seed crystal. The results showed that the tertiary branches grow unevenly like windmills when magnetic fields higher than 2T are applied (see FIGS. 2 and 3 in the Reference).
  • Non-Patent Reference 12 has shown that, when an axial static magnetic field higher than 2T is applied in the DS process of Ni-based superalloy DZ417 alloy (specimen diameter 4 mm ⁇ length 180 mm), columnar dendrites break down to yield an equiaxed grain structure. This tendency becomes more pronounced as the withdrawal speed, i.e., the growth rate is slowed down and the magnetic field is increased (see FIGS. 2 and 3 of the Reference).
  • the purpose of the present invention is to clarify the mechanism for the formation of macrosegregation by a rigorous computer simulation on solidification assuming real directional solidification process of Ni-based alloys, and, as described in Paragraph 0013, to clarify the means for eliminating the macrosegregation defect by applying static magnetic field.
  • the outline of a general-purpose simulation system (named CPROTM of EBIS Corporation, Sagamihara, Japan) for solidification is described below which has been developed by the present inventor to analyze solidification phenomena.
  • the physical variables to be analyzed are the temperature, the solute concentrations of alloying elements redistributed in the liquid and solid phases during solidification (the number of alloying elements is n), liquidus temperature giving the relationship between the temperature and volume fraction solid, and liquid flow vectors and pressure in the liquid and mushy phases. These variables are referred to herein as the macroscopic variables.
  • These n+6 variables and their corresponding governing equations are listed in Table 1.
  • the vector V denotes the interdendritic liquid flow velocity, ⁇ the viscosity of liquid, g L the volume fraction liquid, K the permeability, P the pressure of liquid phase, and X the body force vectors such as gravity or centrifugal force.
  • X also includes the thermoelectromagnetic driving force and the electromagnetic braking force introduced in the present invention.
  • K is determined by the geometrical structure of dendrites and is given by the Kozney-Carman equation below (refer to Non-Patent Reference 8s).
  • S b is the surface area per unit volume of the dendrite crystals (called specific surface area), and is determined by morphological analysis during the growth of the dendrite crystals (the scale is microscopic). Since solidification is regarded as a kind of diffusion rate-controlled process in the liquid and solid phases, the dendrite is modeled with cylindrical branches and a trunk and hemispherical tips, and the solute diffusion equation in the solid and liquid phases are solved to obtain S b (refer to Appendix B of Non-Patent Reference 6). g s the volume fraction solid. K is assumed isotropic. The value of the dimensionless number 5 in the formula was determined by flow experiments in porous media.
  • thermoelectromagnetic driving force and the electromagnetic braking force due to the static magnetic field were incorporated into the aforementioned numerical solution. This allows a complete description of the solidification phenomena taking these forces into account. It was assumed that the solid phase in the mushy zone is stationary. When a uniform static magnetic field Bz is applied only in the axial direction, the Lorentz forces acting on the bulk liquid zone and the liquid phase in the mushy zone are specifically written down as follows.
  • f ⁇ x - ⁇ ⁇ ⁇ ⁇ ⁇ y ⁇ Bz - ⁇ ⁇ S ⁇ ⁇ T ⁇ y ⁇ Bz - ⁇ ⁇ V X ⁇ B Z 2 ( 9 )
  • fy ⁇ ⁇ ⁇ ⁇ ⁇ x ⁇ Bz + ⁇ ⁇ S ⁇ ⁇ T ⁇ x ⁇ Bz - ⁇ ⁇ V y ⁇ B Z 2 ( 10 )
  • fz 0 ( 11 )
  • MV1 method strong cooling+axial magnetic field
  • H GCC 1000 ⁇ 2000 W /( m 2 ⁇ K ) (12)
  • H GCC 1800 W/(m 2 ⁇ K) in Table 2 was set considering this effect.
