Investigation of geometrical features on mechanical properties of AA2198 refill friction stir spot welds

Geometrical features such as stir zone area, hook and bonding ligament of refill friction spot welds produced from AA2198 3.2 mm-thick sheets were analyzed along with their lap shear strength (LSS) and failure mode. Macro and micrographs of weld cross-sections were analyzed using optical microscopy for the understanding of microstructure development and measurement of stir zone area (SZA) and hook height (H). The comparison between SZA and LSS variation indicates a good correlation between these two variables while the hook effect does not present an obvious correlation with the LSS along the different welding conditions. Furthermore, analysis of variance of SZA considering rotational speed (RS), welding time (WT) and plunge depth (PD) shows that the variance of PD is responsible for more than 85% of the variation in SZA. A close relationship between defects and fracture type was also established since the hook and bonding ligament are preferential regions for nucleation and growth of circumferential and annular crack, respectively. All the welds presented the through-the-weld type of fracture, in which the nugget tears apart under loads. Circumferential crack growth was observed to play an important role in weld resistance as it prevents the annular crack growth, which is responsible for the final fracture. Lastly, the association between circumferential crack growth and PD shows a major influence on LSS since the path created by the sleeve plunge is a region of preferential crack growth.