FI93322C - Foam modeling method for metal parts by pressure - Google Patents

Description

93322

The present invention relates to a method of foam model molding of metal bodies by pressure, comprising the steps of: using a molded body pattern consisting of foam of organic matter and coated with a film of refractory material; 10 immersing said pattern in a mold formed of dry sand without a binder; filling the mold with molten metal to burn said pattern; removing vapors and liquid residues from said model; allowing the molten metal to solidify to obtain said body; is applied to the casting mold before the solidified part of the metal exceeds 40% by weight of isostatic pressure with a maximum value between 0.5 and 1.5 MPa. In particular, the invention relates to foam model molding of aluminum and aluminum alloy bodies under pressure.

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For example, according to the teachings of U.S. Pat. No. 3,157,924, it is known to use foam models for forming metals which are an organic substance, such as polystyrene, which is embedded in a mold which is dry sand which does not contain any binding agent. In industry, these designs are usually coated with a film of refractory material to improve the quality of the shaped bodies. In such a method, the metal to be formed, which has been pre-melted, is brought into contact with the model by means of a feed nozzle and sand-through channels and progressively displaces said model by burning it and converting it mainly to vapors leaving sand grains.

Compared to the classical formula in a non-permanent casting mold, this technique avoids the need to pre-compress and agglomerate powdered refractories into rigid castings which are more or less complexly combined with the casting core and can easily recover the formulations. tracks as well as effortlessly cycle into the builders.

5 It is therefore simpler and more economical than classical technology. In addition, it offers designers of patterned objects greater freedom as to the shape of said pieces. Therefore, this technology has proven to be increasingly attractive from an industrial point of view.

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However, it has several drawbacks, two of which are due to classical metallurgical mechanisms, namely: relatively slow solidification, which promotes the formation of casting pits due to hydrogen dissolved in the liquid aluminum alloy, and the relative weakness of thermal gradients, which promotes the formation of microcavity cavities.

To avoid such disadvantages, FR patent application 2 606 588 proposes an invention in which an isostatic gas pressure having a maximum value of between 0.5 and 1.5 MPa is applied to the mold after filling and before the solidified part of the metal exceeds 40% by weight. .

The method according to the present invention comprised the classical steps of foam modeling, namely: using a model of a body to be molded, consisting of foam of organic material and coated with a film of refractory material; • · | immersing said model in a mold formed of dry sand without a binder; filling the casting mold a molten state to burn the metal of the specimen 35 and the filling is carried out through the supply nozzle, which is placed in communication with the model of the casting mold the outer side; . . ·

Il: 3 93322 removes vapors and liquid residues from said model during its combustion and allows the molten metal to solidify to give a body.

5 But the invention had been improved in the sense that when the mold had become completely full, i. when the metal had completely displaced the model and most of the vapors had escaped, gas pressure was applied to the mold; this operation could be performed by placing the mold in a chamber that could be kept under pressure and connected to a source of pressurized gas.

This operation could be carried out immediately after filling when the metal was still completely liquid, but it can take place even later as long as the solidified part of the metal in the mold does not exceed 40%, above which the effect of pressure would be negligible.

The pressure used was preferably at most between 0.5 and 1.5 MPa; less than 0.5 MPa has an ineffective effect and a value above 1.5 MPa causes high operating costs.

It was then found that the tightness of the bodies could be significantly increased by removing or at least reducing the casting pits and microcast cavities caused by the gas and thus improving their typical mechanical properties. However, this caused the manifestation of a new disadvantage called "runoff."

30 If pressure is applied to the mold of the foam modeling without additional precautions, said pressure is applied directly to the metal feed nozzle, from which it is transmitted virtually in the blink of an eye to the whole liquid metal mass, and to the sand surface, from which it . Thus, a pressure imbalance arises, which causes an overpressure of the metal Δ P with respect to the sand at the interface, i.e. where model 4 93322 was in contact with the sand. This imbalance is momentary and occurs shortly after the pressure is applied and is subsequently resorbed.

5 If this overpressure is too high, it will cause metal to penetrate between the grains of sand and cause the surface of the part to deform. This is precisely the phenomenon called "runoff." Thus, in order to prevent it, an attempt had to be made to reduce this overpressure as much as possible and succeeded in 10 main requirements by applying a pressure which progressively increases over time from 0 to the desired maximum value, after which this pressure was maintained until the metal solidified completely. The weaker the pressure at the beginning, the smaller the imbalance. Thus, a sufficiently low rate of pressure increase must be determined to obtain a lower overpressure.

But in addition to the disadvantages of this flowable immi and the classical metallurgical mechanisms mentioned above, for which a solution has been found, two other disadvantages have been observed, which are due to mechanisms specific to the foam modeling method: formation of gas formation associated with oxides and resulting from contact of the liquid aluminum alloy with the carbonaceous residues of the foam.

30 SU 1,079,353 (inventor's certificate) describes solidification casting in a conventional clay-sand mold and shows that solidification of the casting at elevated pressure reduces porosity and results in a higher ingot density. However, the increased pressure results in mechanical entrapment of the sand due to the pressure difference caused by the pressure coming through the molten surface and the pressure generated through the pores of the casting sand at the melt-sand interface. In order to reduce the burning of the sand, it is proposed in this publication 5 93322 to increase the pressure periodically with the crystallization of the ingot, increasing the pressure in steps of 0.1 to 0.2 MPa and every 0.2 to 0.4 seconds, keeping the pressure for 1 to 5 seconds. Successive pressure increases are carried out when the system pressure is the same as at the metal-mold interface, whereby the pressure difference is zero. The number of pressure injection steps is selected so that the pressure difference never rises above the critical pressure and after each pressure increase the pressure is maintained until the system pressure causes the pressures in the system and the metal-mold interface to be equal.

