What are the causes of pinhole formation in aluminum alloy casting process?

2020-01-09 09:40:46 155

Aluminum alloy casting process is the main preparation method of traditional aluminum alloy, but it is difficult to meet the needs of high-performance aluminum alloy casting.

First, the strength, plasticity, rigidity, heat resistance and corrosion resistance of the traditional process are difficult to be further improved;

Second, in the process of pursuing high performance, the cost of casting process increases rapidly due to the increase of equipment and the decrease of output; third, the increase of alloy content and plasticity tend to decrease, so the subsequent pressure processing cost increases and the yield decreases. Therefore, the use threshold of aluminum alloy casting is greatly increased, which seriously affects the development of the whole market scale. In these aspects, spray forming process has the double advantages of performance and comprehensive cost, which can reduce the use threshold of advanced aluminum alloy and further improve the performance. In a certain range, aluminum can replace steel, thus rapidly cultivating the advanced aluminum alloy market. In turn, aluminum alloy casting promotes the spray forming process to gain the advantage of scale cost. Therefore, spray forming process will become the main production process of cast aluminum alloy. In the process of production, shrinkage cavity, sand hole, porosity, slag inclusion and other defects are easy to appear. How to repair defects such as porosity in aluminum alloy castings? If electric welding, argon arc welding and other equipment are used for repair welding, due to large heat dissipation, it is easy to produce side effects such as thermal deformation, which can not meet the repair welding requirements.

The cold welding repair machine adopts the principle of instantaneous discharge of high frequency electric spark and non hot surfacing to repair the casting defects of aluminum alloy castings. Due to the small heat affected zone of cold welding, it will not cause annealing deformation of the base metal, no cracks, no hard spots and hardening phenomenon. It is an ideal method to repair small defects such as pores and sand holes in aluminum alloy castings. What is the reason for the formation of elephants?

In the process of casting aluminum alloy, a large amount of hydrogen is absorbed in the melting and pouring process, and it is continuously precipitated due to the decrease of solubility in the cooling process. The results show that the solubility of hydrogen in aluminum alloy increases with the increase of liquid temperature and decreases with the decrease of temperature. The solubility of hydrogen in aluminum alloy decreases 19 times when it changes from liquid to solid. Therefore, in the process of cooling and solidification of aluminum alloy liquid, the hydrogen content exceeds its solubility in a certain period of time, that is, it precipitates in the form of bubbles. Hydrogen bubbles formed by supersaturated hydrogen precipitation form fine and dispersed pores during solidification, which are usually called "gas pores". The supersaturation before the formation of hydrogen bubbles is a function of the nucleation number of hydrogen bubbles, while oxides and other inclusions play the role of bubble core. In general production conditions, especially in thick sand castings, it is difficult to avoid pinholes. When aluminum alloy melts and casts in the high relative humidity environment, the pinhole in the casting is particularly serious. This is why we often suspect that pinhole defects in aluminum alloy castings are less in dry season than in rainy season.

Generally speaking, for aluminum alloy, if the crystallization temperature range is large, the probability of producing network pinholes is much higher. This is because under the general casting conditions, die-casting aluminum has a wide solidification temperature range, which makes the aluminum alloy easy to form developed dendrites.

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