"Casting" is a liquid metal forming process. It is well known that a liquid metal at a high temperature is oxidized in the atmosphere and generates an oxide film. However, for a long time, the effect of this oxide film on the quality of the aluminum alloy castings basically considers only the problem of entangling non-metallic inclusions in the molten metal, and is rarely discussed further. J. Campbell et al. (Birmingham University, UK), based on years of research, discovered that macroscopically and microscopically, folded bi-films have a very important influence on the quality of aluminum alloy castings. Campbell et al. believe that the understanding of bi-films is a more exciting discovery. At present, we briefly refer to the preliminary conclusions and opinions obtained by Campbell et al. as 'bi-films'. The influence of the oxide film entrapped in the liquid aluminum alloy on the quality of the casting can be roughly divided into two aspects: First, in the macroscopic aspect, in addition to cutting the metal matrix to reduce the mechanical properties, it also induces blowholes and small shrinkage holes. Casting defects such as; the other is the microscopic aspects, have an important influence on the grain size, dendrite spacing, the effect of Na and Sr metamorphism in Al-Si alloy. 1. Characteristics of the oxide film on the surface of the liquid metal The characteristics of the oxide film cannot be considered at the same time in consideration of the density and melting point of the metal mother liquid to which it is attached. In steel and iron, the production of cast steel parts is used as an example. The FeO produced by the oxidation of liquid steel has a much lower melting point and density than molten steel, and it is highly active at high temperatures and is virtually impossible to exist alone. FeO can be combined with SiO2 to form a low-melting FeO.SiO2, which can react with silicon and manganese in steel to form MnO and SiO2, which in turn combines with MnO.SiO2. It can also react with carbon in steel to produce CO, with a small fraction of it. Soluble in molten steel. If the deoxidation is not properly handled, or the molten steel is secondarily oxidized after tapping, non-metallic inclusions in the steel will increase, or defects such as blowholes or surface inclusions will be generated in the casting. However, the oxides produced on the surface of molten steel have a lower melting point than the temperature of the molten steel and can only accumulate. It is impossible to fold into an oxide film interlayer suspended in the molten steel. Therefore, there are no problems caused by oxide film interlayers. . The situation of aluminum alloys and magnesium alloys is completely different from this, and aluminum alloys are briefly described as follows: Aluminum is very active in the liquid state, and the surface of the aluminum liquid can easily react with oxygen in the atmosphere to form an Al2O3 film. The melting point of Al2O3 is much higher than that of liquid aluminum alloys and it is very stable. Al2O3 is slightly more dense than aluminum. Therefore, the Al2O3 film is easily suspended in the aluminum liquid and does not aggregate and separate from the aluminum alloy liquid. When the aluminum alloy liquid is disturbed, the Al2O3 film on the surface will be folded into a sandwich and will be entrained in the molten metal, which will cause many problems unique to aluminum alloys. II. Formation of Oxide Film Interlayer and Its Harmful Effect Aluminum alloys will be strongly affected during the smelting process, during the pouring from the furnace, during the modification process, during the blast cleaning process at high air velocity, and during the pouring process. Disturbances. The disturbance of the liquid metal surface will pull the oxide film on its surface to expand, fold and break. The surface of the cleaning alloy exposed at the disconnected oxide film is oxidized to produce a new oxide film. The folding of the oxide film makes it adhere to the dry surface on the atmospheric side, and it encloses a small amount of air between the two dry surfaces to become an oxide film sandwich. The oxide film sandwich is easily entrapped in the molten metal and can also be squeezed into small clusters under the action of a perturbed molten metal. Since the melting point of Al2O3 is over one thousand degrees Celsius higher than the temperature of the aluminum alloy solution, and because of its high chemical stability, the small clusters will not be fused and will not dissolve in the aluminum alloy. Although the density of Al2O3 is slightly higher than that of the aluminum alloy solution, the density of the oxide film sandwiched by air is relatively close to that of the aluminum alloy solution. Therefore, in addition to the possibility that the oxide film interlayer may sink during long-term standing in a large-scale holding furnace, under the general casting production conditions, it will be relatively stable suspended in the aluminum alloy solution. The aluminum alloy solution, which has been suspended with an oxide film interlayer, will again generate more oxide film interlayers when it is disturbed again. During the production of castings, alloy smelting, decanting, deterioration, purification, and pouring of alloys will cause a strong disturbance in the aluminum alloy solution. In addition to retaining the original oxide film interlayer, the aluminum alloy solution will also cause problems. Once again disturbed and constantly adding new oxide film interlayers. Therefore, the molten metal that enters the cavity contains a large number of tiny oxide film sandwiches. After the molten metal is filled with the cavity, it is at a standstill, and the oxide film sandwiched by the crush will gradually stretch into small pieces. After the metal liquid is cooled below the liquidus, the nucleation and growth of the dendrite is a factor that promotes the expansion of the oxide film sandwiched by the extruded mass. After the casting is solidified, a large number of small pieces of oxide film sandwich itself is a small crack, which plays the role of cutting the metal matrix, of course, will reduce the mechanical properties of the alloy, and