Device for controlling the local temperature of a magnesium part during its manufacture by molding, and method using this device

Abstract

A method for molding parts made of magnesium or magnesium alloy in a permanent metal mold, in which, successively and cyclically, (i) a quantity of liquid metal, which may be liquid magnesium or a liquid magnesium alloy, is injected into a permanent mold, (ii) said liquid metal is allowed to solidify to obtain a part made of magnesium or aluminum alloy, (iii) and said part is removed, said method being characterized in that: - said mold includes at least one zone cooled by a conduit into which a quantity of liquid is injected which evaporates upon contact with the hot wall of said conduit; - at least once during step (ii) a quantity of liquid is injected, with a pressure PL, into said conduit, so that said liquid can evaporate at least partially, to locally cool the wall of the mold;- after step (iii) and before step (i) of the next cycle, a quantity of gas is injected into the same conduit, with a pressure PG, and the presence of a leak in said conduit is checked; knowing that said gas is an inert gas with respect to said liquid metal.;

Description

Title of the invention: Device for controlling the local temperature of a magnesium part during its manufacture by molding, and method using this device Technical field of the invention

[0001] The invention relates to the field of foundry, and more specifically to the field of manufacturing magnesium or magnesium alloy parts by casting molten metal into a permanent mold. More specifically, the invention relates to a device and method for controlling the local temperature of the mold during the introduction and cooling of liquid magnesium or magnesium alloy into a metal mold. The invention relates in particular to a new method for manufacturing magnesium or magnesium alloy parts by casting which allows the local temperature of the material introduced into the mold to be controlled. The invention applies in particular to injection molding, at high pressure or at low pressure, and to gravity casting. State of the art

[0002] Casting is a very old metallurgical shaping technique. Today, the process of injection molding in a permanent mold is widely used to manufacture complex-shaped parts in large series. Thus, parts made of various light alloys for the automotive sector, such as cylinder heads and crankcases, are produced by casting in metal molds; complex-shaped parts are also produced for the aeronautical industry and for mechanical engineering, such as structural parts, heat exchangers, mechanical or fluid connection parts. Apart from so-called light alloys (aluminum, aluminum-silicon, magnesium), the alloys shaped by injection molding include copper alloys and zinc alloys (such as the family of zinc alloys known as zamak). In the field of ferrous metals, cast iron casting is particularly known.

[0003] Permanent molds generally have at least two parts, called shells, which, by closing on each other, define the external shape of the casting; they may include cores to form orifices or depressions in the part. The wall thickness of complex castings can vary significantly from one location to another. Since the latent heat of solidification is extracted by the mold, this implies a variation in the cooling rate of a volume of cast metal depending on its distance from the nearest wall of the mold. The cooling rate influences the metal structure lurgical within the casting, both on macrosegregation, on microsegregation and on precipitation. It follows that the mechanical characteristics of the casting material can vary from one place to another, depending on the local cooling rate. This effect is typically small for small castings, but it can become significant for large parts; the consequence is that the part designer cannot take full advantage of the possibilities of the alloy with which he designs the casting, because he must take into account the weakest point of the part. This situation also complicates the modeling of castings during their design. And finally, it increases the risk of defects appearing in critical areas of the part: these defects can notably be shrinkage or porosities.

[0004] The handling and casting of liquid metal presents numerous risks. These risks are linked to the temperature, the liquid nature of the metal, and possibly to its chemical reactivity. Generally speaking, the contact of a hot molten metal with water can lead to violent evaporation phenomena, in the absence of any chemical reactivity between the water and the metal. This contact can also generate a risk of explosion if the liquid metal can react with the water; this double risk is known in particular from the aluminum foundry. Among the light metals, magnesium has the characteristic of being a combustible metal, even in the massive state: not only can it react violently with water, humidity and oxygen in the air in the liquid state, but it can also burn in the solid state in the event of a fire, developing a very intense white light and white smoke; in the presence of humidity a magnesium fire is particularly violent.The equipment and processes used in a magnesium foundry must ensure that this cannot happen. For this reason, the use of water (or other reactive liquids) and the presence of water pipes for any purpose are minimized in a magnesium foundry.

