| Combined the National Assembly Pre-research Project of China, two kinds of Al-5.4Zn-2.0Mg-0.35Cu (wt.%) alloys with and without Sc and Zr additions (0.25Sc+0.10Zr(wt.%)) were investigated comparatively by the methods of process optimizing and microscope observations. Effects of Sc and Zr microalloying additions on the microstructures and properties of Al-Zn-Mg alloys during the processing of homogenization, hot-deformation and solution-aging were studied. Based on this, the mechanisms of the effects of Sc and Zr minor additions on the recrystallization, anisotropy, corrosion and welding of Al-Zn-Mg alloys were studied in detail. The main results of the research are as follows:(1) The as-cast grains of Al-5.7Zn-2.0Mg-0.35Cu (wt.%) ingot were refined from78μm into58μm by adding0.10Zr+0.25Sc (wt.%).(2) Under the condition of semi-continuous casting, the matrix is closest to supersaturated solid solutions, non-equilibrium eutectic phases containing Zn and Mg and indissoluble impurity phases containing Fe and Si are concentrated on grain boundaries, and thus it is necessary to homogenize. With the increase of homogenization temperatures, MgZn2and T(Mg32(Al, Zn)49) phases firstly precipitated and then dissolved into matrix, and the non-equilibrium phases distributed on the grain boundaries dissolved. Meantime, in the alloy with Sc and Zr additions, lots of fine and disperse Al3(Sc, Zr) particles precipitated during homogenization, and the proper homogenization process for this alloy is350℃/8h+470℃/12h.(3) During the hot deformation of the two studied ingots, with the increase of deformation temperatures, the strength decreased, the plasticity increased firstly and then decreased, and the fractures transformed from trans-granular fractures into intergranular fractures. Deformed between375℃and400℃, the alloys have stable deformation resistance and higher hot working plasticity. The actual production of hot rolling proved that375℃-400℃was the proper hot working temperatures for the studied ingots. Besides, during hot deformation, lots of disperse Al3(Sc, Zr) particles in Al-Zn-Mg-Sc-Zr alloy strongly pinned dislocations and subgrain boundaries, inhibiting the movement of dislocations and subgrain boundaries, improving hot deformation resistance.(4) Added0.25Sc+0.10Zr into Al-5.7Zn-2.0Mg-0.35Cu alloy, under the optimal homogenization treatment, hot deformation process and solution-aging treatment (470℃/1h, followed by water quenching,+120℃/24h), the ultimate tensile strength and the yield strength increased by12%and22%, meantime, the elongation remained above12%. The strengthening mechanisms of minor Sc and Zr are substructure strengthening, grain refining strengthening and precipitation strengthening of Al3(Sc, Zr) particles.(5) During recrystallization annealing,0.10Zr+0.25Sc additions changed the recrystallization textures of Al-5.7Zn-2.0Mg-0.35Cu alloy from cube textures into rolling textures. A recrystallization nucleation model was established successfully where two kinds of nucleation sites were considered:(i) nucleation sites that have a recovery advantage (or have a boundary energy advantage) and (ii) nucleation sites which require a considerable static recovery period in order to reach the critical size. Based on the model, the store energies for different orientations were calculated. Affected by the Zener drag from Al3(Sc, Zr) particles, the mechanisms of recrystallization texture changed from cube nucleation into high stored energy nucleation and the recrystallization temperatures increased from below350℃into above550℃.(6) The textures of the cold rolled sheets consisted of Cube, β-fiber rolling and Goss textures. After solution-aging treatment, Cube textures were the dominant textures for Al-Zn-Mg alloy, whereas the main textures of Al-Zn-Mg-Sc-Zr alloy were β-fiber rolling textures. Sc and Zr additions increased the in-plane anisotropy values of yield strength from2.1%into7.2%. Based on the measured texture data, Taylor factors were calculated under different force axis. Considered the effects from grain boundary strengthening, precipitation strengthening and solution strengthening, the model of yield strength in-plane anisotropy for the sheet products was successfully established, exhibiting that textures were the main reason for strength anisotropy.(7) Sc and Zr microalloying additions and increasing aging time at120℃can both effectively improve the resistance of stress corrosion cracking, intergranular corrosion and exfoliation corrosion of Al-5.7Zn-2.0Mg-0.35Cu alloy.Aged at120℃for36h, the minimum value of stress corrosion cracking susceptibility was larger than95%, the maximum corrosion depths were smaller than34μm, and the rank of the exfoliation corrosion reached PA in Al-5.7Zn-2.0Mg-0.35Cu-0.25Sc-0.10Zr alloy. The improved corrosion resistance is from inhibiting recrystallization, narrowing PFZ, coarsening grain boundary precipitates and increasing the spacing of grain boundary precipitates.(8) Added minor Sc and Zr into Al-5.7Zn-2.0Mg-0.35Cu matrix, ultimate tensile strength and yield strength of the welding for sheet products increased by4%and23%, respectively. The increased welding strength caused by Sc and Zr is from inhibiting recrystallization of the heat affected zones by Al3(Sc, Zr) particles. |
PDF Full Text Request