Microstructure, microhardness and corrosion susceptibility of friction stir welded AlMgSiCu alloy

• The BM was found to be more susceptible to IGC than that of welded zone.
• IGC of BM owes to the continuous intergranular precipitation and presence of PFZ.
• Elemental Si and Q-phase were found at grain boundaries.
• The coarsening and dissolution of intragranular phases can improve IGC resistance.
• The evolution of intragranular precipitates resulted in the variations in hardness.

Microstructure, microhardness and corrosion susceptibility of friction stir welded joint in an AlMgSiCu alloy were investigated. It was found that the joint exhibits different corrosion susceptibility among the microstructural zones. The base material is the most susceptible to intergranular corrosion because of the presence of continuous cathodic precipitates (Si and Q phases) at grain boundaries and the precipitate free zone along the grain boundaries. The coarsening of intergranular precipitates and the precipitation of Q′ phases in the grain bodies reduce intergranular corrosion susceptibility but introduce pitting corrosion in the heat-affected zone. The significant elimination of intergranular corrosion both in nugget zone and thermo-mechanically affected zone is related to the low volume fraction of intergranular precipitate. Microhardness variations depend on the evolution of intragranular precipitates. The dissolution and/or coarsening of the strengthening precipitates result in the softening within the welded zone.