[0005] Liquid or hot magnesium must also be protected from contact with air because, unlike aluminum, magnesium does not form an oxide skin that is impenetrable to oxygen. Thus, liquid magnesium reacts with air, whereas the oxide skin that forms on liquid aluminum is stable. While those skilled in the art of aluminum casting have known for decades techniques for local cooling (also called "directed cooling") of a mold, which are based on the use of locally cooled walls and / or cooled inserts, these techniques are not used in magnesium foundries. There are essentially two reasons for this: until one or two decades ago, magnesium castings had not reached a size that really required the use of these techniques (even if the unavailability of these directed cooling techniques may have generate constraints for the design of magnesium castings even of small or medium size), and moreover, these directed cooling techniques generally use a heat transfer liquid which risks reacting violently with the magnesium in the event of accidental contact.

[0006] A first problem that the present invention seeks to solve is to advance magnesium casting technology to facilitate the production of large parts and / or parts with a large variation in thickness, with homogeneous mechanical characteristics. Another problem is to make directed cooling possible in magnesium foundries, under optimal safety conditions, and to derive maximum benefit from it to optimize existing casting processes. Objects of the invention

[0007] According to the invention, the problem, which arises in a process for molding magnesium or magnesium alloy parts in a permanent metal mold, in which, successively and cyclically, (i) a quantity of liquid metal, which may be liquid magnesium or a liquid magnesium alloy, is injected into a permanent mold, (ii) said liquid metal is allowed to solidify to obtain a magnesium or magnesium alloy part, (iii) and said part is removed, is solved by the combination of the following technical characteristics:

[0008] - said mold comprises at least one zone cooled by a conduit in which is injected a quantity of liquid which evaporates in contact with the hot wall of said conduit;

[0009] - at least once during step (ii) a pressure PL is injected, quantity of liquid in said conduit, so that said liquid can evaporate at least in part, to locally cool the wall of the mold;

[0010] - after step (iii) and before step (i) of the following cycle, in a step called "pressure test", a quantity of gas is injected into the same conduit, with a pressure PG, and the presence of a leak in said conduit is checked; knowing that said gas is an inert gas with respect to said liquid metal.

[0011] In particular embodiments of this method, which can be combined with each other:

[0012] - the presence of a leak is checked by detecting the decrease in this pressure when said conduit is closed; - when a leak is detected in the said conduit, the continuation of the process is immediately stopped; - before carrying out the pressure test, in a step called "purging", said conduit is purged with an inert gas injected with a pressure PP, or said conduit is placed under vacuum, in order to rid said conduit of steam of said liquid; - said conduit into which said liquid is injected is arranged in a metal core in thermal contact with said mold, in particular in the form of a rectilinear channel or a helical channel. Detailed description

[0013] In the present description, the term "magnesium" includes magnesium alloys.

[0014] Localized cooling of a casting after the introduction of the liquid metal into the mold is known from aluminum foundry; this is described in Aluminum-Taschenbuch, 14th edition, Düsseldorf 1983, p. 419. According to this technique, a mold core is formed by a blind tubular body (also called a "cooling finger" or more commonly "core pin") having a conduit for a fluid. Said tubular body is cooled from the inside by the evaporation of a quantity of water injected into said conduit; this process is known as "jet cooling". Different variants of devices of this type are described for example in documents JP 2008 073 722, JP 4 509 684 and WO 03 / 037547 of the company Ahresty.

[0015] The applicant markets jet cooling devices which can be connected to a foundry machine comprising cooling fingers, to implement a casting process comprising, for each casting cycle, a three-stage cyclic cooling process. In this cyclic cooling process, a quantity of water is first injected through a first inlet into the tubular body; in a second stage, this water evaporates at least in part (and it is mainly the evaporation enthalpy which causes the cooling), and the water vapor is then expelled, in a third stage, by a jet of air injected through a second inlet. This cooling cycle, which is carried out when the mold is closed and the liquid metal to be solidified has been injected into said mold, can be repeated several times for each molding cycle.The term "molding cycle" as used herein means the complete cycle of molding a part, which begins with closing the empty mold, includes injecting the liquid metal to be solidified into the mold, opening the mold, removing the molded part, and typically ends with preparing the internal surfaces of the mold, for example, blowing off with compressed air and spraying with a release agent (potting).

