US20090165989A1 - Casting method and apparatus - Google Patents

Casting method and apparatus Download PDF

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US20090165989A1
US20090165989A1 US12/288,805 US28880508A US2009165989A1 US 20090165989 A1 US20090165989 A1 US 20090165989A1 US 28880508 A US28880508 A US 28880508A US 2009165989 A1 US2009165989 A1 US 2009165989A1
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phase
magnetic field
mushy
liquid
macrosegregation
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Yoshio Ebisu
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EBIS Corp
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22F—CHANGING THE PHYSICAL STRUCTURE OF NON-FERROUS METALS AND NON-FERROUS ALLOYS
    • C22F3/00—Changing the physical structure of non-ferrous metals or alloys by special physical methods, e.g. treatment with neutrons
    • C22F3/02—Changing the physical structure of non-ferrous metals or alloys by special physical methods, e.g. treatment with neutrons by solidifying a melt controlled by supersonic waves or electric or magnetic fields
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
    • B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
    • B22D11/10—Supplying or treating molten metal
    • B22D11/11—Treating the molten metal
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
    • B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
    • B22D11/10—Supplying or treating molten metal
    • B22D11/11—Treating the molten metal
    • B22D11/114—Treating the molten metal by using agitating or vibrating means
    • B22D11/115—Treating the molten metal by using agitating or vibrating means by using magnetic fields
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
    • C22B9/00—General processes of refining or remelting of metals; Apparatus for electroslag or arc remelting of metals
    • C22B9/16—Remelting metals
    • C22B9/18—Electroslag remelting
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
    • C22B9/00—General processes of refining or remelting of metals; Apparatus for electroslag or arc remelting of metals
    • C22B9/16—Remelting metals
    • C22B9/20—Arc remelting
    • C—CHEMISTRY; METALLURGY
    • C30—CRYSTAL GROWTH
    • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
    • C30B28/00—Production of homogeneous polycrystalline material with defined structure
    • C30B28/04—Production of homogeneous polycrystalline material with defined structure from liquids
    • C30B28/06—Production of homogeneous polycrystalline material with defined structure from liquids by normal freezing or freezing under temperature gradient
    • C—CHEMISTRY; METALLURGY
    • C30—CRYSTAL GROWTH
    • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
    • C30B29/00—Single crystals or homogeneous polycrystalline material with defined structure characterised by the material or by their shape
    • C30B29/10—Inorganic compounds or compositions
    • C30B29/52—Alloys
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
    • C22B9/00—General processes of refining or remelting of metals; Apparatus for electroslag or arc remelting of metals
    • C22B9/003—General processes of refining or remelting of metals; Apparatus for electroslag or arc remelting of metals by induction
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P10/00—Technologies related to metal processing
    • Y02P10/25—Process efficiency

Definitions

  • This invention is concerned with the casting technologies for primarily improving macrosegregation defects in unidirectionally solidified castings which possess 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), and in the remelting-processed ingots such as Electroslag Remelting (ESR) and Vacuum Arc Remelting (VAR).
  • ESR Electroslag Remelting
  • VAR Vacuum Arc Remelting
  • FIG. 1(A) shows schematic diagram of a typical directional solidification apparatus in use of today. After pouring molten metal into the ceramic mold cavity (of complicated 3D shape) placed on a water-cooled copper chill, the mold is withdrawn in gravitational direction out of the radiation heating zone, thereby allowing directional solidification to obtain polycrystalline columnar dendrite structure (termed DS material) or single crystal dendrite structure (termed SX material). In the figure shown are the liquid, the liquid-solid coexisting (mushy) and the solid phases during withdrawal (the details of casting chamber, vacuum chamber and so on are omitted).
  • Ni-base superalloys with excellent thermal resistances such as high temperature strength are used as a major material.
  • casting defects such as channel segregation (so called freckles), misoriented grains, microporosity tend to occur in these blades, thus lowering the yield of the products (for example, refer to p. 321 of Ref. (1)).
