Metal founding – Process – Shaping liquid metal against a forming surface
Reexamination Certificate
1999-07-13
2001-01-23
Pyon, Harold (Department: 1722)
Metal founding
Process
Shaping liquid metal against a forming surface
C164S113000, C164S253000, C164S312000, C164S457000
Reexamination Certificate
active
06176294
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a die-casting method for production of die-cast products useful not only as structural members but also as functional members and die-cast products manufactured thereby.
In a conventional die-casting method, molten aluminum or aluminum alloy (hereinafter referred to as “molten metal”) poured into a sleeve is forcibly injected into a cavity of a die-casting mold by a plunger. Most of gases such as air and water vapor are purged from the cavity in response to injection of the molten metal, but some of the gases remain as such in the cavity even after the injection. Especially, die-casting molds designed for production of thin-walled products or products having complicated configurations have portions acting as bottlenecks against gas flow, so that it is difficult to completely remove gases from the cavity.
Gases trapped in the cavity are included in a cast product, when the injected molten metal is cooled and solidified in the cavity. Inclusion of gases causes defects such as blowholes and porosity in die-cast products. Therefore, the die-cast products obtained in this way have been regarded as members unsuitable for functional uses, e.g. scrolls, pistons, cylinder blocks, connecting rods or suspension parts, due to poor mechanical properties. If cast defects derived from inclusion of gases are suppressed, a die-casting method excellent in productivity can be applied to various fields of technology.
In order to eliminate harmful influences derived from inclusion of gases, a vacuum die-casting method was proposed. According to the vacuum die-casting method, a cavity of a die-casting mold is evacuated before injection of molten metal, so as to remove gases from the cavity. The cavity is held at a degree of vacuum in the range of 200-500 millibar by evacuation. However, an internal pressure of the cavity can not be reduced less than said value, due to invasion of air through narrow gaps of dies. Invasion of air also occurs during the pouring of molten metal into a sleeve. As a result, cast defects such as porosity caused by inclusion of gases are detected even in products obtained by the vacuum die-casting method, although inclusion of gases is somewhat decreased as compared with products obtained by a conventional die-casting method. In this regard, the products are not good enough for use as functional members.
An oxygen die-casting method has been developed in order to eliminate defects in the vacuum die-casting method. According to the oxygen die casting method, as disclosed in JP B 50-21143, a cavity of a die-casting mold is filled with oxygen at a pressure higher than the atmospheric pressure so as to replace gases by oxygen prior to injection of molten metal. Since oxygen gas fed into the cavity is effused through narrow gaps of dies as well as an injection hole, invasion of atmospheric gas through the narrow gaps or the injection hole can be prohibited. In addition, the oxygen gas fed into the cavity is reacted with molten metal, and a reaction product Al
2
O
3
is dispersed as fine particles in a cast product without harmful influences on an obtained die-cast product.
However, complete replacement of gases from the cavity of a die-casting mold by oxygen injection is substantially impossible, even when oxygen is fed into the cavity at a pressure higher than the atmospheric pressure. Gases often remain at difficult portions for the replacement in the cavity. A die-casting mold designed for production of a product having a complicated configuration has difficult portions to which oxygen is hardly reached, so that gases such as air and water vapor can not be replaced by the fed oxygen but remain as such. The residual gases and water vapor from parting agents are included in products and cause defects.
Residual air can be efficiently removed from the cavity by oxygen blowing during evacuation, as disclosed in JP B 57-140. However, simultaneous oxygen blowing with evacuation is not effective for removal of water vapor. In fact, cast defects caused by inclusion of gases are still detected in a cast product obtained by this method. JP B 1-46224 discloses another die-casting method, wherein oxygen blowing is performed after evacuation. However, some cast defects are also detected in a cast product, since a cavity of a die-casting mold is held at a decompressed pressure during the oxygen blowing.
Inclusion of the trapped gases also causes blisters in die-cast products, when the die-cast products are heat-treated in such as T6 treatment (i.e., solution heating, quenching and then aging) for improvement of mechanical properties. In order to avoid such blisters, most of die-cast products are not used with heat treatment.
SUMMARY OF THE INVENTION
The present invention is aimed at elimination of such problems as above-mentioned. The objective of the present invention is to remarkably reduce inclusion of gases by combining advantages of both the vacuum die-casting and the oxygen die-casting for die-cast products useful as functional members.
A die-casting method according to the present invention is characterized by evacuating a cavity of a die-casting mold to a degree of vacuum not higher than 100 millibar for removal of gases as well as water vapor from the cavity, followed by blowing oxygen gas into the cavity, and then forcibly injecting molten metal into the cavity at a time when an internal pressure of the cavity exceeds the atmospheric pressure.
At first, the cavity of the die-casting mold is evacuated to a degree of vacuum not higher than 100 millibar. Gases are effectively discharged from the cavity, especially when the suction speed is higher than 500 millibar/second. The cavity is then filled with oxygen gas at a pressure a little higher than the atmospheric pressure. When the internal pressure of the cavity exceeds the atmospheric pressure, injection of molten metal into the cavity is started.
Since molten metal is injected into the cavity conditioned in this way, gases to be trapped in a cast product are remarkably reduced to a level less than 1 cc/100 g-Al. Consequently, obtained die-cast products have excellent mechanical properties required for functional members. In addition, the die-cast products can be heat-treated in T6 treatment without blisters caused by the trapped gases.
Gases included in a die-cast product are derived from air remaining in a cavity of a die-casting mold in a conventional die-casting method. Such residual air can be substantially reduced by vacuum or oxygen die-casting. However, cast defects caused by trapped gases inevitably occur in an obtained die-cast product, even when the residual air is substantially reduced.
The inventors have researched and examined an effect of gaseous components on cast defects and their origins from various aspects, with respect to a die-cast product from which harmful influences derived from residual air are eliminated by the vacuum or oxygen die-casting method. As a result, the inventors have found that water for diluting parting agents adhering onto an inner surface of a die-casting mold is a main reason for generation of the cast defects and that the influence of water on the cast defects becomes more apparent as residual air in the cavity decreases.
Water with a parting agent is vaporized and discharged as a vapor from the cavity, when the cavity is evacuated. However, commonly used water-based parting agents will take some time to dry up even under vacuum condition. When the cavity is merely evacuated, vaporization of water is likely limited to a surface of the parting agent without vaporization from the interior of the parting agent so that the parting agent is not sufficiently dried up. In addition, generated water vapor is partially left in the cavity and consequently included in molten metal injected into the cavity.
According to the present invention, water with the parting agent is mostly discharged as a vapor from a cavity of a die-casting mold by vacuum evacuation, and the parting agent is sufficiently dried up. Water vapor, which still remains
Ikari Takaaki
Kuramasu Yukio
Lin I.-H.
Nippon Light Metal Co. Ltd.
Pyon Harold
Webb Zeisenheim Logsdon Orkin & Hanson, P.C.
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