Microstructure, mechanical properties and stress corrosion behavior of friction stir welded joint of Al-Mg-Si alloy extrusion

Microstructure, mechanical properties and stress corrosion behavior of friction stir welded (FSWed) Al-Mg-Si alloy were investigated. The average grain sizes of shoulder-affected zone (SAZ), nugget zone (NZ), heat-affected zone (HAZ) and base material (BM) are 6.03, 4.80, 168.30 and 127.24 mu m, respectively. The thermo-mechanically affected zone (TMAZ), which is generated on the edge position between HAZ and weld nugget zone, has a narrow width of 400 mu m. The ultimate tensile strength (UTS) of FSWed joint is 232.20 MPa, about 91.04% with respect to that of base material of 255.06 MPa, and the joint fracture occurs at HAZ on advancing side (AS). The FSWed joint is more susceptive to stress corrosion cracking (SCC) than base material, and the SCC susceptibility increases with the rise in temperature. The residual UTS of FSWed joints in constant loaded tests at the load levels of 90, 105 and 120 MPa is 89.97%, 67.50% and 54.75% of the UST of FSWed joint in air, respectively. The increase of the load in constant loaded tests and four-point beam-bent tests accelerates the SCC of FSWed joints.