  • solutal instability is larger than the thermal stability, inversed density profile forms, and the liquid phase in the mushy zone induces upward flow due to this density difference, thus leading to the formation of channel segregation (or so called freckles). Also, dendritic growth tends to break down and misoriented grains are likely to form. This kind of alloy is called ‘upward type of buoyancy’ in this description. It has been understood that the freckles formed in Ni-base superalloy blades are caused by the above-mentioned upward flow due to liquid density difference within the mushy zone.
  • the remelting processes for ingot making such as ESR and VAR are characterized by relatively shallow shapes of melt pool and mushy zones.
  • the solidification from the side wall of mold is retarded.
  • these remelting processes differ in the point that the solidification proceeds from the side wall of mold as well (usually water-cooled copper mold is used).
  • the freckles (channel segregation) and other macrosegregations take place in Ni-base superalloy ingots produced by ESR and VAR (for example, refer to Ref. (2)), and that these macrosegregation defects can occur in the alloys of ‘downward type of buoyancy’ where interdendritic liquid density increases as solidification proceeds, as well as in the aforementioned alloys of ‘upward type’ of buoyancy.
  • This invention is concerned with unidirectional solidification process and remelting processes such as ESR and VAR, and provides with casting technologies for producing high quality castings and ingots without such macrosegregation defects as freckles caused primarily by the liquid flow within the mushy zone during solidification.
  • this invention has clarified for the first time that the interdendritic fluid flow with extremely low velocity can be suppressed by exerting high magnetic field onto the whole mushy zone, and thereby the formation of the macrosegregation defects such as freckles can be eliminated.
  • FIG. 1A shows the schematic diagrams of a conventional unidirectional solidification process.
  • FIG. 1B shows the schematic diagram of unidirectional solidification process applied to Specific Example 1 of this invention.
  • FIG. 2 shows the relationships between temperature and volume fraction solid during solidification of Ni-10 wt % Al and IN718 alloys (with respect to IN718, refer to FIG. 1 of Ref. (10)).
  • FIG. 3 shows the variation of Al solute composition in liquid during the solidification of Ni-10 wt % Al alloy.
  • FIG. 4 shows the variations of solute compositions in liquid during the solidification of IN718 alloy (refer to FIG. 2 of Ref. (1)).
  • FIG. 5 shows the variations of liquid densities during the solidifications of Ni-10 wt % Al and IN718 alloys.
  • FIG. 6D is the contours of Al concentrations showing the effects of magnetic field densities (the effect of Bz at RR′ cross section (91.9 mm from the bottom)) exerted in the axial direction on the freckle segregates of unidirectionally solidified round ingots of Ni-10 wt % Al alloy.
  • FIG. 7A shows the whole section of the contours of the vol. fraction solid during solidification (after 18 minutes) of the same ingot as that of FIG. 6A .
  • FIG. 7B shows the magnified section at O.D.
  • FIG. 9A is the contours of Al concentrations showing the freckle segregates of unidirectionally solidified Ni-10 wt % Al square ingot of Specific Example 2 at the cross section XX′ (86.6 mm from the bottom).
  • FIG. 9B is the contours of Al concentrations showing the freckle segregates of unidirectionally solidified Ni-10 wt % Al square ingot of Specific Example 2 the freckles in the vertical section at the end of Y dir.
  • FIG. 10A shows the contours of Al concentration (freckle segregates) and vol. fraction solid.
  • FIG. 10B shows the liquid flow pattern in liquid and mushy zones and the contours of vol. fraction solid (after 20 minutes, at the end of Y dir. of the same ingot as that of FIG. 9 ).
  • the contours in the figure denote the fraction solid of 0.2, 0.4, 0.6 and 0.8 respectively.
  • the gray zone in the upper part of background indicates the liquid zone, the light gray zone in the middle the mushy zone and the dark gray zone at the bottom the solid.
  • the velocity vectors are normalized.
  • FIG. 11D shows the suppressing effects of macrosegregations at XX′ cross section 91.9 mm from the bottom at the end of Y dir.
