Precipitate Structure, Microstructure Evolution Modeling and Characterization in an Aluminum Alloy 7050 Friction Stir Weld

Novel use of differential scanning calorimetry (DSC), with 1.7 mm specimen spacing intervals across a friction stir weld, coupled with microhardness, electrical conductivity, transmission electron microscopy (TEM), and novel three-dimensional thermal modeling of temperature profiles were used to characterize precipitate structure as a function of position across a friction stir welded and post-weld stabilized aluminum alloy 7050. The results show excellent agreement with predictions of existing FSW microstructural evolution models for 7XXX aluminum alloys. The DSC scans and thermal modeling accurately predicted the locations and peak temperatures at which transitions from 1) slow precipitate dissolution to 2) rapid dissolution, coarsening, and transformation of η′ to η precipitates to 3) increasing η dissolution and matrix supersaturation occur along the weld. These results are correlated to significant changes in the microhardness and electrical conductivity profiles. Following a 12-year period after the initial post-weld stabilization treatment, the closely spaced DSC scans were able to show that the initial stabilization treatment, (a standard T6 heat treatment), had not fully stabilized the weld near the heat affected zone (HAZ) hardness minimum. A 2-step stabilization method is proposed to fully stabilize the material in this region of the weld.