The relationship between grain size and the surface roughening behavior of Al-Mg alloys
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I. INTRODUCTION
ONE of the most significant technological obstacles impeding the widespread use of lightweight materials and the development of more complex shapes is the formability of metal sheet. The demand for components with more specific materials properties (e.g., strength-to-weight ratio) has revealed many limitations in the scientific understanding of the behavior of metal sheet during forming. Decades of research have led to the development of a broad knowledge base of engineering solutions that address most of the issues surrounding the forming of steel sheet; however, the knowledge base is not as extensive for many of the aluminum alloys currently under development for automotive applications. In comparison to the traditional unalloyed, low-carbon sheet steels, the structure and the property differences in aluminum alloys, such as 6111 and 6022, may generate considerably different mechanical responses for the same forming conditions. This is largely due to the strain rate–induced property variability and the higher sensitivity to small variations both in the alloying content and in the metallurgical processing conditions exhibited by many aluminum alloys. Aluminum alloys with high multiaxial ductility do exist, but they are often unsuitable for automotive applications because they either lack sufficient strength or they develop surface finishes with undesirable features such as orangepeel, banding, or roping during metal forming.[1] In addition to merely creating cosmetically unacceptable surfaces that require additional finishing operations, the inhomogeneous M.R. STOUDT, Research Engineer, and R.E. RICKER, Senior Scientific Advisor, are with the Materials Science and Engineering Laboratory, National Institute of Standards and Technology, Gaithersburg, MD 208998553. Contact e-mail: [email protected] Manuscript submitted September 24, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
deformation mechanisms that generate the surface roughening also initiate strain localizations that induce necking, tearing, or wrinkling in the component during forming.[2,3] This inhomogeneous deformation can also accelerate die wear by increasing the friction and abrasion between the metal sheet and the die faces.[4,5,6] As a result, the surface roughening behavior becomes a factor that not only determines the quality of the final product, but also can be a measure of the suitability of a particular alloy for an application. For this reason, fundamental studies that relate metallurgical factors for a particular alloy to a performance limiting parameter, such as surface roughness, are needed to develop better predictions of the formability. The goal is a broad-based understanding that enables accurate prediction and control of the mechanical behavior for any alloy in a given forming condition. While it may appear homogeneous on a macroscopic level, deformation in a polycrystalline material occurs by highly complex and nonuniform processes. At low levels of plastic strain, each grain deforms by different amounts depending on
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