JPH0626748B2 - Pressurized lost foam casting method for metal products - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for casting metal products, especially products of aluminum or aluminum alloys, by a pressure lost-foam process.

As is known to those skilled in the art, for example by the teaching of USP No. 3 157 924, in metal casting, a pattern of foamed organic material, such as expanded polystyrene, is formed from dry sand without binder. It can be used by embedding it in the prepared mold. Industrially, these models are typically coated with a thin layer of refractory material to improve the quality of the cast product. In this type of method, pre-melted metal to be cast is brought into contact with a model through supply holes and passages penetrating the sand, and the model is burned to be converted mainly to steam. Since this steam flows out from between the sand grains, the molten metal gradually fills the space in the sand instead of the model.

This method differs from the general casting method using non-permanent molds in that it does not require pre-forming a rigid mold that is connected to the core in a fairly complicated way by compression and consolidation of refractory metal powders, Easy to take out and recycle the mold material. Therefore, it is simpler and more economical than the conventional method. Also, the designer can more freely design the shape of the cast product. For this reason, this method is becoming more and more important industrially. However, this method has some drawbacks.
The following two are derived from conventional metallurgical mechanisms.

1) Since the solidification rate is relatively slow, pinholes are likely to occur due to gas discharge due to hydrogen dissolved in the liquid aluminum alloy.

2) Due to the relatively small thermal gradient, micro shrinkage (micro-s
hrinkage) is likely to occur.

In order to overcome these drawbacks, the present applicant has proposed No. 2 606 6
The invention of the French patent application published at 88, which, after filling, has a maximum of 0.5-1.5 before the solidified part of the metal exceeds 40% by weight.
It was proposed to apply a static gas pressure of MPa to the mold.

The method of the invention uses a step commonly used in lost foam casting, namely, a cast product model consisting of a foamed organic material coated with a thin film of refractory material, and dried without binder. The model is embedded in a mold made of sand, the mold is filled with molten metal through a supply hole that communicates the model with the outside of the mold, the model is burned, and steam generated from the model by the combustion and Draining the liquid residue and solidifying the molten metal to obtain the product.

However, as an improvement, the above-mentioned invention applies gas pressure to the mold when the mold is completely filled, that is, when the metal is completely replaced by the model and most of the vapor is discharged. . This operation can be performed by placing the mold in a pressure resistant chamber connected to a source of pressurized gas.

This pressing operation may be carried out immediately after the filling operation, i.e. while the metal is still completely liquid, but may be carried out after the solidified portion of the metal in the mold does not exceed 40%. .
When the solidified portion exceeds 40%, there is almost no effect even if pressure is applied. It was judged that the maximum value of the applied pressure is preferably 0.5 to 1.5 MPa. If less than 0.5MPa, sufficient effect cannot be obtained, and 1.5M
This is because if it exceeds Pa, the operating cost will increase.

In this way, pinholes and fine shrinkage cavities due to the gas are eliminated or at least reduced, so that the density of the product is significantly increased and thus the mechanical properties are also improved. However, this does not include metal penetration.
It was found to be accompanied by another drawback known as netration). That is, when the lost foam casting mold is pressurized without considering the conditions other than the above conditions, when the pressure is directly applied to the metal supply hole, the pressure is almost instantaneously transmitted to the entire molten metal, while sand is applied. The pressure is transmitted to the surface of the slab due to the pressure drop across the sand grains. Therefore, a pressure imbalance results and the pressure of the metal is over the sand by ΔP in the interface region, i.e. the region where the model contacts the sand. This imbalance is temporary and occurs shortly after pressurization and then erodes.

When the excessive pressure becomes too large, the metal penetrates between the sand grains, so that the surface of the product is deformed. This is the so-called metal penetration phenomenon. Therefore, in order to eliminate this drawback, it is necessary to make the excess pressure as small as possible. The applicant proposed in the patent application to solve the above problem by gradually increasing the pressure from 0 to the desired maximum value and maintaining that maximum value until the metal has completely solidified. This is because the pressure imbalance decreases as the pressure initially applied decreases. In this case, the pressure increase rate must be sufficiently small in order to reduce the excess pressure. Although the above-mentioned drawbacks caused by the metal infiltration phenomenon and the general metallurgical mechanism can be solved in this way, the present applicant has discovered two drawbacks specific to the lost foam method in addition to these drawbacks. That is, 1) the formation of blowholes due to the vaporized residue of the foam material, and 2) the formation of carbonaceous inclusions bound to the oxide, which is caused by the contact between the liquid aluminum alloy and the carbonaceous residue of the foam material. is there.

Therefore, as a result of further research, the following conclusions were obtained.