  • FIG. 15 shows the macrosegregations of each element at RR′ cross-section 1068.8 mm from the bottom which corresponds to the case with no magnetic field of FIG. 14 .
  • FIG. 16 shows the suppressing effects on the macrosegragation of Nb at RR′ cross-section of FIG. 14 (1068.8 mm from the bottom).
  • FIG. 17A shows schematic examples of this invention applied to remelting processes, which is an example applied to ESR.
  • FIG. 17B shows a combined example of VAR+slag refining+magnetic field.
  • FIG. 18A shows schematic examples of DC coil (solenoid coil) 5 configurations of this invention.
  • FIG. 18B shows schematic examples of DC coil (1-unit coil) 5 configurations of this invention.
  • FIG. 18C shows schematic examples of DC coil (2-unit coils (Helmholtz-type or as such)) 5 configurations of this invention.
  • FIG. 18D shows a race track type 1 unit-coil with the magnetic field exerted in the direction so as to cross the direction of gravity.
  • FIG. 18E shows race track type 2 unit-coils.
  • FIG. 19A shows the distributions of Al concentration with no magnetic field for unidirectionally solidified Ni-10 wt % Al thin plate ingots of the Specific Example 3 of straight, 126 mm long.
  • FIG. 19B shows the distributions of Al concentration with no magnetic field for unidirectionally solidified Ni-10 wt % Al thin plate ingots of the Specific Example 3 of tapered, 126 mm long.
  • FIG. 19C shows the distributions of Al concentration with no magnetic field for unidirectionally solidified Ni-10 wt % Al thin plate ingots of the Specific Example 3 of straight, 252 mm long.
  • FIG. 20 shows liquid flow vectors within the mushy zone after 1005 seconds from the start of withdrawal (45 mm withdrawn from the bottom) of the tapered ingot of FIG. 19 .
  • the contours denote the vol. fractions solid of 0.2, 0.4, 0.6, 0.8 and 1.0 respectively.
  • the cross-sectional diagrams are the distributions at the position XX′.
  • the cross-sectional diagrams are the distributions at the position XX′.
  • the cross-sectional diagrams are the distributions at the position XX′.
  • ⁇ electric conductivity of molten metal (1/ ⁇ m)
  • v flow velocity vector of molten metal (m/s)
  • B externally applied magnetic flux density vector (Tesla)
  • E induced electric field strength vector (V/m)
  • J induced electric current density vector (A/m 2 ).
  • the outline of the simulation system for solidification (system name CPRO) will be described bellow which was developed by this inventor to analyze solidification phenomena.
  • the physical variables for describing solidification phenomena are defined by temperature, compositions of alloy elements redistributed in the liquid and solid phases during solidification (take n for the number of elements), liquidus temperature of the relationship between temperature and volume fraction solid, and liquid flow velocity (3 vector components) and pressure of liquid in the bulk liquid and mushy zones. These are called the physical variables in macroscopic scale in this description.
  • the vector V denotes the interdendritic flow velocity, ⁇ the viscosity of liquid, g L the volume fraction liquid, K the permeability, P the pressure of liquid phase, X the body force vector such as gravity and centrifugal force. Furthermore, note that X includes the electromagnetic baking force introduced in this invention as well.
  • K is determined by dendrite morphology and given by Kozney-Calman equation (Ref. (4)) as
  • Sb is the surface area of dendrite crystals per unit volume (i.e. specific surface area), and the dimensionless number f has been found to have the value of 5 by the fluid flow experiment using porous media.
  • the permeability K is obtained by the morphological analysis during dendrite growth in microscopic scale: Considering that solidification is one of diffusion-controlled processes in liquid and solid phases, and assuming that dendrite crystals are modeled to consist of cylindrical branches and trunks, and half-sphere tips, Sb can be obtained by solving the diffusion equations of solute elements in the liquid and solid phases. In doing so, no anisotropy of K by dendritic orientation is assumed.
  • FIG. 1(A) Schematic diagram of a conventional unidirectional solidification apparatus is shown in FIG. 1(A) .
