Modeling of Friction Stir Processing Using Computational Fluid Dynamics (CFD) Analysis

  • Ahmed N. Albakri

Student thesis: Master's Thesis

Abstract

Vital sectors such as transportation, construction and manufacturing have increased the demand on developing advanced class of materials with superior mechanical properties. Light weight alloys, such as Al and Mg alloys, have great potential in the future of sustainable transportation and aviation. However, the difficulties in welding and forming of such materials have limited their utilization. Recently Friction Stir Welding (FSW) has evolved as an innovative welding technique which can produce strong welds that have fine grains and homogenous microstructure. Based on FSW, Friction Stir Processing (FSP) was developed for grain refinement and homogenization of metals, resulting in enhanced formability and durability. FSP is a solid state process in which a non-consumable tool with a specially designed shoulder and a pin is plunged into a sheet of metal, and then translates along a linear path. The generated heat due to friction and severe plastic deformation is enough to soften the material without melting, which yields dynamically recrystallized fine grains and almost defect-free homogenous microstructure within the processed zone. In this work, FSP is simulated using Computational Fluid Dynamics (CFD) approach with particular concentration on cooling during the process. The parameters of the process in terms of rotational and translational speeds, tool geometry and cooling are investigated to study the resulting distributions of temperature, strain rates, velocity fields, calculated grain size and hardness. The simulation results showed that the resulting microstructure is very sensitive to the rotational and translational speeds, tool design and cooling rates. For controlled parameters applied in this work, relatively lower rotational speeds and faster translation resulted in finer grains and higher hardness values. Furthermore, tapered pin with relatively small shoulder diameter was found to produce lower heat input and higher average strain rate around the pin zone, which resulted in smaller grain size. Finally, ultra fine grains in the range of nano-meters were achieved by using liquid Nitrogen as coolant through Copper backing plate below the processed sheet combined with internal cooling through FSP tool. Combined cooling was found to be promising method not only to enhance the processed sheet, but also to improve the tool life by reducing the thermal cycles encountered during FSP. The results of this work were compared with experimental data wherever possible and good matching was achieved.   This research was supported by the Government of Abu Dhabi to help fulfill the vision of the late President Sheikh Zayed Bin Sultan Al Nayhan for sustainable development and empowerment of the UAE and humankind.
Date of AwardDec 2011
Original languageAmerican English
SupervisorMarwan Khraisheh (Supervisor)

Keywords

  • Mechanics
  • Analytic
  • Fluid Dynamics
  • Friction

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