EP0461052B1 - Lost foam low-pressure casting of aluminium-alloy pieces - Google Patents

Abstract

The invention relates to lost-foam casting of metal pieces, particularly from aluminium alloy. <??>It relates to aluminium-alloy pieces having a wide solidification range (gap) and having a high ratio of length to modulus over the path between the point where the contraction cavity forms and the riser (feeder) and consists in applying a gaseous pressure to the mould of between 0.1 and 0.5 MPa. <??>It permits the production, in particular, of motor-vehicle cylinder heads having no pinholes (pitting) or porous (spongy) contraction cavity, and of all pieces with high-performance mechanical properties. <IMAGE>

Description

The invention relates to a process for molding lost foam and under low pressure of metal parts made of aluminum alloy and constitutes an improvement of the process as described in French patent No. 2606688 filed on November 17, 1986.

It is known to those skilled in the art, for example from the teaching of USP No. 3,157,924, to use for molding metals foam models of organic material such as polystyrene as immersed in a mold consisting of dry sand containing no binding agent. Industrially, these models are generally coated with a film of refractory material intended to improve the quality of the molded parts. In such a process, the metal to be molded, which has been previously melted, is brought into contact with the model via a feed orifice and channels passing through the sand, and gradually replaces said model by burning it and by transforming it mainly into vapors which escape between the grains of sand, hence the designation of lost foam molding process.

Compared to conventional molding in a non-permanent mold, this technique avoids the prior manufacture, by compacting and agglomeration of powdery refractory materials, of rigid molds associated in a more or less complicated way with cores and allows easy recovery of the molded parts as well as easy recycling of molding materials.

• la relative lenteur de la solidification qui favorise la formation dans les pièces moulées obtenues de piqûres de gazage provenant de l'hydrogène dissous dans l'alliage d'aluminium liquide
• la relative faiblesse des gradients thermiques qui favorise la formation de microretassures malgré la présence de masselottes.

It is therefore simpler and more economical than the conventional technique. In addition, it offers designers of molded parts greater freedom with regard to the shape of said parts. This is why this technique is proving more and more attractive from an industrial point of view. However, it is handicapped by several drawbacks, two of which result from conventional metallurgical mechanisms, namely:

• the relative slowness of solidification which promotes the formation in the molded parts obtained of gassing pits from the hydrogen dissolved in the liquid aluminum alloy
• the relative weakness of the thermal gradients which favors the formation of microretassures despite the presence of weights.

It is with the aim of avoiding such drawbacks that the applicant has proposed in French patent No. 2606688 an invention consisting in applying to the mold after filling and before the solidified fraction of the metal exceeds 40% by weight, a pressure isostatic gas of maximum value between 0.5 and 1.5 MPa.

• mettre en oeuvre un modèle de la pièce à mouler formé d'une mousse en matière organique revêtue d'un film de matériau réfractaire;
• immerger ledit modèle dans un moule formé de sable sec sans liant;
• remplir le moule avec le métal à l'état fondu pour brûler ledit modèle, ce remplissage s'effectuant par l'intermédiaire d'un orifice d'alimentation mettant en relation le modèle avec l'extérieur du moule;
• évacuer les vapeurs et résidus liquides émis par ledit modèle pendant sa combustion;
• permettre au métal fondu de se solidifier pour obtenir la pièce;

procédé que la demanderesse a amélioré en ce sens que lorsque le moule est rempli complètement, c'est-à-dire quand le métal s'est substitué entièrement au modèle et que la majeure partie des vapeurs a été évacuée, elle applique une pression gazeuse sur le moule; cette opération pouvant être réalisée en plaçant le moule dans une enceinte apte à tenir à la pression et reliée à une source de gaz sous pression.Thus, the method according to this request comprises the conventional stages of lost foam molding, namely:

• implementing a model of the part to be molded formed from an organic material foam coated with a film of refractory material;
• immerse said model in a mold formed of dry sand without binder;
• filling the mold with the metal in the molten state to burn said model, this filling being effected by means of a supply orifice connecting the model with the outside of the mold;
• evacuate the vapors and liquid residues emitted by said model during its combustion;
• allow the molten metal to solidify to obtain the part;

process which the Applicant has improved in the sense that when the mold is completely filled, that is to say when the metal has entirely replaced the model and the major part of the vapors has been removed, it applies a gas pressure on the mold; this operation can be carried out by placing the mold in an enclosure capable of withstanding the pressure and connected to a source of pressurized gas.