This has led to further studies which have led to the following conclusions. The problems described above can be solved by a method according to the invention, which is characterized by what is defined in the characterizing part of claim 1.

As noted above, in the industrial practice of foam model formulation, the models are coated with a film of refractory material, usually consisting of particles of aggregate-agglomerated ceramic material. This film works as follows: at the time of pouring the liquid metal, the foam made of polystyrene is usually removed in both gaseous and liquid form. The function of the refractory layer ..., t is to control the exit of the gaseous form by its pulse and to absorb the liquid form. In general, the body must have a permeability to ensure that the gas cushion remains between the liquid metal and the foam, and the absorbency must be maximized to remove liquid residues.

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Upon completion of the filling of the mold, the refractory layer is impregnated with the residues and the excess relative to the impregnation is removed to the sand. The casting mold thus has a metal having a temperature of 600 to 800 ° C in contact with this layer impregnated with organic matter, which can cause gasification of the liquid, which creates a pressure such that 6 93322 gas penetrates the metal and forms gas bubbles and causes carbon to form. the appearance of inclusions resulting from incomplete combustion of foam residues.

5 To avoid this disadvantage, it is therefore necessary to create a sufficient overpressure in the liquid metal relative to the point in the sand behind the film, so as to allow gaseous and liquid residues to escape towards the sand 10 and thus prevent them from entering the metal.

But this is in contrast to the solution made to avoid the flow, which was to reduce the rate of pressure increase as much as possible in order to reduce this overpressure as much as possible.

Thus, it has been found that it is absolutely essential to keep the overpressure within the limits of overpressure in order to avoid casting failure and the penetration of residues into the metal, which means an improvement according to the invention involving a pressure increase rate which, depending on the sand granulometry and model immersion depth. rapidly and momentarily due to the loss of gas supply through the sand with an overpressure of molten metal relative to the sand at their distribution surface, and this overpressure reaches a value within two limits and then decreases as said pressure increases and then keeps said pressure constant until complete solidification.

The velocity of this is preferably between 0.003 and 0.3 MPa per second and is lower the greater the thickness of the body. And velocity values outside this fork cause either disadvantage to prevail.

35 This speed must, of course, take into account the loss of gas supply through the selector, i.e. sand granulometry and also the immersion depth of the model in the sand.

7 93322 It is therefore selected according to these parameters and so as to obtain overpressure values between 0.001 and 0.030 MPa and preferably between 0.002 and 0.010 MPa.

5 This overpressure is only necessary during the critical phase immediately following the filling of the body, in which case the membrane is not yet completely impregnated with the evaporated substances. This overpressure is preferably reached in less than 2 seconds from the start of the pressure, at the moment when the casting failure phenomenon 10 is at its strongest.

The invention can be illustrated by the following application examples concerning the casting of an internal combustion engine collector and cylinder head under conditions which take into account the granulometry of the sand and the immersion depth of the model to remain within the required overpressure limits.

These conditions and the parameters of the molds used are given in the following table 1: 20 1 4 8 93322 Using the Ifi_1_2_3 Ifli_1_2_3 Ifeti Filling Immediately Filling

Part type Vibratory collector Cylinder head Cylinder _cover_

Granulcometry of sand in 48 48 100 AES *

Jafrnefctation time 60 240 in 240 seconds

Piece thickness 4 8 8 in millimeters second growth duration 12 46 80

between 0-0.8 MS

in seconds. ^ second growth rate 0.066 0.017 0.01 M / s tflexinal P in mounds 0.0097 0.0046 0.0030

Model immersion depth 250 450 450. nrillirretreissä

The time required to reach a maximum of 0.9 0.6 to 0.4 overweight cooocc in seconds i 1AFS = American internationally known gramlcnretrinai norm.

II

9,9322 The pieces thus formulated had very few gas bubbles and no carbon hardening, indicating the effectiveness of the improvement according to the invention. 1 «

Claims (3)

93322 A method of foam molding a metal body by pressure, comprising the steps of: using a molding body model consisting of foam of organic matter and coated with a film of refractory material; immersing said model in a mold formed of dry sand without a binder; 10 filling the mold with molten metal to burn said pattern; removing vapors and liquid residues from said model; allowing the molten metal to solidify to obtain said body; is applied to the mold before the solidified part of the metal exceeds 40% by weight, an isostatic pressure having a maximum value of between 0.5 and 1.5 MPa, characterized in that said pressure rises at a rate of between 0.003 and 20 0.3 MPa / s and the lower the thickness of the body, that depending on the granulometry of the sand and the immersion depth of the model, it quickly and momentarily generates an overpressure of molten metal with respect to the sand at their interface due to loss of gas supply and reaches a value between 0.001 - 0.030 MPa and then decreases as said pressure increases and then said pressure is kept constant until complete solidification. A method according to claim 1, characterized in that said overpressure is between 0.002 and 0.010 MPa. • · Method according to Claim 1, characterized in that the maximum overpressure is reached in less than 2 seconds. 11 93322