Friction Stir Welding of Al Alloy 2219-T8: Part II-Mechanical and Corrosion

In Part I of this series, abnormal agglomerations of theta particles with size of about 100 to 1000 A mu m were observed in friction stir welded AA2219-T8 joints. In this work, the effects of these agglomerated theta particles on the mechanical and corrosion properties of the joints are studied. Tensile testing with in situ SEM imaging was utilized to monitor crack initiation and propagation in base metal and weld nugget zone (WNZ) samples. These tests showed that cracks initiated in the theta particles and at the theta/matrix interfaces, but not in the matrix. The WNZ samples containing abnormal agglomerated theta particles had a similar ultimate tensile stress but 3 pct less elongation than other WNZ samples with only normal theta particles. Measurements using the microcell technique indicated that the agglomerated theta particles acted as a cathode causing the dissolution of adjacent matrix. The abnormal theta particle agglomerations led to more severe localized attack due to the large cathode/anode ratio. Al preferential dissolution occurred in the abnormal theta particle agglomerations, which was different from the corrosion behavior of normal size theta particles.