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I. INTRODUCTION

THE use of tailor welded blanks (TWBs) in the automotive industry has risen substantially in the last five years, in an effort to decrease vehicle weight and improve fuel economy.[1,2,3] A TWB consists of two or more sheets of different gages, grades, or surface treatments joined together and then formed to create a part with optimal properties. For example, thicker or specialized material can be used where strength, ductility, or some other property is needed, and thinner, or nonspecialized material where specifications are not as strict. The overall stiffness of these blanks is typically better than that of conventional reinforced structures, providing an opportunity to downgage these parts,[4] particularly for aluminum alloys, where there is a cost penalty over steel. The TWBs have been fabricated using neodymium : yttrium aluminum garnet (Nd : YAG) laser welding,[5–8] mash seam welding,[5] nonvacuum electron beam (NVEB) welding,[6] and gas tungsten arc welding[2,8,9] depending on the alloy. Even with an added joining process, the weight reduction from using TWBs can be sufficient to make their use worthwhile from a cost standpoint. In some automotive applications there is potential to replace steel with aluminum alloys, which have the advantages of corrosion resistance, high strength-to-weight ratio, and recyclability. However, the difficulty in using aluminum TWBs is that the weld-affected material can be the weakest area in the part, making it necessary to redesign the shape, move the placement of the weld, or increase material thickness. The purpose of the current work is to understand and compare the mechanical behavior of friction stir welded (FSW) aluminum alloy sheets with gas tungsten arc welded (GTAW) sheets, under stretch forming conditions. Gas tungsten arc welding has been used to make TWBs and is considered a viable process not only for lab-scale experiments, but also for industrial production.[2,10] In the ideal case the ductility of the weld region of the blank will not limit its formabiM.P. MILES, Assistant Professor, Manufacturing Engineering Technology, and T.W. NELSON, Associate Professor, Mechanical Engineering, are with Brigham Young University, Provo, UT 84602. Contact e-mail: mmiles@ byu.edu B.J. DECKER, formerly Graduate Student, Manufacturing Engineering Technology, Brigham Young University, is with TrimMasters, Inc., Modesto, CA. Manuscript submitted September 12, 2003. METALLURGICAL AND MATERIALS TRANSACTIONS A

lity. The friction stir welding process has been shown to provide superior weld properties, especially in heat-treatable, thick-plate aluminum alloys.[11,12,13] Therefore, the application of friction stir welding to the joining of thin sheets was investigated and the results were compared to gas tungsten arc welding results. Same-gage sheets will be used in the current investigation to evaluate the mechanical properties of FSW blanks. Development of tooling and process parameters for welding dissimilar-gage sheets is planned for future efforts. II. EXPERIMENTS