WO2001055464A1 - Diecasting method and device for carrying out the same - Google Patents

Abstract

The invention relates to a diecasting method used to produce cast parts from a semi-solidified alloy melt. According to the inventive method, the alloy melt is transformed to a semi-solidified state by stimulating the crystallization process, it is then introduced into a casting chamber and the cast pieces are produced by applying pressure. First an exogenous metal suspension is produced outside the casting chamber in a closed-off space that functionally communicates with the casting chamber and forms a casting unit therewith. The metal suspension is set into motion before it is introduced into the casting chamber, thereby forming a hollow rotating body that is maintained rotating until the suspension homogeneity for the casting reached. The invention further relates to a device for carrying out the inventive method. Said device comprises a vertical casting chamber with a plunger and a treatment container disposed outside the casting chamber. Said treatment container is functionally rigidly linked with the casting chamber and forms a casting unit therewith.

Description

 Die casting process and device for carrying it out

description

The invention relates to a die casting method for producing castings from a semi-solidified alloy melt, the alloy melt being brought into the semi-solidified state by stimulation of the crystallization process, introduced into a casting chamber and the castings being produced under pressure, and a device for carrying out the method.

It is known that the thermodynamic state of the metal plays a crucial role in the different machining processes. The associated effects can not only be used to facilitate the deformation process, but can also be used to significantly influence the morphology of the crystalline shapes and properties of the casting. As the temperature increases, the alloy loses its strength, which is of fundamental importance for the shaping process, and at the same time gains the ability for plastic formability as a structure-dependent property. In the thermodynamic transition phase in the liquidus-solidus temperature range, the alloy melt is in the semi-solidified state. This condition is therefore referred to as a "metallic suspension", which is used as a raw material for new die casting processes. The invention is now concerned with the phenomenological behavior of the crystallizing liquids and their rheological properties and uses them to produce castings from the semi-solid state. A given melt volume is subjected to a special treatment before entering a casting chamber, the process being designed in such a way that the pre-metered metal can be fed into the die casting system without loss of quality.

From preliminary tests it was known that the semi-solid state can be influenced in its properties by various chemical-physical methods and by controlled heating or cooling. However, it should now be found out how the starting material can be produced as a homogeneous metallic suspension and poured into the die casting system without loss of quality.

A method for producing castings from the semi-solidified melt state is known from EP 0 841 406 AI. There, a multi-stage process is described, the aim of which is to produce the parts from the semi-rigid alloys in a vertical die casting machine. The transport of the raw material in the casting chamber is carried out after a Keimbil ^¬ up phase in the overheated melt followed by granulation. A vibration device was used for this is used to stimulate the appearance of germs in the crystallizing melt. In order to maintain the suspension state thus obtained, the semi-rigid primary material was heated in a controlled manner. After the casting chamber was docked to the container for the primary material, this was pressed into the printing form.

The known method is similar to thixoforming and therefore has the following disadvantages:

1. It is not a procedure that can be performed in a closed work space. As a result, it is not suitable for reducing the gas content in the solidified casting.

2. As the building weight increases and the solidification interval increases, there is a risk that segregation will take place in the primary material. As a result, undesirable morphological precipitations can occur, so that the isotropy of the castings and their mechanical properties are impaired.

3. Since the primary material constantly changes its properties in the suspension state due to cooling or heating, the filling process is decisive for the casting quality. In the known method, vortices are formed in the freely falling metal stream when it is poured into the casting mold, which significantly reduces the casting quality. EP 0 733 421 A1 discloses a further casting method in which the casting chamber is filled with a casting plunger after the predetermined melt volume has been metered. The predetermined melt volume is brought into a suspension state by additional cooling and mixing in the electromagnetic field and then pressed into the die.

The known method can improve the quality of the melt volume before pouring in, but it is not possible to keep the desired quality constant over a longer period of time. Rather, it was found that coarse structural deposits and associated low values of mechanical properties occur as a result of changes in the crystallization conditions in the melt.

The inventors have now set themselves the task of achieving an improved structural homogeneity - compared with the products manufactured according to the prior art - the quantity of columnar crystals or transcrystallites produced and the quantity of coaxial or globulitic crystals being used as the quality standard , The experimental results showed that the improved isotropy of the castings can be demonstrated by the type of crystalline shapes that occurred, and that in particular high strengths and very good toughness behavior can be demonstrated.

