EP0206967B1 - Process and device for isostatic low-pressure casting - Google Patents

Description

The present invention relates to a new low-pressure casting process according to the preamble of claim 1 intended in particular to allow with the same installation the casting of alloys subjected to very different conditions such as in particular the alloys of aluminum and magnesium and a low pressure casting machine according to the preamble of claim 6.

• - on remplit par le bas un moule, métallique ou non, avec un alliage liquide contenu dans un four hermétiquement clos. Cet alliage peut remonter dans le moule par l'intermédiaire d'un tube d'injection,
• - on effectue ce remplissage à l'aide d'un fluide de refoulement sous une pression de quelques déci- bars,
• - après remplissage du moule, on maintient une surpression de masselottage pendant la durée de solidification de la pièce,
• - on récupère l'alliage non solidifié, situé dans le bas du moule dans les canaux d'injection, dès solidification de la pièce et après cessation de la pression de refoulement.

The low pressure casting process has been known since the beginning of the century. In this technique:

• - a mold, metallic or not, is filled from below with a liquid alloy contained in a hermetically closed oven. This alloy can go back into the mold via an injection tube,
• this filling is carried out using a delivery fluid under a pressure of a few decibars,
• - after filling the mold, a weighting overpressure is maintained for the duration of solidification of the part,
• - The non-solidified alloy is recovered, located at the bottom of the mold in the injection channels, as soon as the part solidifies and after the discharge pressure has ceased.

Such a method and such a machine are for example known from DE-B 2 703 657, on which the preambles of claims 1 and 6 are based.

The molds used can be permanent (in this case, they are made of cast iron or steel, or graphite) or non-permanent and they are destroyed, in this case, after casting to release the part. These non-permanent molds are made of chemical sands, ceramic or plaster.

The alloys used are generally those of aluminum, the applications for the automobile being very important, but also those of magnesium and copper (brasses, bronzes). In addition, significant developments are taking place for cast irons and steels.

For aluminum alloys, the gas used to pressurize the oven is generally air, it could be nitrogen to prevent oxidation of the surface. Furthermore, the development of the aluminum alloy and the operation in air can introduce hydrogen into the alloy due to the humidity of the fusion products, that of the ambient air and that of the air in the oven. It is therefore necessary to degas the metal at the end of production and in the course of the casting process.

The operation can be done by chemicals (special fluxes, chlorine, axote ...) or by the application of a vacuum of a few millibars on the surface. In this case, it may therefore be advantageous to carry out the vacuum in the casting furnace under low pressure.

In order not to interrupt the flow of the flows by this vacuum degassing, two ovens can be available, one in the casting operation, the other in the degassing operation and which replaces the first when the latter is empty. The two ovens are therefore mobile on rails.

Mobile ovens are known and there are also low pressure aluminum casting furnaces designed to support the vacuum. The pressure-tight outer shell also ensures vacuum resistance. One of the drawbacks of this formula is that the mass of refractory and insulating material which supports the resistances of the furnace and ensures its thermal insulation must reach the desired level of vacuum. However, it is porous, stores a large capacity of gas, even humidity, and it must be permanently maintained at a temperature above 80/100 _° C after having dried it for a long time, for several days before the first setting service. Furthermore, these materials are frequently fibrous or powdery and are entrained towards the vacuum pumps.

These drawbacks, linked to the fact that the seal is ensured by the external envelope, are compensated by this other fact that this envelope is cold (50 to 80 ° C.), since it is protected from resistance by the insulators and it is therefore not subjected to thermal fatigue forces at each pressurization cycle. These efforts would require a significant strengthening of this structure. Similarly, the application of vacuum to an envelope at high temperature would cause deformations in the absence of the appropriate reinforcements. There is therefore only an open sheet in front of the resistors to provide mechanical protection for the resistors.

Low-pressure casting machines with the possibility of vacuuming for aluminum are therefore of this type, with a single sealed envelope, situated outside the furnace, as shown in FIG. 1 of the accompanying drawings.

In this figure 1 is shown in 1 the outer envelope, waterproof; in 2 the fixed cover linked to this envelope; at 3 the movable cover; in 4 resistances; in 5 the refractories carrying the resistances; in 6 the insulators; in 7 the protective sheet open at its lower part; in 8 the crucible; in 9 the metal; at 10 the injection tube; at 11 the entry of pressurizing air; at 12 the air outlet for decompression; at 13 the air outlet for evacuation; at 14 the metal discharge outlet in the event of the crucible breaking; insulating the cover; at 16 and 17 seals; at 18 a connecting nozzle; at 19 the fixed plate of the casting machine, and at 20 a mold.