  • Susceptor is heated by induction coil and the ceramic mold is then radiation-heated by the susceptor.
  • the bottom of the ceramic mold is cooled by water-cooled chill, and the mold is withdrawn gradually downward to establish unidirectional solidification (instead, it is also possible to rise the heating furnace while the mold is fixed).
  • Giamei and Kear have shown that the channel segregation generally termed ‘freckles’ appears at O.D. of unidirectionally solidified Ni-base superalloy monocrystal round ingots of the type upward buoyancy (refer to FIGS. 1 to 4 of Ref.
  • ⁇ L ⁇ L ( C 1 L ,C 2 , . . . , T ) (8).
  • FIGS. 6 to 8 show the contours of macrosegregation distributions, and FIG. 6(D) the distributions in the radial direction at the position RR′ (91.9 mm from the bottom).
  • the freckles were formed at O.D. as shown FIG. 6(A) .
  • C/Co C is calculated solute concentration and Co initial composition (wt %)):
  • the maximum value of C/Co is 1.14 of positive segregation at the freckled O.D.
  • Al concentration depleted zone
  • FIG. 6 (D) the adjacent of the freckle
  • FIG. 8(A) shows the flow pattern in the bulk liquid and the mushy zone after 18 minutes (solidification time is 29.2 minutes), indicating that the profile of the mushy zone is influenced by the heat extraction from the outside.
  • the fluid flow pattern in the mushy zone is such that as a whole the liquid flows from the center toward the O.D. under the influence of ‘upward buoyancy’ due to lower liquid density at higher vol. fraction solid region (the bottom side).
  • the flow occurs from low temperature, high solute concentration liquid of the bottom side to higher temperature, lower solute concentration liquid of the upper side.
  • FIGS. 9 to 11 The macrosegregations of the conventional ingot with no magnetic field are shown in FIGS. 9 to 11 .
  • the freckles take place in vertical sections at outsides with roughly equal intervals.
  • the maximum value of C/Co is about 1.18.
  • FIGS. 10(A) and 10(B) show the states of macrosegregation being formed after 20 minutes during solidification (solidification time is 28.5 minutes).
  • solidification time is 28.5 minutes.
  • the liquid adjacent to freckles flows into and ascends along the channels as shown in FIG. 10(B) , retarding solidification as shown by vol. fraction solid contours in the same figure.
  • the freckles were formed by these ascending flows (refer to FIG. 10(A) ).
  • FIGS. 11(A) , 11 (B) and 11 (C) show the macrosegregations in the vertical sections at the ends of Y dir.
  • the flow patterns in the mushy zones have become essentially the same as in the Specific Example 1 so that the flows in traverse directions have been suppressed with only axial downward flows left (not shown for brevity).
  • the configuration of the real turbine blades mentioned in the Background Art is fairly complicated partially with thin-wall thickness (for example, refer to FIG. 1 , p. 320 and FIG. 5 , p. 321 of Ref. (1)).
  • the casting conditions are given in Table 2.
  • the same casting conditions were employed as before excepting that the heat extraction rate from the side wall was reduced to one tenth compared to the above-mentioned round and square ingots in order to retard the solidification from the side wall of mold.
  • the computations were done for 1 ⁇ 4 cross-section considering symmetricity.
  • the number of elements for ingot is 6390 (18 in X dir. ⁇ 5 in Y dir. ⁇ 71 in Z dir.).
  • the results are shown in FIG. 13 .
  • the freckles take place at the corner of the ends of X and Y directions (refer to FIG. 13(A) ).
  • the influences of casting conditions and the shape of blade were investigated.
  • the withdrawal rate of the mold was reduced down to 1.667 mm/min from 5 mm/min of Table 2; and the temperature of the susceptor was held at 1773K, ⁇ (emissivity) in the radiation heating region 0.05, ⁇ (emissivity) in the radiation cooling region 0.02, h (heat transfer coefficient) at the bottom 0.001 cal/cm 2 sec° C. so that the temperature gradient in solidification range became about 45° C./cm at the middle of the blade length (these conditions match with practical operating range, and were set here for the convenience of computations).