[0016] The cooling finger is made of a metal alloy that resists contact with the molten material. However, its service life is limited, due, on the one hand, to the reactivity of the liquid metal, and, on the other hand, to thermomechanical constraints. Given the risk of the cooling finger being pierced by wear, the tightness of the system is checked after each molding cycle; this is one of the functions of the jet of air. This leak test is called here "pressure test". The other function of the air jet is to expel all traces of water from the tubular body at the end of the molding cycle, before the arrival of the liquid metal in the mold, taking into account the risk of explosion in the event of direct contact between the water and the liquid metal. This operation is called here "purging". Typically, the pressure test is carried out after purging the tubular body.

[0017] This technique cannot be directly transposed to the magnesium foundry, because the risk of explosion of liquid magnesium in contact with traces of water or water vapor is even greater than in the case of the aluminum foundry; moreover, unlike aluminum, solid magnesium is also combustible in the event of fire, which increases the risk. For this reason, until now it has not been considered to use the jet cooling technique in the magnesium foundry.

[0018] The applicant has found that the use of water as a heat transfer fluid is in principle compatible with the use of the jet cooling technique in magnesium foundry, but on condition that any risk of contact between the water and the magnesium, and also between oxygen and the magnesium, is prevented. Thus, it has made two modifications to the jet cooling technique known from aluminium foundry.

[0019] According to an essential aspect of the invention, the pressure test is carried out with a fluid which must be inert with respect to liquid magnesium. This fluid must have a viscosity significantly lower than that of water; a gas is therefore used. This pressure test aims to detect a leak in the cooling finger at a very early stage (for example at a porosity stage where the leak leads to a measurable drop in the pressure of a gas, but would not yet lead to a water leak, even at a water injection pressure higher than that of the gas during the pressure test, taking into account the difference in viscosity between these two fluids).

[0020] Another purpose of this pressure test is, in an extreme case, to cause the rupture of a degraded cooling finger which is still sealed against the pressure of the purge gas or liquid during the water injection. The overall objective being to avoid rupture during the water injection step or during the water vapor purge step, it is preferable to cause the rupture of the conduit during the pressure test, that is to say at a time when this does not cause an explosion, rather than to suffer this rupture at a time when its consequences will be serious.

[0021] More precisely, during the pressure test, a quantity of inert gas is injected into the same conduit as that used to inject the liquid, with a pressure PG, and the presence of a leak in said conduit is checked by detecting a possible drop in pressure after the injection.

[0022] Said inert fluid used for the pressure test can be any gaseous fluid that does not react with liquid magnesium. For this purpose it must be free from traces of moisture and oxygen. It can firstly be a noble gas, preferably argon. SF6 can also be used (which is already used as an inert gas in magnesium foundries to protect the surface of the liquid metal, but which is likely to pose ecological problems). SO2 can also be used (which is also used in magnesium foundries, to replace SF6, but which is likely to pose safety problems for workers); it can be used pure or in a mixture with nitrogen or argon. CO2 or CO (the latter not being preferred because of its toxicity) can also be used, pure or in a mixture with nitrogen or argon. Nitrogen can also be used. Mixtures of these fluids can be used.These mixtures may contain hydrogen, preferably not more than 10% by volume.

[0023] Among all these fluids, argon is preferred.

[0024] The applicant has found that direct contact between the indicated gases (for example argon) and liquid magnesium does not lead to chemical reactions suggesting the risk of dangerous situations; in particular, the purging of the conduit, which first contains liquid water and then water vapor, by the purging gas can be total, so that in the event of piercing the conduit during the pressure test, which aims to render harmless a conduit whose wall has become fragile, the penetration of the gas used for the pressure test into the mold does not create a risk in contact with any remains of liquid or solid metal in the mold or near the mold.It should be noted that the pressure test is always carried out when the mold is empty of all metal, i.e. at the end of the molding cycle after the solidified part has been removed (in other words before the mold is filled with liquid metal during the next molding cycle).

[0025] The purge gas may be air. Alternatively or beforehand, a vacuum may be applied to the cooling circuit comprising the tubular body. The purge gas may be a gas inert to liquid magnesium, for example nitrogen. It may be the same gas as that used for the pressure test, or it may be a different gas.

