M. A. Garcia-Bernal, D. Hernandez-Silva, Instituto Politécnico Nacional-ESIQIE, D.F., Mexico; R. S. Mishra, University of Missouri – Rolla, Rolla, MO; R. Verma, General Motors, Warren, MI
In this work, the effect of friction stir processing on the superplastic properties of continuous cast AA5083 Al sheets have been investigated, optimizing the Mn content. Three continuous cast AA5083 alloys with different Mn (0.5, 0.75 and 1.0wt.%) concentrations were friction stir processed using several parameters and pins. Ultra-fined microstructures with averages grain sizes of 0.8-2.5μm were obtained. Tensile test revealed that the maximum tensile elongation of 810% was achieved at 530 ºC and 3 x 10-2 s-1 in the alloy with the lowest Mn content. The stability of the microstructure at elevated temperature was the most important issue to overcome.
Summary: The present investigation was motivated by the well-established observation that, although high superplastic elongations are generally confined to a relatively narrow of operating strain rates, a reduction in the grain size has the potential of both decreasing the temperature and increasing the strain rate associated with optimum superplastic flow. Recent reports have demonstrated that it is possible to attain ultrafine grain sizes in bulk polycrystalline metals, with grain sizes generally in the nanometer and submicrometer ranges, by intense plastic deformation of the material using procedures such as thermomechanical processing (TMP), equal-channel angular pressing, torsional deformation under pressure, and friction stir processing (FSP). Among these, friction stir processing (FSP) is particularly attractive because it provides a very simple and effective approach for obtaining ultrafine-grained regions for specialized applications. Recent experiments have shown that the FSP of Al alloys is capable of producing a grain size < 5ìm. On the other hand, there is a considerable interest in the possibility of achieving high strain rate superplasticity (HSRS) in conventional commercial AA5083 Al alloys processed by casting. These alloys are currently widely investigated for substitution of steels in automobiles in order to manufacture lighter cars and thus reduce energy consumption. Other valuable properties of these alloys are their good corrosion resistance and a medium strength. Besides, the cost of superplastic grade aluminum sheet is still relatively high. Continuous casting holds promise for low cost superplastic sheet.
