The aging characteristics and the weld-solidified structure of weld Al-Zn-Mg alloys, and the effects of grain and dendrite cell sizes on the aging characteristics of rapidly solidified Al-Zn-Mg alloys were examined by hardness measurements, the X-ray small-angle scattering and by optical- and transmission electron microscopies. Differences in macrostructures in weld Al-Zn-Mg alloys have no effect on their aging characteristics. Differences between microstructures of weld alloys and those of heat-affected zones in the alloy welds affect their aging characteristics. Sub-grains in the weld metals of Al-Zn-Mg alloy welds decrease their age-hardening capacities at 80°C, but increase them at 150°C. These results are discussed in terms of solute segregations and preferred precipitation η′ phases at sub-grain boundaries or on dislocations in the weld metals of Al-Zn-Mg alloy welds
