EP0457502B1

EP0457502B1 - Method and apparatus for precision casting

Info

Publication number: EP0457502B1
Application number: EP19910304192
Authority: EP
Inventors: Noboru Demukai; Shingo Hitotsuyanagi
Original assignee: Daido Steel Co Ltd
Current assignee: Daido Steel Co Ltd
Priority date: 1990-05-15
Filing date: 1991-05-09
Publication date: 1995-10-11
Anticipated expiration: 2011-05-09
Also published as: DE69113676D1; DE69113676T2; JP2541341B2; EP0457502A1; JPH0422562A; ES2080897T3

Description

This invention relates to a method and an apparatus for precision casting, in particular for obtaining precision castings of titanium or titanium alloy which have excellent heat and corrosion resistance properties, in addition to lightness and very high strength.
Titanium and titanium alloys are light, and have excellent heat resistance, corrosion resistance and mechanical strength. It is expected to obtain useful products which have not been used so far, by precision casting base metals, such titanium or titanium alloy, in accordance with the present invention.
However, because titanium or titanium alloy has a melting point higher than 1400°C and is also active, there is a problem in that there are great difficulties in melting and casting titanium or titanium alloy in the majority of cases.
Namely, when an ordinary ceramic crucible is used in which to melt titanium or titanium alloy and obtain the quantity and temperature of molten metal suitable for casting, there is a problem in that oxide ceramics forming the crucible can be easily reduced by titanium at a high temperature. If a graphite crucible is used, there is another problem in that it is only possible to carry out the melting in a small quantity for a short time from a standpoint of preventing the titanium or titanium alloy from being contaminated. This is because carbon dissolves into the titanium or titanium alloy. Furthermore, in regard to a mold for casting the molten metal of titanium or titanium alloy thereinto, a reaction sometimes takes place between the mold and the molten metal. In such a case, it is necessary to reduce the casting temperature as much as possible. However, by doing so the molten metal is apt to solidify before the molding cavity is filled sufficiently. Moreover, there is a different problem in that misrun of the molten metal is caused in precision castings having thin-walled and complicated shapes.
GB-A-2204816 discloses an apparatus for and method of countergravity casting of metals which comprises the induction melting of a metal in a crucible and vacuum casting into a gas permeable mould.
This invention aims to solve or at least partially to alleviate the above-mentioned problems of the prior art.
The present invention provides a method of precision casting comprising:
establishing molten base metal by induction heating;
casting said molten base metal into a permeable mold disposed above the molten base metal by vacuum casting;
characterised in that the base metal is selected from titanium, tungsten, molybdenum, vanadium, zirconium, lithium, or alloys thereof; and in that the base metal is heated by induction in an assembly formed with a plurality of water cooled copper segments disposed circlewise on the inside of an induction heating coil in a state in which said copper segments are insulated from each other.
In the preferred aspects according to this invention, the base metal may be melted in an atmosphere of an inert gas such as argon and the molten metal may be cast into the permeable mold through a tubular sprue. The base metal alloy may be fed continuously into the assembly formed with the water cooled segments from the underside of the assembly.
The present invention also provides a precision casting apparatus comprising:
an induction heating coil;
a permeable mold for vacuum casting;
characterised in that an assembly formed with a plurality of water cooled copper segments is disposed circlewise on an inside of said induction heating coil in a state in which said copper segments are insulated from each other, said assembly being fed with a base metal from an underside thereof; and said base metal being melted by induction heating on the inside of said assembly.
In the preferred aspects according to this invention, the permeable mold may be provided with a plurality of tubular sprues for conducting the molten base metal thereinto at the time of vacuum casting, and a closed feeder head in an upper part thereof. The permeable mold may be a ceramic shell mold.
The precision casting apparatus of the present invention can be used for casting a base metal selected from titanium, tungsten, molybdenum, vanadium, zirconium, lithium, or alloys thereof.
Embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which:-

Figure 1 is a vertical sectional side view illustrating a precision casting apparatus according to an embodiment of this invention; and
Figure 2 is a horizontal sectional view of the assembly formed with the water cooled copper segments in the precision casting apparatus shown in Figure 1.