However, in the case of use with magnesium alloys, this device cannot be used because the gas for setting in motion the magnesium must not cause oxidation of the metal and the currently known solution is hexafluoride. sulfur (SF6) used in dilution of the order of 0.5 to 1% in air or carbon dioxide.

However, this gas which is a fluoride breaks down the materials of the refractories and insulators which are generally based on silicon compounds (silico-aluminous in particular). It is therefore necessary to isolate the SF6 from the refractories and the low-pressure magnesium casting machine must have a sealed inner casing which can serve as a crucible containing the metal.

The diagram of such a machine is given in FIG. 2 on which the envelope has been represented at 1 ' outside of the oven which no longer performs a sealing role; in 2 'the crucible which contains magnesium; in 3 'the cover; in 4 'resistances; in 5 'the refractories; in 6 'the insulators; in 7 'the molten magnesium alloy; at 8 'the injection tube; in 9 'the special layer nozzle with protective gas; in 10 'the entry of the SF6-based gas diluted in the connection nozzle; in 11 ′ the entry of SF6 into the furnace for setting in motion of the metal; in 12 'the outlet of the decompression gas; at 13 'the seal; in 14 'the fixed plate of the machine, and in 15' the mold.

In such an oven, the crucible 2 ′ contains the metal at around 750 ° C. and its outer wall is heated by the resistors. At each cycle it withstands thermal fatigue forces with an internal overpressure which is generally from 0.6 bar to 1 bar. Added to these efforts are the risks of corrosion, which are limited for diluted SF6 only by the use of steel with a high chromium content. We are therefore led to crucibles of great thickness which can be of the order of 20 mm, expensive, difficult to handle, working in conditions which are not without danger.

It should be noted, however, that the magnesium alloys are not subjected to degassing under vacuum because they distill and pass into the vapor state under still notable pressures, frequently of the order of 60 millibars.

These characteristics therefore mean that the current low-pressure casting machines for aluminum and for magnesium are different and that in each of the two cases the solutions provided for the vacuum for aluminum and for the pressurization under diluted SF6 do not are not satisfactory.

However, foundries which pour parts for the aerospace industries use both aluminum alloys and magnesium alloys and these parts which are generally of large dimensions, small thicknesses and high metallurgical qualities, represent cases of typical applications of low pressure casting.

• - des mises sous vide de l'ordre de 1 millibar ;
• - des fonctionnements sous des pressions pouvant atteindre 2 bars ;
• - avec des gaz différents non corrosifs comme l'air, l'azote ou corrosifs comme le SF6 ;
• - sans être soumis à des efforts de fatigue thermique lors des mises en pression ou à des efforts de fluage dûs à l'aspiration lors des mises sous vide.

The aim of the present invention is therefore to define a structure for a new type of low-pressure casting machine capable of withstanding:

• - vacuum packing of around 1 millibar;
• - operations under pressures up to 2 bars;
• - with different non-corrosive gases such as air, nitrogen or corrosive such as SF6;
• - Without being subjected to thermal fatigue forces during pressurization or creep forces due to suction during vacuum applications.

To this end, the subject of the invention is a low-pressure casting process in which a mold is filled from below with an alloy or a liquid metal contained in a hermetically closed heating oven, the alloy rising in the mold by through an injection tube and under the action of a discharge fluid pressure, said method being characterized in that it consists in separating, inside the furnace, the crucible containing the alloy or liquid metal, electrical heating and thermal insulation elements by a metallic, waterproof envelope supporting said crucible, to be ensured during the various phases of the casting until the solidification of the casting is sent fluid on the side of said envelope opposite to the crucible and to vary the pressure of this fluid in correspondence with that prevailing above the level of the crucible liquid so as to permanently establish a pressure balance on either side of said envelope .

