Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
  • B22D18/00—Pressure casting; Vacuum casting
  • B22D18/04—Low pressure casting, i.e. making use of pressures up to a few bars to fill the mould
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
  • B22D19/00—Casting in, on, or around objects which form part of the product
  • B22D19/14—Casting in, on, or around objects which form part of the product the objects being filamentary or particulate in form

Definitions

• the invention relates to a method for manufacturing, by pressure foundry, parts made of a magnesium matrix composite material.
• magnesium is to be understood as also including all magnesium alloys.
• composite material with a magnesium matrix designates any material comprising a reinforcing structure, generally formed of long fibers such as carbon fibers, alumina, etc., embedded in a magnesium matrix.
• the volume rate of fibers contained in the material is generally between approximately 40% and approximately 60%.
• the method according to the invention can be advantageously used to manufacture any foundry part which must have both good mechanical characteristics and a reduced mass. It finds in particular a privileged application in the aeronautical and space industries.
• the pressure foundry technique (generally between around 30 bars and around 100 bars) has been known for a few years, for manufacturing parts from a metal matrix composite material. According to this technique, a crucible containing blocks of metal intended to form the matrix of the part is placed in the same hermetic container, comparable to an autoclave, as well as a mold into which a fibrous preform has previously been introduced.
• a vacuum is created inside the container and the mold, the crucible containing the metal blocks is heated and the mold is preheated.
• the metal in the crucible is completely melted, it is transferred inside the mold. This transfer is carried out automatically by pressurizing the container at a pressure level, generally between approximately 30 bars and approximately 100 bars.
• the cooling of the part is accelerated by bringing a refrigerating member into contact with a wall of the mold.
• the pressure is maintained in the container in order to complete the natural shrinkage of the metal.
• the crucible containing the metal blocks is placed above the mold and the latter is provided in the upper part with a receptacle at the bottom of which opens the imprint of the part to be manufactured.
• the metal flows into the receptacle through an orifice formed in the bottom of the crucible and initially closed.
• the molten metal is then transferred into the mold cavity under the effect of the pressurization of the container.
• the cooling of the part is then obtained by means of a cooling piston brought into contact with the bottom of the mold.
• This first technique in which the crucible is placed above the mold, has the advantage of allowing the use of a simple mold and therefore inexpensive to produce. It is therefore relatively economical.
• this technique is difficult to apply to the production of composite parts with magnesium matrices, despite the advantage that such parts would present in certain industries, such as the aeronautical and space industries.
• the preliminary transfer of the molten metal into the receptacle formed at the upper part of the mold is carried out under vacuum and without any special precautions.
• the magnesium may then evaporate and come to settle in the entire installation, which would make part of it unusable. .
• no precaution is taken in order to avoid an explosive reaction of magnesium with oxygen, in particular during the pressurization of the enclosure.
• the crucible containing the metal blocks is placed below the mold and the bottom of the latter is equipped with a feed tube which initially opens above the crucible. Vacuuming is carried out by a vacuum tube which opens directly into the mold. When the metal is melted, the crucible is lifted so that the mold feed tube plunges into the molten metal. The transfer of the molten metal inside the mold is then obtained by pressurizing the container. The cooling of the part is ensured by a cooling block which is brought into contact with the upper face of the mold.
• This technique in which the crucible is placed below the mold, is more expensive than the previous one because the mold must include a supply tube. On the other hand, it eliminates an intermediate step of transfer of the molten metal.
• this technique is also unsuitable for the production of composite parts with a magnesium matrix. Indeed, the melting of the metal takes place entirely under vacuum as in the previous technique, so that vacuum evaporation of magnesium is practically inevitable. In addition, no special precautions are provided to avoid explosive contact with oxygen.
• the subject of the invention is precisely a method of manufacturing a composite part with a magnesium matrix generally implementing the known techniques of pressure casting, but whose original characteristics make it possible to eliminate any risk of an explosive magnesium reaction. / oxygen, while avoiding evaporation under vacuum of magnesium.
