The Relationship Between Corrosion Characteristics And Microstructure In Al-Mg-Si-Cu Alloys

Due to their low density, excellent formability and weldability, and medium tohigh strength obtained after heat treatment, the Al-Mg-Si alloys have great applicationprospects in lightweight vehicles, which are extremely important for energy-savingand emission-reduction. Cu addition to Al-Mg-Si alloys can enhance the peakhardness and alter the precipitation, especially the grain boundary precipitates andprecipitate free zone (PFZ) which are intimately correlated with the inter-granularcorrosion (IGC) susceptibility of Al-Mg-Si-Cu alloys. The heat treatments alsostrongly affect the IGC resistance, which can be improved by T78temper whoseoperation mechanism is still not fully understood. Moreover, the grain boundarycharacteristics, which can determine the IGC, are influenced by the experienceddeformation process and grain size. In the present work, the effect of alloycomposition and thermo-mechanical processing on the IGC in Al-Mg-Si-Cu alloys areinvestigated. The major results are summarized as follows:Accelerated corrosion testing indicates the peak-aged samples are highlysusceptible to IGC, while over-aged samples mainly appear pitting corrosion.Mg/Si ratios have no obvious effect on IGC in samples with a given Cu content.Scanning electron microscopy (SEM) observations reveal that there are a lot of grainboundary precipitates in peak-aged samples, and the space between precipitates issmall, while few coarsening precipitates exist in the grain. The susceptibility to IGCis attributed to micro-galvanic coupling between grain boundary precipitates and theadjacent depleted zone.T78tempers involved a first step aging (180℃,5h) followed by a second stepaging at195,205and215℃, respectively. With the prolonging of second step aging,the hardness firstly decreased then increased, while the conductivity increasedgradually. The optimum T78process was (180℃,5h)+(195℃,2h). TEMobservation results show that it was mostly needle-like β″phase in the Al matrix forthe peak-aged sample. After T78treatment, a large amount of lath-like precipitatesformed in the matrix, while the precipitate free zones (PFZ) broadened sl ightly. Thesegregation of Cu was found at the interface between lath-like precipitates and thematrix, so more Cu precipitated out from the Al matrix and thus reduced theelectrochemical potential difference between the PFZ and the matrix. The abovefinding may explain why T78temper can desensitize inter-granular corrosion withoutsacrificing strength. The accelerated corrosion test results suggest that the depth of IGC and theextent of corrosion reduce with decreasing grain size. The electrochemical ana lysisshows that as grain size decreases, the corrosion potential is shifted slightly, but thecorrosion current density is gradually reduced. Only a small number of pitting attackswere observed in the aged samples after cold-rolling. The resistance to corrosiondecreases with the aging time. This phenomenon may be attributed to the segregationof alloying elements along sub-grain boundaries.