EP2158986A1 - Lost-wax pattern casting process and lost-wax pattern for this process - Google Patents

Abstract

The method comprises casting a molten metal in a mold (2), where the metal sublimates or liquefies a lost wax pattern so that the volume occupied by the pattern is gradually replaced by molten metal, and fixing an element within the lost pattern during solidification of the metal to enhance the formation of small grains in molded part. An independent claim is included for a lost wax pattern.

Description

The invention relates to a lost model molding method, a lost model for promoting certain properties of the molded part.

In the metallurgical industry, it is common to evoke the "metallurgical health" of a part, characterizing the greater or lesser presence of defects or more generally of different properties that affect the characteristics of the part, in particular these mechanical characteristics.

For aluminum alloys, one way to evaluate the metallurgical health of a part is to look at grain size. It is recalled that when a metal is examined under a microscope, the surface of the metal appears to be composed of juxtaposed crystalline polyhedra. Each of these polyhedra is commonly called the "grain" of the metal. These grains correspond, at their origin, that is to say each one or more nuclei or germs from which the crystallization of the metal occurred during its solidification. It's grain are also known as "crystallite".

The measurement of the grain size is performed by image analysis under an optical microscope or with a scanning electron microscope. The largest grain width is typically between 1 and 100 microns. The size of the grains and their shape have consequences on the mechanical characteristics of the solid phase metal. For example, coarse grains that is to say grains whose size is close to 100 microns correspond to fragile and brittle metals. Conversely, small grains, that is to say, the largest width is less than 50 microns and preferably less than 10 microns, correspond to more solid metals.

Other defects, for example segregation type, eutectic, shrink, can be formed.

The Applicant Knows Lost Pattern Casting Processes Including Melt Casting of a Molten Metal That Sublimates or Liquefies the Lost Pattern so that the volume occupied by this lost model is gradually replaced by the molten metal.

Such known methods do not always make it possible to have optimum solidification of the molded part. The molded part undergoes slow cooling of the mold and non-directed solidification. Thus, this molded part often has internal structural defects which limit its mechanical characteristics and therefore its functional strength. Here, by mechanical characteristic means in particular the strength of the molded part.

The invention aims to remedy this drawback by proposing a lost pattern molding process by which it is possible to avoid the appearance of these internal structure defects of the molded part.

It therefore relates to a lost model molding process comprising fixing inside the lost model of at least one element which is incorporated inside the molded part during the solidification of the metal.

Rather than incorporating an element directly into the molten metal, it is therefore fixed in the lost model and incorporated in the room not homogeneously but essentially in relation to the areas of the model in which this fixation was made. . In other words, it is possible to modify the metallurgical health of a molded part where it is desired, which enables parts of better quality, different properties which may be desired according to the regions of the molded part, or of equal quality of a molded part. lower cost, the location to reduce the total amount of additives.

In a variant of the invention, the element added to the lost mold is such that it promotes the formation of small grains. Attaching the element promoting the formation of small grains in the lost model can place these elements exactly where you want in the mold. In particular, it is possible to place this element inside the footprint that will be filled by the molten metal. It thus becomes possible to act on the size of the grains which are located inside the molded part and not only on the size of the grains located on the outer periphery of this part. In addition, this element can be positioned very precisely in the model which allows to act only locally on the size grains and no longer on the size of the grains of the whole of the molded part. For example, this makes it possible to make certain zones of a cylinder head of an engine more solid without having to make the whole of this cylinder head more solid.

The invention also relates to a lost model adapted to be implemented in the molding process above. This lost model comprises at least one element promoting a given property during the solidification of the metal, this element being fixed inside the lost model so that the element is permanently incorporated inside the molded part obtained. using this lost model.

• l'élément est uniquement fixé dans une zone du modèle perdu correspondant à une zone de la pièce moulée dont les caractéristiques mécaniques doivent être améliorées ;
• l'élément fixé est un insert réalisé dans un matériau absorbant la chaleur et dont la température de fusion est strictement supérieure à la température du métal en fusion coulé dans le moule, la plus grande largeur de cet insert étant au moins supérieure à 0,5cm ;
• l'insert est réalisé dans un métal dont la capacité thermique massique est supérieure à 350 J.Kg-1.K-1 à 25°C et sous la pression atmosphérique ;
• l'élément fixé est un insert réalisé dans un matériau favorisant l'apparition de grain de petite taille lors de la solidification du métal et dont la température de fusion est inférieure à la température du métal en fusion coulé dans le moule, la plus grande largeur de cet insert étant au moins supérieure à 0,5cm ;
• l'élément fixé dans le modèle perdu est un additif favorisant la formation de grains de petite taille dans le métal lors de sa solidification ;
• l'additif se présente sous la forme de particules sub-micrométriques dispersées, de préférence de façon non homogène, à l'intérieur du modèle perdu.
• l'additif est un borure de titane.

