EP0972593A1

EP0972593A1 - Pressure die-casting method and products obtained thereby

Info

Publication number: EP0972593A1
Application number: EP99810629A
Authority: EP
Inventors: Yukio Kuramasu; Takaaki Ikari
Original assignee: Alusuisse Lonza Services Ltd; Alusuisse Technology and Management Ltd; Nippon Light Metal Co Ltd
Current assignee: 3A Composites International AG; Nippon Light Metal Co Ltd
Priority date: 1998-07-14
Filing date: 1999-07-13
Publication date: 2000-01-19

Abstract

After a cavity 2 of a die-casting mold 1 is evacuated to exclude gas, oxygen gas is blown into the cavity 2, and then an molten metal 5 is forcibly injected into the cavity 2. The cavity 2 is decompressed to a degree of vacuum not more than 100 millibar by evacuation through a suction nozzle 11. The oxygen gas is blown through a nozzle 14 into the cavity 2 so as to increase a pressure of the cavity 2 higher than the atmospheric pressure. When the molten metal 5 is injected into the cavity 2 clarified in this way, inclusion of gas is perfectly prohibited. As a result, an obtained die-cast products is free from defects such as blowholes or porosity caused by inclusion of gas and so useful as a functional member as well as a structural member.

Description

BACKGROUND OF THE INVENTION

The present invention relates to a die-casting method for productions of die-cast products useful not only as structural members but also as functional membere 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 gas such as air or water vapor are purged from the cavity in response to injection of the molten metal, but some of the gas remain as such in the cavity even after the injection. Especially, die-casting molds designed for productions of thin-walled products or products having complicated configurations have portions acting as bottlenecks against metal flow, so that it is difficult to completely remove gas from the cavity.
Gas trapped in the cavity is included in a cast product, when the injected molten metal is cooled and solidified in the cavity. Inclusion of gas causes defects such as blowholes and porosity in die-cast products. Therefore, the die-cast products obtained in this way have been regarded as a member unsuitable for functional uses, (scroll, piston, cylinder block, suspension parts), due to poor mechanical properties. If cast defects derived from inclusion of gas 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 gas, 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 gas from the cavity. The cavity is held at a degree of vacuum in the range of 200-500 millibar by the evacuation for instance. However, the degree of vacuum can not be reduced less than said value, due to leakage of air through narrow gaps of dies. Leakage of air also occurs during pouring molten metal into a sleeve. As a result, cast defects such as porosity caused by indusion of gas are detected even in products obtained by the vacuum die-casting method, although inclusion of gas is somewhat decreased as compared with products obtained by a conventional die-casting method. In this regard, the product is not good enough for use as a functional member.
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 Japanese Patent Application Laid-Open 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 gas by oxygen in 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 Al₂O₃ is dispersed as fine particles in a cast product without harmful influences on an obtained die-cast product.
However, complete replacement of gas 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. Gas often remain at difficult portions for the replacement in the cavity. Because most widely used water based parting agents will take some time to dry up under relatively higher atmospheric pressure. A die-casting mold designed for production of an product having a complicated configuration has difficult portions to which oxygen is hardly reached, so that gas such as air or water vapor can not be replaced by the fed oxygen but remain as such. The remained gas and water vapor from parting agents are cast into products to produce defects.
Inclusion of the trapped gas 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 used without 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 gas 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 remove gas 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 first, the cavity of the die-casting mold is preferably evacuated to a degree of vacuum not higher than 100 millibar, Pressure of the cavity is then increased to a value higher than the atmospheric pressure by oxygen gas. When molten metal is injected into the cavity conditioned in this way, gas trapped in a cast product is remarkably reduced to a level less than 1cc/100g-Al. Consequently, die-cast products obtained have excellent mechanical properties required for functional members. In addition, the die-cast products can be heat treated in T6 treatment without blisters derived from the trapped gas.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 a schematic view illustrating a die-casting machine to which the present invention is applied.
Fig. 2 is a view for explaining blowing oxygen through a sleeve into a cavity of a die-casting mold.
