Friction stir welding of Al-Cu-Mg-Ag alloys

Butt welds of 4 mm thick plate of Al-Cu-Mg-Ag alloys in the peak-aged T6 temper were produced using the friction stir welding process. Microstructure, hardness, tensile properties and the corrosion behaviour of the joints were investigated in the as-welded condition. Metallographic examinations revealed a nugget consisting of fine equiaxed grains in the weld centre and an adjacent zone with plastically deformed grains. Precipitation hardening in the base alloys resulted mainly from the Omega phase; small amounts of Theta' and S' phases were also observed. In the fine grain area of the weld nugget precipitates were not found, indicating dissolution of the strengthening particles during welding. An overaged microstructure was observed in the adjacent thermomechanically affected and heat affected zones. Hardness profiles revealed softening in the weld region. Minima occurred in the zone with highly deformed grains. Joint efficiencies approaching 80% were achieved. However, there was a great scatter in the ultimate tensile strength of the joints related probably to an inhomogeneous microstructure of the plate material. The base alloys were found to be sensitive to intergranular corrosion and exfoliation corrosion. Except for the fine grain area, a similar corrosion behaviour was observed for the welds, exhibiting a slightly higher intergranular attack in the heat affected zone. The fine grain region was sensitive to pitting and revealed a high exfoliation corrosion resistance.