Continuous dynamic recrystallization during friction stir welding of high strength aluminum alloys

Abstract

Abstract : Friction stir welding (FSW) is a solid state joining process 1,2,3 that uses a rapidly-rotating, non-consumable high strength tool-steel pin that extends from a cylindrical shoulder (Figure 1). The workpieces to be joined are firmly clamped to a worktable; the rotating pin is forced with a pre-determined load into them and moved along the desired bond line. Frictional heating is produced from the rubbing of the rotating shoulder with the workpieces, while the rotating pin deforms (i.e. 'stirs') the locally-heated material. To produce a high integrity defect-free weld, process variables (RPM of the shoulder-pin assembly, traverse speed, the downward forging force) and tool pin design must be chosen carefully. FSW can be considered as a hot-working process in which a large amount of deformation is imparted to the workpiece through the rotating pin and the shoulder. Such deformation gives rise to a weld nugget (whose extent is comparable to the diameter of the pin), a thermomechanically-affected region (TMAZ) and a heat-affected zone (HAZ). Frequently, the weld nugget appears to comprise equiaxed, fine, dynamically recrystallized grains whose size is substantially less than that in the parent material. The objective of the present research was to develop a basic understanding of the evolution of microstructure in the dynamically recrystallized region and to relate it to the deformation process variables of strain, strain rate, and temperature. Such a correlation has not been attempted before perhaps because of the difficulty in quantifying the process variables. To overcome such difficulties, recent work 4 to measure and model the local temperature transients during FSW was utilized, and an approximate method was employed to estimate the strain and strain rate in the weld nugget.