Method of casting metal and apparatus therefor

Abstract: In an apparatus for casting a metal, particularly an active metal such as Ti, a gas-permeable mould (2) is provided above a levitation melting furnace (1) in a mould chamber, preferably comprising an inner and an outer mould chamber (41,42). Once the metal (5) in the furnace has become molten, the pressure within the apparatus is reduced and a suction pipe (3) connected to the mould (2) is lowered into the molten metal. The pressure in a space (A) above the furnace is then raised by a blown inert gas, thereby causing the molten metal to rise up the suction pipe (3) and be cast in the mould. Atmospheric pressure is then returned to separate the mould chamber from the furnace.

Description: The present invention relates to a method of casting a metal and an apparatus therefor. More particularly, the present invention relates to a novel method of casting metal and an apparatus for the application of such method, which permits achievement of a high-quality casting free from entrapment of impurities, gas contamination or gas defects, and is particularly, but not exclusively, useful for casting active metals such as titanium. Various methods and apparatus for casting a metal have conventionally been known. For these casting methods and apparatus, diverse and various means for obtaining high-quality products free from entrapment of impurities, gas contamination or gas defects have been examined and industrialized. The present inventors carried out extensive studies on means for obtaining such high-quality casting products, and have established a method permitting casting of an active metal, in particular for active and high-melting-point metals such as titanium and alloys thereof, which allows casting at a high efficiency with a high productivity. The method thus established is essentially characterized by the combination of semi-levitation melting a metal, which involves melting a metal while partially magnetic-floating the metal, and reduced-pressure suction casting. This permits melting and casting of an active metal such as titanium by the semi-levitation melting process which is different from conventional processes using a refractory crucible, thereby inhibiting deterioration of quality caused by refractory contamination. It also makes it possible to conduct continuous melting and casting, and to carry out casting of highly uniform castings and ones with complicated shapes by the application of the reduced-pressure casting process. The semi-levitation melting process forming a feature of this method will be described in further detail below. This process is a kind of levitation (magnetic floating) melting process which comprises induction-heating a material charged in a melting crucible, and holding the resultant molten material without causing contact with the inner wall of the melting crucible. More specifically, the levitation melting process is one of the melting processes which allow, when melting a metallic material charged in a melting crucible, the prevention of contamination of the molten metal from chemical reactions caused by contact of the metal with the inner wall of the crucible, thereby achieving an improvement in quality. The process of levitation melting can be further divided into two processes: full-levitation melting process, which comprises fully floating a molten metal within the furnace by the action of electromagnetic force; and semi-levitation melting which comprises using a water-cooled copper crucible, and causing the molten metal to float by electromagnetic force while keeping the bottom of the material in a solidified state. In the full-levitation melting process, although contamination from the melting crucible can be fully prevented because the molten metal is fully floated, it is difficult to keep the molten metal in that floating state. Also the quantity of molten metal capable of being floated is too small to be applied industrially. For industrial purposes, therefore, the semi-levitation melting process is commonly applied. An outline of the semi-levitation melting process is as follows. In the water-cooled copper crucible used in this melting process, a peripheral wall of the main body, which is formed into a cylindrical shape and has a bottom, is circumferentially divided to form a plurality of segments into which cooling water is supplied in circulation. The individual segments are insulated from each other with an insulating material. Doughnut-shaped high-frequency induction coil turns are arranged at prescribed annular intervals on the outside of the water-cooled copper crucible. The material is induction-heated upon charging the material into the crucible by supplying high-frequency current to the induction