Prediction of bending limits in friction-stir-processed thick plate aluminum
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I. INTRODUCTION
THE bending of thick metal plates, defined as greater than about 12 mm in thickness, is typically governed by the bend radius and the ductility of the material. A thick 6061-T6 aluminum plate is difficult to bend to any significant degree, because of its high strength and its relatively low ductility, as measured by its total elongation in a tensile test. However, the advantages to bending thick plates include the resulting structural properties, which are better than welded structures, and the reduced fabrication costs. Plates bent to the proper shape avoid the degradation in properties that accompanies welding. One new method that can be used to increase the bending limits of thick plates is friction-stir processing (FSP). The FSP method is a variation of traditional friction-stir welding (FSW), but in this case, it can be used to locally modify the properties in a part.[1–9] The effect of processing the pretensile surface of a heat-treatable aluminum-alloy plate is to locally anneal and refine the grains in the material. This increases the surface ductility to a depth equivalent to the FSP penetration and significantly enhances its bending limit. This ductility increase can be attributed to both the refined and more homogeneous microstructure and to the annealed condition of the surface material. The separate contribution of each has not been determined. This would be a worthwhile exercise, but it is beyond the scope of the research reported in this article. However, in a separate research effort, attempts were made to create an annealed surface in an aluminum plate by rapid, localized induction heating. Frequencies were adjusted to provide shallow, skin-depth heating.[10] Further, the goal was to minimize the loss of strength in the remainder of the plate, by rapid extraction of heat from the opposite side. This would have provided an annealed surface without the severe deformation associated with FSP, which M.P. MILES, Assistant Professor, is with the Manufacturing Engineering Technology Department, Brigham Young University. Contact e-mail: mmiles@ byu.edu M.W. MAHONEY and C.B. FULLER, Scientists, are with Rockwell Scientific Company. Manuscript submitted February 4, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
leads to the recrystallization and the fine microstructure. Unfortunately, due to the very high thermal conductivity of aluminum alloys, it was not possible to anneal a reasonable depth of material (at least 3-mm deep) without excessively overaging the remainder of the plate. Regardless of the rates of heating and cooling, a static thermal loading eventually resulted in a steady-state temperature profile, which caused softening to a much greater degree than was desired. Similar results were obtained using transient temperature profiles, i.e., induction power was cycled per a predetermined theoretical best-condition cycle. Based on these experiments, it does not appear that conventional heating approaches can selectively anneal thick aluminum plate without significantly reducing mec
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