Claims
1. An apparatus for centrifugal casting, said apparatus comprising :
a rotor (1) having a shaft (2) extending in an essentially vertical direction therefrom and being rotatable around an axis (A) defined by the shaft (2) ,
the rotor (1) having at least one fixing means for release- ably fixing at least one mold (9) in a first radial distance from the axis (A) , and a means for accommodating at least one crucible (6, 7, 23) being associated with the mold (9) so that an outlet opening (8) of the crucible (7, 23) is ar- ranged opposite an inlet opening of the mold (9) ,
a drive device (13) for driving the shaft (2) in order to rotate the rotor (1) ,
characterized in that
an auxiliary acceleration device (14, 16, 25, 26, 27) for generating a force to overcome a moment of inertia of the rotor (1) is provided.
2. The apparatus of claim 1, wherein the auxiliary acceleration device (14, 16, 25, 26, 27) comprises a flywheel (14) and clutch (16) for drivingly connecting the flywheel (14) with the shaft (2) .
3. The apparatus of one of the preceding claims, wherein the flywheel (14) is drivingly connected for creating a rota- tional movement thereof with the drive device (13) provided for driving the shaft (2) .
4. The apparatus of one of the preceding claims, wherein the clutch (16) is provided in the drive chain between the flywheel (14) and the shaft (2) .
5. The apparatus of one of the preceding claims, wherein the drive device comprises an electric motor (13) .
6. The apparatus of one of the preceding claims, wherein the auxiliary acceleration device (14, 16, 25, 26, 27) comprises a jet propulsion (25) .
7. The apparatus of one of the preceding claims, wherein the jet propulsion comprises at least one nozzle (25) being mounted at the outer circumference of the rotor (1) .
8. The apparatus of one of the preceding claims, wherein the jet propulsion comprises a tank (27) with pressurized gas to be expelled through the at least one nozzle (25) .
9. The apparatus of one of the preceding claims, wherein the rotor (1) comprises a gas-tight housing (1, 10) in which the mold (9) and the crucible are accommodated.
10. The apparatus of one of the preceding claims, wherein the rotor (1) is accommodated within a gas-tight housing.
11. The apparatus of one of the preceding claims, wherein a vacuum source is provided to create a vacuum in the housing.
12. The apparatus of one of the preceding claims, wherein the outlet opening (8) of the crucible (7) is arranged in a second radial distance (r2) from the axis (A) .
13. The apparatus of one of the preceding claims, wherein the second radial distance (r2) is larger than the diameter of the crucible (7) .
14. The apparatus of one of the preceding claims, wherein for melting a metal in the crucible a heating device comprising an induction-coil (4) is provided.
15. The apparatus of one of the preceding claims, wherein a device (12) for moving the induction-coil (11) from a first position surrounding at least partly the crucible (7) in a second position in which it does not interfere with a rotational path of movement of the crucible (7) is provided. |
APPARATUS FOR CENTRIFUGAL CASTING
The invention pertains to an apparatus for centrifugal casting, in particular for the production of castings made of ti- tanium aluminides .
US 2001/0045267 Al describes an apparatus for centrifugal casting in accordance with the preamble of claim 1. In the known apparatus a melt taken up in a first crucible is poured into a second crucible or gate, respectively, which is part of a rotor which can be rotated around a vertical axis. The gate has a plurality of radial outlet openings opposite of which there are arranged inlet openings of molds extending in a radial direction. The melt being poured into the gate is forced by centrifugal forces through the outlet openings thereof into the molds. By use of this apparatus castings with a simple geometrical shape like valves for internal combustion engines and the like can be produced.
However, when producing castings from titanium aluminides or titanium grade 2 having thin walls and a complex geometry, e.g. shrouded turbine blades or turbo charges wheels, one is encountered with several problems, like the formation cold- runs, hot tears, shrinkholes, pores, voids and the like.
It is an object of the present invention to overcome the disadvantages in the art. According to an aim of the invention there shall be provided an apparatus for centrifugal casting by which castings having a complicated geometry can be cast with an improved quality. According to a further aim of the invention by the proposed apparatus a production of high- quality castings made of titanium aluminides shall be possible.
This object is solved by the features of claim 1. Advantageous embodiments of the invention are described by the features of claim 2 to 15.
