Die-casting method and die-castings obtained thereby

Details Die-casting method and die-castings obtained thereby Die-casting method and die-castings obtained thereby

1999-12-30

2002-11-12

Elve, M. Alexandra (Department: 1725)

Metal founding

Process

Shaping liquid metal against a forming surface

C164S066100, C164S485000, C164S900000

Reexamination Certificate

active

06478075

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a die casting method and apparatus for manufacturing high quality castings having excellent mechanical properties.
BACKGROUND ART
As is well known, a die casting method is a casting method in which molten metal within a casting sleeve is pressure-charged into the cavity of a die and is solidified to thereby manufacture a casting.
The die casting method has advantages that obtained castings have high dimensional accuracy, mass production is possible because the method allows high speed operation, and fully automatic operation is possible through use of a computer. Therefore, the die casting method is frequently used for casting of low-melting-point metals such as aluminum alloys.
However, there have been pointed out the following problems in relation to the die casting method.
A first problem relates to strength. That is, unless a casting obtained through use of the die casting is subjected to reforming such as heat treatment, the casting is generally inapplicable to high-strength members that must have high strength. The reason for this is as follows.
In general, when die casting is performed, molten metal poured into the casting sleeve is rapidly cooled by means of the inner wall of the casting sleeve, and thus solidification scale is generated. Since the solidification scale is cast together with said molten metal, the resultant product contains the solidification scale, resulting in a decrease in the mechanical strength of the product.
Further, when molten metal is injected from the sleeve into a die, air within the casting sleeve becomes caught in said molten metal and is mixed into a resultant casting. In this case, when the casting is heat-treated, swelling called a blister is generated, which becomes a cause of deterioration of quality.
In order to solve the above-described problems of the die casting method, various types of special die casting methods have been proposed. Among them, a hot sleeve method is a die casting method in which casting is performed while a casting sleeve is heated in order to prevent generation of solidification scale at the inner wall of the casting sleeve.
Also, a vertical-injection die casting method is performed in order to suppress catching of air within the casting sleeve.
However, the above-described various types of special die casting methods have the following problems to be solved.
That is, when the speed of injection from a casting sleeve into a die cavity is increased in order to enhance productivity, molten metal within the casing sleeve undergoes turbulent flow, so that the amount of air caught in said molten metal increases, and in addition solidification scale that is produced by rapid cooling and solidification of molten metal at the inner surface of the die is taken into a product. This causes deterioration of the mechanical properties of the obtained product.
Meanwhile, when molten metal is injected from the casting sleeve into the die cavity at a slow speed in order to prevent catching of air, run of molten metal within the die cavity becomes poor, which becomes a cause of a product defect such as misrun.
Japanese Patent Application Laid-Open No. 8-257722 discloses a die casting method that attempts to solve the above-described problems involved in the various kinds of conventional special die casting methods.
In the die casting method disclosed in Japanese Patent Application Laid-Open No. 8-257722, primary crystals of molten metal are granulated within a casting sleeve, charged under pressure into the cavity of a die in a semi-molten state, and solidified therein. According to the die casting method disclosed in Japanese Patent Application Laid-Open No. 8-257722, die casting is performed in the steps described below.
First, as shown in
FIG. 8
, molten metal maintained at a temperature near the liquidus line is poured into a casting sleeve
2
. Subsequently, as shown in
FIG. 8
, the temperature of said molten metal within the casting sleeve
2
is decreased at a predetermined cooling rate, from the temperature near the liquidus line to a predetermined temperature that is below the liquidus line but higher than the solidus line or eutectic line, in order to substantially granulate primary crystals of said molten metal, thereby bringing said molten metal into a semi-molten state. With this operation, there can be obtained thixotropic fluid composed of granular primary crystals and liquid having a temperature not less than the eutectic temperature.
Subsequently, as shown in
FIG. 8
, the semi-molten metal is charged from the casting sleeve
2
into a die
1
. At this time, the semi-molten metal charged from the casting sleeve
2
into the die
1
undergoes laminar flow due to its thixotropy, so that the amount of gas caught in the semi-molten metal decreases. That is, when the metallographic structure is granulated with resultant formation of a solid phase, even if some force would be added, movement of the granulated solid phase and movement of the liquid phase occur simultaneously, so that there occurs a phenomenon in which the solid and liquid phases move together. As a result, catching of gas occurs to a lesser extent, and therefore the amount of gas contained in a casting decreases with the result that blisters are not generated even when heat treatment is performed.
However, the die casting method disclosed in Japanese Patent Application Laid-Open No. 8-257722 has the following drawbacks that must be overcome.
In the die casting method disclosed in Japanese Patent Application Laid-Open No. 8-257722, as shown in
FIG. 8
, molten metal is poured into the casting sleeve
2
from above through use of a ladle or the like. Therefore, when said molten metal falls into the interior of the sleeve
2
, it undergoes turbulent flow within the sleeve
2
and air may be caught in said molten metal. In this case, the amount of gas contained in said molten metal increases and oxide film tends to be formed on the surface of said molten metal, so that gas holes are produced. When strict quality control is performed in order to prevent generation of such gas holes, yield decreases. Further, since casting must be controlled in order to prevent oxides produced in said molten metal from being caught in said molten metal, which oxides would otherwise affect the mechanical properties, the production cycle time may increase, and yield may decrease due to strict quality control.
FIG. 9
shows an example of oxide film
30
and a gas hole
31
which decrease the yield of products as a result of performance of strict quality control.
The die casting method of the present invention was accomplished in view of the forgoing problems of prior art techniques, and an object of the present invention is to provide a die casting method which can minimize the amount of air caught in molten metal when fed into a casting sleeve in order to reduce the amount of gas contained in said molten metal to thereby prevent generation of oxide film or gas holes, while solving problems such as air catching occurring at the time of injection into the cavity of the die and molten metal run defect, thereby enabling efficient production of defect-free perfect castings and increasing the yield. Another object of the present invention is to provide die castings obtained through use of the die casting method.
DISCLOSURE OF THE INVENTION
To solve the above-described problems, the present invention provides in a die casting method that after molten metal is fed into a casting sleeve through its side portion, said molten metal is cooled in order to granulate crystallized primary crystals,
the die casting method characterized in that said molten metal is fed into a casting sleeve through its side portion in the vicinity of the bottom portion thereof, and an inert gas is supplied near a molten-metal feed port into a molten-metal feed pipe.
In the die casting method of the present invention, primary crystals of molten metal within the casting sleeve are substantially granulated and thus said molten metal is brought into a semi-molten

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