Deformation mechanisms in superplastic AA5083 materials
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INTRODUCTION
THE transportation industry and the automotive industry, in particular, have made increasing use of aluminum alloys in new vehicle construction.[1–4] The primary incentive for using aluminum is the reduction in vehicle mass, which it makes possible. In automotive applications, mass reduction improves performance and fuel economy. An important factor limiting the introduction of aluminum sheet materials into vehicle components, such as body closure panels, is low formability in cold stamping operations. Steel sheet materials can provide elongations of up to 50 pct, while aluminum sheet materials typically provide a maximum of 30 pct. Superplastic aluminum sheet material offers a potential solution to this dilemma. Superplasticity is the ability of a material to exhibit very high tensile ductility, from a few hundred percent to thousands of percent, typically at elevated temperatures and slow strain rates.[5] Two requirements exist for a material to be superplastic: (1) a high strain-rate sensitivity is necessary to reduce the rate of flow localization, i.e., necking, and (2) a low rate of damage accumulation, e.g., cavitation, is necessary to allow large plastic strains to be reached. The deformation mechanism generally accepted as responsible for fine-grain superplasticity in aluminum alloys is grain-boundary-sliding (GBS) creep, which supplies a high strain-rate sensitivity (m 0.5) while producing the low flow stress typically necessary for successful commercial superplastic forming (SPF) operations.[6–15] Aluminum alloy 5083 is the most common material in SPF operations, and it deforms by GBS creep at the elevated temperatures and slow strain rates typical of SPF operations.[16–21] AA5083 MARY-ANNE KULAS, Materials Science and Engineering Program, and W. PAUL GREEN and ERIC M. TALEFF, Department of Mechanical Engineering, are with the University of Texas at Austin, Austin, TX 787120292. Contact e-mail: [email protected] PAUL E. KRAJEWSKI is with the Research and Development Center, General Motors Corp., Warren, MI 48090-9056. TERRY R. McNELLEY is with the Department of Mechanical Engineering, Naval Postgraduate School, Monterey, CA 93943-5146. Manuscript submitted August 26, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
also offers the additional benefits of moderate strength, good weldability, and acceptable corrosion resistance, which make it of prime interest for application to new vehicle construction.[16] More recently, data have been presented that suggest AA5083 deforms by solute-drag (SD) creep at temperatures below and strain rates faster than those at which GBS creep dominates deformation.[17,19,22–25] The present investigation offers conclusive proof that this is, indeed, the case. The SD creep produces a high strain-rate sensitivity (m 0.33 to 0.25), although not as high as that of GBS creep, and can provide very good tensile ductility.[26–34] A proprietary variation on SPF, called quick-plastic forming (QPF), takes advantage of SD creep in AA5083 to increase forming rates an
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