Evaluation of microstructure and superplasticity in friction stir processed 5083 Al alloy
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Friction stir processing (FSP) has been developed as a potential grain refinement technique. In the current study, a commercial 5083 Al alloy was friction stir processed with three combinations of FSP parameters. Fine-grained microstructures with average grain sizes of 3.5–8.5 m were obtained. Tensile tests revealed that the maximum ductility of 590% was achieved at a strain rate of 3 × 10−3 s−1 and 530 °C in the 6.5-m grain size FSP material, whereas for the material with 8.5-m grain size, maximum ductility of 575% was achieved at a strain rate of 3 × 10−4 s−1 and 490 °C. The deformation mechanisms for both the materials were grain boundary sliding (m ∼0.5). However, the 3.5-m grain size material showed maximum ductility of 315% at 10−2 s−1 and 430 °C. The flow mechanism was solute-drag dislocation glide (m ∼0.33). This study indicated that establishing a processing window is crucial for obtaining optimized microstructure for optimum superplasticity.
I. INTRODUCTION
A material that can exhibit >200% tensile elongation prior to its failure is termed as superplastic. The commercial application of superplasticity materializes through superplastic forming technologies. The advantages of superplastic forming are several and rewarding, when compared with conventional forming techniques.1 Al alloys are gaining more usage in transport applications with the growing demand for lighter, more fuelefficient vehicles. One of the most widely used alloys in the automotive industry is 5083 Al. Thus, there has been much interest in investigating superplastic characteristics in 5083 Al because of its good corrosion resistance, good weldability, lower density, and moderately high strength. For more than a decade now, two broad approaches have been underway to produce better superplastic 5083 Al. First, various processing techniques have been applied on the base composition of 5083 Al. There have been a number of efforts following conventional rolling-based thermomechanical processing techniques (TMP),2–7 equal channel angular pressing (ECAP),8–11 and accumulative roll bonding (ARB).12 The drawbacks encountered in these approaches have been slower forming rates and/or instability of grain structures at likely superplastic a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2004.0429 J. Mater. Res., Vol. 19, No. 11, Nov 2004
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temperatures. Second, new alloy design concepts have been employed to modify the composition of the alloy for achieving superplasticity. Various alloying additions like Cu,13,14 Mn,15 Mn + Sc,16 Sc + Sn,17 Mn + Zr,16,18 Zr,19–21 and Sc16,22 in base-5083 compositions have been reported. There have been significant improvements in ductility in the modified alloys, however, with the likely complexities during their production and deterioration of other useful properties. The use of these modified alloys also limits easy use of the vast knowledge base of 5083 Al. Hence, the current approach was to use a commercial 5083 A
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