Corrosion control of friction stir welded AA2024-T351 aluminium alloys

Friction stir welding (FSW) is a modern solid state welding technique developed at the welding institute (TWI) in 1991. The joining is achieved by heat generation, material softening and plastic deformation following the travelling of non-consumable pin through the gap between the two work pieces to be joined. In present study, joining of AA 2024-T3 aluminium alloy, is achieved by FSW. The influence of the FSW on the alloy microstructure and corrosion behaviour is determined. The effect of laser surface melting (LSM) treatment on the improvement of corrosion resistance of friction stir welded alloys is investigated. Further, heat treatments to simulate the welding process with controlled cooling rate are performed to assess the effect of cooling rate on the microstructure, consequently, the corrosion performance of the welds. It is revealed that FSW process introduces elevated temperatures at the weldment, resulting in distinct regions with modified microstructures. The regions are named as the TMAZ (thermomechanically affected zone) and the HAZ (heat affected zone). TMAZ, positioned at the weldment centre, is featured by a central nugget with dynamically recrystallised fine, equiaxid grains, that is surrounded by heavily deformed grains. HAZ, positioned as narrow bands just outside TMAZ, has grain size similar to parent alloy. Corrosion testing shows that the as-welded alloy is highly susceptible to corrosion, particularly at the bands just outside the TMAZ (i.e. HAZ). Welding process resulted in the preferential precipitation of copper and magnesium rich particles at the grain boundaries within the HAZ, which reduces the corrosion resistance as a result of the galvanic coupling of the sensitised grain boundaries and the adjacent matrix. Laser treatment resulted in a melted near-surface layer, up to 6 μm thick, where normal constituent particles are absent. Corrosion testing showed that laser treatment reduces the degree of localized corrosion due to the removal constituent particles. However, scrutiny of the melted near-surface layer revealed continuous segregation bands, approximately 10nm thick, containing mainly copper. The presence of such segregation bands promoted localised corrosion of the laser melting layer due to microgalvanic action. From the areas where melting layer is corroded, localised corrosion propagated further into the weld intergranularly. The severe intergranular corrosion beneath the laser melting layer undermines the laser melting layer, resulting delamination of the surface layer from theunderlying bulk alloy. The simulated heat treatments show that the cooling cycle of the welding process has a significant influence on the alloy's microstructure and corrosion behaviour. Slow cooling can result in formation of a continuous network of second phase particles at the grains boundaries, leading to significantly reduced corrosion resistance. Rapid cooling tends to prevent the formation of second phase particles at grains boundaries, resulting in improved corrosion resistance.EThOS - Electronic Theses Online ServiceLibyan embassyGBUnited Kingdo