The Relationship between Grain Size and the Surface Roughening Behavior of Al-Mg Alloys M.R. STOUDT and R.E. RICKER The inhomogeneous surface deformation generated during metal forming presents significant chal- lenges to the use of high-strength, lightweight alloys in automotive applications through the initiation of strain localizations that produce both tearing during metal forming and increased friction between mating die surfaces. Thus, a generic understanding of the relationships between plastic strain, grain size, and deformation-induced roughness at the free surface is needed before forming models can be fully developed to accurately predict the behavior and, ultimately, the changes in the friction within the dies. This research examines the roughening behavior of a solid solution strengthened, commercial Al-Mg alloy. The results of this evaluation indicate that the standard roughness measures increase with uniaxial plastic strain in a manner that can be represented by a simple linear estimate. The results also demonstrate that the roughening rate (dR a /d« Pl ) is dependent on the grain size in this alloy, and the relationship between the roughening rate and grain size also appears to be linear for the range of grain sizes included in this evaluation. However, examinations of the roughened surfaces reflect that the roughening process is a highly complex combination of mechanisms and it is strongly influenced by grain size. As a result, representing the complex changes that occur during roughening of a free surface by plastic deformation with a single number calculated from profilometry scans may be too coarse of a measure to fully describe these changes when modeling roughness-dependent behavior or properties. I. INTRODUCTION deformation mechanisms that generate the surface rough- ening also initiate strain localizations that induce necking, ONE of the most significant technological obstacles tearing, or wrinkling in the component during forming. [2,3] impeding the widespread use of lightweight materials and This inhomogeneous deformation can also accelerate die the development of more complex shapes is the formability wear by increasing the friction and abrasion between the of metal sheet. The demand for components with more spe- metal sheet and the die faces. [4,5,6] As a result, the surface cific materials properties (e.g., strength-to-weight ratio) has roughening behavior becomes a factor that not only deter- revealed many limitations in the scientific understanding of mines the quality of the final product, but also can be a the behavior of metal sheet during forming. Decades of measure of the suitability of a particular alloy for an applica- research have led to the development of a broad knowledge tion. For this reason, fundamental studies that relate metal- base of engineering solutions that address most of the issues lurgical factors for a particular alloy to a performance surrounding the forming of steel sheet; however, the knowl- limiting parameter, such as surface roughness, are needed edge base is not as extensive for many of the aluminum alloys to develop better predictions of the formability. The goal is currently under development for automotive applications. In a broad-based understanding that enables accurate prediction comparison to the traditional unalloyed, low-carbon sheet and control of the mechanical behavior for any alloy in a steels, the structure and the property differences in aluminum given forming condition. alloys, such as 6111 and 6022, may generate considerably While it may appear homogeneous on a macroscopic level, different mechanical responses for the same forming condi- deformation in a polycrystalline material occurs by highly tions. This is largely due to the strain rate–induced property complex and nonuniform processes. At low levels of plastic variability and the higher sensitivity to small variations both strain, each grain deforms by different amounts depending in the alloying content and in the metallurgical processing on the individual orientation, the local Schmid factor, and conditions exhibited by many aluminum alloys. the constraints imposed by neighboring grains at or below Aluminum alloys with high multiaxial ductility do exist, the surface. [7,8,9] That is, in a grain with a favorable orienta- but they are often unsuitable for automotive applications tion for slip, the deformation will primarily occur by single because they either lack sufficient strength or they develop slip in the interior regions of that grain. However, in a surface finishes with undesirable features such as orange- grain where the slip conditions are not as favorable, the peel, banding, or roping during metal forming. [1] In addition deformation will tend to localize in the grain boundary to merely creating cosmetically unacceptable surfaces that regions because of the additional shear displacements require additional finishing operations, the inhomogeneous required to produce grain rotation and to maintain grain-to- grain contiguity. The resulting anisotropy produces a defor- mation that is a mixture of both single slip and near grain M.R. STOUDT, Research Engineer, and R.E. RICKER, Senior Scientific boundary deformation. [9] Using superposition, Ashby [9] Advisor, are with the Materials Science and Engineering Laboratory, showed how a homogeneous macroscopic strain can produce National Institute of Standards and Technology, Gaithersburg, MD 20899- significant variations in the amount of localized deformation 8553. Contact e-mail: stoudt@nist.gov Manuscript submitted September 24, 2001. within and around the individual grains in a polycrystalline METALLURGICAL AND MATERIALS TRANSACTIONS A U.S. GOVERNMENT WORK VOLUME 33A, SEPTEMBER 2002—2883 NOT PROTECTED BY U.S. COPYRIGHT