  • FIG. 19 shows the contours of Al concentration of the above three unidirectionally solidified conventional ingots with no magnetic fields exerted.
  • the diagrams of the vertical sections show the contours at the centers of wall thickness and those of the cross-sections the contours at the positions XX′.
  • the freckles do not take place at the peripheral surfaces formed in the Specific Example 1 (round ingot of FIG. 6 ) or in the Specific Example 2 (square ingot of FIG. 9 ), but rather take place inside the ingots. This is considered attributed that the temperature difference between surface and inside of the ingot was almost vanished by radiation heating from the susceptor (the temperature at the outside is only slightly higher). It was also found that the freckles were considerably shortened as compared to those in the Specific Examples 1 and 2 where the freckles were elongated in the vertical direction reaching the top of the ingots.
  • the freckles tend to take place more markedly in the inside, particularly in thick-walled region.
  • the profile of the mushy zone and the interdendritic liquid flow at the central vertical section, after 1005 sec from the start of withdrawal is shown in FIG. 20 (the flow pattern only in the mushy zone shown).
  • the lines in the figure denote the contours of vol. fractions solid with the interval of 0.2 from 0.2 to 1.0.
  • the flow velocity at the origin or the starting point of freckle (45 mm from the bottom) is the order of 3 ⁇ 10 ⁇ 2 cm/s, and the velocity in traverse direction the order of 10 ⁇ 3 cm/s.
  • the mushy zone profile is slightly inclined from the central thicker walled portion toward the end, and the interdendritic liquid flows from the end toward the center and a strong upward flow is seen at the origin of freckle. Also, it can be seen from the contours that the solidifications are delayed in the freckling sites compared to the surroundings. In the 252 mm long straight ingot (blade length doubled), the freckles formed more prominently and their lengths became longer (refer to FIG. 19(C) ).
  • the ingots there was no freckle formation and the negative segregation at thinner walled side (right end) was decreased with increasing magnetic field. This is because the very slow flow in the traverse direction as shown in FIG. 20 was effectively suppressed.
  • the site of the formation and the morphology of freckles vary depending on casting parameters such as heating/cooling conditions, withdrawal rate, and the shape of blade.
  • the macrosegregations can be suppressed by exerting strong magnetic field.
  • Van Den Avyle, et al. reported in the aforementioned Ref. (2) that freckles formed in the middle of radial direction and ‘central’ freckles formed at the center in remelting-processed IN718 and Alloy 625 Ni-base superalloys ingots respectively.
  • the depths of the mushy zone tend to deepen toward the center because of heat extraction from the side, leading to channel segregation even in the case of downward type alloy of buoyancy like IN718.
  • the chemical compositions of IN718 studied in this description are regarded approximately equal to those of the Alloy 625 rather than those of the IN718 of Ref. (2). In the light of Ref.
  • the number of elements for ingot is 4800 (40 in radial dir. ⁇ 120 in axial dir.).
  • FIGS. 14 to 16 show the contours of Nb concentrations and distributions respectively.
  • FIG. 15 shows the distributions of alloy compositions in radial direction (1068.8 mm from the bottom) with no magnetic field.
  • the central freckles reported in the Alloy 620 of the above Ref. (2) have formed as shown in FIG. 14(A) .
  • conventional type of freckles did not take place in the middle of the radial direction.
  • this inventor has shown for the first time that the extremely slow interdendritic fluid flow responsible for the formation of macrosegregation can be suppressed by exerting high magnetic field onto the whole mushy zone, and thereby made it clear that the macrosegregations such as freckles can essentially be eliminated.
  • the electromagnetic braking effect on molten metals has long been recognized, there has been no literature, to the best of this inventor's knowledge, showing that the macrosegregation can be eliminated by the application of magnetic field.
  • V V (m/s), ⁇ (1/ ⁇ m), B (Tesla), ⁇ (Kg/m3) and t (sec) respectively.
