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FRICTION-STIR surfacing (FSS) is a special type of friction-stir processing that utilizes a processing tool without a pin, i.e., only the tool shoulder contacts the workpiece. FSS, therefore, specifically modifies the microstructure of surface layers within monolithic specimens to achieve specific and desired properties. Just as in typical FSP (or FSW), the tool induces plastic flow during FSS, but depending on the process parameters, i.e., applied force, tool velocity and rotation speed, the material flow can yield a modified microstructure that is beneficial to the performance of the material. In their review article, Threadgill et al.[1] present friction-stir processing as an exciting technique for microstructural development and property enhancement and provide an excellent account of past and current friction-stir research. The mechanical properties of cast aluminum alloys are significantly limited by porosity, coarse acicular silicon phases, and coarse aluminum dendrites. These three factors can significantly degrade the fracture toughness and fatigue resistance of the alloy. Various foundry and heat treatment schedules are traditionally

CARTER HAMILTON, Associate Professor, is with the Department of Mechanical and Manufacturing Engineering, College of Engineering and Computing, Miami University, Oxford, OH. Contact e-mail: [email protected] MAREK STANISłAW WE ˛ GLOWSKI Research Scientist, is with the Department of the Testing of Materials Weldability and Welded Construction, Institute of Welding, Gliwice, Poland. STANISłAW DYMEK, Professor, is with the Faculty of Metal Engineering and Industrial Computer Science, AGH University of Science and, Technology, Krako´w, Poland. Manuscript submitted July 14, 2014. Article published online April 11, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS B

employed to modify the aluminum microstructure in order to minimize the impact of these factors. Frictionstir surfacing, however, offers the ability to locally modify the microstructure and reduce, in particular, the porosity, thus potentially improving ductility, fracture toughness, and fatigue as demonstrated by Mahmoud and Mohamed.[2] Numerous investigations of friction-stir welding and friction-stir processing have sought to model the material flow behavior, the temperature distribution and the microstructural evolution within the stir/process zones. The foundational work by Colegrove and Shercliff[3] focused only on the temperature distribution during welding and studied its potential influence on the weld microstructure and precipitation kinetics. Today, researchers seek to model both the complex material flow behavior and temperature characteristics during friction-stir processes. Robson and Campbell[4] developed a grain growth and recrystallization model of FSW that predicts the weld nugget size during the joining of 2524 aluminum alloy plates. Utilizing a steady-state Eulerian formulation of friction-stir welding applied to aluminum 2024, Jacquin et al.[5] successfully modeled the material flow behavior and captured its conv