coil. Upon heating the material to a prescribed temperature, the material partially melts, while the lower material in contact with the bottom of the water-cooled copper crucible is kept in its solidified state. The molten material is kept floating in the non-contact state relative to the inner wall of the crucible under the effect of electromagnetic force generated by the penetration of the molten material into the crucible. An example of a semi-levitation melting process is known from JP-A-04/182056. The levitation melting process as a melting process having the features as described above is simply called "levitation melting" hereafter. It is known to use levitation melting in a casting method as shown in Fig. 5 for example, in which a suction pipe (d) connecting with a mould (c) arranged above a melting furnace (a) is immersed in a molten metal (b) in said furnace (a), and the molten metal (b) is sucked up for casting into the mould (c) through the suction pipe (d) under conditions including reducing pressure in the mould (c) and a mould chamber (e) to below atmospheric pressure. In the conventional method and apparatus, however, there were left points to be improved in terms of improvement of casting efficiency along with reduced-pressure suction and uniform casting into the mould. More specifically, it would be desirable to improve productivity of levitation melting and reduced-pressure suction casting and to permit more uniform casting into the mould while avoiding occurrence of defects resulting from casting. The present invention provides a method of casting a molten metal comprising the steps of: levitation-melting a metal in an inert atmosphere in a levitation melting furnace under atmospheric pressure, thereby forming molten metal; providing a mould chamber positioned above the furnace, and a gas-permeable mould housed within said mould chamber, said mould chamber further including a suction pipe for delivering molten metal to said mould; reducing the pressure in said mould chamber and a space above the levitation melting furnace to below atmospheric pressure; lowering said mould chamber and said gas permeable mould toward said levitation melting furnace to immerse an end of said suction pipe into the molten metal, the other end of the suction pipe being in communication with the mould; raising said mould chamber to withdraw the suction pipe from the molten metal; and returning the pressure in the space above the levitation melting furnace and in the mould chamber to atmospheric pressure; characterised in that said mould chamber is a double-structure mould chamber comprising an outer mould chamber having an outer cover to close the outer mould chamber, an inner mould chamber having an inner cover to close the inner mould chamber, said gas-permeable mould being housed within said inner mould chamber and said inner mould chamber including said suction pipe for delivering molten metal to said mould, and in that said method comprises the steps of: lowering said outer cover to close said outer mould chamber and to bring said outer mould chamber into close contact with said levitation melting furnace; lowering said inner cover to close said inner mould chamber; lowering said inner mould chamber toward said levitation melting furnace to immerse the end of said suction pipe into the molten metal; blowing an inert gas at an increased pressure into the space above the levitation melting furnace to cause molten metal to rise up through the suction pipe and to be cast into the mould; withdrawing the suction pipe from the molten metal by raising said inner mould chamber; and raising said outer mould chamber from said levitation melting furnace. Preferably the pressure within the mould chamber and the space above the levitation melting furnace is reduced to below 26.7 kPa (200 torr), most preferably below 13.3 kPa (100 torr). Preferably, in order to cast the molten metal into the mould, the pressure is increased by between 6.67-66.7 kPa (50 to 500 torr) in the space above the levitation melting furnace by an atmosphere control mechanism. The present invention also provides apparatus for casting a molten metal comprising a levitation melting furnace, a mould chamber arranged above the levitation melting furnace and a gas-permeable mould housed within said mould chamber, said mould chamber further including a suction pipe for delivering molten metal to the mould having one end in communication with the mould and the other end arranged to be lowered into the levitation melting furnace, characterised in that said mould chamber comprises a double-structure mould comprising an outer mould chamber having an outer cover to close the outer mould chamber, an inner mould chamber having an inner cover to close