  • V decays exponentially with time, and that the higher the ⁇ and the lower the ⁇ , the lower the V.
  • ⁇ is a dimensionless number representing EM braking effect on the interdendritic fluid flow within the mushy zone.
  • V decays hyperbolically with increasing ⁇ .
  • K and gL change depending on the location and the time.
  • ⁇ L g r is a driving force to cause convective flow in the mushy zone and is determined by the chemical compositions of the alloy. There are upward type and downward type of buoyancies along with the mixture type of these two. The factors II and III are determined by the cooling condition in each casting process. Thus, there exist a variety of cases. To take a couple of examples, the most important factor for the formation of freckles in unidirectionally solidified turbine blade of upward type alloy of buoyancy is ⁇ L g r (factor I). And thus the freckles take place above a certain value of ⁇ L or ⁇ L g r . On the contrary, the contribution from the factors II or III is relatively small.
  • the dimensionless numbers ⁇ or ⁇ defined respectively by Eq. (14) or Eq. (15) are meaningful:
  • B can be determined so as to satisfy ⁇ C or ⁇ C . It is economical to evaluated ⁇ C or ⁇ C in scaled down experiments, where the numerical analysis described in this description can be very useful.
  • FIG. 1(B) The schematic diagram when exerting static magnetic field for unidirectional solidification is shown in FIG. 1(B) .
  • typical directional solidification apparatus comprises various components such as chill, mold heating furnace, withdrawal unit, vacuum chamber, etc. Beside, there are various configurations. For example, it is possible to make use of zone melting where at first a small amount of metal is remelted from once solidified ingot and then slowly move this molten band from one end to the other (For example, refer to p. 2 of Ref. (3)).
  • this invention is aimed at the method and apparatus for unidirectional solidification of castings and ingots, where solid, mushy and bulk liquid zones are made to form and then solidification is done by moving these zones from one end to the other.
  • this invention can be applied to all directional solidification processes.
  • Real unidirectionally solidified turbine blades posses complicated shapes.
  • Ref. (11) describes a technology to make single crystal structure (SX) in thin blade region and polycrystalline columnar dendrite structure (DS) in platform region.
  • SX single crystal structure
  • DS polycrystalline columnar dendrite structure
  • This invention can be applied to such a mixed grain structure as well. It is also possible, as already mentioned, to apply to the cases where solidification is done in horizontal direction perpendicular to gravity or in the opposite to gravity (i.e.
  • DC coils 5 used in this invention are shown in FIG. 18 .
  • Solenoid type (A), 1-unit coil (B), 2-units coils (C), etc. are available for the case to exert the magnetic field in vertical direction.
  • Race track typed 1-unit coil (D), race track typed 2-unit coils, etc. can be used for the case to exert in horizontal direction.
  • Superconductive coils are highly recommended for these coils. In practice, various designs for coils are possible and the most suitable design may be employed depending on the shape of casting, the direction of solidification, the required magnitude of magnetic field and so on.
  • the molten slag pool 2 is heated by the heat of Joule generated by the electric current between the electrodes, and thereby the electrodes are melted. Further, the slag pool is heated and maintained at a prescribed temperature by the heater 4 placed at the O.D. of insulating refractory sleeve 3 . The molten droplets melted from the electrodes flow down through the slag pool, are refined and are solidified. The ingot 7 is withdrawn downward by flexible platform 9 while being cooled from the bottom by water-cooled chill 8 . Because the molten slag gets into the opening between the ingot and the water-cooled mold 6 and also air gap forms there (not shown), the cooling rate from the O.D. is relatively low.
  • FIG. 17(B) Another desirable example is shown in FIG. 17(B) which combines VAR and slag refining and the high magnetic field by this invention.
  • Space 10 is of vacuum or inert gas atmosphere.
  • the electrodes are remelted by high electric current arc (usually DC).
  • High quality ingots with high purity and no segregation can be produced by the application of high magnetic field in addition to the beneficial effects of VAR and slag refining.
  • the magnetic shield 11 is installed in order to insulate the high electric current field around the electrodes from the magnetic field.