  • Mold thickness (ceramic mold): 10 mm Baffle height: 45 mm Withdrawal rate: 2.5 mm/min (15 cm/h) and 5 mm/min (30 cm/h) Casting temperature: 1753 K (1480 C.) (superheat 80 K) Initial temperature of mold: 1723 K Initial temperature of chill: 573 K Radiation heat exchanges between mold and heater (heating zone) and between mold and inner surface of furnace (cooling zone): I.D. of the heater and I.D. of the furnace in the cooling chamber were both set to 300 mm diameter, and the heating zone and the cooling zone were assumed insulated by the baffle (height 45 mm).
  • Ai Area of the mold surface element i Ag: Surface area corresponding to Ai (the heater or the inner surface of the furnace or the mold surface itself) Fig denote view angles which require a very large memory.
  • T 1 Surface temperature of the solid T 2 : Inner surface temperature of the mold ⁇ 1 : 0.4 Emissivity on the surface of the solid ⁇ 2 : 0.35 Emissivity on the inner surface of the mold
  • Heat flux at ingot -chill boundary: q h (T i1 ⁇ T C2 ) (W/m 2 ) h: Heat transfer coefficient 418 W/(m 2 ⁇ K)
  • T i1 Ingot temperature at the bottom
  • T C2 Chill temperature at the upper surface
  • T Chill temperature at the bottom Tw: Water temperature 293 K
  • the mold withdrawal speed was adjusted by preliminary calculation so that the position of the mushy zone was at approximately the same horizontal position as the insulating baffle.
  • the results were summarized in Table 4.
  • the standard deviation ⁇ (wt %) (i.e., square root of the sum of the squares of the differences between the Al concentrations of each computational element and the average value) was used as an index for evaluating the degree of segregation.
  • ⁇ reduces to 1.553E ⁇ 02 wt % (No. I-2).
  • FIG. 6 is the corresponding histograms showing approximately normal distributions. It can be clearly seen that as ⁇ decrease, the widths of the variations decrease. Also, the dendrite arm spacing (DAS) and Al concentration distributions in Z direction at cross-sectional center are shown in FIGS. 7 ( a ) and 7 ( b ) , respectively. In the case of No. I-1, the DAS ⁇ 250 ⁇ m and the variation width 30 ⁇ m, whereas in No. I-5, the DAS ⁇ 190 ⁇ m and the variation has almost disappeared. As for the Al macrosegregation, as shown in FIG. 7 ( b ) , almost no variation is observed in the optimum solution, compared with the variation of 9.95-10.05 wt % for the standard Bridgman (No. I-1).
  • FIG. 8 A schematic diagram of Al macrosegregation is shown in FIG. 8 .
  • the freckles that often develop in the axial direction do not occur, but rather band-like macrosegregation extending roughly in horizontal directions is observed.
  • DAS dendrite arm spacing
  • FIG. 8 A typical segregation pattern by the Standard Bridgman method has already been shown in FIG. 8 (longitudinal section at the center in thickness direction). Since other longitudinal section shows similar aspects, we will discuss the longitudinal section at the center in thickness direction as follows.
  • the segregations repeat positive and negative (greater than or less than 10 wt % Al) in the longitudinal direction. Horizontal sections also show the same way, but the frequency of positive and negative segregation (number of repetitions) is less than that in the longitudinal direction.
  • Such form is called “band segregation” or so-called “banding” in this description.
  • FIG. 9 ( a ) (Standard Bridgman method, no magnetic field).
  • the heat pulses are expressed by contours of temperature variation at time step t ⁇ t to t.
  • the mushy zone is constantly affected by these heat pulses, causing fluctuations in its temperature, volume fraction solid, dendrite morphology, shape of the mushy zone and ultimately the liquid flow pattern (see FIG. 9 ( b ) ). As a result, band-like macrosegregation (see FIG. 8 ) is formed.
