KR20010113858A - Diecasting method and device for carrying out the same - Google Patents

Abstract

The present invention relates to a die casting method used for producing cast parts from semi-solidified alloy melts. According to the method of the present invention, the alloy melt is brought into a semi-solidified state by stimulating the crystallization process and then introduced into the casting chamber and a cast part is produced under pressure. Exogenous metal suspensions are first made in a closed chamber outside the casting chamber. The closed chamber is functionally connected with the casting chamber and together with the casting chamber forms a casting unit. The metal suspension moves before entering the casting chamber, thereby forming a hollow rotating body. The rotor is kept in rotation until suspension homogeneity for casting is obtained. The invention also relates to a device for practicing the method of the invention. The apparatus includes a vertical casting chamber with a plunger and a processing vessel disposed outside the casting chamber. The processing vessel is functionally connected with the casting chamber and together with the casting chamber forms a casting unit.

Description

Die casting method and device for carrying out the same}

It is known that the thermodynamic state of metals plays an important role in many treatment processes. The effects associated with this are not only used to facilitate the deformation process, but can also substantially affect the morphologies of the crystalline forms that occur and the properties of the cast part. As the temperature increases, the alloy changes its strength, which is important for the molding process, while at the same time obtaining the possibility of plastic deformation as a property of the structure.

In the thermodynamic change phase, the alloy melt is semi-solidified in the liquid-solid temperature range. Thus, this state is referred to as the "metal suspension" used as a preliminary material for the new die casting process. The present invention analyzes the phenomenological and rheological properties of crystallized liquids and uses them to produce cast parts from a semi-solidified state. The melt is specially treated before entering the casting chamber. The method is designed so that the pre-doped metal can be changed in the die casting apparatus without degrading the quality.

It is known from preliminary experiments that the properties of the preliminary material are affected in a semi-solidified state by various chemical and physical methods and by controlled heating or cooling. However, it should now be found how the preliminary material can be prepared as a homogeneous metal suspension and introduced into the die casting apparatus without degrading quality.

A method for producing a cast part from a melt in semi-solidified state is known from EP 0 841 406 A1. A multistage method is described here, the purpose of which is to produce parts from semi-solidified alloys in vertical die casting machines. The transfer of the preliminary material into the casting chamber takes place after the nucleation step and subsequent granulation process in the superheated melt. For this purpose, vibration devices are used which stimulate the generation of nuclei in the crystallization melt. In order to maintain the obtained suspension state, the semi-solidified preliminary material is heated. After the casting chamber is docked in the container for the preliminary material, the preliminary material is pressed into the mold.

Known methods have the following disadvantages because they resemble thixoforming:

(1) There is no method that can be implemented in a closed chamber. Thus, it is not suitable for reducing the gas content of solidified cast parts.

(2) As the weight of parts increases, and as the solidification interval increases, there is a risk of separation in the preliminary material. Thus, unwanted morphological separation may appear, thereby degrading the isotropy of the cast part and its mechanical properties.

(3) The filling process is important for casting quality because the preliminary material in suspension is constantly changed in its properties by cooling or heating. In known methods, swirling occurs in the freely flowing metal stream when poured into the casting mold, which significantly reduces casting quality.

EP 0 733 421 A1 discloses another casting method in which the casting chamber is filled with a casting plunger after dosing a given melt. The melt is pressed into the die casting mold after it is brought into suspension by additional cooling and mixing in the electromagnetic field.

Although known methods improve the quality of the melt prior to casting, it is difficult to maintain a consistently desired quality over a long period of time. Rather, coarse structural separation and hence low mechanical properties result from variations in crystallization conditions in the melt.

The present invention relates to a die casting method for producing a cast part from a semi-solidified alloy melt in such a way that the alloy melt is semi-solidified by the stimulation of the crystallization process, introduced into the casting chamber, and the cast part is manufactured under pressure. An apparatus for carrying out a method is provided.

1 is a schematic diagram of a die casting apparatus according to the present invention that can produce a cast part from an exogenous homogeneous metal suspension.

FIG. 2 is an enlarged view of section A of FIG. 1 in the region where the transfer chamber 6 extends into the casting chamber 7.

It is an object of the present invention to obtain improved tissue homogeneity compared to a product prepared according to the prior art, in which the quality measure is the amount of columnar crystals or mouth grains formed and the amount of coaxial or globulin crystals used. Experience has shown that the improved isotropy of cast parts can be given by the type of crystal shape that occurs, whereby particularly high strength and very good toughness can be obtained.

