Study On Microstructures And Mechanical Property Of Al0.2Mn0.3Fe Wing-foil

With the development of air-condition characterized by low energy consumption, large power and miniature, the high-powered, miniaturized and low-cost heat exchanger must be developed. Therefore, Al foil with thinner and higher comprehensive property are required. To meet this requirement, this paper developed a new A10.3Mn0.4Fe-H26 heat exchange foils produced by casting following rolling, whose comprehensive property is better than that of traditional pure Al foils. In addition, this new Al foils can meet the requirement of coining processing, such as attenuation, stretching and ECO, and its output reaches 500t per month. However, during the practical process of production, there existed the fluctuation of mechanical properties and abnormal coarsening of some grains. By means of TEM, SEM, XRD, ODF, OM, DSC, Cupping Test and Tensile Test at different temperature, the paper investigated the key processing technology in producing A10.3Mn0.4Fe-H26 foils by casting following rolling, including the homogeneous treatment of the as-forged slab and the heat treatment of the wing foils as well as the change trend of the mechanical property, microstructures, texture and anisotropic property of the foils. The effects of different treatment on the property and microstructures were revealed. Based on these work, the paper drew several wonderful results and revealed some new principals in A10.3Mn0.4Fe-H26 heat exchange foils, such as the critical recrystallized temperature in the annealing of Al foils, the thin-wall effects and its inhabitation of the recrystallization and coarsening of the grains, the increasing elongation ratio of Al-based H26 wing foils with the decreasing temperature and the turning of fibrous grains to fine equiaxed grains at low temperature. The paper tried to explain these phenomena and drew some common conclusions as following:1. The study of evolution of the property, texture and microstructures and the anisotropic property of the A10.3Mn0.4Fe-H26 foils during the last annealing showed:(1) For the A10.3Mn0.4Fe-H26 foils annealed at the optimal temperature 255â–¡for 7～22h, the tensile strength (σ_b) is about 140～145MPa, the elongation ratio (5) is about 13～15.1%, and the Cupping value (Er) is GE 5.8mm, which can satisfy the needs of new techniques such as attenuation, stretching and ECO.(2) There existed the Critical Recrystallized Temperature Effects (CRTE) in the A10.3Mn0.4Fe-H26 foils during annealing.Being annealed at 330(?) for 22h, the mechanical properties of A10.3Mn0.4Fe-H26 foils are not so good. Microstructures analysis reveals that the recrystallized grains are abnormally coarsening. This result is named Critical Recrystallized Temperature Effects (CRTE) in this paper.The paper confirmed that CRTE originates from two main reasons. Firstly, if the recovery time is too long, the drive energy will be consumed. Therefore, the residual energy for nucleation and recrystallization is inadequate and the amount of nuclei in unit volume is less. Secondly the orientation difference between the recrystallized grains and fibrous grains is bigger and during the grains coarsening, the residual energy saved in fibrous grains can drive the constantly growth of recrystallized grains until they intersect. This is similar to the occurrence of coarse grains and low mechanical properties when the amount of cold-deformation is below critical value. The paper confirmed that the critical recrystallized temperature is a little lower than the traditionally critical recrystallized temperature.(3) The paper revealed the thin-wall effects during annealing of the A10.3Mn0.4Fe-H26 foils. Being annealed at higher temperature, the grains become finer at the higher temperature and the mechanical properties are better. There are two reasons for the result. Firstly, the higher of the annealing temperature, the more of recrystallized nuclei is. Secondly, because the size of the recrystallized grains is greater than thickness of the foils, during annealing the recrystallized grains exposed on the two rolling planes are changed from spherical to cylindrical and along the cross-section of the foil, and turned into the groove. As a result, the growth of grains is inhibited.(4) The anisotropic property of the Al-based wing foils is sensitive to the annealing temperature. Being annealed at 225～260(?), the anisotropy of strength and elongation is minimum, which can improve the deep cupping property and decrease the tendency of cracking.2. The study of the mechanical property and microstructures of A10.3Mn0.4Fe foils deformed at high temperature and low temperature showed:(1) There existed remarkable difference between the mechanical properties at different temperature of the same Al-based foils subjected to different heat treatment. The yield strength and tensile strength of Al-based foils subjected to H19, H26 and 0 treatment both decrease with the increasing temperature, but the tensile strength of Al-based foils subjected to H19 and H26 is higher than the yield strength when the deformation temperature is below -66(?). When the deformation