A major source of error in the previous method was that the melt was subjected to a structural change separately from the casting chamber by electromagnetic stirring in order to produce the suspension. desired structure depended on the crystallization process taking place under the influence of the electromagnetic field. As soon as the liquid metal is pressed against the walls by the centrifugal forces from the center of the chamber, strong acceleration forces occur which lead to the formation of a liquid cylindrical or conical melt jacket in the casting chamber. The printing form cannot be filled uniformly with such a preliminary material, so that the production of homogeneous castings with isotropic properties was not possible at the moment. There was only the possibility of reducing the field strength, which then led to quality losses and then to a structural anisotropy of the primary material, which was responsible for the low mechanical properties of the casting.

The aim of the present invention is a die casting method which reliably ensures a homogeneous suspension state of the starting material. This primary material is then filled into a pressure chamber under pressure in this state. Another aim is that the primary material is produced from an exogenous metallic suspension. The die casting process developed by the present invention is intended to develop a continuous casting process, with all work stages being able to take place in a closed casting system. With the new die casting process, castings of excellent quality can be produced from the metallic suspension.

According to the invention, the aforementioned objects are achieved with a method of the type mentioned in the introduction in that the predetermined melting volume outside the casting chamber in one closed processing container brought into rotation and a homogeneous metallic suspension of exogenous type is produced, from which a casting is made after filling in the casting chamber. During the course of the process, the casting chamber always remains in the docked state and the entire process, which consists of individual work stages, is designed as a closed casting system.

In order to implement the manufacturing process in this way, it is intended during the filling process to first fill the processing container with the predetermined melting volume, to produce a homogeneous metallic suspension there in an exogenous manner and to transport it into the casting chamber by means of a special transport chamber, for which the vertical arranged casting chamber is provided with a filling opening.

A particular distinguishing feature of the method according to the invention is the way in which the metallic suspension is prepared. In comparison with the previously mentioned processes, in which the predetermined melting volume is brought into the suspension state by external cooling, which gives the suspension an endogenous character, a fundamentally different technological approach was chosen in the present invention, according to which an exogenous metallic suspension suddenly occurs , The core of the approach is the rapid and even melt cooling, which is only possible by introducing additional micro-heat processes into the specified melt volume. A powder made from the same alloy as the melting volume is used as the coolant in order to achieve the necessary cooling effect in the shortest possible time To be able to reach time. The cooling effect consists of a decrease in the excess superheating temperature, with two parallel crystallization processes taking place:

The melt is cooled from the inside to the predetermined temperature or at least the liquidus temperature and crystallization nuclei are generated by the additional cooling. These determine the structural morphology. Their uniform distribution in the resulting metallic suspension is of primary importance in order to ensure the same crystallization conditions in the entire suspension volume.

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

A special configuration of the processing container is required to produce metallic suspension after conditioning. According to the invention, this is a closed space, the outlet opening of which can be closed with a stuffing rod. The processing container is placed in an electromagnetic field and the filled melt is set in motion in this closed space and held there until the conditioning is complete.

The movement of the melt leads to the appearance of a sheared material in the area of the solidification front, the melting of which in the liquid area stimulates the crystallization process. This broadens the range from the state of suspension. This suspension area is characteristic of the entire material volume in the proposed process. From the start, the forces acting on the molten metal due to the electromagnetic field are necessary to shear off the solidifying material in the narrow solidification area and to set the entire suspension volume in such a movement that the suspension maintains its ability to flow over a longer period of time and thereby achieving the homogeneity desired for the quality of the casting.

During this process, hollow, partially fluid rotating bodies are formed. The suspension flows onto the walls of the processing container and remains in the moving state until the required casting temperature and homogeneity is reached. It was found on an experimental basis that the homogeneity of the suspension primarily depends on the value of the so-called "gravitational coefficient (K)" which is formed on the free surface of the hollow, partially fluid rotating body. Its minimum limit was determined and, on the basis of the quality analysis, it was found that with a reduction in coefficient in which K is less than 10, the desired morphological isotropy in the cast structure cannot be achieved.

The inventors have also succeeded in obtaining an experimental equation, after which the homogeneity of metallic suspensions is determined by a uniformity coefficient (X), the lower limit for efficient ^¬ aluminum alloys at the value 3.8 x 10 ^{8 2} A / sec lies.

The homogeneity coefficient can be determined from the following ratio: X = N x H ^ x R '

Mean

N = the number of revolutions on the free surface of the partially fluid rotating body;

H = the magnetic field strength;

R = the average wall thickness of the partially fluid rotating body.

The special design of the processing container allows the predetermined melting volume to be placed under constant technological conditions in a metallic suspension, which leaves the container as a homogeneous suspension raw material through the outflow opening and then flows into the casting chamber.

A particularly preferred embodiment of the invention is characterized by a transport chamber provided specifically for the suspension transport. This connects the processing container and the casting chamber, is equipped with a transport piston and can be attached to the vertically arranged casting chamber either at right or at an acute angle.

