Suppressing abnormal grain growth and restoring hardness of a bobbin tool friction stir welded Al–Zn–Mg–Cu alloy

The hardness, grains and second phase particles of an Al-Zn-Mg-Cu alloy subjected to bobbin tool friction stir welding and post-weld heat treatment were investigated by hardness measurements, electron backscatter diffraction analysis, transmission electron microscopy and energy-dispersive spectroscopy. The as-welded alloy showed a steep decrease in hardness from 183 HV to ∼120 HV in the stir zone, in spite of grain refinement and introduction of high-density dislocations. Such a softening was attributed to the dissolution of η′ precipitates in the stir zone. Cr and Mn elements were added in the alloy to form face centered cubic structure dispersoids with the stoichiometry of Al 18 Mg 3 (Cr,Mn) 2 in the base material. High volume fraction of dispersoids were retained in the stir zone after welding. These dispersoids were stable and effectively pinned grain boundaries during subsequent solid solution treatment, which avoided abnormal grain growth. Fine grains with average size of ∼4 μm were evenly distributed throughout the entire thickness of the stir zone after solution treatment. There is almost no change in grain size but dislocation density significantly decreases from 1.63 × 10 14 m −2 to 8.15 × 10 13 m −2 in the stir after solution treatment. Abundant nano-sized η′ reprecipitated during artificial aging. Dense distribution of η′ precipitates in the matrix of fine-grained structure enabled the stir zone to fully restore the hardness to ∼180 HV, comparable with that of the base material.