Magnesium pressure casting

Details Magnesium pressure casting Magnesium pressure casting

2000-06-28

2003-10-21

Elve, M. Alexandra (Department: 1725)

Metal founding

Process

Shaping liquid metal against a forming surface

C164S900000

Reexamination Certificate

active

06634412

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to an improved metal flow system, for use in the production of pressure castings made from magnesium alloys in a molten or thixotropic state and suitable for use with existing machines in various forms including hot and cold chamber die casting machines.
BACKGROUND OF THE INVENTION
An understanding has developed throughout the international pressure casting industry that, because of the lower heat capacity of magnesium alloys compared to zinc and aluminium alloys, it is necessary to use large runners and gates to prevent premature freezing of the molten magnesium alloy metal. Indeed, this is considered best practice by the industry, although interpretations vary considerably.
Within the industry, there are many different design methods which are thought to provide satisfactory castings from magnesium alloys. However, the magnesium alloy pressure castings produced by these methods generally exhibit a greater degree of surface defects, when compared to zinc or aluminium pressure castings, although castings may be of servicable quality.
BRIEF SUMMARY OF THE INVENTION
We have found that it is possible to produce high quality pressure castings of magnesium alloys with use of the present invention. The castings so produced are able to be of a quality comparable to that obtainable with castings of aluminium or zinc alloys. Moreover, we have found that casting quality is able to be enhanced by the use of metal flow systems having runners and gates which are small relative to current best practice. The metal flow systems of the invention enable a substantial improvement in the casting yield; that is, in the percentage ratio of casting weight to total shot weights. Thus, the weight of metal which needs to be recycled and reprocessed is able to be substantially reduced, with resultant reduction in production costs.
The present invention enables a method of calculating metal flow systems for the production of magnesium alloy castings which exhibit improved quality and with significantly less metal in the feeding systems, with consequent reduction in cost compared to prior practices.
The present invention provides or uses, for the pressure casting of magnesium alloy in a molten or thixotropic state with a pressure casting machine having a mould or die which defines a die cavity, a metal flow system which includes a die or mould tool means which defines at least one runner from which molten magnesium alloy is able to be injected into the die cavity. In a first form of the invention, the metal flow system is of a form providing for control of metal flow velocities within the flow system, whereby substantially all of the metal flowing throughout the die cavity is in a viscous or semisolid state.
The invention also provides a process for producing a casting of a magnesium alloy, wherein the magnesium alloy is cast in a molten or thixotropic state, using a pressure casting machine having a mould or die which defines a die cavity, and using a metal flow system which includes a die or mould tool means which defines at least one runner of the system from which molten magnesium alloy is injected into the die cavity, and wherein the flow system is of a form whereby it provides for control of metal flow velocities therein whereby substantially all of the metal flowing throughout the die cavity is in a viscous or semi-solid state.
Our findings indicate that, with the attainment of a viscous or semi-solid state, filling of the die cavity proceeds progressively by semi-solid fronts of metal moving away from a gate or other site of injection. This form of filling with magnesium alloy is a major departure from the highly complex liquid peripheral fill, followed by back-filling, encountered with die casting of aluminium or zinc alloys and first described by Frommer in 1932 (see the reference text “Die Casting” by H. H. Doehler, published 1991 by McGraw-Hill Publishing, Inc.
In the first form of the invention, the flow of magnesium alloy from the runner is via at least one controlled expansion region of the metal flow system in which region the metal flow is able to spread laterally, with respect to its direction of injection, with a resultant reduction in its flow velocity relative to its velocity in the runner. In a preferred arrangement, the controlled expansion region of the flow system comprises a gate through which the metal flows from the runner to the die cavity. In that preferred arrangement, the gate and runner are such that an effective cross-sectional area of flow through the gate exceeds an effective cross-sectional area of flow through the runner, whereby the molten metal has a velocity through the effective cross-sectional area of flow through the runner which exceeds its velocity through the gate. This is contrary to current recommended practice.
In that preferred arrangement according to the first form of the invention, the cross-sectional area of flow through the gate preferably exceeds the effective cross-sectional area of flow through the runner to an extent providing for a ratio of those areas in the range of about 2:1 to 4:1.
The effective cross-sectional area of flow through the runner may prevail throughout the full longitudinal extent of the runner. However, the effective area may prevail over only part of that longitudinal extent. Thus, in the latter case, there may be a larger cross-sectional area of flow through the runner up-stream from the part of its longitudinal extent in which the effective cross-sectional area of flow prevails.
In an alternative arrangement according to the first form of the invention, the controlled expansion region is defined at least in part by and within the cavity, by surfaces defining the cavity adjacent to the site at which the metal enters the cavity. In this alternative arrangement, there may be an in gate at that site, through which metal flows from the runner to the cavity. In that case, the gate need not define a controlled expansion region due to it having a larger effective cross-section than the runner, and the gate may simply comprise the outlet end of the runner at the cavity. However, the gate may define part of a controlled expansion region of which a further part is defined by and within the die cavity.
The alternative arrangement, in which the metal flow system has a controlled expansion region, defined at least by and within the die cavity, is not suitable for all die cavity shapes. Also, attainment of such region is dependent upon the flow direction as the metal enters the cavity relative to adjacent surfaces of the cavity. In general, the surfaces need to alloy expansion while controlling it, so as to function in the cavity in a manner similar to a gate providing controlled expansion. As such, a controlled expansion region defined by the cavity can be regarded as a pseudo gate and, in general, a reference in the following to a gate is to be understood as covering both an actual gate and such pseudo gate. However, the die cavity surfaces which define a pseudo gate, through which metal flows on entering the cavity, usually will not contain the flow on all sides, although substantial containment such as on three sides is preferred.
A controlled expansion region may be achieved by a sharp, step-wise increase in cross-section from the effective cross-section of the runner. However, it is preferred that the controlled expansion region progressively increases in cross-section in the direction of metal flow therethrough. Thus, where the expansion region is defined by an actual gate, the gate preferably increases in cross-section to a maximum cross-section where the gate communicates with the die cavity.
The invention is applicable to either hot-chamber or cold-chamber die casting. In each case, the invention enables very substantial cost savings in the production of castings of magnesium, as illustrated later herein, as it enables a substantial improvement in the casting yield. Hence the weight of runner/sprue metal which needs to be recycled and re-processed is substantially reduced, a matter o

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