Effect of aging on the dissolving rate of Al-Mg₂Si alloys

Abstract

The dissolving rate and the hardness were measured upon isothermal aging, reversion, and re-aging of Al-Mg₂Si alloys. The dissolving rate was measured by hydrogen evolution when specimens were immersed in 1/10N-NaOH solution at 20°C. The results obtained were as follows:
(1) It was found that the amount of evolved hydrogen was proportional to immersion time.
(2) The increase of hardness was rapid at the early stage of the aging and became slower as the aging proceeded. The dissolving rate depended on aging time in a complex manner; it was largest in the as-quenched state, decreased with aging, and again showed a slight increase after showing a minimum.
(3) Reversion treatment softened the aged specimen almost completely. The dissolving rate was increase by reversion except cases when it had been at or close to a minumum upon the first aging.
(4) The specimen, which had been aged to the minumum dissolving rate state, showed a significantly slow rate of hardness increase and dissolving rate decrease upon re-aging. The change of the hardness was not consistent with the change of the dissolving rate upon reversion. However, good correspondence betweer the hardness and the dissolving rate was found at re-aging. It was shown by the dissolving rate measrement that there existed a special state with respect to a structure of Al-Mg₂Si alloys.