In the method and apparatus for precision casting in accordance with the present invention having the aforementioned construction, eddy currents are induced on the inside of the assembly formed with water cooled copper segments disposed circlewise on the inside of the induction coil. The water cooled copper segments are insulated from each other at the time of melting the base metal in the assembly.
The base metal is melted by an eddy current induced in the outer layer thereof by the above-mentioned eddy currents which are alternating currents. The molten base metal is detached from the assembly formed with the water cooled copper segments by repelling forces caused by currents which have opposite phases to each other flowing in the outermost layers of the assembly and the molten metal. A gap is formed between the molten metal and the inner periphery of the assembly.
Accordingly, thermal transmission from the molten metal to the assembly is suppressed by the formation of the gap. A thick-walled skull (a layer of solidified metal) is scarcely formed, differing from the cases of conventional furnaces of a water cooled hearth type such as an arc skull crucible furnace. The base metal which is melted thus has a better yield. It is easy to regulate the temperature of the molten metal by controlling the electric energy supplied to the induction coil. The molten base metal is not contaminated substantially because a ceramic crucible composed of oxides is not used, and precision castings of good quality can be obtained.
A precision casting apparatus in accordance with an embodiment of the the present invention which is used for precision casting of titanium or titanium alloy is shown in Figures 1 and 2. The precision casting apparatus 1 is provided with an assembly 2 in the center part thereof which is formed with a plurality of water cooled copper segments 2a, 2b,...2h disposed circlewise in the state insulated from each other through insulations 7. The respective water cooled copper segments 2, 2b,...2h are provided with water pipes 3a, 3b,...3h. The assembly 2 is provided continuously with a magnetic shield 4 on the upper side thereof.
The assembly 2 has a radio-frequency induction coil 5 disposed on the outside thereof and is designed to allow base metal 6 of titanium or titanium alloy to be fed to the inside from an underside thereof.
The magnetic shield 4 is provided with a circular base 11 through a seal 12 on the upper side thereof. A sleeve 13 is provided on the inside of the circular base 11, and a mold chamber 14 is provided on the inside of the sleeve 13. A melting space 15 is formed in a part surrounded by the bottom face of the mold chamber 14 and inner peripheries of the assembly 2 and the magnetic shield 4. It is possible to replace the atmosphere in the melting space 15 with an inert gas by, for example, supplying argon through a gas intake 16 provided on the circular base 11.
A permeable mold 21 which is a ceramic shell mold is disposed in the mold chamber 14. A turbine wheel-shaped molding cavity 21a in the permeable mold 21 and the melting space 15 are connected by a gate 22 formed in the permeable mold 21. A tubular sprue 23 communicates with the gate 22.
The permeable mold 21 is provided with a closed feeder head 21c in the upper part thereof, and has a heat insulator 24 having gas permeability disposed on the outer surface thereof.
Further, the mold chamber 14 is provided with an upper plate 27 through a seal 26 on the upper end thereof. The permeable mold 21 is held with a support 29 piercing the upper plate 27 through a seal 28, and the upper plate 27 is provided with a suction hold 27a.
In the precision casting apparatus 1 according to this embodiment which is provided with the assembly 2 on the inside of the radio-frequency induction coil 5 as described above, eddy currents are formed on the inside of the assembly 2 by radio-frequency induction of the radio-frequency induction coil 5. The base metal 6 of titanium or titanium alloy is melted by an eddy current induced in the outer layer of the base metal 6. The eddy currents are alternating currents. The molten metal 31 of titanium or titanium alloy is slightly separated from the inner periphery of the assembly 2 by repelling forces caused by the currents flowing in the outermost layers of the assembly 2 and the molten metal 31 which have opposite phases to each other. A gap is formed between the molten metal 31 and the assembly 2.
Accordingly, thermal transmission from the molten metal 31 of titanium or titanium alloy to the assembly 2 is suppressed by the formation of the gap. Thereby, a thick-walled skull which is formed in conventional furnaces of a water cooled hearth type, such as an arc skull crucible furnace, is scarcely formed. It becomes possible to melt the base metal 6 of titanium or titanium alloy with a better yield. It also becomes possible to regulate the temperature of the molten metal 31 of titanium or titanium alloy easily by controlling the electric energy supplied to the radio-frequency induction coil 5. Furthermore, there is practically no contamination of the molten metal 31 since a ceramic crucible composed of oxides is not used.
By reducing the pressure through the suction hole 27a provided in the upper plate 27, gas existing in the molding cavity 21a and the feeder head 21c of the permeable mold 21 is discharged into the mold chamber 14 by passing through the permeable mold 21 (as shown with arrows). This occurs by virtue of the difference of internal pressures between the mold chamber 14 and the melting space 15. The molten metal 31 of titanium or titanium alloy is thus sucked and cast into the molding cavity 21a through the tubular sprue 23 and the gate 22. In this way, the molten metal 31 is drawn by suction up to the feeder head 21c which allows for shrinkage which accompanies solidification of the molten metal 31 of titanium or titanium alloy in the molding cavity 21a.
