INJECTION TIP FOR CASTING MACHINE, CASTING MACHINE AND METHOD USING SUCH A TIP

Abstract

Disclosed is an injection tip for injecting liquid metal under pressure, suitable for being inserted between a liquid metal supply conduit and an injection opening of a mould, the injection tip having a tubular in shape, made from electrically insulating refractory material and in that it includes a channel suitable for the flow of liquid metal between a first end referred to as the upstream end, suitable for being connected to the conduit, and a second end, referred to as the downstream end, suitable for being connected to the mould, and an electromagnetic coil, positioned between the ends, the axis of which merges with at least a portion of the axis of the channel. Also disclosed is a casting machine using such a tip and a method for casting metal using the machine.

Claims

1-10. (canceled)

11. Tip (20) for injecting liquid metal (13) under pressure, said tip being tubular in shape, made from electrically insulating refractory material, suitable for being inserted between a conduit (12) for supplying liquid metal and an injection opening (33) of a mould (30), said tip comprising an injection channel (22) suitable for the flow of liquid metal between a first end, named upstream end (23), suitable for being connected to said conduit and a second end, named downstream end (24), suitable for being connected to the mould, wherein: said tip comprises an electromagnetic coil (26) placed between said ends, having an axis merged with at least one axis portion of the channel, suitable for being supplied with a current pulse from a high-voltage generator (40) and generating an electromagnetic pinch in the injection channel (22), the electromagnetic coil (26) is embedded in the material of the tip.

12. Tip according to claim 11, wherein the injection channel (22) has a diameter comprising a narrowing (27) between the upstream end (23) and the electromagnetic coil (26).

13. Tip according to claim 11, wherein the electromagnetic coil (26) is a multi-turn coil.

14. Tip according to claim 11, wherein the electromagnetic coil (26) is a single-turn coil.

15. Casting machine (10) comprising a liquid metal reservoir (11), a conduit (12) for the supply of liquid metal connected to said reservoir and fitted with an electromagnetic pump (50) suitable for causing the liquid metal to flow in said conduit in the direction of a mould (30), wherein the machine comprises a tip (20) according to claim 11 between said conduit and an injection opening (33) of said mould.

16. Machine according to claim 15, wherein the electromagnetic pump (50) comprises a plurality of induction coils (51) which are coaxial to the conduit (12) for supplying liquid metal and which are suitable for induction heating of the metal flowing in said conduit.

17. Machine according to claim 16, wherein the induction coils (51) are supplied with polyphase current so as to generate a moving magnetic field and to drive the liquid metal in the direction of the tip (20).

18. Machine according to claim 16, wherein a device (52) for cooling the induction coils (50) is interposed between said coils and the conduit (12).

19. Method for casting liquid metal under pressure in a metal mould (30), wherein: a casting machine (10) is used which comprises a liquid metal reservoir (11), a conduit (12) for supplying liquid metal, which is connected to said reservoir and fitted with an electromagnetic pump (50) suitable for causing the liquid metal to flow in said conduit in the direction of a mould (30), the electromagnetic pump (50) is supplied with a polyphase current so as to displace the liquid metal from the reservoir to the injection opening of the mould, wherein the casting machine (10) comprises a tip (20) comprising an electromagnetic coil (26) surrounding an injection channel (22) between said conduit and an injection opening (33) of said mould and in that, at the end of injection, the electromagnetic coil (26) of the tip (20) is supplied with an electric current pulse from a high-voltage generator (40) in order to generate an electromagnetic pinch in the injection channel (22) and to propel the liquid metal under pressure towards the injection opening (33) of the mould.

20. Tip according to claim 12, wherein the electromagnetic coil (26) is a multi-turn coil.

21. Tip according to claim 12, wherein the electromagnetic coil (26) is a single-turn coil.

22. Casting machine (10) comprising a liquid metal reservoir (11), a conduit (12) for the supply of liquid metal connected to said reservoir and fitted with an electromagnetic pump (50) suitable for causing the liquid metal to flow in said conduit in the direction of a mould (30), wherein the machine comprises a tip (20) according to claim 12 between said conduit and an injection opening (33) of said mould.

23. Casting machine (10) comprising a liquid metal reservoir (11), a conduit (12) for the supply of liquid metal connected to said reservoir and fitted with an electromagnetic pump (50) suitable for causing the liquid metal to flow in said conduit in the direction of a mould (30), wherein the machine comprises a tip (20) according to claim 13 between said conduit and an injection opening (33) of said mould.

24. Casting machine (10) comprising a liquid metal reservoir (11), a conduit (12) for the supply of liquid metal connected to said reservoir and fitted with an electromagnetic pump (50) suitable for causing the liquid metal to flow in said conduit in the direction of a mould (30), wherein the machine comprises a tip (20) according to claim 14 between said conduit and an injection opening (33) of said mould.

