EP1046444A1 - Pressure diecasting method - Google Patents

Abstract

A horizontal chamber die casting process comprises forming a stabilized and homogenized cylindrical melt volume for feeding additional compression of the solidifying cast product in the die. A horizontal chamber die casting process comprises applying a vacuum to the chamber and piston, accelerating the melt befor entry into the die and subjecting the die to pressure before or when the melt reaches the ingate opening. Before acceleration, t melt is formed to a cylindrical shape which is retained until achievement of hydrodynamic stabilization, temperature equalizatio and uniform pressure distribution in the cylindrical material volume and which is fed into the solidifying metal after filling o the die to provide additional compression during solidification of the cast product.

Description

The invention relates to a method for producing Castings from Al and Mg alloys, in which in one horizontal casting chamber equipped with a casting piston is a vacuum from the start of the casting process exists and that introduced in the casting chamber Melt volume cooled to the semi-solid state and stirred by means of an electromagnetic field becomes. The metallic suspension is to be introduced into the casting cavity accelerates and before or at the latest when reaching the gate of the die put under pressure.

In principle, the casting chamber can be arranged horizontally or vertically. This arrangement results in specific advantages and disadvantages for the casting process, which are derived from the respective characteristic flow of the melt from the pouring opening to the mold cavity.
A distinguishing feature that can be regarded as typical for the die casting process with the horizontal casting chamber is the formation of the hydrodynamically unstable melt flow, which fills the die with very high speed. As a result, the filling process can often be viewed as spraying rather than flowing. As a result of this type of mold filling, air and oxidic inclusions are introduced into the casting, which subsequently produce cracks in the interior of the casting and bubbles on the surface. A short process analysis shows that such an idea is correct and has a very important meaning in practice.

As is practically always the case, the cast metal only occupies part of the horizontal cylindrical casting chamber and forms a movable non-cylindrical geometric Figure, one side of which is delimited by the casting piston or shaped and the other in the direction of flow can be called open space because there is none there are geometrically pronounced dimensional limits. A conclusion of which is that the predetermined constant hydromechanical Force on the unstable contact surface "Piston - melt flood" acts, in no case evenly distributed on them and thereby a movement causes the hydrodynamic congestion before the Gates lead and as an outflow obstruction works. The hydrodynamic process involved in this Process section not brought into a stable state has no chance on the next stage, to achieve such stability. The one from the outflow cross-section emerging vortex-like jet is crushed even more in the next phase of work, during which already turbulent flood with the corresponding pressure piston speed on the wall of the movable mold half beats. An increase in the inflow velocity occurs due to the narrowing of the cross-section on.

From EP 0733421 A1, which has all the features of the generic terms having independent claims is known that the laminar inflow type when filling reached the die with the melt is that the melt, whose temperature is slightly higher than the liquidus temperature is in the casting chamber by cooling and then heating in the metallic suspension is brought and then into the mold cavity Pressure is injected.

It goes without saying that the conditions for a laminar flow are not sufficient to maintain the extraordinarily low die filling times required for die casting (from 5 to 100 ms), not even when the melt has a very thin layer in it Gun chamber flows. This is because there is no closed space and there is no time to hydrodynamically stabilize the melt (according to the known method) before the mold is filled. As far as the type of inflow is concerned, or any effects that could transform the turbulent flood into a laminar one, there are no such according to the invention, and there are no indications as to whether and how a compensation of the flow velocity could take place in the casting chamber. So there is a turbulent character in the flow flowing along the casting chamber, which manifests itself in the area of the anvil in a vortex formation and a metal jam. The melted material hits the wall of the movable mold half (corresponding to the arrangement shown) and flows into the die casting mold as a scattered free jet.
An improvement in quality, which depends to a greater extent on post-compression of the casting which is already crystallizing, is also not possible according to the arrangement described. All of the disadvantages listed limit the practical use of the method introduced, according to which substantially improved end products cannot be produced.

The invention is therefore based on the object Die casting process in the preamble of the main claim to create described type, in which the Filling the die with a hydrodynamically balanced Melt flood occurs and the crystallization the casting under an additional compression pressure is effected without dispersing the inflow jet.

According to the invention, this object is achieved with a method of the type mentioned solved in that the melt entering the casting chamber before Acceleration a liquid cylinder is designed and its shape up to hydrodynamic stabilization, temperature compensation and even pressure distribution is obtained in the starting material, the crystallizing Metal after filling the die fed with the specially formed melt volume and the solidification of the casting under the additional targeted compression pressure is carried out.

