Retrogression forming and reaging of two high strength aluminum alloys

Rader, Katherine Elizabeth
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Retrogression forming and reaging is a new scientific approach to warm forming high strength aluminum alloys designed to produce automotive components with tensile strengths equivalent or superior to those of the T6 temper. Retrogression forming and reaging is investigated for two high strength aluminum alloys, AA7075-T6 and AA6013-T6. A retrogression and reaging response is identified in AA6013-T6. Retrogression and reaging behaviors are characterized in AA7075-T6 and AA6013-T6. The activation energies for retrogression are measured as 97 kJ/mol for AA7075-T6 and 160 kJ/mol for AA6013-T6. These data are used to develop a method for predicting appropriate times and temperatures for retrogression. The tensile behaviors of AA7075 and AA6013 are fully characterized across conditions appropriate for retrogression forming. Plastic flow of both materials at the warm temperatures examined is determined to be thermally activated. The activation energies of plastic flow are measured as 221 kJ/mol for AA7075 and 253 kJ/mol for AA6013. Ductility improves at elevated temperatures because fracture is delayed until after necking. Retrogression forming conditions are recommended that provide enhanced ductility and allow a single reaging heat treatment to restore strength to that of the T6 condition after forming. Forming experiments implement retrogression forming to produce parts with a geometry representative of automotive structural components. In-plane major strains of up to 22 % are measured in these parts. Accumulated reduced retrogression time is used to successfully design retrogression forming processes and enable a single reaging heat treatment to restore T6 strength. Retrogression forming followed by a single reaging heat treatment produced strengths equivalent or superior to those of the original T6 condition. Simultaneous plastic deformation during a retrogression heat treatment does not impede the ability of the recommended reaging heat treatment or the automotive paint-bake to restore T6 strength in AA7075. Reaging with the automotive paint-bake, a heat treatment process applied to the vehicle body-in-white, might be effective for restoring T6 strength in retrogression formed automotive components.