EP1380369B1 - Method for casting using a casting core, method for producing the core and core - Google Patents

Abstract

Casting of metal component in mold enclosing at least one casting core consists of casting molten metal into the mold, removing the component after cooling and eliminating the core from the cast component. Core is produced in a material made up of a mixture of at least two water soluble salts corresponding to eutectic or close to eutectic composition and core is eliminated by dissolving in water. Independent claims are also included for the following: (a) a method for the production of the casting core; and (b) the casting core made from at least two water soluble salts.

Description

The invention relates to a method of molding a metal part in a mold which encloses at least one molding core, a method of producing the core and the molding core obtained by this method.

In some industries, for example in the automotive industry, foundry parts are made of metals such as aluminum and its alloys, which may have complex shapes, both as regards the external surface of the workpiece. internal surfaces delimiting cavities inside the room.

Such parts are made in molds whose internal imprint makes it possible to obtain the external shape of the part and in which one or more cores are placed in order to obtain the desired shape of the inner surface of the molded part which may comprise cavities of complex shape.

Usually, the cores are obtained industrially by injecting, using air under pressure, a mixture of sand grains coated with a resin, in cores boxes having shapes adapted to obtain the core required for molding. The resin is then cured by chemical reaction to provide mechanical cohesion between the core sand grains.

Generally, the cores comprise, by weight, a mixture of 98% to 99% of siliceous sand grains to which is added 1% to 2% of organic resin, for example the chemical family of formophenolic substances. In terms of volume, the cores comprise 45 to 65% of sand grains and the balance, ie 55 to 35%, consists of air filling the porosities of the core. This inclusion of air between the grains of sand in the porous cores is permeable to the pyrolysis gases and alloys which are cast. During casting, the alloy heats the core and the resin, so that the resin thermally and mechanically degrades and undergoes pyrolysis which generates unwanted gases and residues.

The various phenomena mentioned above related to the porosity and the pyrolysis of the resin are at the origin of difficulties and technical limits which will be indicated below.

Firstly, the cores can undergo deformations and breaks at ambient temperature, during transport, storage or during casting in a mold.

On the other hand, the elastic limit at 20 ° C of the sand cores obtained in the foundry is generally between 0.4 MPa and 0.8 MPa and the breaking stress of these cores is between 2 MPa and 2.5 MPa. Because of these relatively low values, the size of the cores must be less than 200 mm to reduce the bending they undergo during handling.

The cores are also subjected to hot deformation and breakage during casting, since their breaking stress between 200 ° C. and 400 ° C. is generally only between 1.5 MPa and 3.5. MPa, the corresponding elastic limit varying from 0.2 MPa to 1.4 MPa. Given these weak mechanical properties, the thickness of the cores must be greater than 8 mm, in order to obtain sufficient stiffness when hot during casting.

In addition, during casting, the gassing due to the thermal degradation of the resin produced, within the room, gas inclusions that are not desired. The evolution of gas can be bulky, this gas evolution generally being 5 cm ³ / g of the core at 700 ° C.

There are also inclusions of carbon in the part which are related to the pyrolysis of the resin. Indeed, the organic type resin generates significant amounts of carbon residues during its pyrolysis. Thus, at 700 ° C., the resin emits carbonaceous residues with a mass greater than 15% of the mass of the resin of the cores.

On the other hand, during casting, the core is impregnated by the liquid metal cast in the mold, the overpressure is generally greater than 0.15 bar. This phenomenon is due to the high permeability of the nucleus which is between 120 and 200 Georg Fisher units.

In addition, the roughness of the surface of the molded parts obtained can be strongly marked, because it is highly dependent on the particle size of the sand grains. It has been observed that the measurement of the roughness average (Ra) varies from 150 μm for a sand-blasted particle of 55 AFS, to a minimum value of 20 μm for a sand having a minimum particle size of 90 AFS.

Finally, gaseous releases of volatile organic compounds or pyrolyzed from resins at room temperature or hot during casting cause air pollution molding workshops that can be dangerous or unpleasant for the staff of these workshops, because of odors and toxicity of certain compounds.

Because of these drawbacks and technical limitations, the molding cores according to the known technique can not be used in certain cases.

In particular, it is not possible to use them in the context of foundry processes in which mold overpressures above 0.15 bar are used, for example as in the case of "squeeze casting", the casting with depression of at least part of the mold space or thixomolding.

The current manufacturing technique of resin-bonded sand cores can not be applied either to the production of very fine cores having a thickness of less than 8 mm and / or to cores having a great length, for example greater than 200 mm. A fortiori, the current technique can not be used for the realization of nuclei both very thin and long, regardless of the casting processes used.

The cores of the known technique can not also be used to obtain parts having a very smooth skin, whose roughness is less than 15 microns.

Finally, in general, it is desirable to reduce aerial pollution of molding plants.

It is also known, for example by US 4840219 cores for molding metals, especially aluminum, consisting of a mixture of 50% to 90% of water-soluble salts and 10% to 50% of refractory material not reacting with the soluble salt; the composition of the soluble salt mixture is selected to provide a melting point of the nuclei greater than a fixed limit, for example 1225 ° F (or 663 ° C).

