Three-dimensional heat and material flow during friction stir welding of mild steel

Three-dimensional viscoplastic flow and heat transfer during friction stir welding of mild steel were investigated both experimentally and theoretically. The equations of conservation of mass, momentum and energy were solved in three dimensions using spatially variable thermo-physical properties and a methodology adapted from well-established previous work in fusion welding. Non-Newtonian viscosity for the metal flow was calculated considering temperature and strain rate dependent flow stress. The computed results showed significant viscoplastic flow near the tool surface, and convection was found to be the primary mechanism of heat transfer in this region. Also, the results demonstrated the strong three-dimensional nature of the transport of heat and mass, reaffirming the need for three-dimensional calculations. The streamlines of plastic flow indicated that material was transported mainly along the retreating side. The computed temperatures were in good agreement with the corresponding experimentally determined values.