  • the macrosegregation has been reduced to a level where there is no practical problem due to the synergistic effect of the forced cooling by GCC and the relatively low magnetic field less than 1T (i.e., by relatively low cost of superconductive coil). Furthermore, since heat pulses are eliminated and solidification is stabilized, misoriented grain defects should be suppressed. It also brings about advantages by refining the microstructure (i.e., increased creep rupture strength and reduced solution heat treatment time). Note that the discrepancy between computed values and the initial content in Table 4 is considered to be a background error generally associated with such complex numerical analysis.
  • ⁇ Stefan-Boltzmann constant
  • a 1 is surface area of heater
  • a 2 is surface area of mold
  • T 1 is temperature of heater 1500 C.
  • T 2 is surface temperature of mold
  • ⁇ 1 is emissivity of heater 0.35
  • ⁇ 2 is emissivity of mold 0.3
  • Heat flux due to air gap formation at ingot-mold boundary see Non- Patent Reference 2): Same as those shown in Table 2
  • Heat flux at ingot -chill boundary: q h (T i1 ⁇ Tc 2 ) (W/m 2 ) Same as those shown in Table 2 except for h h: Heat transfer coefficient 168 W/(
  • the effect of Bz on the standard deviation ⁇ (wt %) of Nb by the M method and MV2 method is shown in FIG. 12 .
  • FIGS. 12 shows the effect of Bz on the standard deviation ⁇ (wt %) of Nb by the M method and MV2 method.
  • thermoelectromagnetic force TEMF
  • FIG. 15 shows the comparison of DAS distributions for each process (in Z-direction at the XY cross-sectional center).
  • FIG. 16 shows the comparison of the Nb distributions at the same locations as FIG. 15 .
  • the segregation variations in No. II-10 and No. II-13 with Bz applied improved to a large extent and at the same time approached the initial content of 4.85 wt %, indicating that the homogeneities were improved.
  • the above is the mechanism for suppressing convection by the static magnetic field, which accordingly reduced the ⁇ of Nb from 0.1537 wt % (No. II-1) to 0.0204 wt % (No. II-10) (see Table 7).
  • the above magnetic fields are referred to as low magnetic fields herein.
  • such a field range is referred to as the medium field.
  • the flow pattern within the mushy zone is determined by the balance between the thermoelectromagnetic force (TEMF) as a driving force, the electromagnetic braking force (EMBF), and the force generated by the electric field strength and Bz ( ⁇ B).
  • TEMF thermoelectromagnetic force
  • EMBF electromagnetic braking force
  • ⁇ B the force generated by the electric field strength and Bz
  • the refinement and homogeneity of the microstructure improves creep strength, and shortens the time required for solution annealing (i.e. heat treatment for solutionizing microsegregation of the dendrite arm spacing range or the second phases such as ⁇ ′ phase (gamma prime) and carbides into the ⁇ matrix) and subsequent aging time (i.e. heat treatment to precipitate ⁇ ′ phase from ⁇ matrix) after the casting of Ni-base alloys.
  • the time required for solution annealing is roughly proportional to DAS 2 /Ds (Ds are the diffusion coefficients of the alloying elements in the solid phase), so that, if DAS is reduced to 1 ⁇ 2, the time required is reduced to 1 ⁇ 4 (see p. 332, Eq. (10-6) of Non-Patent Reference 7).
  • Liquid flow within the mushy zone is caused by solidification contraction due to the density difference between the liquid and solid phases (the treatment of flow in the mushy zone is described in Paragraph 0042, C. Method of Solidification Analysis, but here we focus on the solidification contraction).
  • the driving force for the flow is the suction force associated with solidification contraction, which is transmitted sequentially from the root of the dendrite to the tip of the dendrite. Therefore, (1) the higher the cooling intensity of the solid phase region and the faster the moving speed R of the mushy zone, the stronger this tendency becomes, and as a result, the flow pattern is considered to become stronger in the axial direction.
  • the reason why the segregation standard deviation ⁇ decrease with increasing cooling intensity and increasing R is because the flow pattern tend to align in the axial direction, which theoretically and quantitatively proves the validity of the above mechanism.
