Implementation of Design of Experiments on Interlayer Friction Stir Spot Welding (IL-FSSW) for 304 Stainless Steel and 6061 Aluminum Alloy Joints

Interlayer Friction Stir Spot Welding (IL-FSSW) represents an advanced variant of the conventional Friction Stir Spot Welding (FSSW) technique, designed for the purpose of joining materials of both similar and dissimilar alloys. The Friction Stir Spot Welding (FSSW) process emerges as the pre-eminent welding methodology for the fusion of metallic substrates and alloys that present inherent challenges in achieving successful fusion through traditional welding techniques. The judicious selection and application of an interlayer material assumes a pivotal role in mitigating the formation of undesirable keyholes on the upper surface of the weld spot. The resulting weld joint, crafted through the utilization of the Interlayer Friction Stir Spot Welding (IL-FSSW) technique, exhibits a visually striking surface finish devoid of any discernible distortions or irregularities emanating from the weld spot. The incorporation of an intermediate layer (IL) assumes a pivotal role in enhancing the lap shear force (LSF), surpassing the prescribed threshold defined by the American Welding Society (AWS) standards by a factor of 1.5. To systematically examine the influence of operational parameters on both the microstructural characteristics and mechanical properties of a dissimilar Al6061/AISI304 weld joint, employment of a Design of Experiments (DOE) approach based on the Taguchi L9 model is carried out. In this investigation, three key parameters — namely, tool rotation speed, plunge depth, and the diameter of the intermediate layer (IL) — have been chosen as variables, while keeping all other relevant factors constant. Analysis conducted through the DOE L9 model reveals that, among the considered parameters, tool rotation speed exerts the most substantial influence. In the context of lap shear tensile tests, it has been observed that there exists a direct correlation between tool rotation speed and plunge depth with the resultant lap shear force (LSF). Specifically, an increase in tool rotation speed and plunge depth leads to a concurrent increase in LSF. Notably, the relationship between plunge depth and LSF exhibits a linear trend up to a certain threshold, beyond which a diminishing return is observed. This diminishing return can be attributed to the reduction in the thickness of the spot weld, resulting in a subsequent decrease in LSF. Optical Microscopy examinations of the dissimilar Al6061/AISI304 joint has proven a vital role in distinguishing the key regions, including Stir Zone (SZ), Thermomechanical Affected Zone (TMAZ), Heat Affected Zone (HAZ), and Base Metal (BM). This differentiation is attributed to the variations in average grain sizes across these structural regions.