This study aims to improve Friction Stir Spot Joining (FSSJ) parameters for AA4045 by examining their effects on energy use and mechanical strength. Also, it proposes a sustainable FSSJ strategy for AA4045 aluminum alloy through multi-parameter optimization and energy-based process evaluation. A Taguchi L9 experimental design was used to study how the rotational speed, plunge depth, and dwell time affect joint performance. Lap-shear tests showed that speeds between 900 and 1200 rpm with a plunge depth of 0.5 to 0.8 mm gave the best bonding. In contrast, a shallow plunge depth of 0.2 mm did not allow enough penetration, resulting in weak adhesion at the interface. Sample No. 9 (1200 rpm, 0.8 mm, 2 s) had the highest shear strength at 2.7 kN, while Sample No. 1 (600 rpm, 0.2 mm, 1 s) had the lowest. The sustainability assessment found that higher rotational speeds increased energy use from 0.06 to 0.11 MJ per weld, but 900 rpm gave the best strength-to-energy ratio. Life-Cycle Assessment (LCA) with ecoinvent data showed a Global Warming Potential (GWP) of 0.004–0.007 kg CO₂-eq per weld, confirming that FSSJ of AA4045 is a low-emission alternative to traditional spot welding. These findings show that FSSJ is an eco-efficient joining method for lightweight applications. This research offers a practical guide for using energy-efficient manufacturing in lightweight structures, which is important for reducing emissions in transport and supports SDG 9 (Industry, Innovation, and Infrastructure) and SDG 12 (Responsible Consumption and Production). Optimised parameters not only lower environmental impact but also improve joint strength.
