Stress Corrosion Cracking in Welds of Al-Zn-Mg System 7NO1-T4 Alloy(Materials, Metallurgy, Weldability)

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An investigation has been made into the effect of microstructure on the stress corrosion cracking (SCC) in 3.5% NaCl+0.5% H_2O_2 aqueous solution for Al-Zn-Mg system 7NO1-T4 alloy welds. Its susceptibility to SCC is known to be affected by microstructure. The microstructure consisting of G.P. zones has high susceptibility to SCC, and that consisting of η' precipitates and η phase has low susceptibility to SCC. The microstructure in which G.P. zone and a few η' precipitates or η' phase are mixed, has lower susceptibility to SCC than that consisting of G.P. zones only. SCC of the welds occured at the area of G.P. zone microstructure such as the weld bond area and base metal of non-heat affected zone, but the SCC hardly occured at the area where only a few η' precipitates or η phase existed in the weld heat affected zone. The fracture morphology of SCC in the microstructure consisting of G.P. zones showed intergranular fracture with plain surface, while that of SCC in the microstructure consisting of η' precipitates or η phase seemed to show intergranular fracture assosiated with grain boundary precipitates.
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