The influence of artificial aging on the microstructure, mechanical properties, corrosion, and environmental cracking susceptibility of a 7075 friction-stir-weld

The influence of overaging to the T7451 temper on the micro-structure, the mechanical properties, and the corrosion susceptibility of friction-stir-welded 7075 aluminum alloy was investigated by means of transmission electron microscope (TEM), SEM investigations, constant extension rate tests (CERT), alternate immersion tests, and potentiodynamic scans. The overaging that occurs during welding within the heat-affected zones of the friction-stir-welded 7075-O tempered plates promotes a slight formation of intragranular and grain boundary precipitates and increases the mechanical as well as the corrosion resistance properties as compared to the T7451 welded plates. The "double" overaging treatment, consisting of the T7451 temper and the thermal transient experienced by the heat-affected zones of the 7075-T7451 welded plates, increases the size of the intragranular and the grain boundary precipitates as well as the precipitate-free zones. These facts decrease the mechanical and corrosion properties of the 7075-T7451 weld. The T7451 thermal treatment applied after the welding of the 7075-O plates promotes the high presence of small precipitates and reduces the size of the precipitate-free zones. This fact increases the general corrosion resistance, but decreases the mechanical properties. Therefore, the corrosion as well as the mechanical properties are greatly correlated with minute changes in the microstructure, which can arise by short-term heat exposure as for instance during welding. It was also found that the environmental susceptibility measured by means of CERTs may be influenced by "solution-strengthening" corrosion mechanisms which increase the strain. This fact is present in weld microzones particularly susceptible to corrosion.