A cast product is obtained by shakeout after solidification of the molten metal 31 in the permeable mold 21.
In this embodiment, Ti-Al intermetallic compound which is light and excellent in mechanical strength at high temperature was chosen as a base metal 6, and cast into a turbine wheel for a turbo charger which is 1200g in finished weight with an outside diameter of 140mm.
The high-frequency generator used in this embodiment for supplying a high-frequency wave to the induction heating coil 5 is a small and comparatively simplified type having capacity of 60kW. The frequency is high, as much as 30kHz, so that it is possible to melt materials with small diameters efficiently.
The turbine wheel has twelve turbine blades and twelve gates 22 having diameters of 8mm provided near the lower parts of respective turbine blades.
The base metal 6 composed of Ti-Al intermetallic compound was fed from the underside of the assembly 2 formed with water cooled copper segments 2a, 2b,...2h, and heated by supplying the high-frequency wave of 60kW with frequency of 30kHz to the induction heating coil 5. The base metal 6 is melted by forming eddy currents on the inside of the assembly 2 and inducing an eddy current in the outermost layer of the base metal 6 of Ti-Al alloy.
The casting temperature was determined at 1580°C by making the temperature of the molten metal 31 higher than the melting point 1520°C of the Ti-Al alloy by 60°C (superheat).
The degree of superheat is remarkably low as compared with that of top poured conventional precision casting (150∼250°C), and is effective for inhibiting the reaction between the permeable mold 21 and the molten metal 31.
By reducing the pressure at the aforementioned casting temperature by a pressure of 350mm Hg or so through the suction hole 27a, the gas in the molding cavity 21a can be discharged through the permeable mold 21 according to the difference of the internal pressures between the mold chamber 14 and the melting space 15. The molten metal 31 of Ti-Al alloy is drawn by suction into the molding cavity 21a and the feeder head 21c through the tubular sprue 23 and the gates 22. The turbine wheel is obtained by solidifying the molten metal 31 in the molding cavity 21a.
As the molten metal 31 is drawn into the molding cavity 21a by vacuum casting, the molten metal 31 spreads well into every nook and corner of the thin-walled turbine blade mold. It is thus possible to obtain a turbine wheel with high accuracy in shape.
Adopting the method and apparatus for precision casting according to this invention, it becomes possible to manufacture complicated and large-sized precision castings which previously have been almost impossible to make. The invention will contribute much to the further development of the precision casting of titanium or titanium alloy. In addition, it is possible to apply the method and apparatus to the precision casting of other metals or alloys thereof having high melting points or high activity such as tungsten, molybdenum, vanadium, zirconium or lithium.
As mentioned above, the method for precision casting according to this invention comprises the step of establishing molten base metal of, for example, titanium or titanium alloy by induction heating in an assembly formed with a plurality of water cooled copper segments disposed circlewise on the inside of an induction heating coil in a state in which the copper segments are insulated from each other, and casting the molten base metal into a permeable mold disposed above the molten base metal by vacuum casting. The molten base metal is detached from the assembly. A gap is formed between the molten metal and the assembly by repelling forces caused by currents flowing in the outermost layers of the assembly and the molten metal which have opposite phases to each other. The base metal is melted by an eddy current induced in the outer layer thereof by eddy currents which are alternating currents at the time of melting the base metal. Excellent effects can be obtained in that the yield rate of the base metal is improved remarkably, and the control of the temperature of the molten metal is facilitated. It is also possible to prevent the molten metal from being contaminated and to obtain precision castings of good quality because the thermal transmission from the molten metal to the assembly is suppressed preventing the formation of a solidified metal layer between the molten metal and the assembly.
The precision casting apparatus according to this invention comprises an induction heating coil, an assembly formed with a plurality of water cooled copper segments disposed circlewise on the inside of said induction heating coil in a state in which the copper segments are insulated from each other, and fed with base metal of, for example, titanium or titanium alloy from the underside thereof, and a permeable mold for casting the base metal which is melted by induction heating on the inside of the assembly formed with the water cooled copper segments by means of vacuum casting. Therefore, an excellent effect can be obtained since it becomes possible to manufacture precision castings accurately with a better yield by enabling execution of the aforementioned method for precision casting.
The preferred embodiments of the present invention can provide a method and an apparatus for precision metals with high melting points or high activity by preventing the molten metal from contamination in melting, maintaining the quality and the temperature of the molten metal required for casting, and casting the molten metal under the forced casting condition suitable to prevent the misrun of the molten metal even if the molten metal is cast at low temperature at the time of carrying out the precision casting. The metals typically include titanium, titanium alloy or other metals having high melting points or high activities such as tungsten, molybdenum, vanadium, zirconium or lithium.

