Retrogression and Reaging Applied to Warm Forming of High-Strength Aluminum Alloy AA7075-T6 Sheet
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INTRODUCTION
MASS reduction 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. Medium-strength aluminum alloys, such as AA6111 and AA5182, were previously used to achieve mass reductions. For example, an all-aluminum cradle saved 10 kg over a steel cradle in the 1999 Chevy Impala,[8] and aluminum body panels saved 175 kg over [1–7]
THOMAS A. IVANOFF is with the Sandia National Laboratories, Materials Mechanics & Tribology, P.O. Box 5800, Mail Stop 0889, Albuquerque, NM 87185. Contact e-mail: [email protected] JON T. CARTER and LOUIS G. HECTOR, Jr. are with General Motors, Research and Development, MC 480-106-RL1, 30470 Harley Earl Blvd., Warren, MI 48092-2031. ERIC M. TALEFF is with the Department of Mechanical Engineering, The University of Texas at Austin, Austin, TX 78712. Manuscript submitted 15 September, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
steel panels in aluminum-intensive Mercury Sable vehicles.[9] Additionally, 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. However, AA7075-T6 exhibits poor formability in stamping at room temperature because its ductility is limited to approximately 10 pct tensile elongation. Consequently, AA7075-T6 has not seen extensive use in the complex component shapes currently required by vehicle bodies. Elevated temperatures can significantly improve the tensile ductility of AA7075-T6,[16–19] though the microstructural changes responsible for this improvement are not well understood. Elongations-to-failure as high as 25 pct at 230 C[16,17] and more than 50 pct at 460 C[18] have been reported for AA7075. For additional context, the tensile ductilities observed for AA7075 and
Table I.
Tensile Elongations of Sheet Materials Relevant to the Transportation Industry at Several Temperatures and Strain Rates
Material AA5182 AA5083
AA6061-T6 AA6061-T4 AA6016-T4 AA6111-T4 AA7075-O AA7075-T6 DP Steel Alloys HSLA Steel Alloys
Temperature (C) 300 400 300 400 450 500 21 21 21 21 350 450 21 340 460 21 220 260 21 21
Elongation (Pct) 51 137 100 200 200 150 17 25 28 28 23 85 17 45 60 11 18 16 15 to 30 10 to 25
Strain Rate (s1) 1 1 1 1 1 1
9 9 9 9 9 9
1
10 102 101 101 101 101
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