Aluminum alloy castings stomata and prevention

【China Aluminum Industry Network】1. Due to the serious oxidation and inhalation tendency of aluminum alloy in the pore category, the aluminum alloy is directly in contact with the furnace gas or the external atmosphere during the smelting process. Therefore, if the control during the smelting process is slightly improper, the aluminum alloy can easily absorb the gas and form pores. Pinholes are common. Pinhole (gasporosity/pin-hole), usually refers to the precipitation pores less than 1mm in the casting, mostly round, unevenly distributed throughout the casting section, especially in the thick section of the casting and the cooling rate is relatively small Parts. According to the distribution and shape characteristics of the aluminum alloy's precipitated pores, the pinholes can be further divided into three categories1, namely:

(1) Dot-shaped pinholes: In the low-power tissue, the pinholes are in the shape of circles, the pinholes are clearly outlined and discontinuous, and the number of pinholes per square centimeter can be counted and their diameters can be measured. Such pinholes are easily distinguished from shrinkage holes, shrinkage, and the like.

(2) Mesh pinholes: Pinholes are densely meshed in low-intensity tissue, and there are a few large holes. It is not easy to check the number of pinholes per unit area, and it is difficult to measure the diameter of pinholes.

(3) Synthetic pores: It is an intermediate type of punctate pinholes and mesh pinholes. From the perspective of low-power tissue, there are many large pinholes, but they are not punctate but polygonal.

The practice of aluminum alloy production has proved that the main gas component of the aluminum alloy due to inhalation is hydrogen gas, and its appearance does not have a certain rule to follow. It is often the case that all or most of the castings have a pinhole phenomenon; materials are also Without exception, aluminum alloys of various compositions are prone to pinholes.

2. The formation of pinholes When smelting and casting, aluminum alloys can absorb a large amount of hydrogen, and when cooled, the aluminum alloys continuously precipitate due to the decrease of the solubility. Some materials introduce 2, the more dissolved hydrogen in the aluminum alloy, its solubility increases with the temperature of the alloy liquid, decreases with the decrease of the temperature, and the solubility of hydrogen in the aluminum alloy changes from the liquid state to the solid state. Decrease 19 times. (The relationship between hydrogen solubility in pure aluminum and temperature is shown in Figure 13). Therefore, during the cooling solidification process of the aluminum alloy liquid, at a certain time of hydrogen, the hydrogen content exceeds its solubility, that is, it precipitates in the form of bubbles. Hydrogen bubbles formed by the supersaturation of hydrogen are less likely to float out, and form fine, dispersed pores during the solidification process, which is what we normally call pinholes. The supersaturation reached before the formation of hydrogen bubbles is a function of the number of nucleation of hydrogen bubbles, while oxides and other inclusions act as bubble cores under normal production conditions, especially in thick sand castings. It is difficult to avoid pinholes. The smelting and pouring of aluminum alloys in a relatively high-humidity atmosphere makes the pinholes in the castings particularly severe. This is why we often wonder in the production that there are fewer pinhole defects in the aluminum alloy castings than in the rainy and wet season.

In general, for aluminum alloys, if the crystallization temperature range is large, the probability of mesh pinholes is much greater3. This is because in the general casting production conditions, castings have a wide range of solidification temperatures, making it easier for aluminum alloys to form well-developed dendrites. In the later stage of solidification, the residual aluminum liquid in the dendrite gap portion may be isolated from each other and exist in an approximately closed small space, because they are subjected to external atmospheric pressure and the static pressure of the alloy liquid is less, when residual aluminum liquid further When the cooling shrinks, a certain degree of vacuum can be formed (ie, the feeding channel is blocked), so that supersaturated hydrogen in the alloy is precipitated to form pinholes.

3. The source of hydrogen that forms pores and the generation of pores in precipitated aluminum alloys are formed by the inhalation of aluminum alloys, but gases in the form of gas molecules generally cannot be dissolved in alloy fluids, only when the gas molecules are decomposed into active atoms. It may dissolve. The amount of dissolved gas in the alloy fluid is not only related to whether the molecule is easily decomposed into an active atom, but also directly related to the type of gas atom. In the melting process of aluminum alloys, the furnace gases that are usually in contact with are: hydrogen, oxygen, water vapor, carbon dioxide, sulfur dioxide, etc. These gases are mainly generated by combustion of the fuel, and the refractory materials, metal charge and flux, and the gas are in contact. Tools and so on can also bring in a certain amount of gas, such as new linings, refractories of furnaces, crucibles, etc., which usually takes days or weeks, and the chemically bound hydrogen can be fully released from the binder. . In general, the furnace gas composition is determined by the type of fuel and the amount of air. Common coke oven oven, furnace gas components mainly for carbon dioxide, sulfur dioxide and nitrogen; gas, heavy oil crucible furnace mainly for water vapor, nitrogen; and most of the smelting furnace used by the current electric furnace smelting, the furnace gas composition is mainly hydrogen. Therefore, when smelting using different smelting furnaces, the amount of suction of aluminum alloy and the degree of generation of pores are different.

The practice of aluminum alloy production shows that hydrogen is a gas that can be dissolved in aluminum or aluminum alloys in large quantities, and is the main cause of the formation of pores in aluminum alloys. It is a relatively harmful gas in aluminum alloys and a gas with a relatively high solubility in aluminum alloys. The porosity generated during the solidification process of the casting due to the precipitation of hydrogen not only reduces the actual cross-sectional area of ​​the casting but also is the source of the crack. Inert gases are insoluble in aluminum or aluminum alloys, and other gases generally react with aluminum or aluminum alloys to form aluminum compounds such as Al2O3, AlCl3, AlN, Al4C3, and the like. As can be seen from Fig. 1, the solution of hydrogen in liquid aluminum or aluminum alloy has a large degree of solution, and is almost insoluble in solid aluminum (having a solubility of about 0.003 or less at room temperature).

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