We also know by FR 2,077,555 foundry cores of water-soluble salts comprising magnesium chloride.

The object of the invention is therefore to propose a method of molding a metal part in a mold containing at least one molding core into which a liquid metal is cast into the mold containing the at least one molding core, demolding the piece after cooling and eliminating the core of the molded part, this method both to improve the quality of the parts obtained, in particular their surface condition, to use mold cores of dimensions not achievable until here and having improved mechanical characteristics and to limit the air pollution in the molding workshops, the cores can be manufactured by a process easy to implement, at a moderate temperature ..

For this purpose, the at least one molding core is made of a material consisting of a mixture of at least two water-soluble salts corresponding to a eutectic or eutectic-like composition, and the core is removed, after molding the piece, by dissolution in water.

The invention also relates to methods for producing water-soluble salt molding cores and the resulting molding cores.

In order to better understand the invention, the manufacture of soluble salt molding cores according to several embodiments and the use of soluble salt cores for the manufacture of parts for the production of soluble salt cores will now be described by way of example. automotive industry, such as cylinder heads and engine blocks.

In general, the production of water-soluble salt foundry cores for carrying out the invention can be carried out by one of the following three techniques: hot sintering, casting of the core from a liquid in a steel or refractory mold, injecting a liquid into a steel mold.

The hot sintering technique in a steel mold can be used for single-form cores. Indeed, the cores produced by sintering must be easily compressible and easily removable hot.

The casting of the cores from a liquid in a mold made of steel or of compact refractory material or of a porous refractory material that can be destroyed after casting can be used for cores of very complex shape that are difficult to demold.

The liquid injection of the core material into a steel mold can be used for medium complex cores. This injection technique makes it possible to obtain a very high production rate.

Example of manufacturing a soluble salt core.

In order to carry out the pouring or the liquid injection of the core material, different mixtures of water-soluble salts have been made, in order to optimize the material's flow characteristics, the physical and thermomechanical properties of the cores, to minimize the defects present in the parts after casting around the cores and to solve as well as possible the problems relating to the respect of the environment, in the management of the workshop of molding of parts.

The tests carried out have shown that a particularly satisfactory material for the production of soluble salt cores consists of a mixture of two salts, namely sodium chloride NaCl in a proportion of 43% to 47% by weight and the carbonate of Na ₂ CO ₃ , in a proportion of 57% to 53% by weight.

This mixture of salts has made it possible to lower the melting point of the mixture relative to that of the two salts by forming a eutectic compound.

Thus, the melting point of sodium chloride is 805 ° C, that of sodium carbonate 850 ° C while a mixture of sodium chloride and sodium carbonate containing 45% by weight of sodium chloride and % by weight of sodium carbonate has a melting point of 636 ° C. For the mass proportions given above, a eutectic mixture is produced whose melting temperature is substantially lower than that of the constituents.

It has also been possible to observe a significant lowering of the melting temperature of the mixture, in the whole range of the compositions that are close together. of the eutectic surrounding the composition at 45% sodium chloride and 55% sodium carbonate.

Due to the lowering of the melting temperature, the use of a mixture of salts as indicated makes it possible to obtain operating temperatures of the process for producing the cores which provide substantial savings and a greater degree of ease of industrial implementation, whether the core is made by casting, sintering or injection. Steel molds of the usual type Z35CDV05, called core boxes, can be used.

The salt mixture is introduced into the core box, either in liquid form by casting or by injection, or in the form of particles which are sintered under a pressure of 50 to 1000 bar and at a temperature below the melting temperature of 50.degree. Eutectic or its constituents, in the kernel box.

After molding or sintering and cooling, the cores are extracted from the core box and cooled to room temperature.

The rheological properties and in particular the flowability of the mixture of soluble salts at 45% by weight of NaCl and 55% by weight of Na ₂ CO ₃ are very satisfactory when the cores are made by casting or by injection of the material containing the soluble salts. , in the liquid state in the core box. Indeed, the mixture of salts in the liquid state is very fluid, its viscosity being low, from 0.01 Pa.s to 0.03 Pa.s at 700 ° C., which is generally the casting temperature greater than the temperature. fusion of the eutectic.

In addition, the material of the core consisting of the mixture of salts in the liquid state is non-wetting and therefore does not tend to adhere or cling to the wall of the steel mold (or silica) in which one mold the nuclei. Indeed, the surface tension of the salt mixture at 700 ° C, which is the casting temperature, is sufficiently low and between 0.04 and 0.07 N / m.

In addition, the solidification rate is satisfactory in the case of the core manufacturing process, the duration of solidification of the liquid mixture being from 0.5 to 2 minutes. This solidification time is sufficient to allow the filling of the kernel box without the occurrence of solidification before the end of filling. In addition, this duration makes it possible to carry out sufficiently rapid manufacturing cycles.

The melting or solidification energies of the mixture are between 330 J / g and 350 J / g.