• this result is obtained by means of a process for manufacturing a fiber-reinforced magnesium part, characterized in that it comprises the following steps:
• the neutral gas circulation is established under a vacuum of approximately 100 mb.
• the heating of magnesium is accompanied by an initial pressurization of the container and of the mold to approximately 0.1 mb.
• the neutral gas circulation preceding the pressurization of the container is ensured until the magnesium reaches a maximum temperature, for example around 700 ° C.
• the neutral gas used is argon.
• the container and the mold are evacuated through at least one passage which opens directly into the container.
• the solid magnesium is brought into contact with the feed tube by moving the crucible upwards as soon as the temperature of the magnesium reaches a threshold lower than its melting temperature.
• the mold is cooled by establishing contact between an upper wall thereof and a cooling block placed at the top of the container.
• FIGS 1A to 1D are schematic sectional views which illustrate the main steps of the method according to the invention.
• FIG. 2 shows respectively in I, II, III and IV, the variation curves, as a function of time t, of the average temperature ⁇ (in ° C) of the metal, of the pressure P (in bars) prevailing in the container, the position of the lower cylinder and the position of the upper cylinder.
• the installation used to manufacture a fiber-reinforced magnesium composite part, by foundry under pressure has many similarities with the installations usually used for the manufacture of composite parts with metal matrix. Therefore, no detailed description will be given.
• an airtight container 10 similar to an autoclave.
• This container 10 is a tubular container centered on a vertical axis. It is closed at its upper part by a cover 12, the opening of which gives access to the volume 14 delimited inside the container. When the cover 12 is closed, it cooperates sealingly with the upper edge of the container 10, so as to hermetically seal the volume 14.
• the container 10 and its lid 14 are designed to withstand a maximum pressure of about 100 bar in the volume 14.
• the container 10 is equipped Internal ⁇ quently first means heating 16 placed in the lower part. of the container and second heating means 18 placed in the upper part of the container.
• These heating means 16 and 18 can be constituted by any suitable devices such as electrical resistors. Their implementation is controlled and regulated from the outside of reci ⁇ tainer 10 by a control unit (not shown).
• Thermocouples (not shown) are also arranged inside the container 10, to allow the regulation of the heating provided by the heating means 16 and 18.
• a heat insulator (not shown path) internally covers all the walls of the container 10 in order to provide thermal insulation of the volume 14 from the outside.
• the container 10 is also equipped with several access passages, only one of which has been shown diagrammatically at 22 in FIGS. 1A to 1D. In practice, several passages are generally arranged in the bottom of the container 10 and in the cover 12. As will appear better in the following description, their main functions are to connect the closed volume 14 delimited by the container 10 either to a vacuum circuit (not shown) is at a source (not shown) of a pressurized neutral gas such as argon.
• the bottom of the container 10 is internally equipped with a base (not shown) on which a crucible 26 can rest, which initially contains solid magnesium blocks 28. This crucible 26 is placed inside the first heating means 16.
• the container 10 is provided with at least one support 30 on. which can be placed a mold 32.
• the mold 32 has internally one or more imprints whose shapes and dimensions are identical to those of the part (s) to be produced.
• Each impression is filled with a fibrous preform 34 before the mold is introduced into the container 10.
• the fibrous preforms are generally formed of long fibers of carbon, alumina, or other intended to form the reinforcements of the part to be produced.
• the volume of fiber in the fiber preform 34 is generally between about 40% and about 60% of the total volume of the impression.
• the imprint (s) it delimits communicate with the internal volume 14 of the container only by a single passage, materialized by a supply tube 36. More precisely, the tube feed 36 opens into the bottom of the mold 32 and extends downward, preferably along the vertical axis of the container 10. The lower end of the feed tube 36 initially opens at a level close to that of the upper edge of the crucible 26, as illustrated in FIG. 1A.
• a lower cylinder 38 initially in the low position as illustrated in FIG. 1A, is placed under the bottom of the container 10 so that its rod 38a passes through this bottom in a sealed manner, along the vertical axis of the container. In the initial lower position of the lower cylinder 38, the upper end of its rod 38a occupies a position such that the crucible 26 is not lifted from its base.
