Abstract
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 °C to improve upon the room-temperature ductility of AA7075-T6, approximately 10 pct 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 °C provides a tensile ductility of 20 pct, approximately double that of room temperature, and that subsequent reaging by a simulated paint bake restores hardness to within 10 pct 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 °C and 350 °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 °C) are measured and compared to those possible under retrogression-forming conditions. Plastic flow at temperatures of 460 °C to 520 °C is governed by dislocation-climb-controlled creep, a mechanism different from the behaviors observed under retrogression-forming conditions.
Highlights
MASS reduction[1,2,3,4,5,6,7] is of paramount importance in transportation industries, the automotive industry in particular, as a means to improve the fuel economy and general performance of future vehicles, including those using alternative powertrain technologies
Specimens in all heat-treated conditions were stored at room temperature, and hardness tests were intermittently performed over a period of three years to measure the effects of natural aging
Among retrogression treatments conducted at 200 °C, 225 °C, and 250 °C, each demonstrates a local minimum in hardness and hardness lost during retrogression is 80 to 100 pct recoverable from the vicinity of these local minima by the standard reaging treatment (24 hours at 120 °C)
Summary
MASS reduction[1,2,3,4,5,6,7] is of paramount importance in transportation industries, the automotive industry in particular, as a means to improve the fuel economy and general performance of future vehicles, including those using alternative powertrain technologies. One approach to mass reduction is increasing the use of materials with greater strength-to-weight ratios than conventional materials currently used in the automobile body-in-white. METALLURGICAL AND MATERIALS TRANSACTIONS A steel panels in aluminum-intensive Mercury Sable vehicles.[9] hot forming has been studied as a means to improve formability of medium-strength Al-Mg-Si alloys.[10,11] High-strength aluminum alloy sheet materials potentially can reduce mass in critical vehicle components, such as side-impact beams, pillars, and rails,[12,13] by replacing components formed from steel or lower-strength aluminum alloys.[14] Aluminum alloy 7075 (AA7075) is of interest [13] because it exhibits high strength in the peak-aged condition, 505 MPa yield strength, and 570 MPa tensile strength minimums in the T6 temper,[15] and its density is one-third that of steel. The tensile ductilities observed for AA7075 and VOLUME 50A, MARCH 2019—1545
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