Claims

A method of precision casting comprising:
establishing molten base metal by induction heating;
casting said molten base metal into a permeable mold (21) disposed above the molten base metal by vacuum casting;
characterised in that the base metal is selected from titanium, tungsten, molybdenum, vanadium, zirconium, lithium, or alloys thereof; and in that the base metal is heated by induction in an assembly (2) formed with a plurality of water cooled copper segments (2a-h) disposed circlewise on the inside of an induction heating coil (5) in a state in which said copper segments (2a-h) are insulated from each other.
A method of precision casting as defined in claim 1, wherein said base metal is melted in an atmosphere of an inert gas such as argon.
A method of precision casting as defined in claim 1 or claim 2, wherein said molten base metal is cast into the permeable mold (21) through a tubular sprue (23).
A method of precision casting as defined in any one of claims 1 to 3, wherein said base metal is fed continuously into the assembly (2) formed with the water cooled copper segments (2a-h) from the underside of said assembly (2).
A precision casting apparatus comprising:
an induction heating coil (5);
a permeable mold (21) for vacuum casting;
characterised in that an assembly (2) formed with a plurality of water cooled copper segments (2a-h) is disposed circlewise on an inside of said induction heating coil (5) in a state in which said copper segments (2a-h) are insulated from each other, said assembly (2) being fed with a base metal from an underside thereof; and said base metal being melted by induction heating on the inside of said assembly (2).
A precision casting apparatus as defined in claim 5, wherein said permeable mold (21) is provided with a plurality of tubular sprues (23) for conducting molten base metal thereinto at the time of vacuum casting.
A precision casting apparatus as defined in claim 5 or claim 6, wherein said permeable mold (21) is provided with a closed feeder head (21c) in an upper part thereof.
A precision casting apparatus as defined in any one of claims 5 to 7, wherein said permeable mold (21) is a ceramic shell mold.
Use of the precision casting apparatus as defined in any one of claims 5 to 8, for casting a base metal selected from titanium, tungsten, molybdenum, vanadium, zirconium, lithium, or alloys thereof.