Embodiments of this model may include one or more of the following features:

• the element is only fixed in a zone of the lost model corresponding to a zone of the molded part whose mechanical characteristics must be improved;
• the fixed element is an insert made of a heat absorbing material and whose melting temperature is strictly greater than the temperature of the molten metal cast in the mold, the largest width of this insert being at least greater than 0.5cm ;
• the insert is made of a metal whose specific heat capacity is greater than 350 J.Kg-1.K-1 at 25 ° C and under atmospheric pressure;
• the fixed element is an insert made of a material promoting the appearance of small grain during the solidification of the metal and whose melting temperature is lower than the temperature of the molten metal cast in the mold, the greater width of this insert being at least greater than 0.5 cm;
• the element fixed in the lost model is an additive promoting the formation of small grains in the metal during its solidification;
• the additive is in the form of sub-micrometric particles dispersed, preferably inhomogeneously, within the lost model.
• the additive is a titanium boride.

These embodiments of the model are particularly advantageous because fixing the element only within a zone only of the lost model makes it possible to modify the mechanical characteristics of the molded part only in this zone and not in the whole of the molded part. In addition, fixing inside the lost model an insert allows to increase locally, even within the molded part, the cooling rate of the molten metal and thus to reduce the size of the grains at this location.

Finally, the subject of the invention is the use of an element promoting the formation of small grains during the solidification of the metal in which this element is fixed inside the lost model used in the molding process. above.

• La figure 1 est une illustration schématique d'un moule pour un procédé de moulage à modèle perdu, et
• La figure 2 est un organigramme d'un procédé de moulage à modèle perdu utilisant le moule de la figure 1.

The invention will be better understood on reading the following description given solely by way of nonlimiting example and with reference to the drawings in which:

• The figure 1 is a schematic illustration of a mold for a lost pattern casting process, and
• The figure 2 is a flowchart of a lost model molding process using the mold of the figure 1 .

In these figures, the same references are used to designate the same elements.

In the rest of this description, the features and functions well known to those skilled in the art are not described in detail.

The figure 1 represents a mold 2 for a lost pattern molding process.

As an illustration, this mold 2 is designed for the production of molded parts for motor vehicles. For example, the molded part is a cylinder head. The metal used to cast this part is, for example, an aluminum alloy such as AlSi7Cu3Mg.

The mold 2 comprises a lost model composed here of two parts 6 and 8. The lost model is the reflection of the molded part to obtain. This lost model is made of a sublimable material when in contact with the molten metal. For example, this sublimable material is a pyrolyzable cellular polymer. Here, this material is polystyrene.

The lost model also includes elements promoting the formation of small grains during the solidification of the molten metal. Here, two types of elements are described namely an inoculant and inserts.

More specifically, in this embodiment, submicron particles of an inoculant are uniformly dispersed throughout the volume of the lost model. Here, the particles promote nucleation, that is, the appearance of nuclei or solid seeds in the liquid metal from which the crystals, and more precisely the dendrites, will grow. Generally, the larger the number of nuclei present in the liquid metal, the smaller the grain size of the solid phase metal will be. The largest width of each of these particles is between 10 nanometers and 1000 nanometers. Preferably, the largest width of each of these particles 10 is strictly less than 1 μm or 0.5 μm. These particles are for example based on ferro-silicon strontium.

In addition, inserts 12, 14 and 16 are fixed inside the lost model. The inserts are much larger than the particles 10. For example, the largest width of the inserts is at least greater than 0.5 cm or 1 cm. These inserts promote the formation of small grains by accelerating the cooling of the molten metal with which they are directly in contact. For this purpose, the inserts are made of a material capable of absorbing a large amount of heat and whose melting temperature is strictly greater than that of the molten metal. Here, these inserts are made of a material having a thermal capacity high mass. It is recalled that the specific heat capacity is the amount of energy to be supplied by heat exchange to a unit mass of this material to raise its temperature by one degree. Here, the material chosen has a specific heat capacity greater than 350 and preferably 700 J.Kg-1.K-1 at 25 ° C and under atmospheric pressure. For example, the inserts are made of metal such as aluminum, steel or other.

