Effect of Heating Cycles on Microstructure of Dissimilar Friction Stir Welded Age-Hardenable Aluminum Alloys
The effect of heating on microstructural changes in dissimilar friction stir welding of aluminum alloys AA6022 and AA7075 was studied. The challenge in welding these alloys is the strength loss in the heat affected zone (HAZ) due to the over-ageing of metastable precipitates. Isothermal ageing studies of AA6022 and AA7075 revealed apparent activation energies of 122 and 112 kJ/mol, respectively, which were used in a modified Shercliff-Ashby model incorporating non-isothermal heating to predict hardness evolution in AA7075/AA6022 lap welds. The model predicts the hardness minimum location in the weld with <15% difference between measured and predicted values, which correlates to the precipitate structure observed by transmission electron microscopy. The effect of a 180 °C, 30 minutes post weld heat treatment (PWHT), typical of automotive paint bake cycles, was further studied. The kinetics of the precipitate and dislocation microstructure evolution from PWHT was investigated in different weld regions using laboratory-based small- and wide-angle X-ray scattering (SAXS/WAXS). The precipitate volume fraction evolution determined by SAXS, for ex- and in-situ heating, reveals a fast dissolution stage in the stir zone and thermomechanical affected zones, followed by asymptotic increase, with little change in the HAZ. The measured PWHT precipitation kinetics follows the Lifshitz-Slyozov-Wagner theory, but with a monotonically decreasing rate constant of ~10^-30 m^3s^-1 compared to the analytically predicted 10^-28 m^3s^-1. New mechanisms for the rate constant reduction based on the dislocation density decrease measured by WAXS are proposed. Strain rate sensitivity measurements of the AA7075 stir zone at 294 and 78 K reveals that mechanical mixing via FSW creates a structure containing solute atoms, precipitates and dislocations that linearly contribute to strengthening, but this mixed state contains less solute than obtained by traditional solutionizing. A two-dimensional, two-particle phase-field model simulation of η phase in Al-Mg-Zn predicts a time^1/3 coarsening kinetic during PWHT, with a rate constant that matches the experimental data for the HAZ over the first 100 s only. The work illustrates that structure parameters obtained at macroscopic, mesoscopic and microscopic length scales using a combination of experimental and modeling approaches are necessary to build a predictive understanding of FSW post-weld heat treatment.
URI for this recordhttp://hdl.handle.net/1974/28218
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