Effect of Relatively Insoluble Compounds on Aging Characteristics in Commercial Al-Zn-Mg Alloy

Metallurgical Study on the Weld Heat Affected Zone in Aluminum Alloy (Report I)

Abstract

An investigation has been made into the effect of relatively insoluble compounds on the aging characteristic in commercial Al-Zn-Mg system 7N01-T4 alloy by means of electrical resistivity measurement.
In the commercial AI-Zn-Mg alloy, the relatively insoluble compounds by additional elements such asCr, Mn, Zr so on and impurity such as Fe, Si have already precipitated during homogenization or hot rolling process. When the material is heated above the solution temperature as such as welds, the relatively insoluble compounds dissolve or precipitate more. According to the result of electrical resistivity measurement, the compounds precipitate more up to heating of about 500°C and dissolve above 500°C.
The compounds have hardly an effect on the natural aging characteristic and formation of G.P. zones.
But the compounds considerably influence the artificial aging characteristic at 120°C, for instance, the age hardening by the artificial aging increases with increasing the dissolution of the compounds.
When the compounds dissolve relatively, the age hardening is due to formation of G.P. zones and η' precipitates. But when the compounds precipitate more, age hardening hardly occur, because of the preferential precipitation of gross η'precipitates and the suppression of precipitation of G.P. zone and fine η' precipitates.
At the high temperature aging such as 210°C and 250°C, the compounds accelerate the precipitation of η' precipitates and η phase.