Behavior of Mn added to 6082 aluminum alloys during homogenization of billets

Behavior of Mn in billet of 6082 aluminum alloys was investigated by absolute value measurement of resistivity and direct analysis of solute Mn concentration. In as cast state, 57~75% of added Mn is found to be in solid solution. Heating up to the homogenization temperature, 823 K, was simulated by accumulative isochronal heat treatment with temperature and time interval 50 K/5.16 ks. Precipitation of Mg–Si compounds was detected from 473 K and attained maximum at 573 K, separated from the starting and maximum temperatures for precipitation of Al–Mn–Si compounds, 673 K and 773 K, respectively. Average diffusion distance of solutes, which was estimated from impurity diffusion data, ranged from 10 to 100 nm at the starting temperature of the each precipitation. Solute Mn concentration at 823 K is extremely lower than equilibrium solubility of Al–Mn binary system because of coexistence with Si. Above results suggest that the added Mn precipitates for most part during homogenization, therefore total amount of the insoluble particles containing Mn will be changed little by preheating and hot extrusion, except shape and dispersion of the particles. Thus, it can be predicted that resistivity and hardness changes during cooling after the homogenization or by various heat treatments after the extrusion are mainly caused by the change in Mg+Si concentration or amount of Mg–Si precipitates.