EP0386384B1 - Process for the lost foam casting under pressure of metal pieces - Google Patents

Description

The present invention relates to a lost foam and pressure molding process for metal parts, in particular aluminum and its alloys.

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 mainly by transforming it into vapors which escape between the grains of sand.

• la relative lenteur de la solidification qui favorise la formation 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.

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.
It is therefore simpler and more economical than the conventional technique. Furthermore, it offers designers of molded parts greater freedom as regards 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 of gassing pits from the hydrogen dissolved in the liquid aluminum alloy.
• the relative weakness of the thermal gradients which favors the formation of microretasures.

It is with the aim of avoiding such disadvantages that the applicant proposed in the French patent application published under No. 2,606,688 an invention consisting in applying to the mold after filling and before the solidified fraction of the metal does not exceeds 40% by weight an isostatic gas pressure 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.

Thus, the method according to said invention included 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;
• fill the mold with the metal in the molten state to burn said model, this filling being effected by means of a supply orifice bringing the model into contact 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.

But the Applicant had improved it in the sense that when the mold had been completely filled, that is to say when the metal had entirely replaced the model and the major part of the vapors had been evacuated, it applied 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 could be done immediately after filling while the metal was still completely liquid, but it could still take place later as long as the solidified fraction of metal in the mold did not exceed 40%, value beyond which the pressure would have had a negligible effect.
Preferably, the value of the pressure applied was at most between 0.5 and 1.5 MPa: a value less than 0.5 MPa being of insufficient effect and a value greater than 1.5 MPa resulting high operating costs.
It was then observed that the compactness of the parts was considerably increased by eliminating or at least reducing gassing pits and micro-shrinkage and thus improving their mechanical characteristics. However, this caused the appearance of a new drawback called "watering".
In fact, when a pressure is established on a lost foam molding mold without more precautions, said pressure is exerted on the one hand directly on the metal supply orifice where it is transmitted practically instantaneously to the whole mass liquid metal, on the other hand to the surface of the sand where it is transmitted with an intensity gradually attenuated by the pressure drop effect through the grains of sand. It therefore establishes a pressure imbalance causing an overpressure Δ P of the metal relative to the sand at their interface, that is to say at the place where the model was in contact with the sand. This imbalance is temporary and occurs soon after the pressure is applied and then subsides.

• la formation de soufflures due à des résidus gazéfiés de la mousse
• la formation d'inclusions de carbone associées à des oxydes et consécutive au contact de l'alliage d'aluminium liquide avec des résidus carbonés de la mousse.

D'où des recherches complémentaires qui ont abouti aux conclusions suivantes.
Comme on l'a vu plus haut, la pratique industrielle du moulage à modèle perdu consiste à revêtir les modèles d'un film de matériau réfractaire généralement formé de particules de céramique agglomérées par un liant. Ce film agit comme suit: au moment de la coulée du métal liquide, la mousse fabriquée le plus souvent à partir de polystyrène est éliminée à la fois sous la forme gazeuse et liquide. La couche réfractaire est chargée de réguler l'élimination de la forme gazeuse par sa perméabilité et d'absorber la forme liquide. D'une façon générale, la perméabilité doit être adaptée à la pièce afin d'assurer le maintien d'un matelas de gaz entre métal liquide et mousse et le pouvoir absorbant doit être maximum pour éliminer les résidus liquides.
Ainsi à la fin du remplissage du moule, la couche réfractaire se trouve saturée en résidus, l'excès par rapport à la saturation s'étant échappé dans le sable. On a donc dans le moule le métal ayant une température de 600 à 800°C en contact avec cette couche saturée en matière organique d'où peut résulter une gazéification du liquide qui génère alors une pression telle que du gaz pénètre dans le métal et y forme des soufflures tout en provoquant l'apparition d'inclusions de carbone provenant d'une combustion incomplète des résidus de mousse.If this overpressure is too great, it causes metal to penetrate between the grains of sand and results in deformation of the surface of the part. This is what the phenomenon known as "watering" consists of. To counter this, it was therefore necessary to seek to reduce this overpressure as much as possible and the plaintiff had achieved this in the main request by applying a pressure which gradually increases in time from the value O to the value maximum desired after which this pressure was maintained until complete solidification of the metal. In fact, the lower the pressure at the start of its application, the lower the imbalance. It is thus necessary to define a rate of increase in the pressure low enough to have a reduced overpressure.
But in addition to this watering phenomenon and the drawbacks resulting from the conventional metallurgical mechanisms mentioned above to which a solution had been provided, the Applicant has noticed two other drawbacks resulting therefrom from mechanisms absolutely specific to the lost foam process and which are:

• the formation of blisters due to gasified residues of the foam
• the formation of carbon inclusions associated with oxides and consecutive in contact with the liquid aluminum alloy with carbonaceous residues of the foam.