Fig. 3 is a view for explaining pouring an molten metal into a sleeve.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In a die-casting method, a sleeve 3 attached to a cavity 2 is coupled with a die-casting mold 1. The sleeve 3 hae a pouring hole 4, through which molten metal 5 is poured in the sleeve 3. The molten metal 5 in the sleeve 3 is pressed by a tip attached to a plunger rod 6 and forcibly injected into the cavity 2. After the cavity 2 is filled with the molten metal 5, the molten metal 5 is cooled and solidified to a profile defined by the inner surface of the die-casting mold 1. A die-cast product obtained in this way is taken from the die-casting mold 1 by pushing ejector pins like 8 in the cavity 2 after the die-cast product is cooled.
According to the present invention, a suction nozzle 11 is attached to the die-casting mold 1 at the proper position such as its parting part, to connect the cavity 2 through the suction nozzle 11 to a vacuum pump. When the cavity 2 is evacuated through the suction nozzle 11, atmospheric air may probably invade through parts where the ejector pins like 8 are inserted during evacuation. Such air invasion is prohibited by sealing gaps between the ejector pins and the die parts with a sealing agent 13. On the other hand, the pouring hole 4 is closed with the plunger tip 7, so that atmospheric air can not invade into the interior of the sleeve 3 through the pouring hole 4.
In order to blow oxygen into the cavity after the evacuation, an oxygen nozzle 14 is opened to the interior of the sleeve 3. The oxygen nozzle 14 is connected through a regulator valve 15 such as a regulator valve to an oxygen supply source.
When the cavity 2 is evacuated through the suction nozzle 11, gas such as air and water vapor are excluded from the cavity 2 as well as the interior of the sleeve 3 connected with the cavity 2. Even if the cavity 2 has a complicated configuration, gas are completely excluded from every nook and corner of the cavity 2 by adjusting a suction speed preferably in a range of 500-800 millibar/second.
The evacuation is preferably continued 1-2 seconds or so, under the condition that the pouring hole 4 is closed with the plunger tip 7. The evacuation time period is set relatively longer, compared with a conventional vacuum die-casting method whereby the cavity 2 is evacuated for a time period shorter than 1 second without closing the pouring hole 4, The cavity 2 is evacuated to a degree of vacuum preferably below 100 millibar due to the longer evacuation period. Water vapor derived from a parting agent adhering onto the inner surface of the die-casting mold 1 is separated from the inner surface of the die-casting mold and discharged outside.
Removal of water vapor is more effectively performed by the evacuation compared with blowing oxygen gas into the cavity, since a gaseous stream flows at a higher speed in the cavity 2. However, when the cavity 2 is evacuated to an insufficient degree of vacuum above 100 millibar, a relatively large amount of gas remain in the cavity 2. A large amount of the gas remaining in the cavity 2 are not replaced by oxygen in the following oxygen blowing step but often included in a cast product.
After the evacuation, oxygen gas is blown through the nozzle 14 into the cavity 2. The oxygen supply is continued preferably 3-4 seconds until gasses and oxygen are effused through the parting part of the die-casting mold 1. Since oxygen gas is blown into the cavity 2 in the state decompressed in the former step, the oxygen gas nows as a high-speed stream to every nook and corner of the cavity 2. As a result, water vapor derived from the parting agent is completely washed off by the supplied oxygen gas.
The plunger tip 7 goes back to open the pouring hole 4 during continuation of the oxygen blowing. When the pouring hole 4 is released, oxygen gas is effused through the pouring hole 4, as shown in Fig. 2. Effusion of the oxygen gas effectively inhibits invasion of atmospheric air through the pouring hole 4 into the sleeve 3.
After the pouring hole 4 opens, an molten metal 5 is poured from a ladle 16 into the sleeve 3. Since the oxygen gas is continuously effused during the pouring operation, the effusion of the oxygen gas effectively inhibits inflow of atmospheric air in accompaniment with the molten metal 5.
The die-casting mold 1 preferably preheated to 150-200 °C before the pouring step, in order to reduce thermal shock caused by the poured molten metal 5 and improve productivity.
When the molten metal 5 in an mass necessary for one cycle of die-casting is poured in the sleeve 3, the pouring hole 4 is closed with the molten metal 5. Since the closed state does not permit inflow of atmospheric air through the pouring hole 4 into the sleeve, supply of oxygen gas can be stopped.
After gas such as air and water vapor are completely excluded from the cavity 2 and the interior of the sleeve 3 as above-mentioned, the plunger 6 is forwarded to forcibly inject the molten metal 5 into the cavity 2. The injected molten metal 5 is shaped to a bulk having a profile imitating the inner surface of the die-casting mold 1. The bulk is cooled and solidified to a die-cast products having a predetermined configuration, Hereon, cast defects such as blowholes or porosity caused by inclusion of gas are not generated in the die-cast products, since gas such as air and water vapor are completely excluded from the cavity 2. Oxygen gas remaining in the cavity 2 is reacted with the injected molten metal 5, and the reaction product Al₂O₃ dispersed as fine products in the die-cast products without causing any harmful influences. Consequently, the die-cast products obtained in this way have excellent properties.

