Precipitation behavior and corrosion properties of friction stir welded AA5083 Al-Mg alloy after sensitization

The Mg-rich phase precipitation behavior and corrosion resistance of the friction stir welded (FSW) AA5083 Al-Mg alloy are studied by microstructure characterizations, phosphoric acid immersion tests, nitric acid mass loss tests, and slow strain rate tests. After 175 degrees C/100 h sensitization, the thermo-mechanically affected zone (TMAZ) and the stirred zone (SZ) exhibit 100 mu m thick beta GREEK TONOS-precipitates along the grain boundaries. Grain size, grain boundary characteristics and dislocations, synergistically determine the precipitation behavior among various regions of the FSW joint. Grain refinement and dislocation facilitate the precipitation of the Mg-rich phases. Although the as-welded FSW joint exhibits excellent mechanical and corrosion properties, it can become much more vulnerable to intergranular cracking (IGC) and stress corrosion cracking (SCC) than the base metal after sensitization. The sensitized TMAZ region exhibits the largest SCC tendency, and this can be ascribed to the continuous Mg-rich phase along the grain boundaries, the high dislocation density, and the large low angle grain boundary (LAGB) fraction in this region. The present work strongly suggests that the performance dete-rioration after sensitization should be paid more attention to the Al-Mg structures in the initial stages of design and manufacturing, not just during the period of in-service.