To implement the manufacturing process in such a way, According to the invention, the casting chamber is designed in such a way that they with the back pressure piston and the Provide compression pistons, not before the gate opening ends and at the same time a geometrically restricted Represents space. This is due to special execution the casting chamber has become possible using a T-piece configuration in which the casting piston and the counter-pressure piston are arranged opposite while the compression piston is stored in the vertical channel.

This piston arrangement is of great importance and determines the most important technological process advantages. One of them is that in the Casting chamber located in the melt by displacement of the opposite piston first into the cylindrical one Form is brought so that a necessary uniform Pressure distribution at the "casting piston melt flood" contact surface is secured. Also kick in the compressed cylindrical melt volume the elastic-liquid waves that form of globular primary crystals already in the casting chamber stimulate. In addition, it is the inventive Procedure possible with appropriate Devices in the closed chamber instead of one unstable melt flow one with pronounced rheological Properties hydrodynamically stable metallic To produce suspension. The development and conservation of the rheo effect is brought about by the introduction of a metallic Cooling powder reached.

The crystallization conditions in the casting chamber arise, are such that the morphology of the structure not mainly from heat dissipation through the Walls of the casting chamber depends on new ones solid, exogenous nuclei that melt to the semi-rigid state in a short period of time bring and ensure a crystallization rate those for simultaneous and even Solid phases appear in the entire melt volume lead and for the temperature homogeneity of the occurring provide metallic suspension.

Among the different processes, which of the increasing external pressure in the closed melt volume be influenced, his influence plays on the emergence of microporosity in the casting is extraordinary important role because on this stage the basis for the production of non-porous castings is placed. This is based on the idea that pores, that arise through a nucleation process, only then exist in the melt stable if the difference from the gas pressure and the pressure of the melt larger is called the capillary pressure from the surface tension of the Melt. It follows that - about nucleation to prevent pores during solidification - it it is necessary to increase the melt pressure, what exactly according to the inventive method in the Casting chamber takes place in the compressed volume of material. The one occurring in the metallic suspension Pressure, which is the sum of the existing hydromechanical Pressure and the internal hydrostatic pressure is understood, nucleation of pores is almost impossible and creates crystallization conditions that significantly increase the density of the end product.

In conclusion, develop in the closed, compressed Melt volume thanks to the above Piston insert and the special melt treatment simultaneously running processes, which are characterized by the hydrodynamic Stabilization, temperature compensation and the even distribution of pressure throughout the cylindrical Material volume are marked.

A material acceleration that this stabilization stage succeeds, but is not according to the invention influences the hydromechanical pressure of the casting piston, but it comes from the hydrodynamic back pressure, by the participation of the back pressure piston is effected. This is related to the beneficial Design of the casting chamber and is a result retraction of the counter pressure piston up to the gate or up to the position at which the gate opening is released. Because of a fast piston shift arises on the free contact surface of the liquid Cylinder a pressure drop and the melt strives to to occupy an empty chamber room. The already spatially temperature-balanced and hydrodynamic The melt is stabilized with the counter-pressure piston moved in the direction of the gate. Here such a frontal transfer differs a type of inflow which is used for filling the mouth can be called laminar. Thereby fulfilled this stage plays a very important role around the casting aisles fill with the vortex-free melt jet.

Another advantageous embodiment of the invention The procedure is also that the opposite Surface or end face of casting and counter pressure pistons are designed so that they are concave, ellipsoidal, and represent page-swapped profiles. In this way, such profiles form in the space not a cylinder with a flat base, but one, the base of which each form two spherical sections. Because this shape deviation does not affect the melt transport along the casting chamber the jet formation when filling the pressure chamber exercised, it was assumed in the following, the shape of the compressed melt volume as a cylinder or to be referred to as a cylindrical volume.

At the same time, the special piston design creates further technological advantages.
The short-term pressure reduction - due to the withdrawal from the counter-pressure piston - is leveled by moving the casting piston on a further stage and the die is already filled with the next melt portions under the hydromechanical pressure due to the acceleration of the casting piston. The piston path ends in the mouth area with a piston coupling, according to which - due to the advantageous design - a small cylindrical melt volume occurs in the intermediate space, which is arranged under the gate opening and above the compression piston, is in the compressed state and has a common axis with the mouth and the Compression piston has. As a result, an additional melt volume is formed in the vicinity of the die or the gate, with which the casting which is already crystallizing is fed in before it finally solidifies.

