退火熱處理對加工後2024 Al-Cu-Mg合金軋延板料晶粒成長機構之探討

Abstract

Most of the materials used for an airplane’s front and rear fuselage skins are the 2024 Aluminum Alloy rolled plate material, which sometimes has the phenomenon of overgrowth of crystals when being stretched to take shape. In this experiment, the 2024 Aluminum Alloy Rolled Plate material in the thickness of 2.29 mm was applied to create the trial pieces whose initial statuses were the O material and the T4 material. We made the trial pieces undergo many kinds of experimental combinations such as different amounts of stretch and deformation, annealing, and solution heat treatment, so as to observe the changes of its sizes of crystal grains, in order to explore what factors result in the abnormal growth of 2024 Aluminum Alloy Rolled Plate during the process of stretch and deformation. In this experiment, Besides, through the observations via EBSD (Electron Back-Scatter Diffraction), we knew the microstructure-oriented {001} texture and structure of the trial pieces of O material and T4 material. When the trial pieces of O material received annealing and solution heat treatment after undergoing 5%-20% of stretch and deformation, compared with the gripping area, no occurrence of the growth of its crystal grains would appear when the amount of stretch and deformation was 5%; the maximum growth would appear when the amount of stretch and deformation was 10%, and the vertical and transverse sizes of the crystal grains were respectively 4.3 times and 3.1 times greater than that of the gripping area. Also, the appearance of the crystal grains after the growth was non-equiaxed crystal. When the amount of stretch and deformation was 15% or 20%, the crystal grains would shrink contrarily and get close to the size of the crystal grains in the gripping area. Therefore, if we make the O material of the 2024 Aluminum Alloy Rolled Plate avoid 10% of the amount of stretch and deformation, we can ward off the overgrowth of crystals. In addition, through the Stretch Stress Strain Diagram, we knew that the stretch stress necessary to be imposed on the trial piece of T4 material was about 2 times greater than that on the O material. Besides, from the observations on the trial piece of T4. material with 10% of stretch and deformation via TEM(Transmission Electron Microscopy), we found that high-density dislocations exist in the aluminum base. However, the experiment results showed that the sizes and shapes of the T4 material’s crystal grains nearly have no changes after the T4 material had undergone 5%-20% of stretch and deformation and then received annealing and solution heat treatment. Furthermore, after the T4 material and the O material underwent 10% of stretch and deformation, their neighboring crystal grains would deform in the two closer crystal axial directions each other. Through the observations on the trial pieces via TEM, the T4 material had plastic deformation centering on the crystal boundary, while the O material had plastic deformation both around the crystal boundary and within the crystal grains. Judging from deduction, we consider: When the plastic deformation of a crystal grain spreads all over the crystal grain, such a plastic deformation can be the thrust of the crystal grain to adjust its crystal axial direction. To sum up the observation results mentioned above, when the O material receives annealing and solution heat treatment after undergoing 10% of stretch and deformation, a certain mechanism will appear, in which, There exists a mechanism including adjustions of neighboring grain axis, the driving force of the strain energy as deformation in room temperatureand surface energy decrease of grain boundaries. It leads to the dislocations by the grain boundaries offset mutually and eliminate by rearrangment.The grains by the side of dislocations combine to grow.