Microstructure Evolution Of 7050 Aluminum Alloy During Constraint Deformation Processs At High Temperatures

Two kinds of extrusion experiments at the temperature of 440±5°C were conducted for solution treated and T7451 7050 aluminum alloy, respectively. The former was deformed by combined extrusion with different reductions, and the later was deformed through alternating the pattern draft (1.5°, 3°and 5°) of filling cavity inside the die.Optical microscope (OM), electron backscattered diffraction (EBSD) and transmission electron microscope (TEM) was employed to give a qualitative description and a quantitative characterization of the microstructure in characteristic regions of the profiles. The microstructure evolution during deformation at high temperature was summarized quantitatively.Numerical simulation was used to simulate the flow of the points in the material during deformation and the distribution feature of the effective strain after deformation. For the same extrusion mode, the earlier metal filling in the die becomes rigidity quickly, and the effective strain value in the rigidity sections is low. The regions that the metal fills later show high strain values after deformation. The strain value at the characteristic regions after direct extrusion is higher than that after indirect extrusion. As the increase of of pattern draft, the strain value decreases in the regions of gussets except rigidity sections.For the solution treated 7050 aluminum alloy, strain hardening increases gradually until the effective strain reached 1.1, and then dynamic softening begins. The fraction of deformed microstructure decreases continuously following a parabolic law, because recovered microstructure is generated gradually. At a low strain value, MgZn2 particles distribute dispersively and interact with dislocations intensively. As the strain increases, particles dissolve into the matrices, and their number reduces. Dislocations accumulate on the subgrain boundaries continuously, leading to the transformation of subgrain boundaries into high angle grain boundaries (HAGBs), and the continuous recrystallization (CDRX) occured at the strain of 0.81~0.85.The evolution of microstructure during indirect extrusion is similar to that during direct extrusion. But the velocity of microstructure evolution is different.