Studies on salt fog corrosion behavior of friction stir welded AA7075 T651 aluminum alloy

High strength aluminum alloy 7075 is used in aerospace applications. This alloy is considered unweldable using fusion welding processes. Friction stir welding being a solid state process has been proved to be a suitable process for obtaining sound welds of these materials. Although no solid liquid phase transformation occurs during friction stir welding, relatively high temperatures, up to 475 degrees C for AA7075 aluminum alloy, are generated by friction between the tool and the workpiece. This produces modified microstructures that may be more susceptible to corrosion than the parent alloy. In the current study, 16 mm thick rolled AA7075-T651 aluminum alloy plates were joined using friction stir welding. The corrosion behavior of base material and friction stir welds was investigated using the Salt fog test (ASTM B117). It was observed that the corrosion resistance of the welds in the basic solution is better than that in the acidic and neutral solutions. In contrast, no significant difference has been found in the corrosion resistance of the base material with change in pH value. It was also found that the corrosion rate decreases with increase in time of exposure. It has been observed that corrosion attack is greater in the weld region than in the parent material and within the weld, the heat-affected zone has been found to be more susceptible to corrosion compared to the weld nugget and thermo-mechanically-affected zone regions. Transmission electron microscopy studies showed coarser precipitates and wider precipitate-free zones in the heat-affected zone which are concluded to be reasons for more susceptibility to corrosion.