Mechanism and Inhibition of Grain Coarsening of Al-Mg-Si Alloy in Hot Forming

A high-Ti 6061 alloy was rolled with strains up to 0. 8 - 2. 0 and at 350 - 550 ℃ . Microstructures that developed during deformation and subsequent solution heat treatment (SHT) were observed by using optical and transmission electron microscopy. Microstructure evolution during SHT depends mainly on the initial rolling temperature,and it was found that the higher this temperature is,the coarser the grains are. After rolling at 400 ℃ ,well-defined cells and subgrains were formed, which induced further sites for recrystallization nucleation during subsequent SHT. The recrystallization mechanism was found to be subgrain rotation,with a final grain size smaller than 200 μm. Increasing the rolling temperature to 500 ℃ results in a low density of dislocations distributed uniformly in the deformed matrix and fewer nucleation sites during subsequent SHT. The recrystallization mechanism is grain boundary bulging,while the final grain size approaches several millimeters. Finally,a hot forming process of high-Ti 6061 alloy for inhibiting grain coarsening was proposed,and verified by experiments.

Dynamic recrystallization kinetic of fine grained Mg-Zn-Y-Zr alloy solidified under high pressure

Fine grained Mg_（96.17）Zn_（3.15）Y_（0.79）Zr_（0.18） alloy with an average grain size of 20 μm was prepared by high pressure solidification. The dynamic recrystallization（DRX） behavior of the fine grained Mg alloy solidified under the pressure of 4 GPa was studied via isothermal compression experiments. The tests were performed under the strain rate of 0.001–1.0 s^（–1） and at a deformation temperature of 523–623 K on a Gleeble-3500 D thermal-mechanical simulation machine. The DRX kinetic of the fine grained Mg alloy solidified under high pressure was established, and the microstructures of the alloy under different hot compression conditions were analyzed by electron back-scattering diffraction（EBSD）. According to the experimental results, the DRX kinetic model of the fine grain Mg alloy solidified under high pressure was X_（DRX）=1-exp[-0.75445（（ε-ε_c）/ε~＊）^（1.066208）.The Avrami exponents of n and k were 1.066208 and 0.75445 respectively, higher than those in the conventional casting alloy. The DRX volume fraction of the fine grain Mg alloy solidified under the pressure had a tendency to increase obviously with the strain rate decreasing and the deformation temperature increasing, which is different from the one in the conventional casting alloy. When compressed at 523 K, the DRX volume fraction of the fine grained Mg alloy solidified under high pressure was 85% under the strain rate of 1.0 s^（–1） and could be up to 95% under the strain rate of 0.001 s^（–1）. The DRX volume fraction of the conventional casting alloy was only 67% although under the condition of 623–0.001 s^（–1）. It was shown that the fine grained Mg alloy solidified under high pressure had a strong DRX capacity.