According to the invention there is provided an auxiliary acceleration device for generating a force to overcome a moment of inertia of the rotor. - By the proposed feature it is possible to rapidly accelerate the rotor within a short time to a high rotational speed. This makes it possible to create the melt within a crucible which is accommodated in the rotor. Once having created a melt in the crucible the rotor is accelerated by means of the proposed auxiliary acceleration device within a short time up to a high rotational speed. Due to the acting centrifugal forces the melt is rapidly forced at a high temperature into the molds.
In the sense of the present invention under a "crucible" there is in general understood a container which has suffi- cient heat resistance to take up a metallic melt without being damaged and without undergoing reactions with the melt. A "crucible" in the sense of the present invention may have any suitable shape. In particular it may have the shape of a cylinder the bottom of which has a rounded concave shape. How- ever, a "crucible" in the sense of the present invention may also be formed as ring-like channel. Suitable materials for the production of a crucible are alumina, Y 2 O 3 , magnesia, silica-glass, graphite and the like.
By the proposed apparatus the step of pouring the melt into a cold crucible or gate, respectively, being accommodated within the rotor can be avoided. It is believed that this step is responsible for an undesirable cooling-down of the
melt and therefore for the formation of cold-runs, hot tears, pores and the like.
According to an embodiment of the invention the auxiliary ac- celeration device comprises a flywheel and a clutch for driv- ingly connecting the flywheel with the shaft . By means of the proposed flywheel a high amount of rotational energy can be transferred within less than one second upon the rotor.
There may be provided a separate drive device for creating a rotational movement of the flywheel. According to an advantageous embodiment the flywheel is drivingly connected for creating a rotational movement thereof with the drive device provided for driving the shaft. In this case the clutch may be provided in the drive chain between the flywheel and the shaft. According to the proposed embodiment the drive device, which may comprise an electric motor, can either be used to create a rotational movement of the flywheel as well as for further accelerating and/or driving the rotor after the rota- tional energy of the flywheel has been transferred to the rotor.
According to a further embodiment of the invention the auxiliary acceleration device may comprise a jet propulsion. The jet propulsion may comprise at least one nozzle being mounted at the outer circumference of the rotor. Advantageously there are provided several nozzles at the outer circumference of the rotor. The jet propulsion may comprise a tank with pressurized gas to be expelled through at least one nozzle. Ac- cording to an advantageous embodiment the at least one nozzle is mounted at an outer circumference of the rotor. The proposed jet propulsion may be used in combination with the pro-
posed flywheel. Thereby a rapid acceleration of the rotor can be reached.
The rotor may comprise a gas-tight housing in which the mold and the crucible are accommodated. However, it is also possible to accommodate the rotor within a gas-tight housing. The provision of a gas-tight housing makes it possible to carry out the centrifugal casting process under vacuum or under a shield gas thereby avoiding an undesirable oxidation of the melt.
The apparatus may comprise a vacuum source to create a vacuum within the housing. Alternatively there may be provided a pressurized shield gas supply which may selectively connected with the housing in order to flood the housing with the shield gas like Ar.
According to a further embodiment the outlet opening/s of the crucible is/are arranged in a second radial distance from the axis. The second radial distance is greater than a diameter of the usually cylindrical shaped crucible. The second radial distance may be in the range of 300 to 500 mm, in particular in the range of 320 to 400 mm. The first radial distance, i. e. the distance between an inlet opening of the mold and the axis, is usually larger than the second radial distance. The difference between the first and the second radial may be 0 to 50 mm, preferably 0 to 10 mm.
By the proposed eccentrical arrangement of the crucible the centrifugal forces acting on the melt can be increased remarkably. By use of such an arrangement also the mold is arranged in a larger radial distance from the axis . Therefore the centrifugal force acting on the melt being forced into
the mold can be increased. By this measure the formation of hot tears, hold runs and the like can be counteracted.
According to a further embodiment for melting a metal in the crucible there is provided a heating device comprising an induction-coil. In this case it has been proven to advantageous to use a crucible being made of graphite or to accommodate within the crucible, which may be made of alumina, Y 2 O 3 or the like, a further crucible being made of graphite. By using a crucible or a further crucible being made of graphite a fast melting of an ingot being taken up therein can be effected.