Hence additional research which led to the following conclusions.
As seen above, the industrial practice of lost-model molding consists in coating the models with a film of refractory material generally formed of ceramic particles agglomerated by a binder. This film acts as follows: at the time of the pouring of the liquid metal, the foam produced most often from polystyrene is eliminated in both gaseous and liquid form. The refractory layer is responsible for regulating the elimination of the gaseous form by its permeability and for absorbing the liquid form. In general, the permeability must be adapted to the room in order to ensure the maintenance of a gas blanket between liquid metal and foam and the absorbency must be maximum to eliminate the liquid residues.
Thus at the end of the filling of the mold, the refractory layer is saturated with residues, the excess relative to the saturation having escaped into the sand. There is therefore in the mold the metal having a temperature of 600 to 800 ° C. in contact with this layer saturated with organic matter from which may result a gasification of the liquid which then generates a pressure such that gas penetrates into the metal and there forms puffs while causing carbon inclusions from incomplete combustion of foam residues.

To avoid this drawback, it is therefore necessary to create a sufficient overpressure in the liquid metal relative to the space located in the sand behind the film in order to cause the evacuation of gaseous and liquid residues towards the sand and thus prevent their entry. in metal.
However, this goes against the solution adopted to avoid watering which consisted in reducing as much as possible the rate of increase in pressure to reduce this overpressure as much as possible.
The Applicant has thus found it imperative to avoid watering and the penetration of residues in the metal to be located in an overpressure range hence the improvement according to the invention which consists in using a speed of increase of pressure. such that depending on the granulometry of the sand and the depth of immersion of the model it generates quickly and temporarily by pressure drop across the sand overpressure of the molten metal with respect to the sand at their interface, this overpressure reaching a value between two limits and then decreasing as said pressure increases and then maintaining said pressure constant until solidification complete.

Preferably, the value of this speed is between 0.003 and 0.3 MPa per second and the smaller the thickness of the part, the greater the speed values outside this range, making one or more the other of the two drawbacks.

This speed must obviously take into account the pressure drop across the mold, that is to say the grain size of the sand and also the depth of immersion of the model in the sand.
This is why it is chosen as a function of these parameters and so as to obtain overpressure values of between 0.001 and 0.030 MPa and preferably between 0.002 and 0.010 MPa.
This overpressure is necessary only during a critical period immediately following the filling of the room and when the film is still saturated with products which are not completely vaporized. Preferably, this overpressure is reached in less than 2 seconds after application of the pressure, when the watering phenomenon is most significant.

The invention can be illustrated with the aid of the following application examples which relate to the molding of a manifold and cylinder head of an internal combustion engine under conditions which take account of the particle size of the sand and the depth of immersion of the model in order to fall within the claimed overpressure ranges.
These conditions and the parameters of the molds used are shown in Table 1 below:

The parts thus molded had very little blowing and no carbon encrustation, which shows the effectiveness of the improvement according to the invention.

Claims (5)

1. A process for the lost-foam casting, under pressure, of metal articles involving the following stages:

   - using a pattern of the article to be cast formed by a foam of organic material covered with a film of refractory material;

   - immersing said pattern in a mould formed by dry sand without binder;

   - filling the mould with the metal in the molten state to burn said pattern

   - evacuating the steam and liquid residues emitted by the pattern;

   - allowing the molten metal to solidify so as to obtain said article

   - applying to the mould, before the solidified fraction of metal exceeds 40% by weight, an isostatic pressure having a value in a maximum range of between 0.5 and 1.5 MPa, characterised in that said pressure increases at a rate which is such that, as a function of the grain size of the sand and the depth of immersion of the pattern, it rapidly and temporarily generates, by loss of charge through the sand, an excess pressure of molten metal relative to the sand in the region of their interface, this excess pressure reaching a value which lies between two limits and then decreases as said pressure increases, then keeping said pressure constant until solidification is complete.
2. A process according to Claim 1, in which the rate of increase of pressure is between 0.003 and 0.3 MPa/sec and the thicker the article, the lower the rate.
3. A process according to Claim 1, in which the excess pressure has a value of between 0.001 and 0.030 MPa.
4. A process according to Claim 3, in which the excess pressure has a value of between 0.002 and 0.010 MPa.
5. A process according to Claim 1, in which the maximum excess pressure is reached in less than 2 seconds.