  • FIG. 17 Various designs are possible as illustrated in FIG. 17 .
  • Ref. (12) discloses a technique to reduce a large change in solute composition(s) distribution in growth direction of single crystal semiconductor, which is caused by convection of liquid in front of the flat interface.
  • the crystal is grown from a starting material with non-uniform solute distribution to obtain more uniform single crystal.
  • it aims to produce the single crystal with more uniform composition(s) by growing in the opposite direction to once grown direction or by exerting magnetic field (about 0.2 Tesla) to suppress the convection.
  • this technique of Ref. (12) intends to suppress the convection in front of the growing flat interface in true single crystal growth with no mushy zone. Hence, it is different from this invention.
  • Ref. (13) discloses a single crystal growth technique by Bridgeman's method to obtain better crystal(s) with as less a number of crystals as possible.
  • a material having the positive change of magnetic susceptibility at the melting point of liquid-solid transition i.e. the magnetic susceptibility of solid is higher than that of liquid
  • the technique by Ref. (13) is different from this invention.
  • the stable flat interface growth breaks down with solid intruding into liquid phase, thus resulting in cellular structure.
  • the dendrite structure results.
  • the mushy zone consists of cells and liquid phase, and so this invention can be applied.
  • Ref. (14) discloses a technique for reducing macrosegregation of Al-alloy continuous castings by exerting the magnetic field of maximum 0.15 Tesla onto the molten metal pool.
  • the liquid flow rate within the melt pool is the order of 10 to 100 cm/s.
  • the magnetic field as low as 0.1 Tesla can effectively suppress such high speed flow, but it can not induce the braking force against the extremely slow liquid flow within mushy zone.
  • grain refiners have been added in all the experiments done in this reference. This point deserves attention. With respect to the suppressing effect on the macrosegregation in the case of Ref.
  • this inventor considers as follows: As a result of reduced convection within the bulk liquid pool by the applied magnetic field, the grain refining effect was enhanced compared with the case of no magnetic field. And thus, grain structure became that of finer equiaxed grains leading to less macrosegregation in the central region of the ingot.
  • the essence of this invention is that the macrosegregation such as freckles can completely be eliminated in unidirectional solidification (where in principle grain refiners must not be used) or remelting processes by exerting. the magnetic field onto the whole mushy zone with the magnitude necessary to suppress the extremely slow interdendritic liquid flow.
  • the viewpoint of this invention is thus totally different from that of Ref. (14), and hence different in terms of casting process, exerting region and the magnitude of the magnetic field.
  • the macrosegregation such as freckles can completely be eliminated.
  • this invention makes it possible to produce unidirectionally solidified castings such as high quality turbine blades or remelting-processed ingots, which greatly contributes to the safety of important mechanical components and to the energy conservation by enhancing the efficiency of gas turbine engines.
  • high magnetic field can be obtained at relatively low cast owing to the recent progress in superconductive technology, there seems no barrier to realize this invention. Therefore, the industrial merits are very high.
  • two types of Ni-base alloys were examined as shown in the Specific Examples. However, it is apparent in principle that the similar benefits can be obtained for all alloy productions: For example, directionally solidified AlTi base alloy turbine blades, low alloy steels, etc. as well as all Ni-base alloys.

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US20150209859A1 (en) * 2014-01-28 2015-07-30 General Electric Company Casting method and cast article
US9555471B2 (en) * 2014-01-28 2017-01-31 General Electric Company Casting method and cast article
US10265764B2 (en) 2014-01-28 2019-04-23 General Electric Company Casting method and cast article
US20180236595A1 (en) * 2015-02-11 2018-08-23 Ksb Aktiengesellschaft Flow-Conducting Component
US11033966B2 (en) * 2015-02-11 2021-06-15 Ksb Aktiengesellschaft Flow-conducting component
CN109817284A (zh) * 2019-01-25 2019-05-28 东北大学 一种钢液中枝晶移动的预测方法

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US20130276939A1 (en) 2013-10-24
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