According to a further embodiment there may be provided a "de- vice for moving the induction-coil from a first position surrounding at least partly the crucible in a second position in which it does not interfere with a rotational path of movement of the crucible. This embodiment is directed in particular in an arrangement in which one or more separate crucibles are accommodated in the rotor in an eccentrical position.
According to a further embodiment the crucible may also have the form of a ring-shaped channel being centrally accommodated in the rotor. Such a ring-shaped channel may have a plurality of outlet openings vis-a-vis the inlet openings of radially extending molds. The proposed ring-shaped crucible may be surrounded by induction-coil for heating ingots taken up therein.
An exemplary embodiment of the object of the invention is explain in greater detail below on the basis of Figs. 1 and 3.
Fig. 1 shows a vertical cross section through essential parts of a first apparatus,
Fig. 2 shows a vertical cross section through essential parts of a second device, and
Fig. 3 shows a plot of the rotational speed of the first apparatus over the time.
Fig. 1 shows a first apparatus for centrifugal casting. A rotor 1 has a shaft 2 which extends vertically therefrom. The shaft 2 is rotatable around an axis A. The rotor 1 is accommodated within a housing 3. A lid 4 of the housing 3 can be opened.
The rotor 1 may comprise several arms 5 which extend in a radial direction therefrom. In a bottom B of each arm 5 there is provided an opening which is sealed in gas-tight manner by a first crucible 6 extending the vertical direction from the bottom B. The first crucible 6 may be made of a heat resistant material like alumina, silica-glass or the like. Within the first crucible 6 there is accommodated a second crucible 7 which may be made again of a heat resistant material like alumina, Y 2 O 3 , magnesia and the like. The second crucible 7 also extends beyond the bottom B of the arm 2. The second crucible 7 has in an upper section thereof a radial outwardly protruding outlet opening 8 which is distanced from the axis A with a second radial distance r2.
A mold 9, which may be made of a ceramic material lined with Y 2 O 3 , is arranged vis-a-vis the outlet opening 8 of the second crucible 7 and extends in a radial direction therefrom. An inlet opening (not shown here) of the mold 9 is arranged
opposite to the outlet opening 8. The inlet opening is distanced from the axis A with first radial distance rl . In the embodiment shown in Fig. 1 the first radial distance rl is roughly the same as the second radial distance. However, it has be understood that the first radial distance rl may be larger than the second radial distance r2. The mold 9 is covered by a piston 10 which can be releasably mounted at the arm 5 in gas-tight manner. The rotor 1 being sealed with the pistons 10 mounted at the arms 5 can form per se a gas-tight housing. This housing can be evacuated by a vacuum source
(not shown here) which may be connected with the rotor 1 via the shaft 2. However, it is also possible to provide breakthroughs in the rotor 1 and to evacuate the housing 3 surrounding the rotor 1. In both cases it is possible to carry out the centrifugal casting process under vacuum. Alternatively, it is possible to carry out the centrifugal casting process under a shield gas, like Ar.
An induction-coil 11 is movable in an essentially vertical direction by a lifting device 12 so that the induction-coil
11 can selectively be lifted to surround the first crucible 6 as well as the second crucible 8 accommodated therein. - Within the second crucible 7 they may accommodated a third crucible (not shown here) which may be produced from a mate- rial which couples with induced currents. Such a material may be for example graphite. When using a third crucible the melting of an ingot taken up therein can be accelerated.
A drive chain comprises an electric motor 13 which is con- nected with a flywheel 14 by first V-belts 15. The flywheel 14 can selectively be connected by a clutch 16 with a first pulley 17. The first pulley 17 is drivingly connected by second V-belts 18 with a second pulley 19 being mounted on the
shaft 2. Reference signs 20 designate bearings for rotatably supporting the shaft 2.
A first gear transmission ratio between a motor pulley 21 and a flywheel pulley 22 is around 1:2.5. A second gear transmission ratio between the first pulley 17 and the second pulley 19 is around 1:1.4. It has to be understood that the gear transmission ratio can be adapted in accordance with the power of the used electric motor 13, the radius and the mass of the rotor 1.