To date, there have been important error sources for the melt to be separated from the casting chamber and restructured by electromagnetic agitation for the production of suspensions. The preferred tissue here depends on the crystallization process occurring under the influence of the electromagnetic field. When the liquid metal is pressed from the center of the chamber toward the wall by centrifugal force, a strong acceleration force is generated. The acceleration forces produce a liquid cylindrical or conical melt in the casting chamber. Since the die casting mold could not be uniformly filled with the preliminary material, a uniform casting body with isotropic properties could not be produced. There was only a possibility to reduce the field strength. However, this causes deterioration of quality and structural anisotropy of the preliminary material, which lowers the mechanical properties of the cast part.

It is an object of the present invention to provide a die casting method which reliably obtains a homogeneous suspension of preliminary materials. The preliminary material is filled into the pressure chamber under pressure in the suspension state. Another object is to make a preliminary preparation from an exogenous metal suspension. By the die casting method developed by the present invention, this object is to develop a continuous casting process, whereby all steps of the method can be made in a closed casting system. The new die casting method allows casting parts of excellent quality to be produced from metal suspensions.

The above-mentioned problems are achieved according to the invention by the rotation of the melt in a closed processing vessel outside the casting chamber, whereby an exogenous homogeneous metal suspension is produced and the casting part is produced after filling the casting chamber with the suspension. The casting chamber is always in the form of a casting system in which the entire method consisting of the individual steps is closed while performing the method.

In order to implement the manufacturing method in this way, during the filling process, the processing vessel is first filled with a melt, there exogenously producing a homogeneous metal suspension, and the suspension is transferred into a casting chamber by a special transfer chamber, for which The casting chamber arranged with has a filling port.

A special feature of the process according to the invention is the method of preparing the metal suspension. In contrast to the aforementioned method, in which the melt is brought into suspension by external cooling, whereby the suspension has endogenous properties, the present invention uses a technique in which an exogenous metal suspension is suddenly generated. At the heart of this technique is rapid and uniform melt cooling, which is only possible by introducing additional micro thermal processes into the melt. In order to be able to obtain the required cooling effect in the shortest time, a powder made from an alloy such as a melt is used as the coolant. The cooling effect is the reduction of excessive superheat temperatures, in which two parallel crystallization processes occur:

The melt is cooled from the inside to a predetermined temperature or a minimum liquidus temperature and additional cooling forms crystallization nuclei. The crystallization nucleus determines the shape of the tissue. Its uniform distribution in the metal suspension is of great importance to ensure that the same conditions of crystallization are present throughout the entire suspension.

In this process, the cooling powder is introduced into the melt under pressure, for which air, argon or nitrogen is used as the carrier gas.

In order to form the metal suspension after conditioning, a special design of the treatment vessel is required. According to the invention, this processing vessel is a closed chamber, the outlet opening of which can be closed with a stopper bar. The processing vessel is located in the electromagnetic field, and the filled melt moves in the closed chamber and is maintained there until the conditioning process is complete.

The motion of the melt generates sheared material in the solidification zone, and the crystallization process is stimulated by the melt of the solidification line in the liquid range. For this reason, the range of a suspension state expands.

The suspension range is characteristic of the entire material in the proposed process. The force acting on the metal melt in the form of an electromagnetic field is necessary from the outset to shear the coagulation material in a narrow coagulation region and to bring the entire suspension into motion. During this exercise, the suspension is fluid for a long time and creates the homogeneity necessary for the quality of the cast part.

The form of the partially liquid hollow rotor occurs during this process. In this process, the suspension flows to the walls of the treatment vessel and remains in motion until the required casting temperature and casting homogeneity are obtained. Experience has shown that the homogeneity of the suspension depends mainly on the value of the so-called "gravity coefficient (K)" on the free surface of the partially liquid hollow rotor. Its minimum limit was determined and based on the quality analysis, it was found that if the coefficient decreased, i.

The inventor also obtained an empirical equation for determining the degree of homogeneity of the metal suspension by the homogeneity coefficient (X). The lower limit of homogeneity for aluminum alloys is 3.8 x 10 ⁸ A ² / s.

The homogeneity coefficient can be determined from the following formula:

X = N x H ² x R ²

Where

N = number of revolutions on the free surface of the partial liquid phase rotor

H = magnetic field strength

R = average wall thickness of the partially liquid rotor.

The special design of the treatment vessel makes it possible for the melt to be in a metal suspension under certain technical conditions. This suspension exits the container through the discharge opening as a uniform suspension material and then enters the casting chamber.