• An upper cylinder 40 initially in the high position, is also mounted above the cover 12 of the container 10.
• the rod 40a of this cylinder 40 which passes through the cover 12 in a sealed manner along the vertical axis of the container 10, its lower end a cooler block 42.
• this cooler block 42 In the initial high position of the jack 40, this cooler block 42 is spaced from the upper face of the mold 32.
• Access passages comparable to passage 22 illustrated in FIGS. 1A to 1D can pass axially through jacks 38 and 40 to open into the volume 14.
• a passage 23 passing through the upper cylinder 40 is shown in FIGS. 1A to 1D.
• FIG. 1A illustrates the initial state of the installation, in which magnesium blocks 28 in the solid state have been placed in the crucible 26, the mold 32 containing the fibrous preform 34 has been introduced into the container 10 and the cover 12 has been put in place.
• the lower cylinder 38 is in the low position and the upper cylinder 40 in the high position.
• the magnesium 28 contained in the crucible is then heated simultaneously and progressively and the interior volume 14 of the container 10 is evacuated.
• the heating of magnesium 28 is provided by the first heating means 16 and is accompanied by the preheating of the mold 32 using the second heating means 18.
• the preheating of the mold 32 aims to prevent the molten metal from solidifies too quickly when it is then transferred to the mold.
• the preheating temperature of the mold is therefore relatively close to the heating temperature of magnesium 28 (to within a few tens of degrees).
• the evacuation of the interior volume 14 of the container 10 is ensured by one or more of the access passages which equip the container 10. It is illustrated diagrammatically by the arrow FI in FIG. 1A, opposite the passage 22.
• the other access passage (s) to the container 10 are then closed by valves (not shown).
• the vacuum level in the container 10 is stabilized as soon as the pressure reaches a level of about 0.1 mb corresponding to a primary vacuum. This vacuum level is reached well before the beginning of the melting of the magnesium blocks 28 in the crucible 26, which occurs at a temperature of about 600 ° C (curve I). This temperature level is reached after a time which depends in particular on the quantity of magnesium initially placed in the crucible.
• the evacuation of the interior volume 14 of the container 10 is accompanied by an evacuation of the imprint (s) formed in the mold 32, due to the fact that these communicate with the volume 14 by the tube supply 36.
• the first step of the process which has just been described with reference to FIG. 1A, is followed by a step which makes it possible to avoid the immediate evaporation of part of the magnesium during its melting , while eliminating any risk of an explosive reaction between magnesium and oxygen and while maintaining a primary vacuum inside the mold 32 ⁇
• these three objectives are achieved by the fact that a circulation of a neutral gas such as argon is established inside the container 10, under a vacuum level insufficient to cause evaporation of the magnesium, as soon as its temperature reaches a value close to its melting temperature.
• a neutral gas such as argon
• the beginning of the melting of the magnesium 28 contained in the crucible 26 is detected and the conditions prevailing in the container 10 are immediately modified, on the one hand, by introducing the lower end of the feed tube 36 into the magnesium melted during melting and, on the other hand, by establishing in volume 14 a circulation of argon under a vacuum level of approximately 100 mb.
• the plunger of the feed tube 36 in the magnesium being melted is obtained by actuating the lower cylinder 38 so as to raise the crucible 26 as illustrated in FIG. 1B. This eliminates any communication between the interior volume 14 of the container 10 and the imprint (s) formed in the mold 32. The interior of the latter therefore remains under primary vacuum.
• the circulation of argon is established by injecting argon into the interior volume 14 of the container 10, through one of the access passages, as illustrated by the arrow F2 (opposite the passage 23 formed in the upper cylinder 40) in FIG. 1B, while maintaining in this volume 14 a level of vacuum controlled by at least one other access passage, as illustrated by arrow F3 (opposite passage 22).
• a neutral gas sweep is thus carried out in the container 10, which avoids any risk of oxygen returning to this container.
• the vacuum inside the container is insufficient for the molten magnesium to evaporate.