In this way, the already mentioned melt volume not just pressed in over the mouth, but it also finds a targeted pressing of the crystallizing Casting instead of the compression piston. To the technological operations outlined realize, the compression piston is in the vertical Part of the T-configuration casting chamber installed in such a way that he a vertical shift in the direction of the mouth area can enforce. The between the casting and counter-pressure piston shaped melt volume due to this arrangement by accelerating the Compression piston in the die with the corresponding one hydromechanical force pressed. A the constructive condition for this is that the Diameter of the end face of the compression piston the inner diameter of the cylindrical, from casting and counterpressure piston shaped contour got to. This will not only crush inflow avoided, resulting in indispensable conditions of the conventional Procedure for filling the die heard but both a metal loss is reduced because there is no metal residue in the gate opening arises, as well as a targeted densification of the solidifying Cast performed.

• Fig. 1 eine schematische Darstellung der VACURAL-Druckgießmaschine, in T-Konfiguration ausgerüstet, mit Gießkammer, Gieß-, Gegendruck- und Verdichtungskolben.
• Fig. 2 die Kolbenposition bei der Füllung der Gießkammer mit der Schmelze.
• Fig 3 die Kolbenposition, bei der die Schmelze in eine zylindrische Form und einen zusammengedrückten Zustand gebracht ist.
• Fig. 4 die Kolbenposition, bei der die entstehende metallische Suspension unter dem hydrodynamischen Druck in die Druckgießform einströmt.
• Fig. 5 die Kolbenposition, bei der unter der Mündung ein "Speicher" angelegt wird.
• Fig. 6 die Kolbenposition, bei der die Zuspeisung und die Verdichtung des kristallisierenden Gußstücks durchgeführt werden.
• Fig 7 einen Schnitt durch den gefüllten Formhohlraum nach Abschluß des Füllvorgangs.

Further advantages, features and details of the invention result from the following description of preferred exemplary embodiments and from the drawing; this shows in

• Fig. 1 is a schematic representation of the VACURAL die casting machine, equipped in a T configuration, with casting chamber, casting, back pressure and compression pistons.
• Fig. 2 shows the piston position when the casting chamber is filled with the melt.
• 3 shows the piston position in which the melt is brought into a cylindrical shape and a compressed state.
• Fig. 4 shows the piston position in which the resulting metallic suspension flows into the die under the hydrodynamic pressure.
• Fig. 5 shows the piston position in which a "memory" is created under the mouth.
• Fig. 6 shows the piston position at which the feed and the compression of the crystallizing casting are carried out.
• 7 shows a section through the filled mold cavity after completion of the filling process.

On the die casting machine shown schematically in FIG. 1 a melting tank (1) is provided, which by means of an intake manifold (2) with a T-configuration casting chamber (3) is connected (hereinafter: casting chamber). By a mouth (4) with a die casting chamber (5) connected between a movable (6) and a fixed (7) mold half. In the horizontal branches the casting chamber (3) are casting pistons (8) and counter pressure pistons (9) while in the vertical channel Compression piston (10) is mounted. The suction pipe (2) and the casting chamber (3) are with a powder metering device (11) and an electromagnetic stirrer (12) equipped, the latter around the Casting chamber (3) is arranged in a ring.

• eine zylindrische Form anzunehmen
• sich hydrodynamisch zu richten bzw. hydrodynamisch zu stabilisieren und demgemäß
• in der hydrodynamisch stabilen Flut schon auf dieser Etappe durch den zunehmenden Druck die Kristallisationsprozesse zu aktivieren.

