Strength and hardness enhancement in CNTs-reinforced dissimilar friction stir welds employing Taguchi-assisted Box-Behnken RSM optimization

Dissimilar friction stir welding of AA2024 and AA7075 remains challenging because joint performance is often limited by interfacial heterogeneity, localized softening, and inadequate reinforcement control. The novelty of this study lies in integrating a two-pass interfacial multi-wall carbon nanotubes (CNTs) deployment strategy with sequential Taguchi screening and Box-Behnken response surface modeling to develop statistically validated predictive models for CNTs-reinforced dissimilar AA2024-AA7075 welds. CNTs were introduced through a pre-machined groove and redistributed by reverse direction stirring, after which tensile strength and hardness were optimized as functions of rotational speed, traverse speed, and axial load. CNTs incorporation increased ultimate tensile strength (UTS) by 11.6% (413.31 to 461.21 MPa) and hardness by 26.8% (151.70 to 192.40 HV) relative to the unreinforced joint at the Taguchi optimum. The response surface methodology optimum at 1400 rpm, 80 mm/min, and 1.1 kN yielded 461.56 MPa and 192.70 HV. The developed models showed excellent agreement with experiments, with coefficient of determination values of 0.99981 for UTS and 0.99978 for hardness, while the prediction deviations at the optimum condition were only 0.0065% and 0.0156%, respectively. ANOVA confirmed rotational speed as the principal factor (75.88% for UTS and 65.32% for HV), with traverse speed secondary (23.86% and 34.47%) and axial load minor (