[China Aluminum Network] The final performance of aerospace aluminum alloy material has a lot to do with its microstructure. Therefore, crack-free large billet casting is a key problem to be solved in the production of large-size materials. High-quality billet blanks must not have obvious porosity, porosity, and low content of hydrogen and oxide inclusions, and small crystal grains. In addition to the strict control of hydrogen content, some alkali metals such as Li, Na, K, and alkaline earth Ca must also be strictly controlled. High-strength aluminum alloys have a high content of main elements (Zn, Mg, Cu, etc.), and are not only easily segregated in the melt, but are difficult to distribute evenly, and the nucleation rate is reduced and the crystal grains are coarse. Due to the large size of the ingot, the thermal stress of the shrinkage is large and it is easy to crack. High-strength aluminum alloys have a wide range of crystallization (up to 180K), and the unbalanced solidification has a greater eutectic cracking tendency, which is particularly prominent for the high Zn content 7XXX (7050,7055) alloys. Large wide-gauge spindles can easily crack during casting. Flat ingots are more difficult to cast than ingots. In order to cast high quality large crack-free billets, a series of fused casting technologies were developed. Melt electromagnetic stirring is one of the most important new technologies.
Melt electromagnetic stirring technology is to generate electromagnetic force in the aluminum bath to stir the flow of aluminum melt in the molten pool, so that the melt composition is uniform, to avoid contamination of the iron tools when mixing manually. This technology not only can effectively control the Fe impurity content, but also can reduce the destruction of the oxide film on the aluminum melt surface, reduce the burning of alloying elements and the incorporation of hydrogen. The use of melt electromagnetic stirring technology can shorten the smelting time by about 20%, reduce energy consumption by 10% to 15%, reduce slag volume by 20% to 50%, and shorten slag removal time by 20% to 50%. The introduction of ultrasonic fields, mechanical vibration, etc. is also conducive to refinement of ingot grains and homogenization of components. The advanced on-line degassing and filtration technology can well control the hydrogen content and inclusion content. For example, degassing combined with ceramic filtration can control the hydrogen content in the melt to 0.1 ml/100 g Al. Castings mostly use hydraulic semi-continuous casting machines, which have the characteristics of stable operation, high degree of automation, and high control accuracy. Smooth control of the casting process is critical to the formation of large ingots. Advanced aluminum processing plant through the computer on the casting temperature, casting speed, cooling water injection angle, distribution, flow and strength and other process parameters for fine control, can effectively prevent cracking of the ingot. At the same time, ingots were examined using ultrasonic testing to detect defects such as inclusions, cracks, and pores. At present, foreign countries have produced aluminum alloy round ingots such as 7050, 7175 and 2219 with a diameter of 1066.8mm and a mass of 16 tons, and 2618 alloy slabs with a weight of 4368.8mm × 2438.4mm × 1066.8mm and a weight of about 32 tons.
High-strength aluminum alloy ingots Because of the high content of alloy elements, non-uniformity and supersaturation, the homogenization of ingots becomes a critical process for the preparation of a material immediately after the casting. The homogenization treatment can uniformly distribute the alloy components, eliminate the non-equilibrium crystalline low melting point phase, spheroidize the hard second phase (eg, second phase particles containing impurities Fe, Si), and form a coherent dispersed phase (eg, Al3Zr) as follow-up. The grain structure of the process control material reduces the quenching sensitivity of the alloy and improves the toughness of the material for the preparation of the structure. In order to optimize the distribution of dispersed phase particles after homogenization of the alloy, a two-stage homogenization process may be adopted which is followed by high temperature after low temperature. After two-stage homogenization, the distribution of dispersoids in the alloy is more uniform and smaller. In order to suppress the recrystallization to a greater extent and obtain high mechanical properties, it is necessary to avoid the formation of a coarse second phase during cooling after homogenization. After the ingot is homogenized, it must be cooled at a faster rate. For the 7050 aluminum alloy, the cooling rate should be greater than 0.5°C/s.
Battery operated Light
Ningbo Deamak Star Intelligent Technology Co., Ltd , https://www.deamakstar.com