[0026] According to an advantageous embodiment of the invention, the pressure test is carried out at a pressure PG at least equal to, and preferably greater than, the pressure PP used for the injection of purge gas during the water vapor purge step, and / or at a pressure at least equal to, and preferably greater than, the pressure PL used for the injection of liquid during the cooling step. The condition PG > Pp is more important, because the fluids used for the pressure test (pressure PP) and for the purge (pressure PP) are gases, whereas the liquid injected during the cooling step PP is a liquid whose viscosity is greater than that of gases.

[0027] As an example, PP may be of the order of 2 to 8 bars, and preferably of the order of 2 to 7 bars; an air pressure of 6 bar is typically used, which can be obtained by applying a pressure limiter set at 6 bar to a compressed air distribution network distributing a pressure of 7 bar. If an inert gas is used, the latter will typically be supplied by a mobile tank designed to provide a pressure greater than 6 bar, for example 20 bar. Even in this case, a pressure PP of between 2 and 8 bar (preferably between 2 bar and 7 bar) is suitable: to limit the consumption of inert gas there is no need to go beyond this pressure.

[0028] The pressure PG may also be of the order of 2 to 8 bars, and it is preferably between 4 bar and 7 bar. In an advantageous embodiment, PG > x PP. Preferably, x > 1.05, and preferably x > 1.2, and even more preferably x > 1.3, and most preferably x > 1.4. In another embodiment, which can be combined with the previous one, PG > y PL, where y > 1.05.

[0029] This method allows for casting complex and / or large parts in magnesium and magnesium alloys, while benefiting from the advantages of localized cooling for areas that require particular control of the solidification microstructure. This allows for maximum advantage to be taken of the intrinsic possibilities of a given alloy.

[0030] Said core may be a cooling finger, and in this case advantageously it has a rectilinear shape. If the core is thick, it may be advantageous to provide a helical-shaped conduit which runs along at least part of the wall below its external surface; this allows better thermal homogeneity of the external surface of the core. Such thick cores having a helical-shaped conduit are known as such from the aluminum foundry; they are manufactured in two or more pieces (one of which has an open helical channel machined) and subsequently assembled.

[0031] Said core may be in direct contact with the liquid metal.

Claims

Claims

1. Method for molding magnesium or magnesium alloy parts in a permanent metal mold, in which, successively and cyclically, (i) a quantity of liquid metal, which may be liquid magnesium or a liquid magnesium alloy, is injected into a permanent mold, (ii) said liquid metal is allowed to solidify to obtain a magnesium or magnesium alloy part, (iii) and said part is removed, said method being characterized in that: - said mold comprises at least one zone cooled by a conduit into which a quantity of liquid is injected which evaporates in contact with the hot wall of said conduit; - at least once during step (ii) a quantity of liquid is injected, with a pressure PL, into said conduit, so that said liquid can evaporate at least in part, to locally cool the wall of the mold;- after step (iii) and before step (i) of the following cycle, in a step called "pressure test", a quantity of gas is injected into the same conduit, with a pressure PG, and the presence of a leak in said conduit is checked; knowing that said gas is an inert gas with respect to said liquid metal.;

2. Method according to claim 1, characterized in that said inert gas is preferably selected from the group formed by: a noble gas and preferably argon, nitrogen, carbon dioxide, carbon monoxide, sulfur hexafluoride, sulfur dioxide, a mixture of two or more of these gases, said mixture possibly containing hydrogen.

3. Method according to claim 1 or 2, in which the presence of a leak is checked by detecting the decrease in this pressure when said conduit is closed.

4. A method according to any one of claims 1 to 3, wherein, when a leak is detected in said conduit, the continuation of the method is immediately stopped.

5. Method according to any one of claims 1 to 4, in which, before carrying out the pressure test, in a step called "purging" said conduit is purged with an inert gas injected with a pressure PP, or said conduit is placed under vacuum, in order to rid said conduit of the vapor of said liquid.

6. A method according to any one of claims 1 to 5, wherein PG > x PP, where x > 1.05, and preferably x > 1.2, and even more preferably x > 1.3, and most preferably x > 1.

4.

7. A method according to any one of claims 1 to 6, wherein PG > y PL , where y > 1.

05.

8. Method according to any one of claims 1 to 7, in which said conduit into which said liquid is injected is arranged in a metal core in thermal contact with said mold, in particular in the form of a rectilinear channel or a helical channel.

9. The method of claim 8, wherein said metal core is finger-shaped.

10. A method according to claim 8 or 9, wherein said core is in direct contact with the liquid metal.