Economical High Strain Rate Superplastic Forming via Friction Stir Processing

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Superplastic forming has emerged as an attractive near net shape forming technique for production of complex shaped components and unitized structures for aerospace applications. In spite of many attractive features, the widespread commercial use of superplastic forming outside aerospace sector is hampered by two major drawbacks: (a) slow superplastic forming rates, and (b) high cost of starting material for superplasticity. Fine grain size is a key requirement for materials to exhibit superplastic behavior, i.e., tensile ductility of 200%. The conventional rolling-based thermo-mechanical processing methods for producing aluminum alloys having superplastic properties typically require six steps.The goal of this research is to develop an innovative one-step thermo-mechanical processing technique for grain refinement, Friction Stir Processing (FSP). The basic concept of FSP (an adaptation of friction stir welding) for enhanced superplasticity is based on the very fine microstructure that develops during solid-state friction stirring. Successful development of FSP would provide opportunities to develop new concepts, that include: (a) selective superplastic forming; (b) superplastic forming of thick sheets; and (c) one-step processing for superplasticity from cast sheet or hot-pressed powder metallurgy sheet. This project will focus on developing an understanding of critical microstructural issues, such as: (1) influence of FSP parameters on the microstructural development; (2) the effect on microstructure of overlapping passes during FSP; (3) the response of refined microstructure to elevated temperature exposure; and (4) microstructure-superplastic property correlations in friction stir processed materials. The research involves active collaboration of university (UMR) and industries (the Rockwell Science Center and the Boeing Phantom Works). The direct industrial participation will result in faster technology transition and the overall impact is likely to be very significant.