Interfacial Microstructural and Corrosion Characterizations of Friction Stir Welded AA6061-T6 and AISI304 Materials

The use of aluminum in conjunction of steel can reduce the weight of structures but dissimilar materials welded structure results in the formation of intermetallic compounds and inhomogeneous distribution of grains. Since aluminum is more active than the steel, the structures made from such dissimilar materials can be affected from corrosion medium which needs to be investigated. In the present work, friction stir welding has been used to join AA6061-T6 and AISI304 in lap configuration, each having a thickness of 1 mm under varied process parameters. The detailed investigations have been made which includes understanding the effect of process variables on microstructures, intermetallic compounds and their phases, and thereby on corrosion of the aluminum-steel welded joint. SEM with integrated EBSD detector and XRD analyses have been carried out to characterize the weld interface that revealed the evolution of grain boundaries and existence of phases such as Fe₂Al₅ and AlCrFe₂. The grain size of the weld zone has been found to be decreasing with increase in weld speed and plunge depth. The temperature profiles have shown a faster rate of heating and cooling with increase in welding speed and plunge depth which led to the refinement of microstructure. The evolution precipitates mainly comprised of Al, Mg and Si as the major elements. The corrosion rate was found to be increasing with decrease in grain size. Samples were corroded by pitting corrosion, inter-granular corrosion, and environmental corrosion. Severity of pits have been found to be non-uniform in the along weld cross-section.