Casting method and apparatus

This is a Continuation Application of PCT Application No. PCT/JP2007/059353, filed Apr. 24, 2007, which was published under PCT Article 21(2) in English.

This application is based upon and claims the benefit of priority from prior International Application No. PCT/JP2006/309133, filed Apr. 25, 2006, the entire contents of which are incorporated herein by reference.

1. Field of the Invention

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).

2. Description of the Related Art

A. Unidirectionally Solidified Castings

Typical examples of unidirectionally solidified castings in the technical field of this invention are turbine blades used in aircraft jet engines, power plant gas turbines, and so on. 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molten metal into the ceramic mold cavity (of complicated 3D shape) placed on a water-cooled copper chill, the mold is withdrawn in gravitational direction out of the radiation heating zone, thereby allowing directional solidification to obtain polycrystalline columnar dendrite structure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). Because the turbine blades are used under severe conditions, Ni-base superalloys with excellent thermal resistances such as high temperature strength are used as a major material. However, 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)).

Presently, the mechanism of the formation of freckles has qualitatively been considered as follows: One of the main features of Ni-base superalloys is that they possess γ′ phase (intermetallic compound called gamma prime whose basic composition is Ni₃ (Al, Ti) which precipitates coherently with γ matrix. And generally speaking, the higher the volume fraction of γ′, the higher the high temperature strength. However, as solidification proceeds in an alloy containing such elements as Al, Ti and W lighter than Ni, the interdendritic liquid density is decreased with increased solute concentrations of these lighter elements. Therefore, when solidifying such alloy in the direction opposite to gravitational direction, the liquid density at the bottom of mushy zone, i.e., at the roots of dendrites becomes relatively smaller compared to that at the liquid-mushy phase boundary, i.e., at dendrites tips. Such alloy is called “solutally unstable” against convection in this description. On the other hand, the temperature at the roots of dendrites is lower than that at dendrite tips, and therefore it does not give rise to convective flow due to density difference: Thus ‘thermally stable’. If the 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.

Despite that a vast amount of efforts have been paid to improve these casting defects by optimizing casting parameters such as sustaining temperature in the radiation heating zone, withdrawal rate, radiation cooling rate, etc. or by adjusting alloy compositions so as to add heavier elements than Ni (for example Ta), it still remains insufficient. Thus, a novel technology to eliminate the above-mentioned defects is highly wanted at present.

The remelting processes for ingot making such as ESR and VAR are characterized by relatively shallow shapes of melt pool and mushy zones. In the above-mentioned unidirectional solidification, the solidification from the side wall of mold is retarded. On the other hand, 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). It is well known that 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.

In order to reduce these defects, various attempts have been being undertaken ranging from regulating cooling conditions so as to form as shallow a melt pool depth as possible (especially useful for downward type alloys of buoyancy) to increasing cooling rate, to setting proper melting rate, or to adjusting chemical compositions. However, as the cross-section of the ingots become larger, the formation of these macrosegregation defects is unavoidable. Thus, an innovative technology is strongly desired capable of constantly producing large cross-section ingots with less macrosegregation which are used in Ni-base superalloy turbine disks, Fe-base alloy turbine rotors for power plant and so on.

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. With special attention paid to the above-mentioned liquid flow phenomena within the mushy zone, 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.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1A shows the schematic diagrams of a conventional unidirectional solidification process.

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