IE63394B1 - A process for the lost-foam casting under pressure of metal articles - Google Patents

Abstract

This process consists in applying the increasing isostatic gas pressure to the mould after it has been filled at a rate of increase such that the said pressure generates, rapidly and temporarily as a result of pressure drop through the sand, an excess pressure of the molten metal relative to the sand at the level of their interface, this excess pressure reaching a value lying between two limits and subsequently increasing as the said pressure increases, and then in maintaining the said pressure at a constant level until the part has completely solidified. This invention finds its application in the production of parts, in particular made from aluminium alloys, having, apart from an improved compactness, a surface which is free of blowholes and carbonaceous inclusions.

Description

The present invention relates to a process for the lostfoam casting, under pressure, of metal articles, in particular of aluminium and alloys thereof.

It is known to a person skilled in the art, for example from the teaching of USP No. 3 157 924, that patterns of organic material foam such as polystyrene which are immersed in a mould formed from dry sand containing no binder can be used for the casting of metals. Industrially, these patterns are generally coated with a film of refractory material intended to improve the quality of the castings. In such a process, the metal to be cast, which has previously been melted, is brought into contact with the pattern by means of a supply orifice and channels traversing the sand and is gradually substituted for said pattern by burning it and transforming it mainly into vapour which escapes between the grains of sand.

In comparison with conventional casting in a nonpermanent mould, this method avoids the preliminary manufacture, by compacting and agglomeration of powdered refractory materials, of rigid moulds connected in a fairly complicated manner to cores and allows simple recovery of the castings and easy recycling of the casting materials. It is therefore simpler and more economical than the conventional method. Furthermore, it gives the designers of castings greater freedom with regard to the shape of said castings.

This is why this method has been found increasingly attractive from the industrial point of view. However, it is handicapped by several drawbacks, two of which result from conventional metallurgical mechanisms, that is to say: the relative slowness of solidification which promotes 63ό94 the formation of gassing pin-holes originating from the hydrogen dissolved in the liquid aluminium alloy; the relative weakness of the thermal gradients which promotes micro-shrinkageWith the aim of overcoming such drawbacks, the applicants have proposed, in the French patent application published under No. 2 606 688, an invention consisting in applying to the mould, after it has been filled and before the solidified fraction of metal exceeds 40% by weight, an isostatic gas pressure having a maximum value of between 0.5 and 1.5 MPa .

Thus, the process according to said invention comprised the conventional stages of lost-foam casting, that is to say - using a pattern of the article to be moulded, formed from a foam of organic material coated with a film of refractory material; immersing said pattern in a mould formed from dry sand without binder; filling the mould with the metal in the molten state to burn said pattern, this filling process being effected via a supply orifice connecting the pattern to the exterior of the mould; - evacuating the vapour and liquid residues emitted by said pattern during combustion thereof; allowing the molten metal to solidify to obtain the article.

However, the applicants improved the invention in that when the mould had been completely filled, that is to say when the metal had entirely replaced the pattern and the majority of vapour had been evacuated, they applied a gas pressure to the mould. This operation could be carried out by placing the mould in a chamber which is capable of withstanding pressure and is connected to a source of gas under pressure.

This operation could be carried out immediately after the filling process while the metal was still completely liquid, but it could also take place later providing that the solidified, fraction of metal in the mould did not exceed 40%, beyond which value the pressure would have had a negligible effect. The value of the pressure applied preferably had a maximum range of between 0.5 and 1.5 MPa. A value of less than 0.5 MPa would have been insufficiently effective and a value higher than 1.5 MPa would lead to high operating costs.

It was thus found that the density of the articles was increased considerably by eliminating or at least reducing the pin-holes caused by gas and the micro-shrinkage, and their mechanical characteristics were therefore improved. However, this gave rise to a new drawback known as metal penetration. In fact, when a pressure is established on a mould for lost-foam casting without other precautions, said pressure is exerted, on the one hand, directly on the metal supply orifice where it is transmitted virtually instantaneously to the entire mass of molten metal and, on the other hand, at the surface of the sand where it is transmitted with gradually attenuated intensity by the effect of loss of charge through the grains of sand. The pressure is therefore unbalanced, causing an excess pressure P of metal relative to the sand in the region of their interface, that is to say at the point where the pattern was in contact with the sand. This imbalance is temporary and occurs just after the application of pressure but is then re-absorbed.

