F. D. Ramos, C. P. Mazzaferro, T. Rosendo, J. Mazzaferro, M. Durlo Tier, J. Dos Santos, GKSS- Forschungszentrum, Geesthacht, Germany
Friction joining processes are largely investigated due to their potential to avoid metallurgical and economical problems related to resistance welding processes. Among these friction processes, friction stir spot welding (FSSW) has become a very interesting 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 to mechanical fastening (riveting and clinching), since it produces high strength joints and a reduction of 99% of the energy costs can be obtained (in Al alloys compared to conventional RSW). There are many other 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), environmental friendliness, easy of automation and no need for additional material. Four different pin profiles were employed to produce the joints. The plunge rate, spindle speed, plunge depth and dwell time were kept constant. Samples were produced to perform mechanical testing (shear test), metallographic analysis (optical and scanning electron microscope (SEM) and microhardness profiles. To evaluate the material flow, a cupper sheet was placed between upper and lower sheets prior to FSSW process in order to obtain a good contrast. The mechanical testing indicates that the pin profile has influence on the shear strength. The different pins have also resulted in different microstructures and material flow. Optical microscopy revealed three different microstructural zones: The Stir Zone (SZ), the Thermo-Mechanically Affected Zone (TMAZ) and the Base Material (BM). The presence of the Heat Affected Zone (HAZ) was only revealed by microhardness testing, where the four zones could be observed.
Summary: FSSW of AA6181-T4 was developed in this work aiming to evaluate the influence of the pin profile on the joint performance. Samples were produced to metallographic, material flow, shear tensile and microhardness evaluation. The results showed that the profile of the tool affects metallurgical and mechanical performance of the joint.