As mentioned above, in the industrial investment casting method, the model is coated with a thin film of heat resistant material. This thin film of refractory material usually consists of ceramic particles that are hardened with a binder. This thin film functions as follows. That is, the foamed material, which is usually made of polystyrene, becomes a gas and a liquid when the molten metal is injected and is removed, but the heat-resistant layer adjusts the discharge of the gasified material by its permeability and becomes a liquid material. Absorbs. Generally, the permeability must be compatible with the product so that a gas cushion is retained between the molten metal and the foam material, and the absorbency must be maximal to remove liquid residues. There must be. When the filling of the mold is completed in this state, the heat resistant layer is saturated with the residue, and the excess amount exceeding the saturation is absorbed by the sand.
Therefore, the heat-resistant layer saturated with the organic material is not
Metals at ˜800 ° C. may come into contact, resulting in vaporization of the liquid. When this vaporization occurs, pressure is generated, and as a result, the gas permeates into the metal to form blow holes, and carbon inclusions are generated due to incomplete combustion of the foam material residue.

To overcome this drawback, a pressure in the liquid metal is generated that is sufficiently greater than the pressure in the space in the sand behind the membrane so that gas and liquid residues are expelled towards the sand, Therefore, it must be prevented from penetrating into the metal. However, this method is against the method for preventing the metal infiltration phenomenon, that is, the method for minimizing the pressure increase rate to minimize the excess pressure.

The Applicant has found that it is essential to have a certain overpressure range in order to avoid metal penetration and residue penetration into the metal. Therefore, in the present invention, as an improvement point, the sand grain size and the embedding depth of the model are set so that the pressure of the molten metal in excess of the pressure of the sand in the interface region is rapidly and temporarily generated by the pressure drop through the sand. The overpressure using a pressure increase rate according to reaches a value between two limit values (0.001-0.030MPa), then decreases with increasing static pressure, after which the metal completely solidifies Keep constant pressure constant until.

The speed is preferably 0.003 to 0.3 MPa / sec, and the smaller the product thickness, the smaller the speed. If the speed deviates from the above range, either one of the two drawbacks becomes remarkable.

This speed must, of course, be determined in view of the pressure drop that occurs through the mould, i.e. the size of the sand grains and the depth of embedding the model in the sand. Therefore, the speed is based on these parameters and the overpressure is 0.001-0.030MP
a, preferably 0.002 to 0.010 MPa.
This overpressure is only required during the critical period immediately after the filling operation, when the membrane is still saturated with the material that has not been completely vaporized. This overpressure is preferably achieved within less than 2 seconds of applying the static pressure. This is because the metal penetration phenomenon becomes maximum during this time.

The above-mentioned range of excess pressure to be used, preferable pressure increase rate, and preferable time until the maximum excess pressure is reached are not effective only in special cases (for example, special shape and material of the model). Absent. The specific value of each of them is selected from the above range according to parameters such as the grain size (size) of the sand grains and the embedding depth of the model.

Here, as an example of use of the present invention, a case where an exhaust manifold and a cylinder head of an internal combustion engine are cast under conditions in which the sand grain size and the embedding depth of the model are taken into consideration so that the excess pressure falls within the above range. The conditions and parameters of the template used are as shown in Table 1.

As the gas used for pressurization, a compressed gas, an inert gas, a non-reactive gas or the like can be used. In short, as long as the essential steps can be carried out and the product is not adversely affected, it does not depend on the type of gas used.

The product cast by this method had very few blowholes and no carbon deposits. This proves the effect of the improvement of the present invention.

Claims (4)

[Claims] 1. A model of a product to be cast, which is made of a foamed organic material coated with a thin film of a heat-resistant material, is used, and the model is embedded in a mold formed of dry sand containing no binder, The mold is filled with molten metal, the model is burned, the vapor and liquid residue generated from the model are discharged, the molten metal is solidified to obtain the product, and the maximum solidified portion of the metal exceeds 40% by weight. 0.5-1.5MP
In a method of pressure lost foam casting of a metal product including the step of applying a static pressure of a to a mold, the pressure of the molten metal with respect to the pressure of the sand is changed in the interfacial region according to the sand grain size and the model embedding depth. The static pressure is increased at a rate such that the pressure drop across it rapidly and transiently causes the excess pressure to reach between 0.001 MPa and 0.030 MPa, after which the excess pressure is increased. Is decreased as the static pressure is increased, and then the static pressure is maintained at a constant value until the solidification is completed. 2. A pressure increase rate of 0.003 to 0.3 MPa / sec,
The method of claim 1, wherein the thicker the product, the lower this speed. 3. The method according to claim 2, wherein the overpressure has a value of 0.002 to 0.010 MPa. 4. The method of claim 1, wherein the time to reach maximum overpressure is less than 2 seconds.