Effect of Rotation Speed on Microstructure Evolution During Underwater Friction Stir Processing Al-Cu Alloys

Effect of rotation speed on microstructure evolution during underwater friction stir processing Al−Cu alloys

X L Feng ^{a, b}, H J Liu ^a, S S Babu ^b, A Chaudhary ^c, M Keller ^c

a State Key Laboratory of Advanced Welding and joining, Harbin Institute of Technology, Harbin 150001, China

b Department of Materials Science and Engineering, the Ohio State University, OH 43221, USA

c Applied Optimization, East Monument Ave, Suite 204, Dayton, OH 45402

Abstract: The effect of tool rotation speed on microstructure evolution in 2219-T6 Al alloys, during underwater friction stir processing, was investigated. Different samples were prepared by employing tool rotation speeds ranging from 600 rpm to 1000 rpm at a constant traverse speed of 200 mm/min. The processed samples were examined by electron back-scattered diffraction (EBSD) and transmission electron microscope (TEM). A recrystallized microstructure was observed in the stirred zone. The average grain size of the stirred zone was reduced over an order of magnitude compared to the base metal. Microhardness maps were measured on the polished cross-section of as-processed samples. The Vickers hardness in the stirred zone is lower than that in the unprocessed base metal, and decreased with increasing rotation speed. The above microstructure evolution was rationalized based on the competition between grain boundary and precipitation strengthening. Thermal profiles were obtained using commercial software FSWS as a function of processing conditions. These profiles were used in conjunction with a material model to rationalize the microstructure evolution during friction stir processing.

Keywords: Friction stir processing; Microstructure; Al−Cu alloy; Rotation speed; Microhardness