Preparation, microstructure and properties of Al-Zn-Mg-Sc alloy tubes

The Al-6.0Zn-2.0Mg-0.2Sc-0.10Zr hollow tube ingots, prepared by semi-continuous casting technology, were subjected to homogenization treatment, hot extrusion, intermediate annealing, tension, solution and aging treatment. The microstructures and properties of as-cast Al-Zn-Mg-Sc alloy at different homogenization treatment conditions were studied using hardness measurement, X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analysis. The results showed that the phases in the as-cast Al-Zn-Mg-Sc alloy were α(Al) and T(Mg32(Al,Zn)49). The metastable supersaturation solid solution decomposed firstly. Then the equilibrium T phases and dispersed Al3(Sc,Zr) particles precipitated and dissolved into the solution matrixes at high homogenization temperature. The appropriate homogenization processing was determined to be 470 °C for 24 h. The microstructures after extrusion were non-recrystallized grains because of dispersed Al3(Sc,Zr) particles. After intermediate annealing most of the deformed microstructures were eliminated and softened. During solution treatment the equilibrium phases dissolved into the matrix, leading to the formation of supersaturation solid solution. The η' phases precipitated from the solid solution during aging process. The η' phases in the grain and the equilibrium phases on the grain boundaries were coarsening and the precipitate free zone (PFZ) was wider with increasing of the aging temperature and time. The suitable heat treatment for Al-Zn-Mg-Sc-Zr aluminum tubes was solution treated at 470 °C for 1 h, followed by aging at 120 °C for 24 h. At this condition, the ultimate tensile strength, yield strength and elongation could reach 520 MPa, 510 MPa and 11%, respectively. The high strength was mainly due to the fine grain strengthening caused by the addition of Sc and Zr, the subgrain strengthening and precipitation strengthening of Al3(Sc,Zr) particles and η' phase.