Investigation On The Microstructure And Corrosion Mechanism On The Top Surface Of Friction Stir Welded Joint Of High Strength Aluminium Alloys

The microstructure and properties on the top surface of FSW joint were characterized by optical microscopy and the Vickers hardness tester. SEM and XRD were employed to investigate the types and quantities of secondary phase particles in different regions of FSW joint. Accelerated corrosion immersion test was performed in EXCO solution. SEM with an EDS using element mapping mode was employed to obtain the in-situ corrosion development and the chemical compositions of different intermetallic compound particles in the alloy. The corrosion mechanism of FSW joint was investigated by combining TEM,DSC and electrochemical analysis.The results of metallographic observation show that many regular arc shaped stringers and alternately dark and light waves are presented in shoulder active zone (SAZ). They consist of secondary phase particles and fine grains. The cladding layer of pure aluminium is destroyed during FSW, and the pure Al is enriched in retreating side. The results of SEM and XRD reveal that the main precipitates in the FSW joint are S phase,ηphase and Fe-containing phase. The redistribution of these phase particles is affected by welding parameters. The welding parameters are important factors for the distribution of the microstructure and hardness on the top surface of FSW joint. The microstructure homogenizes and hardness increases with increasing the rotational speed in the SAZ. The hardness value almost equals to base metal's in the SAZ of FSW AA2024 under high welding parameters.The results of in-situ observation of corrosion evolution indicate that the pitting corrosion was observed for all regions of the FSW AA2024 joint's top surface after being immersed in EXCO solution. The pitting corrosion initially originates in dissolving of the S phase. The density and degree of the pitting corrosion in the SAZ are slightly larger than those in other regions. The reason for corrosion is the selective corrosion of S phase. The particles of Fe-containing phase also contribute to the pitting corrosion. The pitting corrosion,intergranular corrosion and heterogeneous general corrosion coincide on the top surface of the FSW AA7075 joint after being immersed in EXCO solution. The corrosion behavior in the SAZ of FSW AA7075 is severer than other regions due to theηphase's partial solution and the presence of precipitate-free zone along the grain boundaries during the FSW process. The results of corrosion immersion test show that the corrosion resistance property of FSW AA2024 joints under different welding parameters increases with increasing the rotational speed, whereas for FSW AA7075 joints this tendency is not obvious. The SAZ corrodes worse than other regions, and the exfoliation corrosion extends from the SAZ to heat-affected zone, which are attributed to the secondary phase particles'solution and precipitation under different welding parameters according to the DSC results. The distribution of the intermetallic compounds can be modified by altering the welding parameters, so the corrosion resistance property of FSW joint could be improved by optimizing the welding parameters.