the inner mould chamber, said inner mould chamber housing said mould and including said suction pipe for delivering molten metal to said mould, and in that an atmosphere control mechanism is provided to blow an inert gas into a space above the levitation melting furnace to cause molten metal to rise up through the suction pipe. In a preferred embodiment, a sliding mechanism is provided to lower the mould chamber toward the levitation melting furnace. The mould chamber comprises an inner and an outer mould chamber which are slidably coupled together. Preferably a mould keeper is arranged to press and keep the mould in the inner mould chamber. Preferably the sliding mechanism has an elastic body, of which compression causes descent and of which repulsive force causes accent. Preferably the atmosphere control mechanism is provided with exhaust pressure reducing means and inert gas supply means. As described above, the interior of the mould is kept under a reduced pressure or under vacuum, the reduced pressure or vacuum being retained due to the double structure of the mould chamber, and the molten metal being cast by pressurising. Using such a system, defects resulting from entrapment by the atmosphere gas do not occur, thus permitting the casting of a uniform structure. In addition, the entire operations of casting including stabilization and fine adjustment of the casting speed are very efficiently accomplished by the generation of a difference in pressure caused by blowing of inert gas. The present invention, furthermore, based on levitation melting, is suitable for casting an active metal such as titanium, and because of the absence of inclusions caused by crucible refractories, is effective for ferrous castings. Some preferred embodiments of the present invention will now be described by way of example only and with reference to the accompanying drawings, in which: Fig. 1 shows a partial perspective sectional view illustrating a preferred embodiment of the casting apparatus of the present invention; Fig. 2 is a sectional elevation of the outer cover above the outer mould chamber during a descending operation in the apparatus shown in Fig. 1; Fig. 3 is a sectional elevation of the apparatus of Fig. 1 after the outer mould chamber has descended and been brought into contact with the levitation melting furnace, following the operation shown in Fig. 2; Fig. 4 is a sectional elevation illustrating descent of the inner mould chamber and casting operation, following the operation shown in Fig. 3; and Fig. 5 is a sectional elevation illustrating a conventional apparatus and method. For example, as shown in Figs. 1 to 4, the preferred casting apparatus of the present invention comprises a levitation melting furnace 1, and a mould chamber 4 which holds a gas-permeable mould 2 in the interior thereof into which the molten metal is to be cast. A suction pipe 3 is also provided for sucking up the molten metal into this mould 2. The mould 2 is not limited to a precision-casting mould based on the lost wax process, but may consist of any of various moulds using sand, a metal or the like, which so long as it is permeable may be used. The mould chamber 4 has a double structure comprising an inner mould chamber 41 having a mould 2 arranged therein which connects a suction pipe 3 to the mould 2, with an outer mould chamber 42 forming the outer periphery thereof. The casting apparatus is provided with a sliding mechanism which moves the inner mould chamber 41 and the outer mould chamber 42 independently up and down, and an atmosphere control mechanism. In this embodiment, furthermore, an inner cover 411 for the inner mould chamber 41 and a mould keeper 412 for pressing the mould 2 from above are made vertically slidable, and an outer cover 421 for the outer mould chamber 42 is provided. The inner cover 411 and the outer cover 421 are made vertically slidable by independently operating cylinder mechanisms 6 and 7. Outer springs 9 are provided between the outer mould chamber 42 and a base 8, and inner springs 10 between the flange portion of the inner mould chamber 41 and the staged portion of the outer mould chamber 42. Casting operations are now described further in detail with reference to Figs. 2 to 4. I . In the levitation melting furnace 1 in the condition shown in Fig. 2, melting of a metal is started by operating an induction coil 11 while supplying an inert gas such as argon (Ar) or the like through a gas supply port 12, in order to adjust the atmospheric conditions in the upper space A. Molten metal 5 is generated substantially at the center, thereby minimizing possible contact with the furnace wall, a feature of levitation melting. II . The outer cover 421 is brought down by the cylinder piston mechanism 1 