If this excess pressure is too great, it causes the metal to penetrate between the grains of sand and causes deformation of the surface of the article. This constitutes the so-called phenomenon of metal penetration. To overcome this, it was necessary to try to reduce this excess pressure as far as possible, and the applicants achieved this in the main application by applying a pressure which increases progressively in time from the value 0 to the maximum desired value, whereupon this pressure is maintained until the metal has completely solidified. In fact, the lower the pressure when it is first applied, the slighter the imbalance. A sufficiently low rate of increase of pressure has thus been defined to achieve a reduced excess pressure. However, apart from this phenomenon of metal penetration and the drawbacks resulting from the conventional metallurgical mechanisms mentioned above, for which a solution had been provided, the applicants have noticed two other drawbacks resulting from mechanisms which are absolutely specific to the lost-foam process, that is to say: the formation of blow-holes due to gasified residues of foam the formation of carbon inclusions associated with oxides and resulting from the contact between the liquid aluminium alloy and the carbonaceous residues of the foam.

Hence the complementary research which has led to the following conclusions.

As seen above, the industrial practice of investment casting involves covering patterns with a film of refractory material generally formed from particles of ceramic agglomerated by a binder. This film acts as follows: at the moment when the molten metal is cast, the foam which is usually produced from polystyrene is eliminated in both gaseous and liquid form. The refractory layer is responsible for regulating elimination of the gaseous form by its permeability and for absorbing the liquid form. Generally speaking, the permeability should be adapted to the article in order to ensure that a cushion of gas is maintained between molten metal and foam and the absorbing power should be at a maximum to eliminate the liquid residues. Thus, when the mould is completely filled, the refractory layer is saturated with residues, the excess over saturation having escaped into the sand. The metal at a temperature of 600 to 800°C in contact with this saturated layer of organic material is therefore in the mould, and this may cause gasification of the liquid which then generates a pressure such that gas penetrates into the metal and forms blow holes there while giving rise to carbon inclusions originating from incomplete combustion of the foam residues.

To avoid this drawback, sufficient excess pressure must therefore be created in the molten metal relative to the space in the sand behind the film to cause evacuation of the gaseous and liquid residues toward the sand and thus to prevent them from entering the metal. However, this goes against the solution adopted to avoid metal penetration, which involved reducing as far as possible the rate at which the pressure was increased in order to reduce this excess pressure to a minimum.

The applicants have therefore found that it was essential, for avoiding metal penetration and the penetration of the residues into the metal, to fall within a range of excess pressure, hence the improvement according to the invention which involves using a rate of increase in the pressure 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 the interface thereof, this excess pressure attaining a value lying between two limits and then decreasing as said pressure increases, then keeping said pressure constant until complete solidification is achieved.

This rate preferably has a value of between 0.003 and 0.3 MPa per second and, the thicker the article, the lower the value, values outside this range causing one or other of the two drawbacks to predominate.

This rate should obviously take into consideration the loss of charge through the mould, that is to say the grain size of the sand and also the depth of immersion of the pattern in the sand. This is why it is selected as a function of these parameters and in order to obtain excess pressure values of between 0.001 and 0.030 MPa and preferably between 0.002 and 0.010 MPa. This excess pressure is only required for a critical period which immediately follows the filling of article and when the film is still saturated with incompletely vaporized products. This excess pressure is preferably reached in less than two seconds after application of the pressure, at which moment the phenomenon of metal penetration is greatest.

The invention can be illustrated by the following examples of application which relate to the casting of exhaust manifold and cylinder head of internal combustion engines under conditions which take into consideration the grain size of the sand and the depth of immersion of the pattern so as to fall within the claimed ranges of excess pressure. These conditions and the parameters of the moulds used are shown in Table 1 overleaf.

The articles cast in this way had very few blow-holes and no encrustation of carbon, demonstrating the efficiency of the improvement according to the invention.

Claims (7)

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 5 excess pressure is reached in less than 2 seconds.

6. - A process for casting metal articles as claimed in Claim 1 substantially as hereinbefore described with reference to the Examples.

7. 10 7. Metal articles whenever made by a process as claimed in any of the preceding claims.