This operation can be done immediately after filling while the metal is still completely liquid, but it can still take place later as long as the fraction of solid dendrites formed during solidification in the mold does not exceed 40%, value beyond which the pressure has only a negligible effect.

Preferably, the value of the pressure applied in this request is at most between 0.5 and 1.5 MPa.

• compaction des piqûres de gazage dont le volume est pratiquement réduit dans le rapport des pressions absolues régnant au cours de la solidification. Ainsi, par exemple, quand on applique une pression absolue de 1,1 MPa alors qu'antérieurement cette pression était celle de l'atmosphère, à savoir 0,1 MPa, cette réduction a lieu dans un rapport voisin de 11;
• meilleure alimentation du moule du fait que la pression, s'exerçant sur les masselottes encore liquides, force ledit liquide à travers le réseau de dendrites qui se forme au début de la solidification d'où une quasi suppression des microretassures.

Under these conditions, it can be seen that the health of the parts obtained is improved and this phenomenon is explained by the following mechanisms:

• compaction of gassing pits, the volume of which is practically reduced in the ratio of absolute pressures prevailing during solidification. Thus, for example, when an absolute pressure of 1.1 MPa is applied while previously this pressure was that of the atmosphere, namely 0.1 MPa, this reduction takes place in a ratio close to 11;
• better feeding of the mold because the pressure, exerted on the still liquid weights, forces said liquid through the network of dendrites which is formed at the start of solidification, hence a virtual elimination of micro-shrinkage.

However, it has been observed in certain cases that the application of a relative pressure greater than 0.5 MPa leads to the appearance of particular defects called "spongy shrinkage" which is explained as follows: if the alloy molded has a relatively large solidification interval, a pasty zone develops within the part; moreover, if the distance between the counterweight and the place where the shrinkage occurs is large relative to the thickness of the part, the pasty zone creates a significant pressure drop on the metal supply to the mold so that the flyweight even under the effect of the external pressure does not manage to play its role, that is to say, to supply sufficiently quickly the recesses which form.

As the solidification interval is relatively large, the "skin" of the part (part located at the metal-sand interface) is fragile much longer and the external pressure exerted by the application of the gas on the sand then pushes this skin towards the inside of the part, allowing a fraction of gas to infiltrate between the dendrites towards the recesses and thereby creating a so-called "spongy" recession which is as harmful as conventional recession to obtaining good mechanical properties.

Therefore, when it is desired to mold parts of an aluminum alloy having a relatively large solidification interval, and when the geometry of said parts leads to having a relatively large distance between the counterweight and the recess area called the critical zone with respect to their thickness, we have sought to avoid these phenomena by eliminating the application of pressure for example.
However, it would be a shame to deprive yourself of this pressure molding technique which also leads to a considerable improvement in the quality of the molded parts.

This is why the applicant has tried to solve this problem and has found that the application of a relative pressure of less than 0.5 MPa makes it possible to eliminate spongy settling while leading to good compaction of the bites.

The invention consists, as in French patent No. 2606688, of a lost foam molding process for metal parts, in particular aluminum alloy, in which a model of the product to be obtained is made, made of a foam of organic material in a mold consisting of a dry sand bath containing no bonding agent, then after filling the mold with the molten metal and before the solidified fraction of metal exceeds 40% by weight, an isostatic gas pressure is applied to the mold, but this invention is characterized in that it is mainly used in the molding of aluminum alloy parts having a solidification interval greater than 30 ° C. and whose geometry is such that the ratio R of the length which separates the counterweight from the critical zone (s) on half of the average thickness of the part over this length is greater than 10 and that the relative pressure applied is between 0.1 and 0.5 MPa.

Thus, the invention consists in applying the basic method to parts made of aluminum alloys having a relatively large solidification interval and having a particular geometry such that the ratio R is greater than 10, that is to say where the distance L between the region in which solidification occurs at the last moment and the counterweight is relatively large compared to the average thickness e of the part over this distance.