A practical use of the invention is clarified with the graphic representation in FIGS. 1-7. The melt (13) or a predetermined melt volume passes from the melt container (1) by means of the suction pipe (2) into the T-configuration casting chamber (3). The molten material is mixed with a cooling powder in the suction pipe (2); this is done by the powder metering device (11) (Fig. 2). The powder introduced has a cooling effect, which not only lowers the superheating temperature, but also creates undercooling areas in the melt, the characteristics of which determine the crystallization process and ultimately the formation of the casting. The cooled melt reaches the casting chamber (3) in front of the casting piston (8), which occupies a position P8-1, and after a short time begins to produce primary crystals, which are preferably round in shape. Before the melt enters, however, the positioning of the counterpressure piston (9) changes. With a displacement provided along the casting chamber (3), it leaves its starting position P9-1 behind to occupy the position P9-2. As a result, a geometrically limited space is formed in the casting chamber (3), in which there is a pressure piston and does not come into contact with the gate opening (4).
The flood of melt that enters the casting chamber (3) in the conventional process is characterized by a pronounced hydrodynamic instability, which develops several times during the pressure increase. According to the invention, the subsequent displacement of the casting piston (8) not only achieves general stabilization, but also, to a certain extent, temperature homogeneity in the melt volume. The casting piston (8) moves forward or counter pressure piston (9) and occupies a position P 8-2, acts on the unstable melted flood with a constant driving force, pushes it in front of itself and forces the melt (Fig. 3) herewith

• assume a cylindrical shape
• to orient itself hydrodynamically or to stabilize hydrodynamically and accordingly
• to activate the crystallization processes in this hydrodynamically stable flood at this stage due to the increasing pressure.

It is the occurrence of elastic-liquid Waves in the compressed melt volume. One phenomenon of this is the emergence of the character variable Pressure in the melt leading to increase of the density and energy fluctuation and thereby the Crystallization development stimulated. However, it requires the solid-liquid melt cylinder in the casting chamber (3) to form and a homogeneous pressure distribution achieve. Such a result is very important because it's a conclusion from the match of contact areas on the boundary between the liquid Cylinder and casting piston (8), so that one to a certain extent homogeneous pressure distribution in the material volume can be ensured.

• im axial verschobenen Material in jedem/r Punkt/ Schicht homogene Druck- und Geschwindigkeitsverhältnisse herrschen.
• die hydrodynamisch stabilisierte Strömung die Anschnittmündung (4) erreicht, ohne einen Aufschlag eines stationären Freistrahls an eine senkrechte Wand der beweglichen Formhälfte hervorzurufen bzw. ohne daß der Strahl zerstiebt.
• der Gießgang ebenso wie die Mündung laminar mit der Metallströmung aufgefüllt werden.

The temperature compensation achieved is maintained by means of electromagnetic stirring, for which purpose the stirring device (12) is arranged in a ring around the casting chamber (3). This creates a forced circular movement of the melt, which has already been cooled with the cooling powder, which leads to temperature compensation in the cylindrical material volume and contributes to the development of crystallization conditions for round (globular) crystals. In the next stage of the process, the counterpressure piston is withdrawn from position P9-2 to starting position P9-1. As a result, the gate opening (4) is opened (FIG. 4), with its concave ellipsoidal profile continuing the opening profile after stopping. Because of a rapid shift, a pressure drop occurs on the free surface of the liquid cylinder and the compressed rotating metallic suspension strives in this direction, and only under hydrodynamic pressure. The temperature-balanced and hydrodynamically stabilized suspension occupies the free chamber space and is pulled with the counter-pressure piston (9) to the gate opening (4). The distinguishing feature of this stage of work is that

• The axially displaced material has homogeneous pressure and speed conditions in every point / layer.
• the hydrodynamically stabilized flow reaches the gate (4) without causing a stationary free jet to strike a vertical wall of the movable mold half or without the jet being destroyed.
• the pouring passage as well as the mouth are laminarly filled with the metal flow.

The short-term pressure reduction - due to the retraction from the piston (9) - is compensated for by the shifting of the casting piston on the next stage (Fig. 5), which begins at the same time as the counter-pressure piston reaches position P9-1. The piston movement is the change in position from P8-2 to P8-3 and ends before the gate (4) after it is coupled to the counter-pressure piston (9). The metallic suspension, which already fills the casting aisle and the mouth, is pressed out into the pressure casting chamber (5) by hydromechanical piston action and fills the cavity in a laminar manner. Since the end face of the casting piston (8), like the counter-pressure piston (9), has the concave, ellipsoidal profile, a small, cylindrical material area is formed in the space, which is directly under the gate (4) and above the compression piston (10) is in an elasto-plastic state and has a vertical axis common to them. This "storage" is used to make up the already crystallizing casting.
The last stage of work, which can be referred to as the post-compression of the end product, is carried out by means of the vertical displacement of the compression piston (10) according to the direction of the mouth, and thus the die-casting process is completed. The compression piston leaves its starting position P10-1 and thus displaces the cylindrical metal portion over the mouth (4) until the new position P10-2 is reached (Fig. 6). As a result, a feed of the already crystallizing casting is pushed through and the semi-rigid casting is pressed through the end face of the compression piston located in the mouth.
In Fig. 7 is an important embodiment of the deformation portion of the invention is shown Metalleinströmung (⊘ _M - diameter of the cylindrical melt volume ⊘ _k -diameter of the outline). Its contour-closed design not only allows the swirl-free die-mold filling, feeding and post-compression of the end product, but it also enables the metal loss (reduction of the press residue) to be reduced considerably.