• the sudden rise in pressure to around 100 mb and the maintenance of the vacuum at this value are illustrated by part IIb of curve II in FIG. 2.
• the start of melting of magnesium which triggers the step illustrated on FIG.
• the heating of the magnesium 28 continues until its complete melting in the crucible 26.
• its temperature is raised to a predetermined value, higher by example of around 100 ° C at its melting temperature.
• the circulation of argon under a vacuum level of approximately 100 mb is maintained.
• the time required to obtain this predetermined temperature for example around 700 ° C., varies as appropriate between around 30 minutes and around 60 minutes.
• the molten magnesium 28 is transferred from the crucible 26 to the mold 32 by the supply tube 36.
• This transfer is carried out by pressurizing the volume inside 14 of container 10, always under an atmosphere of a neutral gas such as argon. Simultaneously, all the heating means 16 and 18 of the container 10 are stopped.
• volume 14 is obtained by interrupting all communication between this volume and the vacuum circuit and by connecting it to a pressurized argon circuit, as illustrated by arrow F4 (opposite access passage 23) on Figure 1C.
• the pressure is thus raised rapidly, for example from about 1 bar / s, to a pressure level generally between about 30 bars and about 100 bars.
• the rise in pressure to a value of approximately 100 bars is illustrated by the part Ile of curve II in FIG. 2. It takes place, for example, in approximately 1 minute.
• the speed of pressure build-up in the internal volume 14 of the container 10 can vary according to the nature and the arrangement of the fibers forming the preform 34. In fact, this speed must be as high as possible in order to ensure a efficient filling of the fiber preform, without exceeding a threshold beyond which the fibers forming this preform may be displaced or damaged.
• the upper cylinder 40 is actuated to accelerate the cooling of the part, as soon as the pressure in the container 10 has reached the predetermined maximum threshold (100 bars in the example represented) .
• the cooling block 42 then comes into contact with the upper face of the mold 32 (FIG. 1D), so that the magnesium begins to solidify from the top of the mold.
• the cooling effect can be obtained by a cooling circuit (not shown) housed in the cooling block 42 as well as by a circulation of a neutral cooling gas, such as argon, injected through the access passage 23 ' which passes through the upper cylinder 40.
• This refrigerant gas then circulates between the refrigerating block 42 and the upper face of the mold 32 in grooves formed radially on the lower face of the cooling block.
• the cooling of the magnesium in the mold 32 is illustrated by the part 1c of the curve I in FIG. 2.
• the pressure of approximately 100 bars is maintained until the solidification of the magnesium in the mold 32.
• the pressure in the container 10 then gradually decreases, while the cooling of the room continues.
• the jacks 38 and 40 are brought back to their initial positions and the cover 12 of the container 10 is opened to allow the mold 32 to be extracted therefrom.
• the demolding of the manufactured part or parts is then carried out .
• the process which has just been described can undergo certain modifications without departing from the scope of the invention.
• the upper cylinder 40 can be omitted.
• the cooling of the part is obtained by using a lower cylinder 38 having a greater stroke.
• the jack 38 is again actuated to raise the crucible 26 beyond the position illustrated in FIGS. 1B and 1C.
• the crucible 26 then comes to bear against the bottom of the mold 32 and lifts the latter until its upper face comes into contact with the cooler block 42, which is then mounted directly under the cover 12.

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• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• Manufacture Of Alloys Or Alloy Compounds (AREA)

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• 1997
  • 1997-03-24 FR FR9703551A patent/FR2760984B1/fr not_active Expired - Fee Related
• 1998
  • 1998-03-23 JP JP10545131A patent/JP2000511826A/ja active Pending
  • 1998-03-23 WO PCT/FR1998/000579 patent/WO1998042463A1/fr not_active Application Discontinuation
  • 1998-03-23 US US09/147,298 patent/US6125914A/en not_active Expired - Fee Related
  • 1998-03-23 EP EP98917209A patent/EP0914221A1/fr not_active Withdrawn
  • 1998-03-23 CA CA002257081A patent/CA2257081A1/en not_active Abandoned

Non-Patent Citations (1)

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