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The Quality Defects and Improvement Measures of Aluminium Alloy Die Casting Parts (Part One)

Source: Junying   Author: Junying   Posted: 2017-07-06 11:23:45   Hits: 21
Introduction: High pressure die casting is widely applicable to many industries and areas as a kind of special forming technology. It has been an indispensable part, especially for scaled automobile, motorcycle, internal combustion engine, electronics, instrument and aerospace industries. 

1. The summary of aluminium alloy die casting technology
Aluminium is the metal element which is most widely distributed and most largely reserved in the earth's crust. Aluminium belongs to reusable resources which are beneficial for the environmental protection. Pure aluminium is slivery white. It has a low melting point, good thermal and electrical conductivity. It is corrosion resistant. Aluminium alloy is suitable to be processed into various sectional profiles because it has low density, small specific gravity, high specific strength, good thermal conductivity, good corrosion resistance and low price and it is easily shaped. It is a kind of non-ferrous metal construction materials used most in current die casting industry, its usage amount only less than the amount of steel used in the industry. Aluminium alloy has the feature of high melting point and light weight. High melting point means that it can be widely applied to all trades and professions as a material with high temperature resistance. Most lunar rovers our country has built are made of high strength aluminium alloy which has the advantage of light quality and can be applied in the aspect of aerospace equipment. There are still many other examples as this. Just because of this, aluminium alloy has become the irreplaceable metal material in the automobile and aerospace industries.

2. The quality defects and improvement measures of aluminium alloy die casting parts

2.1 Blowholes 
Blowholes refer to eyelets and holes of different size inside or on the surface of die casting parts. They have smooth surfaces, and most of them are round. The production of blowholes will cause insufficient hardness of die casting parts and affect surface appearance.

2.1.1 Blowholes around the bolt holes of die casting boxes
There are many bolt holes, oil holes and various mounting holes on the die casting aluminium alloy box. These will directly affect assembly quality and operational performance of engines. In the process of die casting, the quality of these bolt, oil, and mounting holes needs to be controlled strictly.

2.1.2 Causes
When the aluminium alloy box is casted, gases inside the mold cavity can't be discharged easily and will be left in the molten aluminium liquid easily because of the high speed of liquid metals filling the cavity. The residual gas forms little bubbles inside the die casting parts after aluminium liquid is cooled and solidified. The bubbles are just blowholes.

In the production process of aluminium alloy die casting, the temperature of casting aluminium liquid is around 660℃ commonly. However, at this temperature, aluminium liquid contains a large amount of gas (mainly hydrogen). Solubility of hydrogen in aluminium alloy is closely related to the temperature. At this temperature, the gas content is about 0.69cm3/100g which is about from 19 to 20 times as much as that under normal circumtances. Therefore, after aluminium alloy is solidified, the gas will be precipitated massively, which causes a large quantity of blowholes existing in the aluminium alloy die casting parts. Besides, the blowholes caused by air entrapment and gas-forming of the release agent due to techniques can also occupy a considerable proportion.

2.1.3 Improvement measures
If there exist certain structural problems in the gas discharging channel design of molds or gas can't be released smoothly through the gas discharging holes, gases inside the mold cavity will not be released thoroughly in the process of die casting. When the casting system is designed, it is necessary to confirm whether the sectional area is decreasing gradually. Guarantee effective measures of refinement quality during the process of melting aluminium alloy. Choose the suitable refining agent. When it reacts, bubbles are generated evenly and continuously. Then, touch the impurity in aluminium liquid effectively and bring it to the surface by physical adsorption. Adjust techniques and lower speed properly. Make sure whether the release agent is sprayed too much. Based on above reasons, vacuum die casting can be considered to be used. 

Moreover, if the length of blowholes around bolt holes with manufacturing procedures is one third less than that of the thread, and blowholes are not in the thread area, having no impacts on torques and operational performance, the blowhole problem doesn't need to be solved.

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