The Effect of Microstructure and Cu Content on Corrosion Behavior of 6060 Aluminum Alloy
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AlMgSi alloys have gained more popularity in the automobile industry in recent years due to attractive properties such as good corrosion resistance, machineability and high strength-to-weight ratio. Cu is commonly added to the alloys for improved mechanical properties. However, the Cu has an adverse effect on the intergranular corrosion (IGC) resistance of the AlMgSi alloys. Deterioration of the IGC resistance is caused by the confluence of precipitation of cathodic Q-phase and Cu-rich film along the grain boundaries, and the formation of precipitated free zones (PFZs) due to depletion of Cu in the near vicinity of the grain boundaries. High potential differences between the PFZs and the phases along the grain boundaries lead to corrosion attack along the grain boundaries. In this thesis, the effect of microstructure on the IGC susceptibility of AlMgSi alloys with small amounts of Cu additions have been studied. Three variants of an AlMgSi alloy, AA6060, with various Cu contents were used. These variants was subjected to several thermomechanical routes - T5, T6 and T10 temper - to produce a series of microstructures. T5 tempered samples were artificially aged after extrusion directly from the as-extruded samples. The T10 samples was deformed after extrusion, followed by artificial aging. Samples in the T6 temper were deformed after extrusion, followed by solution heat treatment (SHT) and subsequent artificial aging. Both underaged and peak hardened condition were obtained. The samples were deformed to 20, 40 and 60\% thickness reduction. Moreover, the IGC susceptibility for the respective microstructures was investigated by an accelerated IGC test (EN ISO 11846, method B). Characterization of microstructure before and after the accelerated IGC test was done by optical microscopy, whereas investigation of the grain boundary misorientation was carried out by using electron backscatter diffraction (EBSD) in scanning electron microscope (SEM). Vickers hardness measurements of the samples were done to study how the thermomechanical processing affected the strength. The results show that samples in the T10 temper were more resistant against IGC than the samples in the T6 temper. It was suggested that the increased IGC resistance was owed to flattening of the grains during deformation which caused longer propagation path that inhibited IGC. The volume fraction of low-angle grain boundaries (LAGBs) was also significantly higher for samples in the T10 temper. Increased volume fractions of LAGBs were posited to improve the IGC resistance. Higher hardness was also observed in the T10 tempered samples. Based on these findings, the microstructure of the T10 temper is claimed to be more IGC resistant and harder than the microstructure of the T5 and T6 temper.