The thermomechanical properties of the cores consisting of 45% sodium chloride and 55% sodium carbonate are quite satisfactory.

Indeed, the cores obtained are very mechanically resistant, the breaking stress of the material of the cores being 35 to 40 MPa; this value is approximately 50 times greater than that obtained in the case of conventional cores made of sand agglomerated with resin.

These cores are very rigid, the crush value at break being only 2 to 3%.

These nuclei are very compact, their permeability being less than 5 Georg Fisher units, that is to say about 20 times lower than that of a conventional sand core. The cores obtained are sufficiently refractory, since the melting point of the mixture of salts is 636 ° C .; this temperature is generally sufficient to allow the casting of metals usually used in the manufacture of parts, for example for the automotive industry, such as alloys of aluminum, magnesium or zinc.

The cores obtained are overmouldable directly in a conventional stone box filled with sand, because they are very mechanically resistant.

Finally, the cores obtained have a smooth surface, so that during the molding of a workpiece, they generate only a small skin roughness of the workpiece, this roughness being generally less than 15 microns.

In addition, the soluble salt cores greatly reduce gas inclusion defects and carbon residues that are present in molded parts using conventional sand and resin cores. Indeed, these soluble salt cores do not generate hot pyrolysis products. In particular, the salt cores do not create gas inclusions by gassing at the casting and carbon residues during the casting. The volume of gas evolved is less than 2 cm ³ / g at 700 ° C. and the carbonaceous residues are in a proportion of less than 0.1% of the mass of the cores, during a casting at 700 ° C.

After demolding and cooling the cores, they are placed inside a mold, for example a sand mold for casting parts such as cylinder heads or a steel mold. The liquid metal of the part is poured into the mold containing the cores, for example aluminum or an aluminum alloy at a temperature of the order of 700 ° C.

The part is cooled inside the mold and the solidified and cooled part is demolded.

It remains to eliminate the nuclei present in the room.

This removal of the nuclei is carried out by dissolving in water, either water at room temperature or hot water, for example at a temperature of 20 to 70 ° C.

The soluble salt cores are brought into contact with water, either by immersing the pieces in a container containing water, or by circulating water inside the molded part to dissolve the nuclei into salts. soluble.

The soluble salts constituting the nuclei have a good solubility in water, this solubility being from 35 to 45 g / l in cold water or in hot water.

The brine obtained by dissolving the salts in water is not toxic because the salts are not toxic themselves.

The recycling of the salts extracted from the brine is an operation which can easily be carried out, for example by boiling or evaporation of the water.

The operation of removing the cores is carried out without causing air pollution of the molding plants by volatile organic compounds, as in the case of sand and resin cores which are removed by heat treatment.

The salt cores used in the context of the invention are sufficiently resistant to be used in foundry processes using pressure injection of alloys into the casting mold, thixomolding, die casting, squeeze casting, and other molding processes using injection of a metal such as aluminum, magnesium, zinc or other metals and alloys in a mold.

The cores according to the invention can also be made in the form of fine and long cores for use in gravity casting processes.

In particular, it is possible to make the cores in the form of very fine cores with a thickness of less than 8 mm, long cores longer than 200 mm or even very fine and long cores. In particular, the cores used in the implementation of the invention can make it possible to make cavities having a small dimension such as inter-cylinder water passages in V6 engines or recesses between the valves, in the cylinder heads of the cylinders. HDI type motors.

The mixture of soluble salts for the constitution of the cores may comprise more than two salts; in this case also, at least two of the salts may be in proportions forming a eutectic composition.

The invention can also be used in different fields of motor vehicle engine parts production.

The invention may have applications in many fields of industry, for the production of parts by molding in a mold enclosing at least one molding core.

Claims (6)

1. Method for casting a metallic piece in a mould containing at least one casting core constituted by soluble salts, in which liquid metal is poured in the mould containing the at least one casting core, the piece is stripped after cooling and the core of the cast piece is eliminated, characterized in that the at least one casting core is realized by mixing by mass from 43% to 47% sodium chloride NaCl and from 57% to 53% sodium carbonate Na₂CO₃, and that the core is eliminated, after casting of the piece, by dissolving in water.
2. Method according to Claim 1, characterized in that the mixture of sodium chloride and of sodium carbonate contains, by mass, 45% sodium chloride and 55% sodium carbonate.
3. Method according to Claim 1 or Claim 2, characterized in that the at least one casting core is realized by one of the following methods: casting or injection in a mould of the material containing the at least two soluble salts in molten state, calcination of particles of the material containing at least one water-soluble salt, in a mould constituting a core box.
4. Casting core for the casting of a metallic piece in a mould, characterized in that it is constituted by a material constituted by a mixture of 43% to 47% by mass of sodium chloride NaCl and 57% to 53%, by mass, of sodium carbonate Na₂CO₃.
5. Core according to Claim 4, characterized in that it has a thickness less than 8 mm and/or a length greater than 200 mm.
6. Core according to Claim 4 or Claim 5, for the realization of cavities, in cylinder heads of automobile engines, realized by casting.