Characterization of superplastic deformation behavior of a fine grain 5083 Al alloy sheet
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INTRODUCTION
THE present interest in the application of lightweight aluminum base alloys in automotive body panels has led to a resurgence of interest in the formability of A1-Mg alloy sheets. A typical A1-Mg commercial alloy is 5083 aluminum (A1-4.7Mg-0.7Mn). It is widely used as a structural material in applications requiring corrosion resistance and moderate to high strength (155-300 MPa) and has potential as a body sheet alloy. In a recent article, thermomechanical processing approaches for this alloy to produce fine recrystallized grain size and the observation of superplasticity have been discussed.ill Magnesium is attractive as an alloying element since it is known to enhance the recovery process within A1, which may enhance superplastic response of this alloy. Other studies have indicated that 5083 aluminum can possess a modest degree of superplasticityY ,3I This subject has drawn our present interest in a detailed investigation of superplastic behavior of 5083 A1. It is worth recognizing that dynamic grain growth during superplastic deformation can lead to strain hardening,t~TI which can lead to fracture and thereby limit superplastic elongation. In some aluminum alloys, a rapid prestraining step, prior to slower rate superplastic straining, has been shown to enhance overall superplastic elongations.IS,9,1oJ During this rapid prestraining step, a continuous dynamic recrystallization process has been shownt~Ol to develop a fine grain structure which is more stable during subsequent R. VERMA, formerly Visiting Scientist, Department of Materials Science and Engineering, University of Michigan, is Research Metallurgist with the GM Research and Development Center. P.A. FRIEDMAN, Graduate Student, and A.K. GHOSH, Professor, are with the Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109-2136. S. KIM and C. KIM, Senior Research Metallurgists, are with the GM Research and Development Center, Warren, MI 48090. Manuscript submitted June 15, 1994. METALLURGICAL AND MATERIALS TRANSACTIONS A
superplastic deformation than is a statically recrystallized fine grain structure. Another factor limiting superplastic stretching of aluminum alloys is cavitation,t~I which can also have deleterious effects on the postformed mechanical properties of these alloys.[12.~3] The thrust of our present work was a detailed investigation of uniaxial superplastic behavior of a specific heat of 5083 A1 manufactured by Alusuisse Co. (Zurich, Switzerland). The alloy was tested under constant and variable strain rate conditions. Dynamic grain growth and cavitation, which adversely affect superplastic elongation, have been studied. The effect of imposed hydrostatic pressure on suppression of cavitation and on net tensile elongation of the specimen has also been investigated.
II.
MATERIAL FOR STUDY
The specific 5083 AI alloy selected for this work was from Alusuisse Co., hereinafter referred to as alloy 545, and was received in the form of a rolled sheet, 1.6-mm thick. Chemical composition of the all
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