These inserts are placed at model locations to remove internal structural defects of the molded part. For example, if the molded part, in the absence of added elements in the lost model, has mechanical weaknesses in a particular zone, the insert is fixed in the corresponding zone of the lost model to eliminate this molding defect. Thus, the inserts can be as well fixed in the heart of the lost model as near its outer surface.

For example, the portion 6 comprises, in its center, the insert 12. The portion 8 comprises the two inserts 14 and 16 located near its outer surface.

Each portion 6, 8 is integral with several casting attacks 18 through which the molten metal will be introduced within these parts 6 and 8 of the lost model. The end of each casting stroke 18, opposite to the lost pattern, is connected to a casting chute. One end of the descent 20 is connected to a casting cup 22 made of refractory material. The descent 20 and each attack 18 is initially formed of a sublimable material when it comes into contact with the molten metal. For example, this sublimable material is polystyrene.

The assembly formed by the lost model, the casting attacks 18 and the pouring down 20 is incorporated inside a tank 26 containing vibrated sand 28.

Finally, the lost model, the casting attacks and the pouring down are coated with a layer (not shown) suitable for isolating the molten metal from the sand 28 when it is introduced into the mold 2 This layer is, for example, made of refractory material whose melting temperature is very high. higher than the temperature of the molten metal. Typically, this layer has a thickness of between 1 mm and several millimeters.

The implementation of the mold 2 for making a lost pattern molding is now described in more detail with regard to the figure 2 .

Initially, during a step 40, the lost model is realized. More precisely, during this step 40, the two parts 6 and 8 of this lost model are realized. It is during step 40 that the inoculant particles and the inserts 12, 14 and 16 are fixed inside the polystyrene.

For example, during an operation 42, the particles 10 are incorporated in the polystyrene during its manufacture to obtain a uniform spatial distribution of these particles 10 in the entire volume of polystyrene. Then, during an operation 44, this polystyrene incorporating the particles 10 is carved to obtain the parts 6 and 8 of the lost model.

At the end of the operation 44, during an operation 46, the inserts 12, 14 and 16 are fixed inside the parts 6 and 8 obtained. For example, to fix an insert inside the lost model, the polystyrene is first dug to form a housing adapted to receive the insert. Then, the insert is introduced inside this housing. Finally, eventually, the orifice used to introduce the insert inside the housing is closed with a polystyrene plug.

Once the parts 6 and 8 have been made, during a step 48, these parts, the casting attacks 18 and the pouring down 20 are assembled together to obtain the assembly described with reference to FIG. figure 1 . This assembly is called "cluster".

Then, during a step 50, this cluster is coated with refractory material to obtain the layer to prevent the molten metal from mixing with the sand 28.

In a step 52, the cluster covered with the layer of refractory material is placed inside the tank 26. Then, during a step 54, the sand 28 is added to the tank 26 and vibrated for fill all the interstices of the lost model. Then, in a step 56, the cup 22 is installed. At the end of step 56, the mold 2 as represented on the figure 1 is obtained.

Then, during a step 58, the molten metal is cast inside the mold 2. More specifically, the molten metal is poured into the cup 22 and then flows inside the pouring chute 20 . When the molten metal comes into contact with the polystyrene, the polystyrene turns into gas and the gas is evacuated through the same channels that allowed the arrival of the molten metal.

Gradually, the molten metal fills the entire descent 20 and flows into the casting attacks. Once all the polystyrene present in the attacks 18 and in the descent 20 has been sublimed, the molten metal continues to flow inside the lost model. When the molten metal flows inside parts 6 and 8, it mixes with the particles dispersed within the polystyrene forming these parts 6 and 8.

The presence of the particles in the molten metal within the lost model increases the number of solid phase nuclei from which a crystal can grow. This therefore promotes the appearance of small metal grains in the entire volume of the lost model. In addition, when the molten metal comes into direct contact with one of the inserts 12, 14 or 16, this insert absorbs some of the heat of the molten metal so that the molten metal cools faster in contact with the metal. This insert in other parts of the mold 2. Accelerating the cooling of the molten metal also promotes the formation of small grains in the solid phase of this metal. Thus, small grains will preferably be formed near the inserts 12, 14 and 16.