  • Non-Patent Reference 14 proposed a method to enhance cooling capability by using Pyrolytic Graphite (PG, pyrolytic graphite) molds with high thermal conductivity and high thermal diffusivity.
  • PG Pyrolytic Graphite
  • FIG. 18 A schematic diagram is shown in FIG. 18 .
  • the mold is surrounded by a laminated solid consisting of alternating heat transfer layer (PG layer) and insulation layer, inside of which another laminated solid is placed so as to follow up the shape of the blade.
  • the mold itself is coated with a very thin coating.
  • Heating and cooling are done by resistance heaters and water cooling, respectively, located around the periphery of the heat transfer layer.
  • Directional solidification is performed by moving the heating-cooling cycle step by step upward through an electrical network. They state that this method provides much higher cooling capability than GCC or LMC.
  • the SSC mold as an intensified cooling method in the present invention.
  • the heating and cooling means are based on the inventive means of the present application ( FIG. 19 shows an example of using the mold by the SSC method in the MV2 method of the present application).
  • a parallel static magnetic field Bz is applied to the entire mushy zone and the bulk liquid zone for the sake of simulations, but this is not necessarily required for actual operation. It may apply at least onto the whole mushy zone (in this case, the parallel magnetic field effectively covers a fairly wide area above the solidification interface).
  • a desired microstructure can be obtained by adjusting the cooling intensity in the solid phase region and Bz.
  • DAS microstructure
  • the GCC method was used as the strong cooling means in this example, it is clear in principle that the same effect can be obtained by the LMC method having almost the same cooling capability, or by using a mold made of Static Solid Cooling method, which has even higher cooling capability.
  • the present invention is equipped with a real-time solidification monitoring system for monitoring the solidification status when performing directional solidification based on predetermined casting parameters (operating parameters). This enables to efficiently establish optimal casting conditions for manufacturing high-quality blades for each product in a short period of time.
  • FIG. 20 shows an overview of the solidification monitoring system incorporating the solidification simulation system CPRO.
  • 61 is the detection section, which detects each of the operation parameters described below and outputs them as data.
  • 62 is a computer, which processes the data output from the detection section 61 as input conditions to perform a solidification simulation using CPRO, as described in detail in the Specific Examples 1 and 2.
  • the computer has the function of processing the solidification state so that it can be imaged and observed.
  • the 63 and 64 are monitoring devices connected to said computer 62 .
  • the monitoring device 63 is used to display the operating parameters
  • the monitoring device 64 is used to display the images of solidification simulation results.
  • the measurement items of the operational parameters in the detection section 61 of FIG. 20 are as follows.
  • the real-time monitoring items are as follows.
  • the monitoring system thus enables visualization of the solidification phenomena such as temperature change and distribution, the shape of mushy zone, liquid phase flows in the bulk liquid zone and mushy zone, and the formation of macrosegregation, etc. that change from moment to moment, so that it makes it possible to observe in real time the solidification phenomena that were previously unknown as a black box. This enables a deeper understanding of solidification phenomena.
  • Ni—Al alloy and Ni-based superalloy IN7108 it is clear in principle that this invention can be applicable for such alloy systems as Ni-based Superalloys, Titanium alloys, Co-based alloys, Fe-based alloys, and so on that exhibit dendrites or cellular structures in the solidification process. Therefore, these alloy systems are subject to the present invention.
  • this invention enables the production of high-quality directionally solidified castings or ingots such as Ni-based superalloy turbine blades, and will greatly contribute to energy conservation and global warming countermeasures by increasing the safety and longevity of these important components and improving efficiency of gas turbine. It is widely known that the most effective way to increase the combustion efficiency of gas turbines for power generation is to raise the combustion gas inlet temperature of the turbine, and this invention can raise the inlet temperature by enabling the practical use of large single crystal blades that can withstand harsh operating environments (Effects of the single crystallization of the blade material include an increase in the melting point and creep strength).
  • Insulation layer (alumina beads)

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