shown in Fig. 1 toward the outer mould chamber 42, to close the outer mould chamber with this outer cover 421. At the same time, as shown in Fig. 3, the outer spring 9 is further compressed to bring the outer mould chamber 42 composing the double-structure mould chamber into close contact with the levitation melting furnace 1. In this state, an inert gas such as argon (Ar) is blown by the atmosphere control mechanism into the inner and the outer mould chambers, 41 and 42. The inner and the outer mould chambers 41 and 42 are thus filled with an inert gas atmosphere. III. Pressure in the outer mould chamber 42 and the inner mould chamber 41 forming the double-structure mould chamber, and the upper space in the levitation melting furnace 1 is reduced to a pressure lower than the atmospheric pressure (preferably to below 26.7 kPa (200 torr), or more preferably to below 13.3 kPa (100 torr)). Gases are withdrawn through a gas guide 130 by an evacuation pump forming part of the atmosphere control mechanism, from an exhaust port 120 provided in the slide for sliding the inner cover 411. IV . As shown in Fig. 4, the slide 110 is caused to descend by the above-mentioned cylinder piston mechanism 6, and the inner mould chamber 41 is brought down while compressing the inner springs 10 to immerse the suction pipe 3 connecting with the mould 2 into the molten metal 5. The inner surface of the steps of the inner mould chamber 41 and the outer mould chamber 42 are sealed with a packing material 422. The inner mould chamber 41 is closed by the inner cover 411, and the mould 2 is pressed by the mould keeper 412. V . Simultaneously with immersion as described above, an inert gas such as argon (Ar) is blown into the upper space A in the melting furnace 1, and the molten metal 5 is pushed up by the difference in pressure 6.67-66.7 kPa (50 to 500 torr) between the upper space A and the inner mould chamber 41. The molten metal 5 thus uniformly rises up through the suction pipe 3, and immediately cast into the mould 2. VI . After blowing argon gas by means of the atmosphere control mechanism into the inner and the outer mould chambers. 41 and 42 and the above-mentioned space A, and returning to atmospheric pressure, the outer mould chamber 42 is lifted up by the sliding mechanism as described above to separate the outer mould chamber 42 and the inner mould chamber 41 from the levitation melting furnace 1. The suction pipe 3 is consequently pulled out from the molten metal. In the preferred casting method using the preferred apparatus of the present invention, as is clear from these operations I to VI, the mould chamber 4 is more perfectly closed under the effect of the double structure comprising the inner mould chamber 41 and the outer mould chamber 42, and casting of a product free from defect at meniscus, having a uniform structure and containing minimum impurities is made possible by the absence of entanglement by the atmosphere gas and uniform ascent of molten metal, resulting from casting by reduced pressure and inert gas pressurizing. Control of atmosphere is also easier. The outer springs 9, the inner springs 10 and the cylinder piston mechanisms 6 and 7 for ensuring the close contact and sliding property of the inner mould chamber 41 and the outer mould chamber 42 are not limited to particular ones. Inert gas is supplied for sealing the furnace during levitation melting, and is continued throughout the entire period of casting except for the period of pressure reduction before casting. After the completion of a casting operation as described above, wear of the suction pipe 3 is determined, and if the degree of wear is within a tolerable range, the mould 2 is removed and additional materials are charged into the melting furnace 1 to repeat the same operations as above. By using a suction pipe made from the same metal as the molten metal or base metal of the molten alloy, the wear tolerance of the suction pipe is increased. Thus, it is preferable to use a suction pipe made from Ti for melting a Ti-Base-alloy. According to the method and the apparatus described above, it is possible to avoid entanglement of inclusions from crucible refractories resulting from levitation melting, leading to easier casting of an active metal such as titanium. The atmosphere in the mould is controlled by the double structure of the mould chamber and the pressure reduction and casting by pressuring of the inert gas make it possible to cast a product with the least defects end excellent uniformity of structure. Furthermore, the pressure difference achieved by the pressure reducing conditions and inert gas blowing permit efficient casting including stabilization and fine adjustment of the casting speed, with a largely improved productivity.

