Research On Corrosion Protection Of Friction Stir Welded 2024-T3 Aluminum Alloy Joints By Cold Spraying

As an innovative solid state welding technology, friction stir welding(FSW) developed by the Welding Institute of UK in 1991 can avoid many kinds of solidification defects. Since then, FSW has been widely used for joining Al alloys in many industries, such as aviation and ship building industries. However, FSW joints of highly alloyed Al alloys such as 2xxx and 7xxx series are very susceptible to corrosion, which has inhibited the wide use of FSW. Only a few studies about corrosion properties and corrosion protection of FSWs have been performed. The attempts to improve the corrosion resistance by the use of several treatments, such as fast cooling, heat treatment, laser surface treatment and micro arc oxidation, during or after the welding are not satisfying. Cold spraying is a promising technique for overcoming corrosion problems of FSWs, which has been used as a simple technique for surface protection of magnesium alloys and aluminum alloys. So far, there are few reports on corrosion protection of FSWs by cold sprayed coatings.In this work, therefore, 3mm thick 2024-T3 plates with aluminum cladding were used as base metal and the corrosion performance of the FSW joints was studied. Pure Al and 6061 aluminum alloy coatings were deposited on the surface of FSWs by cold spraying. The effect of the cold sprayed coatings on the corrosion resistance and mechanical properties of FSW joints was investigated.Results show that on the surface of FSW joints, different regions show different corrosion behaviors. Because of the presence of aluminum cladding, the base metal and the heat affected zone(HAZ) show higher corrosion resistance. The weld nugget(WN) shows higher corrosion resistance than the thermomechnically affected zone(TMAZ) for the solution of precipitates in the WN decreasing its susceptibility to corrosion. The WN is prone to be homogenously corroded and the TMAZ is found as exfoliation corrosion.Relatively dense pure Al and 6061 aluminum alloy coatings were deposited under the cold spraying parameters: the standoff distance 30 mm and the inlet gas pressure 2.8MPa and temperature 350?. The coatings provide good protection to the substrate. In the 3.5% NaCl solution, the average circuit potentials of cold sprayed pure Al and 6061 coatings are-580.35 mV and-636.25 mV, respectively. When the solution temperature is 40?, the cold sprayed coatings can still protect the 2024-T3 aluminum FSW joints for a long time.The mechanical properties of the 2024-T3 aluminum FSW joints are also improved by the cold sprayed coatings. The average tensile strength of coated specimen increases from 393.3MPa to 420.0MPa and the average elongation increases from 4.28% to 6.91%. When the fatigue life is 1×107 cycles, the fatigue strength corresponding to a 50% survival probability of coated specimen is 138.3MPa, 29.14% higher than the as-welded specimen, and the fatigue strength corresponding to a 95% survival probability of coated specimen is 114.3MPa, 30.92% higher than the as-welded specimen.