The corrosion and environmentally assisted cracking behavior of high strength aluminum alloys friction stir welds: 7075-T651 vs. 7050-T7451

The localized corrosion and environmentally assisted cracking of high strength aluminum alloys 7075-T651 and 7050-T7451 friction stir welds have been investigated in the weld transverse and longitudinal orientation by using conventional electrochemical measurements, corrosion immersion tests as well as constant extension rate testing. The weld micro-zones of both 7075-T651 and 7050-T7451 friction stir welds were found to be more susceptible to corrosion as compared to the unaffected parent metals. The heat affected zones of the weld were found to be the most susceptible to corrosion and environmental assisted cracking for the 7075-T651 FSW. An asymmetrical distribution of the' localized corrosion susceptibility (intergranular corrosion) correlated with the slightly different Cu depletion along the grain boundaries on both sides of the weld. In this alloy, fracture occurred constantly within the "soft" heat affected zones for samples tested in air and in 3.5 wt. % NaCl solution. In 7050-T7451, the TMAZ-nugget boundary exhibited the greatest corrosion susceptibility. In this alloy, the fracture position changed from the "soft" heat affected zones for samples tested in air to the TMAZ-nugget interface for samples tested in 3.5 wt. % NaCl solution. In this case a high environmentally susceptibility was found within the nugget and the TMAZ which exhibited grain boundaries heavily populated with Cu-enriched corrosion susceptible Mg (Zn, Cu)₂. In constant extension rate tests, 7075-T651 FSW generally exhibited better environmental cracking resistance as compared to 7050-T7451 FSW, while 7075-T651 parent metal indicated lower resistance then the parent metal of 7050-T7451.