Localised Corrosion Studies on FSW Al-Li alloys

Abstract

Al-Li alloys were introduced for the use in aerospace applications due to its many advantages over steel such as its low density, good thermal and electrical conductivity and corrosion resistance of these alloys. The current generation of Al-Li alloys were developed to replace the currently used AA2024 alloy in commercial airframes, military and space applications. Traditional joining methods cannot be used to join dissimilar aluminium alloys. Thus, friction stir welding (FSW) was used to join the 2 dissimilar Al-Li alloys - AA2099 T83 and AA2060 T8E30 alloys. FSW causes several changes in the microstructure due to the rotational movement of the tool which results in localised plastic deformation and a thermal cycle in the alloys. This leads to 4 distinct zones in the alloys - the stir zone, thermo-mechanically affected zone, heat affected zone, and the base metal. The differences in microstructure is suggested to cause a change in the mechanical properties and localised corrosion behaviour of the alloys. In this work, the localised corrosion behaviour of the friction stir welded Al-Li alloys were investigated. The effect of FSW on the microstructure and corrosion behaviour was also studied. Microstructural characterisation was done for both the alloys and their respective weld zones. Coarse constituent particles were found in all weld zones with a decreasing trend in average size towards the weld centre. Strengthening precipitates such as the T_1 phase particles were observed on the grain boundaries of the alloys. This had a decreasing trend of distribution density towards the weld centre with virtually no precipitates in the SZ. In order to assess the corrosion performance potentiodynamic polarisation, open circuit potential, linear polarisation resistance, and immersion tests were deployed. Furthermore, scanning electron microscopy with energy dispersive X-ray spectroscopy was used to evaluate the morphology and chemical composition of the of the corroded surface. It was found that for the BM and HAZ regions of both alloys, the attack occurred mainly on the grain boundaries which were sites for the T_1 particles. These particles were suggested to be the controlling factor of localised corrosion behaviour in these regions due to their high electrochemical behaviour, which also resulted in almost no passivity in these regions. A large attack site was also observed on the surface of the matrix which was the site of hydrogen evolution during initial immersion time periods. Pits were also formed on the sites of coarse intermetallic particles. For the SZ regions the dominating attack was due to the coarse particles in the matrix. The effect of anodising the surface and sol-gel coating of the surface on the corrosion behaviour was determined in this project. It was found that the anodised layer did enhance the corrosion performance of the alloys. The SZ of the anodised sample was found to be most prone to corrosion compared to the anodised base metals. The sol-gel coating on the surface was also found to increase the corrosion resistance of the surface due to its self healing properties thus protecting the surface of the alloys.