Evolution of strain-induced microstructure and texture in commercial aluminum sheet under balanced biaxial stretching
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THE replacement of conventional steel sheet used for various automobile components with aluminum alloys would lead to reduced vehicle weight. This would provide one effective approach to reaching the gas-mileage goals set forth by the Partnership for a New Generation of Vehicles. However, widespread application of these lightweight materials by the automotive industry is presently limited, due to their poor formability in terms of limiting strains, wrinkling, and overall final surface appearance. These forming defects arise from both materials issues (i.e., a limited number of slip systems for fcc aluminum compared to bcc ferritic steel; solution-strengthened aluminum alloys strain-rate soften, while steels strain-rate harden) and a lack of experience in forming aluminum. Automotive manufacturers are employing finite-element simulations to contend with these stamping problems by carrying out intensive die-design development prior to die fabrication. Unfortunately, this approach has been relatively unsuccessful at predicting the behavior of the material in terms of the complex stress states occurring within the sheet and the frictional forces occurring along the die-workpiece interfaces. This is believed to be a result of the in-situ development of surface roughness. At room temperature, the surface roughness of aluminum S.W. BANOVIC and T. FOECKE, Materials Research Engineers, are with the Metallurgy Division, National Institute of Standards and Technology, Gaithersburg, MD 20899-8553. Contact e-mail: [email protected] Manuscript submitted November 29, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
alloys has generally been observed to increase in linear proportion to the magnitude of the plastic deformation[1–7] and to the average grain size,[4–9] but is independent of strain rate[4,5] and stress type (mode of deformation).[4–7,9] The predominant mechanism cited for the roughening process is associated with the different orientations of slip systems in adjacent grains.[2,4–7,10–13] This can lead to mismatch strains at the grain boundaries under multiaxial stretching. Upon deformation, neighboring grains, which typically have different crystallographic orientations, will deform by slip on particular crystallographic systems. The differences in the deformation of adjacent grains will lead to strain incompatibilities between them. As the effects of mutual constraint are partially relaxed at the free surfaces, the outermost grains are allowed to move normal to the surface to accommodate the strain at the grain boundaries, resulting in the formation of peaks and valleys. Thus, the higher the amount of deformation or the larger the initial grain size, the greater the surface roughness. The emergence of slip steps is thought to play a lesser role in roughening,[4,11] as their height magnitude is on a much smaller scale when compared to that of grain rotation. Becker investigated the relationship between the initial crystallographic texture of an aluminum sheet and surface roughening as a result of deformatio
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