The function of the first apparatus for centrifugal casting is as follows:
The driving chain is disconnected by means of the clutch 16. Then the flywheel 14 is driven by means of the electric motor 13 until a high rotational speed is achieved. At the same time an ingot consisting of a γ-titanium aluminide which may have in at . % the following composition:
Ti 4 5 -5 2 Al 4 5- 4 βXl l - 3 X2 2 - 4 X3 i ,
where
Xl = Cr, Mn, V X2 = Nb, Ta, W, Mo
X3 = Si, B, C.
For example, the titanium aluminide alloy may contain 30 to 45 wt .% Al, 4 to 6 wt .% Nb and as balance Ti as well as un- avoidable impurities. Further, the alloy may contain one or more of the following constituents: 0,5 to 3,0 wt . % Mn, 0,1 to 0,5 wt. % B, 1,5 to 3,5 wt . % Cr. Further, the titanium aluminide alloy may contain O in an amount of 0 to 1000 ppm, C
in an amount of 0 to 1000 ppm, preferably 800 to 1200 ppm, Ni in an amount of 100 to 1000 ppm and N in an amount of 0 to 1000 ppm.
An ingot of the aforementioned composition is melt by means of the induction heating. During the heating process the induction-coil 11 is in a lifted-up position surrounding the first 6 and second crucible 8. As soon as the ingot has been molten the induction-coil 11 is brought into a lower position in which it does not interfere with the first crucible 6 extending from the bottom B of the rotor 1. Then by means of the clutch 16 the rotational energy saved by the flywheel 14 is transmitted upon the rotor 1. The rotor 1 is accelerated with a high speed. The centrifugal force acting on the melt being taken up in the second crucible 8 forces the melt into the mold 9.
Fig. 2 shows a second apparatus for centrifugal casting. In contrast to the first apparatus there is accommodated a fourth crucible 23 in a centrical position relative to the axis A. Outlet openings of the fourth crucible 23 are designated by reference signs 8. The fourth crucible 23 may be made of alumina, graphite, Y 2 O 3 and the like. In order to melt an ingot being taken up in the fourth crucible 23 it may be surrounded by an induction-coil (not shown here) . Vis-a- vis the outlet openings 8 there are mounted molds 9 with their inlet openings 24 being located opposite the outlet openings 8. At the outer circumference of the rotor 1 there are provided nozzles 25 which are connected via a pressure air line 26 with a pressure air supply tank 27.
The function of the second apparatus for centrifugal casting is as follows:
An ingot taken up in the fourth crucible 23 is molten by an induction heating (not shown here) which is part of the rotor 1. As soon as the melt has been created a valve (not shown here) interrupting the pressure air line 26 is opened so that the nozzles 25 are pressurized. The back stroke created by the nozzles 25 rapidly accelerates the rotor 1. Again the melt being taken up in the fourth crucible 23 is forced by centrifugal forces into the molds 9.
The auxiliary acceleration devices described in the first and second apparatuses can be combined in order to achieve a further enhanced acceleration of the rotor 1.
Fig. 3 shows a plot of the rotational speed of the rotor of the first apparatus above the time. From this plot one can see that in less than one second the rotor 1 of the first apparatus can be accelerated on a speed of around 140 rpm. This initial acceleration is essentially created by a transfer of the rotational energy from the auxiliary acceleration device, e.g. flywheel 14, to the rotor 1. Afterwards, the rotor 1 is accelerated at a lower rate of acceleration by the effect of the electric motor 13. - Although it is not shown in Fig. 3 it has to be understood that with the proposed apparatus the rotor can be rotated at a constant rotational speed after first period of high acceleration and the subsequent period of lower acceleration. There may be provided a control equipment by which the period of constant rotational speed may be limited to 1 to 6 minutes, preferably to 4 to 6 minutes. Af- terwards the movement of the rotor may be stopped. By the control equipment the movement of the rotor may be controlled automatically.
By the proposed auxiliary acceleration device it is possible to rapidly force the melt from a second crucible 7 or a fourth crucible 23 into the mold 9. In particular the melt immediately can be forced into the mold 9 after it has reached a predetermined temperature. An undesirable cooling- down of the melt, which for example is created when a metal melt is poured from further crucible being located outside the rotor 1 into a second 7 or fourth crucible 23 being accommodated in the rotor 1 is avoided. Furthermore, an evapo- ration of volatile constituents of a metal alloy can be minimized.