Metal founding – Process – Shaping liquid metal against a forming surface
Reexamination Certificate
2002-07-10
2003-11-18
Lin, Kuang Y. (Department: 1725)
Metal founding
Process
Shaping liquid metal against a forming surface
C164S065000, C164S113000, C164S312000
Reexamination Certificate
active
06648054
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a die-casting method and machine for manufacturing a die-cast product usable as not only structural members but also functional members by virtue of its suppressed cast defects such as internal porosities or blowholes.
In a conventional die-casting method, molten aluminum or molten aluminum alloy (hereinafter correctively referred to as “molten metal”) poured into a sleeve of a die-casting machine is forcibly injected into a cavity defined in dies by a plunger in the sleeve. While most of gases, such as air or water vapor, residing in the cavity are purged from the cavity in conjunction with the injection of the molten aluminum, a part of the gases can be undesirably left in the cavity after the completion of the injection of the molten aluminum alloy. Particularly, a die assembly designed for a specific product having a very thin thickness or a complicated configuration can involve narrowed portions constraining fluid flow, and thereby it is difficult to achieve complete exclusion of gases residing in its die cavity.
During a cooldown/solidification stage of the injected molten aluminum in the dies, the residual gases in the cavity are took in the molten aluminum and incorporated into the resulting die-cast product as a factor of casting defects such as internal porosities or blowholes. As a result, the obtained die-casting products have suffered from inferiority in mechanical properties such as strength or elongation, resulting in unfavorable evaluation for being inadequate to use as functional components, such as a scroll, piston, cylinder block, connecting rod or suspension member. If such casting defects caused by the residual gases are suppressed, the die-casting method will have enlarged range of applicable fields with its intrinsic excellent productivity.
There has been known a vacuum die-casting method as one of techniques intended to eliminate the adverse effect of the residual gases. In the vacuum die-casting method, a die cavity is evacuated prior to a forcible injection of molten aluminum in order to remove air from the cavity. However, in this method, the internal pressure of the cavity has a limited vacuum ranging from 200 to 500 millibar and it is practically impossible to obtain further reduced vacuum, because some ambient air enters in the cavity through a gap between mating faces of dies and another ambient air is additionally introduced in a sleeve of a die-casting machine during the operation of pouring the molten aluminum into the sleeve. Thus, even though a product obtained from the vacuum die-casting method has a reduced volume of incorporated air as compared with products from another conventional die-casting method, it is still involved with the casting defects such as internal porosities caused by the incorporated gases, and is thereby quite inadequate to use as functional components.
An oxygen die-casting method has been developed to eliminate the disadvantage of the vacuum die-casting method (see Japanese Patent Publication No. 50-21143). In the oxygen die-casting method, oxygen gas is filled in a die cavity with a pressure of atmospheric pressure or more to replace gases in the cavity by the oxygen gas. Thus, an excessive part of the supplied oxygen gas is blown out of the cavity through a gap between mating faces of dies and a pour opening for pouring molten aluminum therethrough to prevent ambient air from entering in the cavity through the gap and the pour opening. The supplied oxygen gas remaining in the cavity reacts with the molten aluminum to form a fine structure of Al
2
O
3
dispersed over the resulting die-cast product without any adverse effect on the properties of the product.
However, even by supplying the oxygen gas into the cavity with the pressure of atmospheric pressure or more, it is difficult to completely remove air from the cavity. Generally, a die cavity having a complicated configuration leads to an increased volume of residual air therein. More specifically, the die cavity having a complicated configuration involves a narrowed portion incapable of receiving therein the supplied oxygen gas, and the narrowed portion keeps gases such as air and water vapor residing therein without replacing the gases by the supplied oxygen gas. These residual gases are incorporated into the resulting die-cast product as a factor causing the casting defects.
The residual air in the cavity as a factor causing the casting defects can be effectively replaced by oxygen gas by injecting the oxygen-gas simultaneously with the evacuation of the cavity (Japanese Patent Publication No. 57-140). However, even if the oxygen gas is injected in synchronous with the evacuation of the cavity, water is not effectively removed from the cavity. In fact, a die-cast product obtained from this technique is still involved with the cast defects caused by the residual gases. A technique of injecting oxygen gas after the evacuation of the cavity has been also known (Japanese Patent Publication No. 1-46224). However, this technique cannot sufficiently suppress the casting defects caused by the residual gases, because the inner pressure of the die cavity is maintained merely at a vacuum ranging from about 200 to 400 millibar.
In view of the above circumstance, the inventors has developed a die-casting method comprising the steps of evacuating a die cavity to a vacuum of 100 millibar or less, then injecting a reactive gas such as oxygen gas into the cavity, and initiating a forcible injection of molten aluminum alloy at a time the internal pressure of the cavity is increased to atmospheric pressure or more (Japanese Patent Application No. 11-154566). Evacuating the cavity to the vacuum of 100 millibar or less accelerates vaporization of water from a release agent attached on the inner surfaces of dies. Subsequently supplying the reactive gas to the evacuated cavity allows the reactive gas to be spread all over the interior of the dies, so as to effectively purge the residual air and the water from the release agent as well as other undesirable gases in the cavity. Thus, a die-cast product can be obtained with a significantly reduced volume of incorporated gases, and thereby the casting defects caused by the residual gases can be suppressed. In addition, the die-cast product can be heated without generation of blisters caused by the residual gas. This allows the die-cast product to be improved in its mechanical properties through a heat treatment such as a T6 treatment.
Most of the reactive gas supplied to the cavity reacts with the molten aluminum ally injected into the cavity to form a fine structure of Al
2
O
3
dispersed over the resulting product, and a part of the reactive gas is pushed out of the cavity by the molten aluminum alloy forcibly injected into the cavity. However, depending on the configuration of an intended die-cast product, a part of the reactive gas can be left in the cavity after the completion of the injection of the molten aluminum alloy. Then, the residual reactive gas is undesirably incorporated into the die-cast product without effective reaction with the molten aluminum alloy or in an unreacted state. For instance, in dies intended to manufacture a die-cast product having a complicated configuration, its cavity is typically designed to have a configuration in which a metal flow channel are branched into plural channels and the plurality of channels are jointed together. This junction area creates a dead-end-like portion where a path for pushing out the reactive gas therethrough is clogged by the respective metal flows in the jointed channels to trap the reactive gas.
A system operable to evacuate a die cavity by opening a bypass passage in fluid communication with the cavity (Japanese Patent Publication No. 1-46224) has a limited vacuum ranging from 200 to 400 millibar in the cavity for the following reasons, resulting in a substantial volume of residual air and insufficient purge of the reactive gas in an unreacted state, which leads to insufficient reduction of gases incorporated i
Ikari Takaaki
Kuramasu Yukio
Tajima Yoshihiro
Yoshida Osamu
Lin Kuang Y.
Nippon Light Metal Co. Ltd.
Smith Patent Office
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