According to the invention, the transport piston fulfills two functions:

Introduce the metallic suspension or its rest quickly into the casting chamber and seal it tightly after filling. For this purpose, the piston is designed so that its frontal contour continues the inner contour of the casting chamber, so that there is no disturbance from the side of the transport piston in the filling opening area during the movement of the casting piston. The process in which the casting chamber is filled with the metallic suspension is characterized by a so-called "rising filling". As the casting chamber fills with the suspension, the casting piston is moved down (synchronously).

The invention is explained in more detail below on the basis of a drawing illustrating an exemplary embodiment. Show it:

Figure 1 is a schematic representation of a die casting device according to the invention, with which castings can be made from a homogeneous metallic suspension of an exogenous type.

2 section A from FIG. 1 in the region of the junction of the transport chamber 6 into the casting chamber 7.

In the die casting machine shown in Figure 1, a processing container 1 is provided, which is equipped with a stuffing rod 2 and a pipe 3 for the introduction of cooling powder. The container is also connected to the holding furnace 5 by means of a melt line 4 and to the vertically arranged casting chamber 7 by a transport chamber 6.

The processing container 1 is also inserted into an electromagnetic stirring device 8, which as an integrating system consists of an induction and control unit in the form of a closed component of the die casting machine. As can be seen from the diagram, the transport chamber 6 is in the acute angle mounted on the casting chamber 7. However, it is possible to bind the components together at right angles and to provide them with the transport piston 9. This not only clears the transport chamber 6 of the metal residue, but also closes the filling opening 10 in the casting chamber 7. As usual, the casting chamber is aligned with a casting piston 11.

An electromagnetic field is generated in the induction device with a control device (not shown on the diagram), which enables a stirring movement of the melt. Then the melt 12 reaches the processing container 1, which is located in the induction coil 8, by means of a melting line 4 from the holding furnace 5. By using the rotating magnetic field, the predetermined melting volume is set in motion in a closed space because the outflow opening of the processing container is sealed off with the stuffing rod 2. Under centrifugal forces, the melt flows onto the container walls and forms a liquid, hollow rotating body (according to the drawing, the rotating body has a conical shape). This creates a gravitational coefficient K on the free surface of the rotating body, the value of which is determined by the speed at which the different liquid layers move against each other. Thanks to an advantageous design of the processing container, the entire melting volume can already be set in motion on this stage of the work, and not only on the solidification front. The entire melt volume is kept in this movement until the desired condition or suspension homogeneity is reached. Shortly after a hollow liquid rotating body has formed from the superheated melt, a quantity of powder is introduced into the rotating melt via a powder metering device 3 (shown as a pipe in the diagram) which is sufficient to bring about a cooling effect. According to the invention, the cooling powder is introduced into the melt under pressure, specifically in a pulsating regime. The exogenous metallic suspension that suddenly occurs in processing container 1 is the result of three converging processes:

The first process is part of the heat exchange process, in which the powder material extracts the excess heat from the overheated melt. Several cooled suspension areas arise, the temperature of which is below the liquidus level.

The second process is related to the way in which the powder is introduced into the melt. Since this process takes place under pressure, the powder particles do not remain on the inner surface of the liquid rotating body, but penetrate deeply into the melt and act as efficient internal heat consumers, whereby an exogenous metallic suspension occurs simultaneously in the entire melt volume. Since the powder is introduced into the melt in a pulsating manner, elastic oscillations develop in the liquid metal, which stimulate the occurrence of new crystallization nuclei in the specified melt volume even more thanks to a microcavitation effect.

The third process is the constant rotational movement of the given melting volume, which takes place during and parallel to the two operations already mentioned takes place. The advantageous embodiment of the method according to the invention makes it possible to keep the entire melting volume in motion in the closed space. First of all, a material is formed from the melt and then from the metallic suspension, which is the hollow rotating body. The gravitational coefficient that arises on its free surface is extremely important because it determines the homogeneity of the metallic suspension. The following parameters are affected:

1) Geometry of the rotating body, which ensures the stability of the set technological parameters.

2) Temperature and chemical uniformity.

3) No additional hydrogen absorption, viscosity accuracy.

4) Acceleration at which the necessary shear force is present and evenly distributed not only on the solidification front, but in the entire predetermined or rotating melting volume.

5) mixing, through which the metallic suspension wins the ge ^¬ desired homogeneity.

