Modified age hardening of Al–Zn–Mg–Cu with minor amount of scandium and zirconium

Two Al–Zn–Mg–Cu alloys were prepared with small amounts of scandium (0.05%, 0.1%) and the same amount of zirconium and the same ratio of Zn/Mg (Zn/Mg = 2) via casting in metal mold. The prepared alloys were subjected to homogenization, warm-rolling, secondary annealing, and dissolution followed by artificial age hardening. The age hardening treatment was performed via two methods. The results indicated that, the dissolution method imposes significant impacts on the ultimate hardness and tensile characteristics of the resultant alloys, so that, compared to conventional T6 heat treatment, quench-controlled age hardening treatment ended up increasing the alloy hardness from 175 HB to 220 HB (i.e. 30% increase) and the tensile strength from 492 MPa to 600 MPa (i.e. 22% increase) also elongation from 8% to 10% (i.e. 25% increase). Enhanced strength and hardness of the alloys was a result of more volume fraction of Al₃(Sc, Zr) and MgZn₂ precipitates and their more uniform distributions across the aluminum matrix in the quench-controlled age hardening treatment, as compared to conventional T6 heat treatment. The results further showed that, with increasing the content of scandium in the alloys, their hardness increased, so that conventional T6 heat treatment on an alloy containing 0.05% Sc resulted in a hardness of 150 HB, while the corresponding figure to the alloy containing 0.1% Sc was 175 HB. Moreover, hardness of the alloys containing 0.05% Sc and 0.1% Sc and subjected to quench-controlled age hardening was found to be 185 HB and 220 HB, respectively, indicating 25% increase in hardness upon increasing the content of scandium. (paper)