• - quel que soit le niveau du métal dans le creuset de la machine ;
• - quel que soit l'état d'avancement dans le cycle de coulée, donc même pour les valeurs élevées de la phase de surpression ;
• - quel que soit le signe de cette pression par rapport à la pression atmosphérique, c'est-à-dire que l'on soit en vide ou en pression ;

By this process, it is thus possible to maintain an equilibrium on either side of the internal envelope:

• - whatever the level of the metal in the crucible of the machine;
• - whatever the state of progress in the casting cycle, therefore even for the high values of the overpressure phase;
• - whatever the sign of this pressure with respect to atmospheric pressure, that is to say that one is in vacuum or in pressure;

In particular, it is possible to keep a residual pressure greater than atmospheric pressure between two flows in order to keep the metal at the top of the injection tube in order to avoid the oxides due to the descent and ascent of the metal in the tube.

The invention also relates to a casting machine for implementing the above method and in particular an improved machine intended for non-permanent molds.

Other characteristics and advantages will emerge from the description which follows of an embodiment of a machine according to the invention, description given by way of example only and with reference to FIGS. 3 and 4 of the appended drawings respectively representing a schematic sectional view of a mobile mold casting machine and an elevation view with axial vertical half-section of a machine according to the diagram in FIG. 3.

In Figure 3 there is shown at 21 the outer shell of a furnace, pressure and vacuum tight. Inside the oven is arranged, supported by an internal sealed envelope 22, a crucible 23 (in graphite or silicon carbide for aluminum, in steel for magnesium) containing the metal 24 in the liquid state (alloy of aluminum or magnesium). The inner casing 22 is made of chrome steel resistant to corrosion by SF6.

The electric heating resistors 25 are arranged outside the casing 22 as well as the refractories 26 and the thermal insulators 27.

The oven cover consists of a fixed annular cover 28 linked to the outer casing 21 and made of chrome steel in its part in contact with SF6 and of a movable cover 29 made of chrome steel, disc-shaped and removably attached to the annular cover 28. The cover 29 is provided on its internal face with an insulator 30.

An injection tube 31 passes through the cover 29, plunges into the liquid metal 24 of the crucible and is surmounted by a connecting nozzle (32 for magnesium, 33 for aluminum).

The oven is overhung by a horizontal fixed table supported by four pillars 35 and above which a mold 36 can move, carried by a carriage 37 circulating on two parallel rails 38 arranged horizontally on either side of the fixed table 34.

As can be seen in more detail in Figure 4 which shows an industrial embodiment of the machine shown schematically in Figure 3, the rails 38 are supported by the rods of four hydraulic cylinders 39 arranged vertically and fixed on consoles integral with the four pillars 35 carrying the table 34.

In FIG. 4 is shown at 40 the wheels of the mold carrier carriage 37, circulating on the rails 38 and fixed laterally to the lower plate 41 on which the mold 36 rests.

A regulating or piloting device symbolized at 42 in FIG. 3 is responsible for piloting and coordinating the operations prior to casting, then the operations specific to casting and finally those subsequent to casting.

To this end, the apparatus 42 is connected to a valve 43 interposed in a pipe for supplying gas for internal pressurization of the space 1 above the liquid of the crucible 23 (or for placing said space under vacuum). The connection with the space 1 is made by a conduit 44 formed in the thickness of the fixed annular cover 28. In reality, there are three conduits such as 44 (and three corresponding valves 43) assigned respectively to the inlet of said gas. pressurization, its extraction and vacuum.

Likewise, the device 42 is connected to a valve 45 for entering and leaving the pressurizing (or vacuum) gases of the space II between the internal envelope 22 and the external envelope 21. The connection with the space It is made by a conduit 46 formed in the thickness of the annular cover 28.

As for space 1, there are actually three separate sets 45-46 for the introduction or extraction of said gas and for the vacuum.

The apparatus 42 is also connected, on the one hand, to a sensor 47 for pressure in space 1 and, on the other hand, to a sensor 48 for pressure in space II.

In line with the lower mold entry orifice 36 is a presence sensor 49 detecting the passage of the metal, constituted for example by an insulated metal wire stripped at its end, and connected to the regulating device 42.

In addition, the upper end of the pouring nozzle 33 is provided with an asbestos washer 50 capable of being interposed between said nozzle and the orifice 51 for the passage of liquid metal in the plate 41 supporting the mold 36.

On either side of the orifice 51 are provided two sensors 52 connected to the apparatus 42 and intended to inform the latter that the mold 36 is correctly and sealingly placed above the pouring nozzle 33.

In the embodiment shown in FIGS. 3 and 4 it is an aluminum alloy casting for which the nozzle 33 is used.