This ratio is in fact that of L on the module M of the part of the room located between the flyweight and the critical zone, parameter used in molding and which corresponds on average to half the thickness, ie e / 2, hence (L / M) = (L / (e / 2)) = 2 L / e.

This ratio makes it possible to set the value of the maximum pressure that can be applied without risk of spongy recession: thus the higher the ratio, the lower the value of the pressure should be. It has been found that for a pressure of 0.5 MPa, the minimum value used in French patent No. 2606688, R is close to 10. Therefore when R is higher, it is necessary to use a pressure less than 0.5 MPa and which can go up to 0.1 MPa, value below which the pressure has only a negligible effect and can therefore be suppressed.

The invention is preferably applicable to alloys with a large solidification interval such as for example Al-Cu, Al-Cu-Mg, Al-Zn-Mg, Al-Si-Cu-Mg, as well as to Al- Si-Mg hypoeutectics whose silicon content is preferably less than or equal to 9% by weight.

• fig. 1, une micrographie d'une pièce en alliage AS5U3G (composition en poids Silicium 5%, Cuivre 3%, Magnésium 1%, solde aluminium) dans laquelle R est égal à 15 et où la pression appliquée au cours du moulage a été de 1,1 MPa.
• fig. 2, une micrographie de la même pièce mais où la pression appliquée a été seulement de 0,30 MPa.

The invention can be illustrated with the aid of the attached figures which represent:

• fig. 1, a micrograph of an AS5U3G alloy part (composition by weight Silicon 5%, Copper 3%, Magnesium 1%, aluminum balance) in which R is equal to 15 and where the pressure applied during molding was 1 , 1 MPa.
• fig. 2, a micrograph of the same part but where the pressure applied was only 0.30 MPa.

We see the presence in Figure 1 of black areas corresponding to the infiltration of dendrites by gas and the formation of spongy shrinkage while in Figure 2 these areas are virtually nonexistent.

The invention will be better understood using the following application examples:

Internal combustion engine cylinder heads were manufactured from the same aluminum alloy (AS5U3G). These cylinder heads had two types of geometry represented by FIGS. 3 and 4 and consisted respectively of a fabric 1 or 4, a trigger guard 2 or 5 corresponding to the critical zone and a counterweight 3 or 6.
On each of these types, the dimensions of the critical zone were measured: the thickness e ′ and the width l ′; the dimensions of the canvas: the thickness e, the width L and determined the ratio L / e and the value of R = L / M.
The cylinder heads of each type were divided into 2 batches and each batch was subjected during molding either to a relative pressure of 0.3 MPa, or to a relative pressure of 1.1 MPa.
After demolding, the health of the cylinder heads was checked with regard to spongy shrinkage.
The results are shown in Table 1.

It can be seen that for a value of R = 7.6, and whatever the pressure applied, no spongy shrinkage appears.
For the cylinder heads of Figure 3, we can therefore apply the conventional method.
On the other hand, for the cylinder heads of FIG. 4 where the L / M ratio is equal to 15.4, the spongy shrinkage appears under 1.1 MPa but not under 0.3 MPa.
These cylinder heads must therefore be molded according to the method of the invention so that they can be used.

The invention finds its application in particular in the manufacture of cylinder heads of automobile engines and of all parts requiring high mechanical characteristics.

Claims (3)

1. Process for the lost foam casting of metallic articles, in particular of aluminium alloy, in which a pattern of the product to be obtained, made from a foam of organic substance, is immersed in a mould formed by a bath of dry sand containing no binder then, after having filled the mould with the molten metal and before the solidified fraction of metal exceeds 40% by weight, an isostatic gas pressure is applied to the mould, characterised in that it is used essentially for the casting of articles of aluminium alloy having a solidification range higher than 30°C and of which the geometry is such that the ratio R of the length separating the feeder from the critical zone or zones over half the mean thickness of the article over this length is greater than 10 and the relative pressure applied is between 0.1 and 0.5 MPa.
2. Process according to claim 1, characterised in that, the higher the ratio R, the lower the pressure applied.
3. Process according to claim 1, characterised in that the aluminium alloy belongs to the group consisting of Al-Cu, Al-Cu-Mg, Al-Zn-Mg, Al-Si-Mg and Al-Si-Cu-Mg alloys.

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