Once the molten metal has filled the entire volume previously occupied by the lost model, in a step 60, it stops pouring the molten metal inside the mold 2. During a step 62 the molten metal is then allowed to cool so that all of this molten metal passes from the liquid phase to the solid phase. In a step 64, when all the metal has solidified and the casting has cooled sufficiently, the sand 26 and the layer of refractory material covering the lost pattern are eliminated.

Thus, at the end of the process of figure 2 , we obtain a molded piece identical to the lost model but made of metal and not polystyrene.

In addition, the mechanical characteristics of this molded part are modified locally by the presence of the inserts 12, 14 and 16 and uniformly throughout the volume of this molded part thanks to the particles 10.

Many other embodiments are possible. For example, the molten metal may be cast iron, steel, or other material that crystallizes when it solidifies.

The element added to promote the formation of small grains may either be melted inside the molten metal, as for example here the particles 10, or otherwise not melted as in the case of the inserts 12, 14 and 16 .

The inserts may have a lower melting temperature than the molten metal. In this case, the insert is melted in the molded part during molding. The material of this molten insert is chosen to promote the appearance of small grain size. For example, for an aluminum alloy, the insert is made of Titanium-Boron, the TiB2 particles making it possible in a well-known manner to slow down the formation of dendrites, which ultimately promotes the formation of small grains.

For an iron melt, an inoculant can be made from other particles such as, for example, Al-5% Ti particles. The inoculant particles may also have the objective of limiting the development of the dendrites of the metal crystals being formed.

A portion of the inserts may be attached within the model, i.e. several millimeters below the outer surface of the model, while another portion of the same insert may be flush with the outer surface of the model. lost model.

The spatial distribution of the particles of the inoculant may be non-uniform throughout the volume of the lost model. For example, the particles 10 are distributed in a particular area of the lost model while the other areas of the lost model are devoid of or less densely provided with such particles. Preferably, the particles are more specifically concentrated in a zone of the lost model corresponding to an area where the mechanical characteristics of the molded part are to be improved.

The distribution of inoculant particles within the lost model can be achieved by various methods.

Here, the lost model has been described as being made from a sublimable material such as polystyrene. However, many other sublimable materials are possible. Typically, there is a very large number of pyrolysable cellular polymeric materials that may be suitable for such an application. Moreover, the material forming the lost model can also be replaced by a liquefiable material when it comes into contact with the molten metal. For example, what has been described above also applies to lost patterns made in wax.

The method described above is applicable to any type of part made by the lost pattern molding process.

Claims (10)

A lost model molding method comprising casting (58) in a mold a molten metal which sublimes or liquefies a lost pattern so that the volume occupied by this lost pattern is progressively replaced by the molten metal, characterized in that this method comprises fixing (42, 46) inside the lost model of at least one element incorporated inside the molded part during the solidification of the metal. A lost pattern molding method according to claim 1, wherein the member attached within the lost pattern is adapted to promote the formation of small grains in the molded part. A lost model adapted for use in a lost pattern molding process, said molding method comprising casting in a mold (2) a molten metal which sublimates or liquefies the lost pattern so that the volume occupied by the lost model is gradually replaced by the molten metal, characterized in that the lost model comprises at least one element (10, 12, 14, 16) fixed inside the lost model and capable of being incorporated into the inside of the molded part during the solidification of the metal. The model of claim 2 wherein the element (10,12,14,16) is unattached in a localized area of the lost model. A model according to any one of claims 2 to 3, wherein the fixed member is an insert (12, 14, 16) made of a heat-absorbing material and whose melting temperature is strictly greater than the temperature of the metal in question. melting cast in the mold, the largest width of this insert being at least greater than 0.5cm. The model of claim 4, wherein the insert (12, 14, 16) is made of a metal having a specific heat capacity greater than 350 J.Kg ¹ .K ^-1 at 25 ° C and atmospheric pressure. Model according to any one of claims 2 to 5, wherein the fixed element is an insert made of a material promoting the appearance of small grain during the solidification of the metal and whose melting temperature is lower than the temperature of the molten metal cast in the mold, the largest width of this insert being at least greater than 0.5 cm. A model according to any one of claims 2 to 6, wherein the element fixed in the lost model is an additive promoting the formation of small grains in the metal during its solidification. The model of claim 7, wherein the additive is in the form of submicron particles (10) dispersed within the lost model. The model of claim 8 or claim 9, wherein the additive is a titanium boride.

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