Corrosion Mechanism On The Friction Stir Welded Jonints Of High Strength Aluminium Alloys

In the present work, the immersion corrosion experiment in3.5%NaCl aqueoussolution is carried out on the surface of friction stir welding joints of2524-T3and7050-T7451aluminum alloy. In situ observation investigated the corrosive evolutionprocess using metallographic microscope and scanning electron microscopy (SEM). AndSEM incidental energy spectrum analysis system and transmission electron microscopy(TEM) were used to analyze the size, type and distribution of the second phase particlesand the corrosion condition. Combined with the electrochemical analysis method of openpotential, dynamic potential polarization curves and electrochemical impedancespectroscopy(EIS), the corrosion mechanism of the weld seam do deep research. Inaddition, the surface of the FSWed joints was remelted by different laser processingparameters in order to try to improve the surface properties. Corrosion resistance andmechanical property were characterized by using vickers hardness tester and opticalmicroscope.In situ observation of corrosion experiment indicated that the pitting corrosionoccurred on the welding joint of AA2524, which start around the bigger secondprecipitates. The pitting corrosion around the smaller second phase precipitates and thegeneral corrosion were followed. The analyzed results of SEM and TEM showed the mainphases of the joint were the S and θ phase particles. And these particles induced thecorrosion of the joint surface. On the surface of the FSWed AA7050, spot pitting occurredat the beginning of immersing experience. Intergranular corrosion was found in the parentmetal and the heat affected zone, but no founding in the SAZ. The surface of SAZ wascorroded seriously and evolved into the general corrosion. These were closely related tothe η phase precipitates.Electrochemical analysis shows that the corrosion resistance of different areas andimmersing times of the welding joints is different. The open circuit voltages and corrosionrates through the voltage-time curve and polarization curve investigating and calculating,which revealed lowest corrosion resistance appeared in SAZ in the early immersion time.With the extension of immersing time, the corrosion resistance of different areas was more and more equalized. What's more, the differences of EIS characteristic were displayed indifferent immersing times and different regions of the surface of specimens. Theelectrochemical impedance spectroscopy analysis revealed the corrosion behavior waschanged during the immersing process, which is concordant with the results of in situobservation.The microstructures and properties of the joint showed that the grains couldtransform into small equiaxed grains after laser remelting treatment. However, coursinggrains was appeared on the subsurface. And the weld joint after laser treatment have holedefects easily, especially on the HAZ. The hardness and corrosion resistance of2524and7050aluminum alloy FSWed joint enhance after laser remelting processing at the laserscanning speed of1200mm/min.