the more harmful is the generation of induced pores and small shrinkage. As the temperature of the liquid metal gradually decreases, the solubility of hydrogen in the molten metal decreases, but it is very difficult for hydrogen to precipitate from the liquid metal in the form of pores. When another homogeneous phase (gas phase) is produced in a homogeneous liquid phase, it is always formed by the aggregation of several atoms or molecules, and its volume is small. This small new phase has a large specific surface area (ie, surface area per unit volume). To create a new interface, it is necessary to work on it. This is the interface energy of the new phase, ie, its surface area and surface tension. The product of. It is virtually impossible to obtain such a large amount of energy during cooling of the aluminum alloy liquid. Even if the core of the new phase is produced, it needs a lot of energy to grow up, and it can grow only if the size of the new phase exceeds a certain critical value. The core of the new phase with a size less than the critical value cannot grow up and will only disappear on its own. In theory, it is very difficult for the gas phase to nucleate and grow in the liquid phase. In fact. If there is no other precipitating factor, it is impossible for a homogeneous aluminum alloy to produce pores due to the evolution of hydrogen under conditions in which the hydrogen content is substantially normal. When the molten metal contains a large number of suspended oxide film interlayers, the situation is very different. Most of the oxide film sandwiches contain a small amount of air. When the temperature of the molten metal decreases and the solubility of hydrogen decreases, the small air bubbles in the oxide film sandwich are a vacuum for hydrogen, and the dissolved hydrogen in the molten metal flows into the air bubbles. Medium diffusion is very convenient. Hydrogen diffuses into the small air bubbles, making the oxide film sandwich large, creating pores in the casting. If the purification treatment of the aluminum alloy liquid is good, the hydrogen content in the molten metal is very low, and the pores generated in the casting will be few. However, if there is no oxide film interlayer in the molten metal, even if the content of hydrogen in the molten metal is high, the hydrogen can only be solid-dissolved in the alloy in a supersaturated state during solidification, and it is impossible to generate pores. If the castings are in poor condition, shrinkage holes will occur during solidification and shrinkage. Since the oxide film interlayer is empty, it is easy to pull open, and the shrinkage pores are mostly formed at the oxide film interlayer. In this case, hydrogen dissolved in the molten metal will also diffuse into it, expanding the pores. In summary, it can be considered that for the aluminum alloy castings, the oxide film interlayer is the main reason for the decrease of the mechanical properties of the material and the generation of pinhole porosity defects in the casting. In order to improve the mechanical properties of the material and increase the density of the casting, it is more important to take measures to eliminate the oxide film interlayer than to strengthen the degassing and purifying operation. III. Measures to reduce or even eliminate oxide film interlayer Because of the recognition of the role of oxide film interlayer, there is currently no mature experience in reducing or eliminating the oxide film interlayer in the aluminum alloy solution. This is exactly what we will face in the future. Subject. According to the current understanding, in principle, we can start from the following aspects: During the alloy smelting process, the disturbance of the liquid surface oxide film should be avoided as much as possible. However, the convection and agitation of the molten metal below the liquid level will not cause the oxide film to become entangled; the use of spray clean-up treatment also has the effect of removing the oxide film suspended in the molten metal, but the flow rate should be reduced as much as possible during processing. The destructive effect of the liquid surface oxide film is reduced to a low degree; when using the 'potting and casting' method, a teapot nozzle type ladle is preferably used to reduce the disturbance to the liquid surface oxide film; if the low pressure casting process is adopted, it can be maintained. The liquid flow smoothly into the cavity, the mechanical properties of the casting body will be significantly higher than the castings manufactured using conventional techniques; process design, we must strive to smooth the metal flow in the pouring system, no turbulence, it is better to use the bottom note the way. In addition, special attention should be paid to the quality of the aluminum alloy ingot as a charge. The recycling and reuse of scrap metal are necessary for a sustainable industrial society. An important advantage of aluminum and aluminum alloy products is that they are easily recycled, recycled, and reused. Recycled aluminum can reduce energy consumption by about 95% compared to primary aluminum. At present, the global amount of recycled aluminum accounts for about one-third of the total amount of aluminum metal. The amount of recycled aluminum ingots in the foundry industry is also considerable. It needs to be emphasized that the quality of recycled aluminum ingots varies greatly. With aluminum ingots with similar chemical compositions produced by different manufacturers, the quality of castings produced can vary greatly. The quality of aluminum ingots supplied by the same manufacturer can vary greatly. In the production process of recycled aluminum ingots, the control of the oxide film interlayer is one of the important reasons for this difference. In addition to vigorously calling for strengthening the quality control in the production process of recycled aluminum ingots, aluminum alloy casting manufacturers must pay special attention to the quality assessment of incoming materials when they use recycled aluminum ingots, and they should have a trial production stage. It is not unreasonable for some manufacturers to purchase raw aluminum ingots at higher prices.