temperature is above 70(?), the tensile strength is lower than the yield strength. For Al-based foils subjected to 0 treatment, the tensile strength is always greater than the yield strength.When the temperature is above room temperature,δof Al-based foils subjected to H19,H26 and 0 treatment increase with the increase of temperature. Below the room temperature, For Al-based foils subjected to H19 treatment,δslightly decreases with decreasing temperature up to about] -66(?) and then increases with the further decreasing temperature and reaches 5.4%, 1.4 times as many as that at room temperature. For Al-based foils subjected to 0 treatment,δkeep constant with decreasing temperature. For Al-based foils subjected to H26 treatment,δincreases remarkably with decreasing temperature and reaches 28.5% at -175(?), 2.4 times as many as that at room temperature, showing abnormal plasticity at low temperature.(2) The paper found that when deformed below -170(?), the fibrous grains in Al-based foils turn into many fine equiaxed grains and dealt with the process and mechanism of fracture of those fibrous grains.The paper argued that at low temperature the distances between atoms decrease, leading to the slipping of dislocations to the grains boundary, phase interfaces and dislocation walls. But at low temperature, unlike dislocations are difficult to offset. Therefore, those dislocations slipped to the grains boundaries and phase interfaces form obvious groove and pedestal sit, widening the grain boundary. Those dislocations aggregated at the DDW increase the orientation decrement of the grains and thus make the DDW turn into grains boundaries. After lots of DDW turn into grains boundaries, the fibrous grains are divided into many finer equiaxed grains. This tendency becomes more obvious with decreasing temperature.The obvious groove and pedestal sit at grain boundaries as well as microcrack will decrease the elongation, but the fine equiaxed grains induced by DDW will increase the strength and elongation, compensate the decrease of strength and elongation induced by enlarged grains boundaries and polw grooves.(3) After the H26 treatment, foils are fully annealed. Therefore, the unlike dislocations completely vanish, only identical dislocations gather and form the dislocation wall, leading to polygonization accompanying with the occurrence of some fine subgrains. At low temperature, the polygonized microstructures turn into fine equiaxed grains. The tendency becomes more obvious with decreasing temperature. At -170(?), most of the fibrous grains turn into fine equiaxed grains. As a result, the fine equiaxed grains have t no polw grooves or steps, increasingδat low temperature.(4)The researches on mechanical property of A10.3Mn0.4Fe foils at high temperature and low temperature are of important engineering value, providing the data of mechanical properties at different temperature and guiding the selection of Al foils. Above 70(?), the fracture strength of H19 and H26 are only 55～95% of its yield strength. During the hydrophilic treatment of H26 foils, the stretching force must be lower than 24MPa. Otherwise, the plastic deformation will occur, leading to the change of the mechanical property and even the fracture.In addition, the difference between the tensile strength at -45(?) andσb at 50(?) of H26 foils amounts to 21MPa, exceeding the permission fluctuation of standard. This reveals that there exists a remarkable difference between mechanical properties of Al foils tested at different temperature. This difference is so big as to bring about business dissension. As a result, we suggest that the government support the study about mechanical properties of Al foils at different temperature and build the standards.3. The study of variations of the microstructures of the casting following rolling during homogeneous treatment showed:The second phases of the as rolled plates before homogenization is tree-like, or acicular, or reticular, or skeleton-like with the sharp interfaces. After being homogenized at 430(?) for 4h, the tree-like, acicular, reticular and skeleton-like microstructures are remarkably reduced and lots of dispersed spherical second phase are produced in the whole matrix. EPMA, XRD and single crystal diffraction analysis determine that the coarse second phase before homogenization is mainly Al₆(Fe Mn). After homogenization, the fine spherical second phase is mainly A19FeO.84Mn2.16Si. Being homogenized at 220～390(?), the solution and spheroidization of Al₆(Fe Mn) are inadequate and thus the A19FeO.84Mn2.16Si is less. With the increasing temperature to 470～610(?), lots of Al₆(Fe Mn) phases are solved into the matrix and the amount of . A19FeO.84Mn2.16Si precipitated decreases. The higher the homogenization temperature, the more obvious this tendency.Being homogenized at 420(?), even period of the annealing time is long to 40h, the size of grains is close to those before homogenization. Being homogenized at 450(?), a few grains begin to coarsen and over 450(?), almost all grains coarsen. As a compromise, the optimal homogenization temperature for A10.3Mn0.4Fe foils is 420～430(?) and the time is 4h.