Particularly preferred embodiments of the invention feature a transfer chamber provided specifically for the suspension transfer. The transfer chamber connects the processing vessel and the casting chamber, includes a transfer plunger, and may be mounted at right angles or acute angles to the vertically disposed casting chamber.

According to the invention the conveying plunger serves two functions:

The metal suspension or residue thereof is rapidly introduced into the casting chamber and closed after filling. For this purpose, since the plunger is formed such that its front contour follows the inner contour of the casting chamber, there is no interference from the side of the conveying plunger in the filling opening region when the plunger is moving.

The process of filling the casting chamber with a metal suspension is called "increase filling". While the casting chamber is filled with the suspension, the plunger is pushed downwards (in particular synchronously).

Hereinafter, with reference to the drawings showing an embodiment of the present invention will be described in detail.

The die casting machine shown in FIG. 1 includes a processing vessel 1. The processing container 1 comprises a stopper bar 2 and a pipeline 3 for introducing cooling powder. The vessel is connected to the heat retention oven 5 by melt line 4 and to the casting chamber 7 arranged vertically via the transfer chamber 6.

The processing vessel 1 is inserted into the electromagnetic stirring device 8. The stirring device 8 consists of an induction and control unit in the form of a closed part of a die casting machine as an integrated system. As shown in the schematic, the transfer chamber 6 is mounted to the casting chamber 7 at an acute angle. However, it is also possible for the parts to be joined at right angles and to include a conveying plunger 9. The conveying plunger not only removes metal residues in the conveying chamber 6 but also blocks the filling opening 10 in the casting chamber 7. The casting chamber has a plunger 11 as is conventional.

In the induction device, an electromagnetic field is formed by a control device (not shown), which enables the stirring motion of the melt. The melt 12 then reaches from the heat retention oven 5 into the processing vessel 1 by the melt line 4. The processing vessel 1 is arranged in an induction coil 8. By the use of a rotating magnetic field, the melt moves in a particularly closed chamber, since the discharge opening of the processing vessel is closed by the stopper bar 2. By centrifugal force, the melt flows to the vessel wall and forms a liquid hollow rotor (rotator in the figure is conical). At this time, there is a gravity coefficient K having a value determined by the speed with which the different liquid layers push each other on the surface without the rotor. According to a preferred embodiment of the treatment vessel, not only the solidification line but also the entire melt can already be moved in this working step. The movement continues until the entire melt reaches the desired state or suspension homogeneity.

Immediately after the hollow liquid rotor is formed from the superheated melt, the powder is introduced into the rotating melt by the powder dosing device 3 (shown as a pipe line). The amount of powder is sufficient to cause a cooling action. According to the invention the cooling powder is introduced into the melt under pressure in a pulsating manner. Exogenous metal suspensions appearing abruptly in the treatment vessel 1 are the result of the following three processes:

The first process belongs to a heat exchange process. In this process, the powder material takes away excess heat from the overheated melt. There are a number of cooled suspension zones with temperatures below the liquidus level.

The second process involves the way in which the powder is introduced into the melt. Because this process takes place under pressure, exogenous metal suspensions appear simultaneously in the entire melt because the powder particles do not remain on the inner surface of the liquid rotor, but penetrate deeply into the melt and act as efficient internal heat absorbers. Since the powder is pulsatingly introduced into the melt, elastic vibrations occur in the liquid metal, which further stimulate the generation of new crystallization nuclei in the melt by the micro activation effect.

The third process is the continuous rotational movement of the melt, which takes place in parallel with the process during the two aforementioned processes. With a preferred embodiment of the method according to the invention, it is possible to keep the whole melt in motion in a closed chamber. First, a preliminary material is formed from the melt and then from the metal suspension to form the hollow rotor. The coefficient of gravity on the free surface of the rotating body has a special meaning because it determines the homogeneity of the metal suspension. The following parameters are affected:

1) Structure of the rotor to ensure the stability of the set technical parameters.

2) Temperature uniformity and chemical uniformity.

3) No additional water absorption, viscosity accuracy.

4) Acceleration so that the required shear force is given and uniformly distributed over the entire rotating melt as well as at the solidification line.

5) Mix giving the desired homogeneity to the metal suspension.

6) Close contact between the melt and the powder particles results in very efficient heat dissipation.

The supercooled metal regions appearing in the melt expand and grow together by continuous mixing. Since this is done under the same technical conditions, the homogeneity is optimally controlled since the suspension occurs simultaneously in the whole melt.

Due to the homogeneity of the obtained suspension, a uniformly distributed round crystal shape is produced in the solidified cast part, which leads to high mechanical properties.

