Retrogression and Reaging Applied to Warm Forming of High-Strength Aluminum Alloy AA7075-T6 Sheet

This study introduces retrogression forming, a new warm-forming methodology for high-strength aluminum alloys such as AA7075-T6. Retrogression forming combines a retrogression heat treatment with simultaneous warm forming at approximately 200 degrees C to improve upon the room-temperature ductility of AA7075-T6, approximately 10pct in tension, and can then use the automotive paint-bake cycle to restore nearly peak-aged strength. Experimental data indicate that retrogression forming at 200 degrees C provides a tensile ductility of 20pct, approximately double that of room temperature, and that subsequent reaging by a simulated paint bake restores hardness to within 10pct of the original peak-aged (T6) hardness. To explore the retrogression-forming concept, the retrogression behaviors of two AA7075-T6 sheet materials from different suppliers are characterized between 200 degrees C and 350 degrees C. The times and temperatures of retrogression treatments suitable for simultaneous warm forming are determined, and tensile ductilities under these conditions are measured. Tensile ductilities during high-temperature deformation (up to 520 degrees C) are measured and compared to those possible under retrogression-forming conditions. Plastic flow at temperatures of 460 degrees C to 520 degrees C is governed by dislocation-climb-controlled creep, a mechanism different from the behaviors observed under retrogression-forming conditions.