Claims: A method of casting a molten metal comprising the steps of: levitation-melting a metal in an inert atmosphere in a levitation melting furnace (1) under atmospheric pressure, thereby forming molten metal (5); providing a mould chamber positioned above the furnace, and a gas-permeable mould (2) housed within said mould chamber, said mould chamber further including a suction pipe (3) for delivering molten metal to said mould; reducing the pressure in said mould chamber (41,42) and a space (A) above the levitation melting furnace (1) to below atmospheric pressure; lowering said mould chamber (41) and said gas permeable mould toward said levitation melting furnace (1) to immerse an end of said suction pipe (3) into the molten metal (5), the other end of the suction pipe being in communication with the mould (2); raising said mould chamber (4) to withdraw the suction pipe (3) from the molten metal (5); and returning the pressure in the space (A) above the levitation melting furnace (1) and in the mould chamber (41,42) to atmospheric pressure; characterised in that said mould chamber is a double-structure mould chamber comprising an outer mould chamber (42) having an outer cover (421) to close the outer mould chamber, an inner mould chamber (41) having an inner cover (411) to close the inner mould chamber, said gas-permeable mould (2) being housed within said inner mould chamber and said inner mould chamber including said suction pipe (3) for delivering molten metal to said mould, and in that said method comprises the steps of: lowering said outer cover (421) to close said outer mould chamber (42) and to bring said outer mould chamber into close contact with said levitation melting furnace (1) ; lowering said inner cover (411) to close said inner mould chamber (41); lowering said inner mould chamber (41) toward said levitation melting furace (1) to immerse the end of said suction pipe (3) into the molten metal (5); blowing an inert gas at an increased pressure into the space (A) above the levitation melting furnace (1) to cause molten metal to rise up through the suction pipe and to be cast into the mould (2); withdrawing the suction pipe (3) from the molten metal (5) by raising said inner mould chamber (4); and raising said outer mould chamber (42) from said levitation melting furnace (1). A method as claimed in claim 1, wherein the pressure in the double-structure mould chamber (41,42) and the space (A) above the levitation melting furnace (1) is reduced to below 26.7 kPa (200 torr). A method as claimed in claim 2, wherein the pressure is reduced to below 1.33 kPa (100 torr). A method as claimed in any of claims 1, 2 or 3, wherein the molten metal (5) is cast into the mould (2) under a pressure increase of between 6.67-66.7 kPa (50 to 500 torr). Apparatus for casting a molten metal comprising a levitation melting furnace (1), a mould chamber (41,42) arranged above the levitation melting furnace and a gas-permeable mould (2) housed within said mould chamber, said mould chamber further including a suction pipe (3) for delivering molten metal to the mould having one end in communication with the mould and the other end arranged to be lowered into the levitation melting furnace, characterised in that said mould chamber comprises a double-structure mould comprising an outer mould chamber (42) having an outer cover (421) to close the outer mould chamber, an inner mould chamber (41) having an inner cover (411) to close the inner mould chamber, said inner mould chamber (41) housing said mould (2) and including said suction pipe (3) for delivering molten metal to said mould (2), and in that an atmosphere control mechanism (12, 120) is provided to blow an inert gas into a space (A) above the levitation melting furnace (1) to cause molten metal to rise up through the suction pipe. Apparatus as claimed in claim 5, wherein a sliding mechanism is provided to lower the inner and outer mould chambers of the double-structured mould chamber (41,42) toward the levitation melting furnace (1). Apparatus as claimed in claim 6, wherein the sliding mechanism is resiliently biased to urge the mould chamber (41,42) away from the levitation melting furnace (1). Apparatus as claimed in any of claims 5, 6 or 7, wherein the inner and outer mould chambers are slidably coupled together. Apparatus as claimed in any of claims 5 to 8, wherein a mould keeper (412) is provided to retain the mould (2) in position. Apparatus as claimed in any of claims 5 to 9, wherein the atmosphere control mechanism (12,120) comprises a pump, an exhaust port (120) and an inlet (12).

Inventors: DEMUKAI NOBORU, YAMAMOTO MASAYUKI, YMADA JUNJI, DEMUKAI, NOBORU, YAMAMOTO, MASAYUKI, YMADA, JUNJI
Applicants: DAIDO STEEL CO LTD, DAIDO TOKUSHUKO KABUSHIKI KAISHA
Application year: 1996
Main classification: B22D18
IPC codes: B22D27, B22D18, B22D23