Textural Development of AA 5754 Sheet Deformed under In-Plane Biaxial Tension
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OVER the past three decades, aluminum alloys have been increasingly used to replace conventional Fe-base components in automotive applications. Their light weight and excellent corrosion properties make them suitable for both internal structural members and outerbody panels. However, some obstacles to widespread use of these alloys exist, which include unfavorable mechanical response (lower forming limit strains compared to Fe-base counterparts, serrated yielding, and yield plateaus), forming defects (wrinkling and tearing), and macroscopic surface imperfections (orange peel and ridging/roping lines). Another issue restricting their application concerns numerical modeling presently used to determine the appropriate processing parameters of a given alloy for a specific part. The predictions of multiaxial behavior are largely inaccurate as these models are based on extrapolations or predictions from basic uniaxial tension tests.[1] Further, the constitutive equations available for use in the models and the data used to develop them are limited in the strain states measured, the level of strains achieved, and accuracy of the measurement. Recent work by this research group has shown that this barrier is being overcome as multiaxial flow surfaces can be experimentally determined, out to failure of the sheet, using an in-situ X-ray diffraction technique.[2,3] S.W. BANOVIC, M.A. IADICOLA, and T. FOECKE, Materials Research Engineers, are with the Metallurgy Division, Technology Administration, Department of Commerce, National Institute of Standards and Technology, Gaithersburg, MD 20899. Contact e-mail: [email protected]. Manuscript submitted October 19, 2007. Article published online May 17, 2008 2246—VOLUME 39A, SEPTEMBER 2008
Typically, in-plane anisotropy is seen in the mechanical response of rolled sheet, with different uniaxial behavior parallel and perpendicular to the rolling direction (Figure 1(a)). These effects are not limited to the uniaxial case, as shown in Iadicola et al.[3] for AA5754-O sheet subjected to biaxial in-plane straining. The plane strain (Figure 1(b)) and equibiaxial (Figure 1(c)) stress-strain data show that typically assumed isotropic (von Mises) transverse responses (i.e., half the first principal stress and equal biaxial stress, respectively) do not actually occur. However, it was seen that by comparing contours of equal plastic work (based on work values at specific values of plastic strain in the uniaxial rolling direction) in stress space (Figure 1(d), for 1, 5, 10, and 15 pct plastic strain) that the initial anisotropy decreased above 4 and 7 pct strain, for plane strain and equibiaxial tension, respectively. Barlat and Richmond[4] have shown that this material anisotropy may be a result of the preferred crystallographic orientation in the as-received sheet. Using the Taylor–Bishop–Hill model, materials with strong initial textures of Goss f011gh100i and Brass (B) f011gh211i components developed flow surfaces similar to that observed early in the deformation process. However, with incr
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