The sensors 52 consist for example of a conductive tip projecting from the underside of the plate 41, inserted without an insulating sleeve and connected by an insulated wire to the device 42. Said tips are capable of passing through the thickness of the asbestos 50 and, in contact with the underlying metal nozzle 33, to supply an electrical signal to the device 42.

Finally, the latter is connected to a device for locking the carriage 37 mold carrier in the casting position, consisting (FIG. 4) of horizontal pins 53 arranged on either side of the table 34, orthogonally to the latter. The pins 53 are moved by jacks 54, or other means of movement, mounted on supports integral with the table-holder frame. In the locking position of the carriage 37 above the oven, the pins 53 are engaged both through holes made in lateral angles 55 integral with the plate 41 of the carriage and in centering holes made in the edge of the table 34 .

The apparatus 42 directs the movement of the carriage 37 from sensors 56 (for example contactors with elastic blades) fixed on the table 34 and the rails.

The start of metal injection, and therefore of the pressurization of the spaces or enclosures 11, 11 requires that the closing of the mold 36 and the sealing on the pouring nozzle 33 be ensured.

When the carriage 37 arrives vertically from the pouring tube, the jacks 39 are lowered, the carriage is applied to the pouring nozzle 33. The device 42 locks the carriage and the mold on the table 34 of the machine with the aid of jacks 54 and pins 53, while the sensors 52 inform the device 42 that the application, therefore the seal, is obtained.

The apparatus 42 then initiates the casting according to the arrangements given below by directing the pressure in the two chambers 1 and II.

As soon as the order is given to start the pouring, the apparatus 42 causes the valves 43 and 45 to open for the admission of gases into the two chambers l, ll. This device 42 may be of the same type as that described in French patent _No. 79 17317 or in French patent _No. 82 00115 and may include a microprocessor, a memory assembly and an input-output assembly. By the pressure sensors 47 and 48 it opens and closes the valves 43 and 45 to maintain the equality of pressure between the two enclosures I, II.

When the metal reaches the level of the presence sensor 49, the device 42 records the pressure of each enclosure 1,11 as a zero reference and then changes the pressures above this reference according to the indications which could have been stored in the appliance 42.

During this pressurization phase, the envelope 22 which has its two inner and outer walls at the same pressure is therefore not subjected to expansion forces due to the pressure.

Its mechanical role is to support the weight of the crucible 23 and the metal 24 which represents a static force.

The same applies to decompression, the device 42 giving the opening-closing orders to the gas outlet valves which are in the same configurations as the valves 43 and 45. The decompression is also controlled by the device 42 to maintain the pressure balance on the two faces of the envelope 22.

When this residual pressure reaches a determined value calculated by the apparatus 42 as a function of the pressure recorded on passage to the sensor 49 and of the weight of metal cast for the part obtained, the apparatus 42 maintains this value so that the metal remains at a constant level 57 at the top of the tube 31. This arrangement prevents the metal from falling into the injection tube 31, towards the crucible, an operation which results in the formation of oxides which are carried to the workpiece during the ascent of the metal in the tube for the next pour.

When the metal is placed under vacuum for degassing, the conduits 44 and 46 are connected to pumping circuits which carry out the vacuum in the two enclosures I, II. The pressure sensors 47 and 48 give the information to the device 42 so that it opens or closes the valves 43 and 45 to maintain the pressure identity on the two faces of the inner envelope 22. This arrangement avoids deformations by "suction" of this envelope.

In the case of permanent molds, the device 42 obviously does not have to control the closing and the sealing of the mold on the pouring nozzle.

The machine shown in FIGS. 3,4 can be used both for the casting of aluminum alloys as illustrated above and for the casting of magnesium alloys. In the case of aluminum, the gas sent to enclosure I is air or nitrogen and, in the case of magnesium, sulfur hexafluoride. In both cases, the gas sent into the enclosure It may be air.

Claims (12)

1. A process for low pressure casting in which a mould is filled from the bottom with a liquid metal or alloy contained in a hermetically sealed heating furnace, the alloy being forced up into the mould by means of an injection pipe and under pressure from a liquid propellant; the said process being characterized in that it consists in separating, within the interior of the furnace, the crucible (23) containing the liquid metal or alloy, from the electric heating and thermal insulation elements (25 to 27) ny means of an impervious metal casing (22) which supports the said crucible; in ensuring that during the various phases of the casting process, until the cast piece solidifies, a fluid is supplied at the side of the casing (22) remote the crucible; and in adjusting the pressure of this fluid in relation to the pressure in the gap above the level of liquid in the crucible in order to maintain a constant equilibrium between the pressures on each side of the said casing.

