The Influence of the Tool Profile in Friction Stir Spot Welding of the Aluminum Alloy AA6181-T4

Friction joining processes are largely investigated due to its potential to avoid metallurgical and economical problems related to fusion welding processes. Among these processes, friction stir spot welding (FSSW) has become a very active field of research. The process is a variation of the friction stir welding where a tool comprised of a shoulder and a pin penetrates into an overlap joint plasticizing the material and consolidating the joint leaving a keyhole after tool removal. FSSW is an alternative process to resistance spot welding, as well as for the mechanical fastening (riveting and clinching). There are many advantages using this process in relation to conventional resistance spot welding and mechanical fastening methods including avoidance of solidification and metallurgical defects (solid state welding process), environmentally friendly, ease of process automation and no additional metal. Twelve different tools profiles and three rotational speeds were tested to perform the welds. The plunge rate, plunge depth and dwell time were kept constant. Four samples were produced to perform the mechanical testing (shear test) and two samples were produced to metallographic characterization (optical and scanning electron microscope). Microhardness profiles were made in the metallographic samples. The result of mechanical testing indicates that higher values of rotational speed give an improvement on the mechanical strength of the welds. The different tools had also shown different results for mechanical testing and metallographies. For the metallographic samples, three different zones could be observed: The Stir Zone (SZ), the Thermo-Mechanically Affected Zone (TMAZ) and the Base Material (BM). The observation of the Heat Affected Zone (HAZ) was only possible by microhardness profile testing, were the four zones were observed.