Effect of thermo-mechanical processing on quench-induced precipitates morphology and mechanical properties in high strength AA7075 aluminum alloy

The effect of a novel thermo-mechanical process route on quench-induced precipitates morphology and mechanical properties of high strength AA7075 aluminum alloy is investigated in the present work. After solutionizing the probed material is hot formed and quenched at different tool temperatures ranging from 24 to 350 ?C to determine the influence of different cooling rates. The strengthening mechanisms and precipitation morphology after aging are examined based on mechanical testing, microstructure observation and differential scanning calorimetry. The results demonstrate that the tool temperature, as a key element of the investigated process, affects significantly the material strength due to a fundamental change in the microstructure and the strain hardening behavior when exceeding a temperature limit. At tool temperatures from 24 to 200 ?C only a slight decrease in tensile strength from 590 to 568 MPa is obtained. When increasing the tool temperature above 200 ?C, up to 350 ?C, the tensile strength drops to 336 MPa and the initial strain hardening rate ?0 increases. This behavior is linked to the change of quench-induced precipitates morphology and shape. TEM investigations revealed high fractions of semi coherent ??-phases for low and coarse ?-phases for high tool temperatures causing a fundamental shift of strengthening mechanisms from predominantly particle shearing to a combination of bypassing and strain hardening by Orowan loops. High diffusion rates and local accumulation of solute atoms at grain boundaries during cooling led to the formation of large grain boundary precipitates during aging, even after high cooling rate using cooled tools.