• Mit der räumlich beschränkten Gießkammer, die T-Stück-Konfiguration hat, und dem konturgeschlossenen Deformationsherd für Metalleinströmung erhöht sich die Funktionsfähigkeit des gesamten Druckvorgangs beträchtlich.
• Im Gußstück dominiert ein homogenes feinzelliges Gefüge.
• Die Entwicklung von typischen Gußfehlem wie verteilte Schrumpfungsporen, Lunker und undichtes Gefüge wird durch die Zuspeisung und das Nachverdichten des kristallisierenden Gußstücks behindert. Der Dichteindex der nach dem erfindungsgemäßen Verfahren hergestellten Gußstücke erhöht sich auf das Fünffache.

Initial experiments with the method according to the invention have clearly shown:

• With the spatially restricted casting chamber, which has a T-piece configuration, and the contour-closed deformation source for the inflow of metal, the functionality of the entire printing process increases considerably.
• A homogeneous fine-cell structure dominates in the casting.
• The development of typical casting defects such as distributed shrinkage pores, voids and a leaky structure is hindered by the feed and the recompression of the crystallizing casting. The density index of the castings produced by the process according to the invention increases five times.

Claims (10)

Die casting process for the production of castings with a horizontal casting chamber and a casting piston, with the vacuum applied from the start the liquid material before it is introduced into the Form accelerated and before or at the latest at Reaching the gate opening of the mold under pressure is set, characterized in that the liquid material before acceleration into the cylindrical Form is brought up to the hydrodynamic Stabilization, temperature compensation and the uniform pressure distribution in the cylindrical Volume of material remains, with the crystallizing metal after filling the die with specially created melt volume and the formation of the casting under the additional, targeted compression pressure is carried out. Method according to claim 1,
characterized in that
the casting chamber is provided with the counter pressure piston and the compression piston in order to bring the liquid material into the cylindrical form, to accelerate it along the casting chamber and to compress the crystallizing metal in the printing form into a casting. Method according to one of the preceding claims,
characterized in that
the casting chamber is in the T-piece configuration and does not end in front of the gate, the casting piston and the counterpressure piston being arranged opposite one another, while the compression piston is mounted in the vertical channel. Method according to one of the preceding claims,
characterized in that
the end face of the casting and counter-pressure pistons is designed so that they represent concave, ellipsoidal, and side-reversed profiles. Method according to one of the preceding claims,
characterized in that
the liquid material, which is brought into the cylindrical form before the acceleration, it is in the compressed state between the casting and counter pressure piston. Method according to one of the preceding claims,
characterized in that
the hydrodynamic stabilization and the temperature compensation of the cylindrical melt volume or of the metallic suspension built up in the casting chamber are activated and maintained by introducing a cooling powder into the molten material, for which purpose the corresponding device is provided. Method according to one of the preceding claims,
characterized in that
the acceleration of material in the direction of the pressure drop in the area between the counterpressure piston surface and the free surface of the liquid cylinder begins after the counterpressure piston quickly retracts up to the gate opening, with its concave, ellipsoidal profile continuing the mouth profile after stopping. Method according to one of the preceding claims,
characterized in that
the already accelerated melt, which is only under hydrodynamic pressure, is acted upon with the hydromechanical pressure by means of the casting piston displacement, which after its coupling with the counterpressure piston and the simultaneous formation of the small, cylindrical melt volume in the intermediate space is completed. Method according to one of the preceding claims,
characterized in that
the melt volume mentioned at the outset is arranged under the gate opening, which has an axis common to the mouth and is in the compressed state. Method according to one of the preceding claims,
characterized in that
said melt volume is pressed by means of the compression piston into the pressure casting chamber via the mouth, and the semi-rigid casting is pressed through the end face of the compression piston located in the mouth, the mouth and compression pistons lying in one axis.

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