| Friction stir welding (FSW) is a new type of solid-state joining technique, which has theadvantages of high efficiency and energy saving. FSW is an environment friendly technique,and it is especially suitable for light metals joining.5083Aluminum alloy has goodmechanical properties, excellent corrosion resistance and good weld-ability, which is widelyused in ship building industries. In this study,3mm thick AA5083plates are friction stirwelded. Effect of welding parameters on the microstructure and mechanical properties ofFSW joints are studied, Especially, tensile behaviors of base materials and FSW joints at77Kare examined. Corrosion behaviors of joints and base material are also investigated.Firstly, effect of various parameters on the weld quality of AA5083plates are studied. Itis found that the weld quality of AA5083plates is comprehensively affected by not only theplunge depth, but also the rotational rate (ω) and welding speed (v) during the weldingprocess. In order to join3mm thick AA5083plates successfully, an unconsumed tool with aspin length of2.85mm is appropriate and the optimum plunge depth is0.05~0.1mm. Flashcan be found on the joints when the high rotational rate or the low welding speed is used,which is the result of excessive heat generation during the process. Tunnel defect can beobserved in the joints when the low rotational rate or the high welding speed is selected.When fixing the rotational rate at800rpm, the optimum welding speed range is60~150mm/min. As increasing rotational rate to1500rpm, sound joints can be made under thewelding speed at60~300mm/min.Microstructure and mechanical properties of the joints welded under various parametersare also investigated. Defect-free joints can be made at a constant rotational rate of800rpmand the welding speed range of60~150mm/min. Tunnel defects are found in the advancingside of joints by increasing welding speed to200~300mm/min. Four typical zones can becharacterized by grain structure, i.e. base materials (BM), heat affected zone (HAZ),thermal-mechanical affected zone (TMAZ) and stir zone (SZ). Onion rings can be observed inSZ of some FSW joints. Under the same rotational rate, the average grain size of stir zone decreases by increasing the welding speed. The average grain size of stir zone is4.3μm whenthe parameter was800rpm and300mm/min. Micro hardness in SZ is higher due to the grainrefinement. The ultimate tensile strength (UTS) of the joint welded under800rpm and60mm/min is316MPa and the elongation is21.2%The failure mode is a mixture of ductileand brittle fracture. At77K, UTS and elongation of the joint is403MPa and33.5%respectively, fracture morphology observations reveals that the failure mode is a ductilerupture.The corrosion behavior of FSW joints and base materials are also examined in this study,and it is found that FSW welds have better corrosion resistance property. Both base materialsand FSW joints have the same degree of corrosion rate during the3.5wt%NaCl immersiontest. As a result of salty spray testing, weight loss of both base materials and joints increasesby increasing testing time and is tend to be stable after144h. The corrosion potentials of jointsare higher than that of BM investigated by electrochemical test. Finer distribution ofprecipitates in SZ results in more excellent corrosion properties. The results obtained byintergranular corrosion testing shows that FSW joints are not sensitive to intergranularcorrosion owing to the smaller size and uniform distribution of second phases, while basematerial is sensitive due to the existence of β phase along the grain boundary. |
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