25. Casting machine (10) comprising a liquid metal reservoir (11), a conduit (12) for the supply of liquid metal connected to said reservoir and fitted with an electromagnetic pump (50) suitable for causing the liquid metal to flow in said conduit in the direction of a mould (30), wherein the machine comprises a tip (20) according to claim 15 between said conduit and an injection opening (33) of said mould.

26. Casting machine (10) comprising a liquid metal reservoir (11), a conduit (12) for the supply of liquid metal connected to said reservoir and fitted with an electromagnetic pump (50) suitable for causing the liquid metal to flow in said conduit in the direction of a mould (30), wherein the machine comprises a tip (20) according to claim 16 between said conduit and an injection opening (33) of said mould.

27. Machine according to claim 16, wherein a device (52) for cooling the induction coils (50) is interposed between said coils and the conduit (12).

Description

[0024] Other aims, features and advantages of the invention will become apparent in view of the following description and the accompanying drawings, in which:

[0025] FIG. 1 shows a schematic, cross-sectional view of a casting machine in accordance with the invention,

[0026] FIG. 2 is schematic, cross-sectional view of tip in accordance with the invention,

[0027] FIG. 3 is a cross-sectional view of a tip in accordance with the invention during pinching of the flow of the metal.

[0028] FIG. 1 shows a longitudinal cross-sectional view of a casting machine 10 comprising a reservoir 11 suitable for containing liquid metal to be injected into a mould 30. The reservoir 11 can comprise heating means (not shown) to maintain the metal at its melting temperature, or be shaped into the form of a hopper in which the liquid metal is poured from a crucible. The liquid metal is then channelled in a conduit 12 for supplying the metal in the direction of a tip 20 fixed between the conduit 12 and the mould 30.

[0029] The conduit 12 is fitted with an electromagnetic pump 50 comprising a plurality of induction coils 51 regularly spaced along the conduit 12. Each of the induction coils 51 is connected to a power inverter 53 adapted to supply the coils 51 with alternating current. The induction coils 51 fulfil a dual role: on the one hand, they operate as induction heating coils permitting the stream of liquid metal flowing in the conduit 12 to be kept in the liquid state and, on the other hand, being supplied with polyphase alternating current adapted to the number and the order of the coils 51, they generate a moving magnetic field going from the reservoir 11 in the direction of the tip 20, this magnetic field making it possible to cause the liquid metal to flow in the conduit in the direction of the mould 30 at a velocity V0. By way of example, the induction coils are supplied with three-phase current at a voltage of 400V at a frequency of the order of 50 Hz to 10 kHz with a current variable between 50 A and 10 000 A.

[0030] The coils 51 also comprise a cooling circuit 52, e.g. using a cooling liquid circulating in copper tubes forming the coils 51, making it possible to limit heating thereof. Of course, the use of an air cooling system with forced convection employing one or more fans and cooling fins fixedly attached to the windings can also be contemplated.

[0031] Reference is made to FIG. 2 to detail the preferred structure of the tip 20. The tip 20 is fixed by means of flanges 25 to the end of the conduit 12 by its upstream end 23 (thus designated in relation to the direction of flow of the liquid metal in the conduit and in the tip). The tip 20 is also fixed to the mould 30 by flanges 25 at its opposite end, named downstream end 24. The tip 20 comprises a body 21 of refractory and electrically insulating material, preferably ceramic, and more particularly zirconium/aluminium nitride. Other refractory materials can also be used, e.g. alumina-based, zirconium-based, yttrium-based, titanium-based or nickel oxide-based ceramics, or even a mixture of these constituents in various proportions.

[0032] The body 21 of the tip is traversed by a channel 22 extending from the upstream end 23 to the downstream end 24 where the channel 22 issues into the injection opening 33 of the mould 30. The channel 22 is preferably of a revolutional cylindrical shape and comprises, at the upstream end 23, a conical part forming a narrowing 27 between a large cross-section with a diameter corresponding to the end of the conduit 12 and a smaller cross-section of the channel 22 corresponding to the cross-section of the injection opening 33 of the mould 30.

[0033] The body 21 also comprises, downstream of the narrowing 27, an electromagnetic coil 26 surrounding the channel 22 and over-moulded in the body 21. The coil 26 is preferably a multi-turn coil of copper or other highly conductive material, e.g. aluminium, cupro-beryllium, copper-chromium-zirconium alloy, tungsten or tungsten-copper alloy . . . . The coil 26 is suitable for connection to a current pulse generator 40 generally comprising a bank 41 of capacitors, which are charged by an external continuous supply source (not shown), and is discharged in the electromagnetic coil 26 via a spark gap 42. The coil 26 can also be formed by a single turn. Whether the coil 26 is a multi-turn or single-turn coil it has an axis of rotation substantially merged with at least a portion of the axis of the channel 21 over the part thereof which it surrounds. In the channel 21, the coil 26 thus defines a zone, named pinch zone 28, within which develops the electromagnetic field created by the coil 26 when this coil is supplied by the generator 40.