After suspension homogeneity is obtained, the suspension is emptied from the processing vessel 1 to the transfer chamber 6 by the stopper bar 2. The suspension can be in motion or flow directly from the treatment vessel 1, which does not affect the quality of the suspension but affects the discharge time.

Since the metal suspension formed in this way has a large kinetic energy potential, it flows very quickly through the transfer chamber 6 in the direction of the casting chamber 7, which is filled with the suspension through the filling opening 10. Increasing filling can be achieved since the plunger 11 is pushed downward in synchronization with the flowing suspension. In particular, in order to protect the transfer chamber 6 from the metal suspension flowing through the filling opening during high acceleration of the plunger, a transfer plunger 9 is installed in the transfer chamber. After the filling process is finished, the conveying plunger 9 is pushed forward and the filling opening 10 and the casting chamber 7 are closed. The conveying plunger 9 is embodied so that its front contour follows the inner contour of the casting chamber 7. The metal suspension in the casting chamber fills the pressure chamber by the acceleration of the plunger, so that the cast part is made from the solidified preliminary material.

In a first embodiment by the method according to the invention, the aluminum alloy cast part produced according to the method of the invention has a refined and isotropic tissue morphology, which is a uniform, round crystalline shape of the primary phase. Has As a result, elevated mechanical properties are obtained in comparison with products manufactured by conventional methods. These values are shown in Table 1 for the casting state of the alloy AlSi ₉ Cu ₃ .

Table 1

Way Homogeneity Factor XA ² / sec Tensile Strength N / mm ² Elongation N / mm ² Elongation at Break% Conventional 1.1 207 126 2.4 new 3.0 214 130 2.5 new 3.8 234 151 2.88 new 5.0 251 157 3.12

Parts already produced according to the new method in the casting state show a marked improvement. There is a fixed connection between the general rise in mechanical properties and the technical conditions under which the metal suspension is made. By use in the method according to the invention, cast parts with high mechanical properties and invariable quality can be produced.

Claims (15)

In the die casting method for producing a cast part from a semi-solidified alloy melt in such a way that the alloy melt is semi-solidified by the stimulation of the crystallization process, introduced into the casting chamber, and the cast part is manufactured under pressure. Exogenous metal suspension is made in a closed chamber outside the casting chamber, the chamber is functionally connected with the casting chamber and forms a casting unit with the casting chamber, the metal suspension moves before entering the casting chamber, Wherein a hollow rotor is formed and the rotor is kept in rotation until a suspension homogeneity for casting is obtained. The die casting method according to claim 1, wherein the stimulation of the crystallization process takes place in a rotating magnetic field. The die casting method according to claim 1, wherein a cooling powder is supplied for thermodynamic stabilization of the rotating melt, the powder suddenly making the melt semi-solidified. 4. The die casting method according to any one of claims 1 to 3, wherein the formation of the melt is made under the elastic vibration action of the liquid metal. The die casting method according to any one of claims 1 to 4, wherein the cooling powder is introduced into the melt in a powder state. The die casting method according to any one of claims 1 to 5, wherein the cooling powder is introduced into the melt under pressure, and air, argon or nitrogen is used as the carrier gas. 7. Die casting method according to any of the preceding claims, characterized in that the outflow metal melt is affected by an electromagnetic field. The die casting method according to any one of claims 1 to 7, wherein a filling increase process is performed by moving the plunger downward in synchronization with the filling of the casting chamber. An apparatus for carrying out the method according to any one of claims 1 to 8, comprising a vertical casting chamber having a plunger and a processing vessel disposed outside the casting chamber, wherein the processing vessel is functional with the casting chamber. And form one casting unit together with the casting chamber. 10. The apparatus of claim 9, wherein the processing vessel is surrounded by a coil, the coil creating an electromagnetic field inside the processing vessel; And the processing vessel is connected to the casting chamber through a transfer chamber. The device according to claim 9, wherein the processing container comprises a closing device, wherein the container can be completely closed during processing or during rotation of the melt by the closing device. 12. The apparatus according to any one of claims 9 to 11, wherein the transfer chamber is installed at right angles to the casting chamber. The device according to any one of claims 9 to 12, wherein the transfer chamber is installed at an acute angle with the casting chamber. 14. An apparatus according to any one of claims 9 to 13, wherein the transfer chamber comprises a transfer plunger, the transfer plunger closing the filling opening of the casting chamber. 15. The device according to any one of claims 9 to 14, wherein the conveying plunger is formed such that its front contour follows the inner contour of the casting chamber at its side towards the filling opening of the casting chamber.