(1) Point-shaped pinholes: In the low-magnification structure, the pinholes are rounded, clearly and discontinuous, and the number of pinholes per square centimeter can be counted, and the diameter can be measured. This kind of pinholes is easily distinguished from shrinkage holes and shrinkage.
(2) Mesh pinhole: In the low-magnification structure, the pinholes are densely connected into a net, with a few large holes. It is inconvenient to check the number of pinholes per unit area, and it is difficult to measure the diameter of the pinholes.
(3) Synthetic pore: It is an intermediate type of point-like pinholes and mesh-shaped pinholes. From the perspective of low-magnification, there are many large pinholes, but they are not round-shaped but polygonal.
It has been found from aluminum alloy production experience that the main gas component of the aluminum alloy porosity due to suction is hydrogen gas, and the reason of porosity does not have a certain rule to follow. It tends to be that all or most of the furnace batch will generate porosity; also the material, aluminum alloys with different compositions are prone to generate porosity.
2．Pinhole formation
When aluminum alloy is smelted and casted, it can absorb a large amount of hydrogen, and when it is cooled, it is continuously precipitated due to a decrease in solubility. According to some data, when there are much hydrogen dissolved in the aluminum alloy, its solubility increases with the temperature of the alloy liquid, and decreases with the decrease of temperature. The solubility of hydrogen in the aluminum alloy will decrease 19times when the liquid is converted into a solid state. (The relationship between the solubility and temperature of hydrogen in pure aluminum is shown in Figure 1). Therefore, during the solidification process of the aluminum alloy liquid, at a certain moment, the hydrogen content exceeds its solubility, that is, it is precipitated in the form of bubbles. The hydrogen bubbles formed by the supersaturated hydrogen evolution, which do not float in time will form fine, dispersed pores during the solidification process, and that is what we usually call the gas porosity. The degree of supersaturation achieved before the formation of hydrogen bubbles is a function of the number of hydrogen bubble nucleuses, while oxides and other inclusions act as bubble cores.
Pinholes are difficult to avoid under normal production condition, especially in thick sand castings. The aluminum alloy is smelted and casted in an atmosphere of relatively high humidity, and pinholes are particularly severe. This is why the frequency of pinholes happened in dry season is often less than the rainy and wet conditions. In general, for aluminum alloys, if the crystallization temperature range is large, the probability of producing mesh pinholes is much greater. This is because, under normal casting production conditions, the casting has a wide range of solidification temperatures, making it easy to form well-developed dendrites. In the late stage of solidification, the residual aluminum liquid in the dendritic gap portion may be isolated from each other and exist in a small space that is approximately closed, because they are less affected by external atmospheric pressure and static pressure of the alloy liquid, when the residual aluminum liquid is further When cooling shrinks, a certain degree of vacuum is formed (the feeding passage is blocked), so that supersaturated hydrogen in the alloy is precipitated to form pinholes.
Source and precipitation of hydrogen forming pores：
The generation of pores in the aluminum alloy is formed by the aluminum alloy suction, but the gas in the gas molecular state generally cannot be dissolved in the alloy liquid, and it is only possible to dissolve when the gas molecules are decomposed into active atoms. The amount of gas that can be dissolved in the alloy liquid is not only related to whether the molecule is easily decomposed into active atoms, but also directly related to the gas atom type. In the process of melting aluminum alloy, the furnace gas usually contacted is: hydrogen, oxygen, water vapor, carbon dioxide, sulfur dioxide, etc. These gases are mainly produced by combustion of fuel, while refractory materials, metal charge and flux, and gas contact Tools and the like can also bring in a certain amount of gas, such as a new furnace lining, a furnace refractory material, a crucible, etc., usually takes several days or several weeks, and the chemically combined hydrogen can be fully released from the binder. . In general, the composition of the furnace gas is determined by the type of fuel and the amount of air. Ordinary coke ovens, the main components of the furnace gas are carbon dioxide, sulfur dioxide and nitrogen; gas, heavy oil furnaces are mainly water vapor, nitrogen; and for most of the current melting furnaces used in electric furnace melting, the composition of the furnace gas is mainly hydrogen. Therefore, when smelting in different melting furnaces, the amount of suction of the aluminum alloy and the degree of generation of pores are different.
It has been proven that hydrogen is the only gas that can be dissolved in aluminum or aluminum alloy in a large amount, which is the main reason for the formation of pores in aluminum alloy. It is the most harmful gas in aluminum alloy, and also the most soluble gas. The pores generated by the precipitation of hydrogen during the solidification of the casting not only reduce the actual cross-sectional area of the casting but also the source of the crack. The inert gas is insoluble in aluminum or an aluminum alloy, and other gases generally react with aluminum or an aluminum alloy to form an aluminum compound such as Al2O3, AlCl3, AlN, Al4C3, etc. It can be seen from Fig. 1 that the solubility of hydrogen in liquid aluminum or aluminum alloy is large, and is hardly dissolved in solid aluminum (the solubility is about 0.003% max. at room temperature).
1.Effect of Porosity on Properties of Aluminum Alloy Castings
The effect of pinhole on the properties of aluminum alloy is mainly reflected in the decrease of the density of the microstructure of the casting and the decrease of the mechanical properties. Therefore, in aluminum alloy casting production, it is very important to strengthen the influence of the pore level on the mechanical properties, and to ensure the quality of the aluminum alloy casting by controlling the pinhole grade. Pinhole grade assessment, low-maganificaion test is carried out according to GB10851-89, when it is controversial, it is carried out according to Table
2; X-ray radiography is carried out according to GB11346-89 aluminum alloy casting pinhole grading standard, which uses two commonly used in industrial production. The alloys ZL101 (Al-Si-Mg) and ZL201 (Al-Cu-Mn), and the test results of бb and σ5 in the T4 state show that the mechanical properties of various pinhole samples of ZL101T4 and ZL201ST4 are shown in the table. 3, Table 4): There is a linear correlation between the mechanical properties of the casting and the pinhole level. As the level of the pinhole increases, the mechanical properties gradually decrease; for each level of the pinhole, the mechanical бb decreases by about 3%, σ5 Drop by about 5%. For the requirements of cutting performance samples of aluminum alloy castings, the pinhole level allowed for castings is detailed in GB9438-8.
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