Influence of process condition on stretch flange-ability of Al–Mg–Si alloys

Hole expanding limit, λ, measured as an index of stretch flange-ability of 6022 aluminum alloy was severely decreased by slow cooling (forced air: 10 K/s) after solutionizing at 823 K for 15 s, in 0.5% Cu added Al–0.6%Mg–0.9%Si alloy sheet which ingot was homogenized at a high temperature, 823 K. Whereas by fast cooling (water quenching: 500 K/s), 1.5 mm thick sheets of both alloys showed the λ higher than 50%. Influences of homogenizing temperature and cooling rate after solution treatment on the λ were investigated by semi-quantitative metallography. Ingot homogenization at higher temperature decreases amount of Mg–Si precipitates and increases solute concentration. Judging from measured conductivity (IACS%) , almost all of added Cu seems to remain in solid solution. Even after hot rolling, cold rolling, solution treatment and T4 treatment for more than 600 ks at room temperature, IACS% of the T4 sheets was in nearly same order as ingots. The severe decrease of the λ in the Cu added alloy sheets homogenized at 823 K and slowly cooled after solution treatment will be explained by coarse Si precipitates on grain boundaries formed during the slow cooling. The Cu addition and high homogenization temperature will lead to a high degree of solute supersaturation and elevate temperature for precipitation starting. Region of high dislocation density was observed nearby the coarse Si precipitates by transmission electron microscopy after the hole expanding test.