Macro/micro mechanical level characterization of friction stir welded joints in aluminum alloys using combined experimental/numerical approaches

Friction stir welding (FSW) is a solid state joining process having key benefits such as low energy consumption, greater environmental friendliness and increased versatility, when compared with the conventional welding processes such as gas metal arc welding or tungsten inert-gas welding. The FSW process has been used successfully to weld high strength aluminum alloys that are used in the aerospace and automotive industries. In general, the heating and plastic deformation that takes place during the FSW process produce local microstructural changes, which lead to local variation in mechanical properties in and near the weld. The local properties control the failure in and along the weld. On the other hand, the global mechanical behavior of the welded structure directly depends on the local mechanical properties in the heterogeneous weld zone.;The objectives of this research were to study the effect of local (micromechanical) vibrational characteristics (stiffness and damping) on the global (macromechanical) vibrational characteristics of friction stir weld (FSW) and spot friction weld (SFW) joints in aluminum alloys and to indirectly determine effective material properties (elastic moduli) in the weld zones of such joints at both global and local levels, using combined experimental/numerical approaches based on both vibration and static tests. Attempts were also made to study the relationships between weld process parameters and weld zone material properties (moduli) and also the distribution of local material properties (moduli) in friction stir welded (FSW) and spot friction welded (SFW) joints in aluminum alloys.;A new approach to characterization of the distribution of local moduli in different regions of the weld zone using modal vibration tests on micron scale cantilever beam specimens with a micro-scanning laser vibrometer was developed. Each microcantilever specimen was taken from a different region in the weld zone. The current work will help to develop a fundamental understanding of modal and material characteristics of FSW joints, and how these characteristics are related to weld joint properties at both the macromechanical and micromechanical levels.