EXAMPLE

A die-casting mold 1 used in this example had a cavity 2 of 150 in diameter and 120 mm in length, a proper water-cooling means was provided at the die-casting mold 1 for partially cooling the die-casting mold 1,
After the cavity 2 was cleaned by air blow, a parting agent was sprayed 5 seconds onto an inner surface of the die-casting mold 1. The die-casting mold 1 was then preheated at 180°C and located at a proper position in a die-casting machine. The surrounding around a ejector pin 8 was sealed with a sealing agent 13, and a suction nozzle 11 was attached to a parting part of the die-casting mold 1.
The pouring hole 4 was closed with a plunger tip 7, and gas were sucked through the suction nozzle 11 from the cavity 2 and the interior of a sleeve 8 by evacuating the cavity 2 at a suction speed 700 millibar/second. A vacuum gage (not shown) provided at a vacuum source 12 indicated 76 millibar.
After the evacuation, a regulator valve 15 was opened to blow oxygen gas through an oxygen nozzle 14 into the cavity 2. Oxygen blowing was continued under such pressure condition that oxygen was effused through the parting part of the die-casting mold 1.
After oxygen blowing was continued 3.5 seconds, the plunger tip 7 went back to open the pouring hole 4. Thereafter, molten aluminum alloy ADC12 prepared by a conventional molten metal treatment was poured through the pouring hole 4 into the sleeve 3. While the molten metal 5 was poured into the sleeve 3 for 5 seconds, oxygen gas was continuously blown through the oxygen nozzle 14 into the sleeve 3.
After the pouring was finished, supply of oxygen gas was stopped, and the plunger 6 was forwarded to forcibly inject the melt 5 into the cavity 2. Injection of the molten metal 5 was completed in a very short time of approximately 0.1 seconds.
It took 5 seconds to solidify the injected melt 5 in the die-casting mold 1. After the die-cast products was cooled, it was taken from the die-casting mold. The die-cast products No.1 obtained in this way was subjected to Ransley test for measuring gas contents included therein and Also to a mechanical test.
For comparison, a die-cast products No.2 obtained by a conventional vacuum die-casting method and a die-cast products No.3 obtained by a conventional oxygen die-casting method from the same aluminum alloy were also subjected to the same Ransley and mechanical tests. In the vacuum die-casting method, the cavity 2 was evacuated 1.5 seconds before injection of the molten metal 5. In the oxygen die-casting method, oxygen gas was blown into the cavity 2, and then the molten metal 6 was injected into the cavity 2 for 6 seconds while blowing oxygen gas.

The test results are shown in Table 1. It is noted from Table 1 that an amount of gas such as N₂ and H₂ in the die-cast products No.1 according to the present invention is extremely reduced as compared with values in the die-cast products Nos.2 and 3. The die-cast products No.1 had ductility and tensile strength superior to those values of the die-cast products Nos. 2 and 3. In addition, the die-cast products No.1 was improved in mechanical properties by T6 treatment (i.e., heating 3 hours at 480°C, water quenching and then aging 5 hours at 160°C) without occurrence of blisters due to the extremely reduced gaseous impurities.

EFFECTS OF A DIE-CASTING METHOD ON PROPERTIES OF DIE-CAST PRODUCTS
Sample No.	Die-Casting Method	Amount of gaseous impurities (cc/100g-Al)	As Cast	After T6 Treatment
			T.S.	EI.	T.S.	El.
1	Present Invention	0.6	32	2.0	40	5.0
2	Vacuum Die-Casting	6	23	0.8	Blisters of 1-2mn in diam.
3	Oxygen Die-Casting	2	27	1.5	Blisters of 0.2-0.5mm in diam.
NOTE: T.S. means tensile strength (kg/mm²) El. means elongation (%).

According to the present invention as above-mentioned, gas such as air and water vapor derived from a parting agent adhering onto an inner surface of a die-casting mold is completely excluded from a cavity of the die-casting mold by oxygen blowing in succession to evacuation. Since an molten metal is injected into the cavity conditioned to the state perfectly free from gas, an obtained die-cast products does not include defects such as blowholes or porosity caused by the gas. Consequently, this new die-casting method is applicable for production of functional members as well as structural members, using advantages of high productivity.

Claims

A method of die-casting aluminum or aluminum alloy, comprising the steps of:

evacuating a cavity of a die-casting mold to discharge gas from said cavity,

blowing oxygen gas into said cavity,

and then forcibly injecting an aluminum or aluminum alloy melt into said cavity.
The die-casting method defined by Claim 1, wherein the cavity of a die-casting mold is evacuated to a degree of vacuum 100 millibar at highest.
The die-casting method defined by Claim 1, wherein oxygen gas is blown into the cavity so as to increase a pressure of said cavity above an atmospheric pressure.
A die-cast products produced by the die-casting method defined by either one of Claims 1 to 3 and including gaseous impurities at concentration below 1cc/100g-Al.