[0034] According to one variation which may be used when injection conditions permit, i.e. if the injection pressure and/or temperature and/or the metal to be injected are compatible with a tip having a body 21 with sufficiently thin walls, it is possible to place the coil 26 around the body 21. For example, for the injection of zinc alloys (without aluminium) having a melting point lower than 450 C., it is possible to use a tip of non-magnetic austenitic refractory stainless steel which permits satisfactory resistance with thicknesses reduced to a few millimetres. A single-turn or multi-turn electromagnetic coil 26 can thus be threaded onto the body 21 and fixed by any suitable means. This variation permits easy removal of the coil while retaining the body 21 of the tip.

[0035] Reference is now made to FIGS. 1 and 3 to describe the operation of the machine 10 and of the tip 20 as well as the casting method of the invention.

[0036] During a casting operation, the reservoir 11 is filled with a liquid metal, e.g. an alloy of zinc or magnesium. The liquid metal flows from the reservoir 11 into the conduit 12. The power inverter 53 supplies the induction coils 51 with a polyphase (e.g. three-phase) alternating current so as to induction heat the liquid metal in the conduit 12 to avoid any initiation of solidification or the formation of lumps. Each induction coil 51 also develops a magnetic field, the field lines of which are orientated along the axis of the induction coils and of the conduit 12. The phase difference in the magnetic field of the induction coils generates a moving magnetic field in the conduit 12 which displaces the liquid metal contained therein towards its end opposite to the reservoir 11 at a substantially constant velocity V0.

[0037] At the inlet of the tip 20, the flow cross-section of the liquid metal is reduced in the narrowing 27 and thus the velocity of displacement of the metal increases as a function of the cross-section ratio between the upstream end 23 of the tip and the cross-section of the channel 22 to reach a velocity V1 at the end of the narrowing, at the inlet into the pinch zone 28 located under the electromagnetic coil 26. The liquid metal progresses at the velocity V1 in the channel 22 then in the injection opening 33 of the mould 30 to fill one or more cavities 32 formed between the dies 31 of the mould 30. When the cavities 32 are filled, e.g. at the end of an injection time which is predetermined as a function of the velocity V1, the cross-section of the channel 22 and/or the injection opening 33 which define the metal throughput and the volume of the cavity or cavities 32, the pulse generator 40 is activated and delivers a current pulse, e.g. of an intensity of the order of 20 kA to 1 MA, a duration of 40 s to 2 ms which flows in the coil 26. The large variation in the flux of the magnetic field generated by the coil 26 when this current pulse passes through it gives rise to a radial magnetic force which acts on the liquid metal 13 in the pinch zone 28. The liquid metal 13 is thus radially compressed and, owing to its incompressibility, is ejected axially on both sides of the pinch zone 28 at an ejection velocity V2. The combination of this ejection velocity V2 with the flow velocity V1 of the liquid metal in the direction of the mould 30 imparts to the metal a velocity corresponding to V1+V2. Taking into account that the cavity 32 is filled, this velocity V1+V2 is manifested as an increase in the injection pressure in the mould 30. By way of example, for a single-turn coil 26 of aluminium having a working zone (pinch zone 28) of a length of 15 mm, supplied by a current pulse Imax=500 kA to 500 Hz (2 ms), the coil generates a magnetic field of 40 T. When this magnetic field is applied to the liquid metal, this metal can achieve a maximum velocity (V1+V2) of the order of 30 m/s in the tip and a maximum pressure of 700 MPa.

[0038] On the other side of the pinch zone 28, the flow velocity of the metal following the combination of the flow and ejection velocities is V1-V2. In general, the ejection velocity V2 is higher in absolute value than the flow velocity V1 which imparts a movement to the liquid metal in the direction of the conduit 12 of the machine. By virtue of the narrowing 27 which behaves as a diverging area in this direction, the velocity of the liquid metal is decreased in the ratio of the cross-sections and does not generate a large shock wave in the conduit 12 likely to damage the machine, but simply a pressure wave which promotes the mixing of the metal in the conduit.

[0039] Of course, this description is given only by way of illustrative example and a person skilled in the art will be able to modify it in numerous ways without departing from the scope of the invention, e.g. by adapting the dimensions of the various elements of the casting machine according to the metal to be cast, the volume of the cavities of the mould etc. Similarly, the metering of the quantity of metal to be injected into the cavities of the mould can be regulated by an injection period prior to triggering the pulse generator or by regulating the velocity of transfer of the liquid metal by the frequency and phase of supply of the induction coils.