| The microstructures of friction stir welded (FSW) joints of 2219-T87 Aluminum Alloys were characterized by scanning electron microsopy (SEM), energy dispersive specture spectrometer (EDS) and hardness analysis. Size and Distributions of the precipitates in different micro-areas of the joints and their relation with the micro-hardness of the weld were investigated. The immersion corrosion experiment in 3.5% NaCl aqueous solution, intergranular corrosion and exfoliation corrosion were carried out on the different micro-areas of the joint. Micro galvanic corrosion effect was investigated by in-situ observation with SEM and metallographic microscope. The macro galvanic corrosion effect between micro-areas was investigated by electrochemical analysis.The results of metallographic observation show that the main presipitates in the joint was Al2Cu phase. The presipitates in the shoulder affected zone(SAZ) was small equiaxed grain structure because of the heat affect and broken by mechanism. The dissolution of the Al2Cu phase made the hardness decresed.The immersion corrosion, intergranular corrosion and exfoliation corrosion experiments indicated that the corrosive evolution process was pitting corrosion, intergranular corrosion along the rolling direction of metal and exfoliation corrosion in turn in BM and HAZ resulted from the galvanic corrosion effect between matrix and coarse precipitate, while, there were only pitts in whole SAZ.Electrochemical analysis proved that there was potential difference between micro-areas of joint.SAZ actted as cathode region with higher potential than base metal. The polarization curve and the results showed during immersion the corrosion behaviours of the different micro-areas would change. At the beginning, the potential of SAZ was higher while the BM was higher after a time of immersion. This might be influenced by galvanic corrosion effect and the change of corrosion behaviours of presipitate during corrosion. |
