Numerical simulation of modified friction stir spot welding processes

Spot welding can be considered a very common joining technique in automotive and generally in transportation industries as it permits to obtain effective lap-joints with short process times and what is more it is easily developed through robots and automated systems. Recently the Friction Stir Spot Welding (FSSW) process has been proposed as a natural evolution of the already known Friction Stir Welding (FSW) process, allowing to obtain sound spot joints that do not suffer from the insurgence of typical welding defects due to the fusion of the base material. In the paper, a variation of the Friction Stir Spot Welding (FSSW) process has been considered. In detail, different peculiar tool paths are given to the tool, after the sinking phase, nearby the initial penetration site, with the aim to enhance the final joint mechanical properties. A continuum based FEM model for Friction Stir Spot Welding process is proposed, that is 3D Lagrangian implicit, coupled, rigid-viscoplastic. This model is used to investigate the distribution of temperature, strain and strain rate in the heat affected zone and the weld nugget. In particular, the large values and gradients of temperature and strain rate found in the weld zone during the process can be used to determine the Zener-Holomon parameter which, in turn, strongly affects the Continuous Dynamic Recrystallization (CDRX) process that takes place in the weld nugget. In this way, the average grain size and the extension of the nugget can be predicted with the ultimate goal to get information on the joint mechanical resistance. The developed model appears an effective tool in order to design effective spot joints