6) Close contact between the melt and powder particles results in very efficient heat dissipation. Overcooled metal areas occurring in the melt are widened thanks to the constant mixing and grow together. Since this takes place under the same technological conditions, the suspension is formed simultaneously in the entire melting volume, so that its homogeneity is optimally adjusted.

An important consequence of the suspension homogeneity obtained is the formation of round, crystalline shapes which are evenly distributed in the solidifying casting, which leads to higher mechanical properties.

After the homogeneity of the suspension is reached, the processing container 1 is emptied of the suspension by the pull-out stuffing rod 2, which is received by the transport chamber 6. The suspension can either be set in rotation or flow directly out of the processing container 1, which has no influence on the suspension quality, but does influence the outflow time.

Since the metallic suspension produced in this way has a large potential of kinetic energy, it flows very quickly towards the transport chamber 6 in the direction of the casting chamber 7, which it fills through the filling opening 10 provided. Here, the injection piston 11 moves simultaneously with the influences such ^¬ sequent suspension synchronous downward to thereby enable the increasing filling operation. In order to be able to protect the transport chamber 6 from the metallic suspension flowing in through the filling opening, particularly at high piston acceleration, the transport chamber is equipped with the transport piston 9. After the filling process ends, the transport piston 9 is pushed forward and closes the filling opening 10 and thereby the casting chamber 7. The transport piston 9 is designed so that its front outline continues the inner outline of the casting chamber 7. The metallic suspension in the casting chamber fills the pressure chamber by means of piston acceleration, so that a casting is made from the semi-rigid starting material.

Initial tests with the method according to the invention have clearly shown that the aluminum alloy castings produced by the method according to the invention have a refined and isotropic structure morphology which is characterized by uniform, round, crystalline forms of primary phases. The result of this is an increase in mechanical properties compared to conventional process products. These values are shown in Table 1 for AlSi9Cu3 alloy cast condition.

Plate 1

Even in the as-cast state, the parts produced by the new process showed considerable improvements. There is a firm connection between the general increase in mechanical properties and the technological conditions under which the metallic suspension was produced. By using the concept according to the invention, it has been possible to produce the castings with high-quality mechanical characteristics and consistent quality.

Claims

claims 1. Die casting process for producing castings from a semi-solidified alloy melt, the alloy melt being brought into the semi-solidified state by stimulation of the crystallization process, introduced into a casting chamber and the castings being produced under pressure, characterized, that an exogenous metallic suspension is produced outside the casting chamber in a closed space, which is functionally connected to the casting chamber and forms a casting unit with it, the metallic suspension being set in motion before entering the casting chamber, a hollow rotating body being formed , which is kept in the rotating state until the suspension homogeneity for pouring is reached. 2. The method according to claim 1, characterized, that the stimulation of the crystallization process takes place in a rotating electromagnetic field. 3. The method according to any one of the preceding claims, characterized, that a cooling powder is supplied for thermodynamic stabilization of the rotating melt, which suddenly brings the melt into a semi-rigid state. 4. The method according to any one of the preceding claims, characterized, that the suspension formation takes place under the action of elastic oscillations of the liquid metal. 5. The method according to any one of the preceding claims, characterized, that the cooling powder is brought into a pulsating state and introduced into the melt. 6. The method according to any one of the preceding claims, characterized, that the cooling powder is introduced under pressure into the melt, air, argon or nitrogen being used as the carrier gas. 7. The method according to any one of the preceding claims, characterized, that the outflowing metallic suspension is placed under the action of an electromagnetic field. 8. The method according to any one of the preceding claims, characterized, that with the filling of the casting chamber, the casting piston is moved downwards synchronously and the increasing filling process is thus enforced. 9. Device for carrying out the method according to one of the preceding claims, consisting of a vertical casting chamber with a casting piston and a processing container arranged outside the casting chamber, characterized, that the processing container is functionally firmly connected to the casting chamber and forms a casting unit with it. 10. The device according to the preceding claim, characterized, that the processing container is surrounded by a coil that generates an electromagnetic field within the processing container and that the processing container is connected to the casting chamber via a transport chamber. 11. Device according to one of the preceding claims, characterized, that the processing container contains a closure device with which the container can be completely closed during processing or during the melt rotation. 12. Device according to one of the preceding claims, characterized, that the transport chamber is attached at right angles to the casting chamber. 13. Device according to one of the preceding claims, characterized, that the transport chamber is attached at an acute angle to the casting chamber. 14. Device according to one of the preceding claims, characterized, that the transport chamber is equipped with a transport piston which closes the filling opening of the casting chamber. 15. Device according to one of the preceding claims, characterized, that the transport piston is formed on its side facing the filling opening of the casting chamber in such a way that its frontal outline continues the inner contour of the casting chamber.