Rheological behavior of the AA6061 aluminum by effect of the thermally activated phase transformations

ABSTRACT

Aluminum alloys at room temperature not experience significant rheological behavior due to its coherent microstructural order, and its high compactness. However, the increase of the temperature generates stress fields, and thereby deformations, that alter the microstructural equilibrium, manifesting the rheological behavior as consequence of the internal friction. In the alloys of Al-Mg-Si, the precipitation of phases cause loss of coherence producing internal friction. The objectives of this work were characterize and identify the microstructural transformations experienced in the AA6061 aluminum by temperature effect; study the kinetic no-isothermal and isothermal, through atomistic models. Analyze the rheological behavior, manifested in the deformation, and in the stress relaxation, considering the postulates expressed by Deborah's Number (D_e). The experimental development has been oriented and organized through the electrical resistivity and microscopy. Four regions of transformations were observed: I: 20-190, II: 220-420, III: 420-520, IV: ≥ 525 °C; with activation energys: ${\mathrm{E}}_{\mathrm{a}}^{\mathrm{I}}$: 20 − 60, ${\mathbf{E}}_{\mathbf{a}}^{\mathbf{I}\mathbf{I}}$: 90 − 120, ${\mathbf{E}}_{\mathbf{a}}^{III}$: 400 − 470 kJ/mol; and formed of solute aggregates and needle and cylindrical phases, respectively. It determined that D_e ≤ 1, associated to a viscoelastic behavior. It observed, between 100 and 200 °C, that: 0.1 ≤ D_e ≤ 0.3, linking to a considerable degree of diffusion, with maximum rate of nucleation at 150 °C. These characterizations will broaden the understanding of the diffusion phenomenon, the precipitation processes, deformation and relaxation; for practical applications and related theoretical developments.

Keywords:
aluminum AA6061; Deborah's Number; electrical resistivity; phase transformations; rheological behavior