2. Process as claimed in claim 1, characterized in that the balanced pressure levels on each side of the said casing (22) may drop below atmospheric pressures and may reach high levels of vacuum.

3. Process as claimed in claim 1, characterized in that between two castings, the pressure on both sides of the said casing (22) is maintained above the level of atmospheric pressure, thus ensuring that the liquid metal remains at the upper part of the injection pipe (31) in order to prevent the formation of oxides.

4. Process as claimed in any one of claims 1 to 3, characterized in that the pressurized fluids are independent fluids of the same type or of different types, used on both sides of the said casing (22).

5. Process as claimed in claim 4, more specifically for use in casting magnesium alloys, characterized in that the liquid propellant used is sulphur hexafluoride mixed with air or carbon dioxide, and the fluid used on the other side of the impervious casing (22) is air.

6. Low pressure casting machine for the working of the process as claimed in claim 1, of a type consisting of an impervious external casing (21) covering the furnace, which houses a crucible (23) containing the liquid metal or alloy (24), and which is itself surrounded by electric heating element (25) and refractory elements (26) and thermal insulators (27), and closed by a movable cover (29), equipped with a casting nozzle (31) and means by which the gap above the level of the liquid in the crucible may be pressurized or depressurized, the said machine being characterized in that it also consists of an internal metal casing (22) which supports the crucible (23) and provides an impervious insulation between the latter and the said heating, refractory and insulating elements, the said casing (22) being joined to the structure of the furnace by means of an impervious seal, means being provided to allow the gap (II) defined by the said metal casing, remote from the crucible, to be connected to a source supplying pressurized liquid or to a vacuum source, by means of valves (45) controlled by a regulating device (42) which is connected to both the said means for pressurizing or depressurizing the gap (I) above the liquid metal in the crucible and to pressure pickups (47, 48) disposed in the furnace on both sides of the internal metal casing (22).

7. Machine as claimed in claim 6, characterized in that the said internal metal casing (22) is joined by its upper part to an annular cover (28) which is fixed on to the external casing of the furnace (21) and houses the said movable cover (29), this annular cover being equipped with ducts (44, 46), which connect the exterior of the furnace with the two gaps inside the furnace (I, II) defined by the internal casing (22), the said ducts being connected to the source of pressurized fluids, or vacuum source by means of pipes equipped with valves (43, 45).

8. Machine as claimed in claim 6 or 7, more specifically for use with non-permanent moulds movable on rails, characterized in that it comprises a horizontal platform (34) fixed above the furnace, in which is fixed the upper part of the casting nozzle (31), rails (38) disposed on each side of the platform and along which the movable moulds (36) may move, a means for moving the said rails in a vertical direction in order to bring the lower opening of a mould against the said casting nozzle and a means for bolting the mould-bearing carriage (37) on to the said platform before casting begins.

9. Machine as claimed in claim 8, characterized in that the said means of vertical movement of the rails (38) are composed of rigidly fixed vertical jacks (39), at the end of which are rods supporting the rails, the jacks being maintained in synchronization by the said regulating device (42).

10. Machine as claimed in claim 8, characterized in that the said means of bolting the mould-bearing carriage (37) comprise pins (53) which can move perpendicularly in relation to the rails (38) and are moved by jacks (54) or similar devices, fixed in such a way that they fit into both the holes formed along both sides of the rigid platform (34), and into the holes formed in the side corners (55) integral with the mould carrying carriage and which can be brought into alignment with the corresponding holes in the platform when the mould (36) is in the correct position for casting, the said jacks (54) or similar devices being maintained in synchronization by the said regulating device (42) from checks made by sensors (56) fixed onto the rails and the platform, which indicate the correct position of the carriage.

11. Machine as claimed in any one of claims 8 to 10, characterized in that the upper end of the casting nozzle (33) is equipped with an annular asbestos disk (50) which can be inserted between the nozzle and the intake port (51) of the mould, this port being equipped with projecting conductive spikes (52) inserted in insulating bushes and turned towards the asbestos disk; these spikes being connected to the said regulating device (42) by means of an insulated conductor, and when the mould is being lowered, the spikes are capable of passing through the asbestos and coming into contact with conductive metal part of the casting nozzle, which is also connected to the regulating device, in order to stop the lowering of the mould, and to start the pre-casting operations and the casting process itself.

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