Abnormal Grain Growth and Recrystallization in Al-Mg Alloy AA5182 Following Hot Deformation
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INTRODUCTION
COMMERCIAL aluminum alloys, which contain magnesium as the primary alloying addition, the 5000series alloys, have gained importance through the hot forming of structural components, particularly for the transportation industry in its drive toward reduced vehicle mass. Among the most common materials used in commercial superplastic forming (SPF) operations is fine-grained AA5083 sheet.[1–4] Fine-grained AA5083 exhibits a low flow stress and very high tensile ductility, classical superplastic behavior,[5,6] when deformed under the grain-boundary-sliding (GBS) creep mechanism. GBS creep occurs at slow strain rates and high temperatures, which are the conditions used in SPF operations. As strain rate increases and temperature decreases, deformation in fine-grained AA5083 transitions to control by solute-drag (SD) creep.[7] Although the high strain-rate sensitivity of SD creep (m 0.25 to 0.33) provides less tensile ductility than is possible under GBS creep (m 0.5), ductility is still sufficiently enhanced to form quite complex components. The quick-plastic forming (QPF) process takes advantage of SD creep to form fine-grained AA5083 sheet at faster rates and lower temperatures than possible in traditional SPF operations.[8] It has been shown that SD creep can provide enhanced ductility in even coarse-grained Al-Mg alloys, which are not subject to deformation by GBS creep at JUNG-KUEI CHANG, Postdoctoral Fellow, and ERIC M. TALEFF, Professor, are with the Department of Mechanical Engineering, The University of Texas at Austin, Austin, TX 787120292. Contact e-mail: taleff@mail.utexas.edu KEN TAKATA, Senior Researcher, is with the Steel Research Laboratories, Nippon Steel Corp., Chiba, 293-8511, Japan. KOJI ICHITANI, Research Engineer, is with Furukawa-Sky Aluminum Corp., Saitama, 366-8511, Japan. Manuscript submitted August 23, 2009. Article published online May 19, 2010 1942—VOLUME 41A, AUGUST 2010
the temperatures and strain rates of interest for commercial forming operations.[9–16] Nonsuperplastic varieties of commercial Al-Mg alloys are now used in commercial production of structural components by hot forming processes such as high-cycle blow forming[17,18] and hot bulge forming.[19,20] The use of nonsuperplastic commercial Al-Mg materials considerably expands the opportunities to use hot forming for production of aluminum structural components. However, a critical problem was encountered in hot-forming trials with these materials, the development of abnormal grains.[18–20] Abnormal grains are isolated large grains in an otherwise much finer microstructure.[21,22] Abnormal grains can significantly lower yield strength through the Hall–Petch effect, which can render a formed part unusable. Fukuchi et al. encountered abnormal grains in alloy AA5052 with a moderate initial grain size during trials of hot bulge forming.[19] This problem was addressed by modifying the alloy with a small Cr addition,[20] which assists the pinning of grain boundaries. A similar issue of abnormal grain production was encountered
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