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The effects of preaging treatments on aging kinetics and mechanical properties in AA6111 aluminum autobody sheet

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Abstract

The automotive paint bake cycle is used to impart a modest artificial aging response, referred to as the paint bake response (PBR), in Al-Mg-Si-Cu autobody sheet alloys following stamping. This aging response acts to increase the dent resistance of exterior automotive panels and allows for greater down gaging. Unfortunately, the short duration of the paint bake cycle (as dictated by production demands) is generally insufficient to exploit more than a small fraction of the age-hardening potential of the alloys. The application of a preaging treatment immediately after quenching from solution heat treatment has been shown to be effective in increasing the PBR by up to a factor of 4, while at the same time improving the formability of the alloy and reducing the natural aging rate. Through the use of atomic resolution microscopy and differential scanning calorimetry, the mechanism of preaging can be shown to be the result of changes in the precipitation sequence of metastable variants of Mg2Si, resulting in an increase in the precipitation kinetics during the aging cycle and a finer distribution of strengthening precipitates. The model proposes that preaging biases the formation of disordered clusters, which subsequently undergo partial dissolution during higher temperature thermal excursions. The cluster dissolution is associated with both an increase in the matrix concentration of elemental solute (Mg and Si) and a transient flux in vacancies that were previously entrained with the disordered clusters. The dissolution results in the formation of an intermediate metastable phase and an enhancement in the aging kinetics of β″ precipitates. The enhancement in aging kinetics allows superior final component strength at higher production rates, as well as allowing for lower automotive paint bake temperatures.

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  1. Advanced Technology/Product Applications, Reynolds Metals Company, 23836, Chester, VA

    J. Daniel Bryant (Manager)

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Bryant, J.D. The effects of preaging treatments on aging kinetics and mechanical properties in AA6111 aluminum autobody sheet. Metall Mater Trans A 30, 1999–2006 (1999). https://doi.org/10.1007/s11661-999-0010-3

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