This study explores Friction Stir Spot Welding (FSSW), a well-established solid-state joining technique, for high-strength aluminum alloys like AA2024-T4, which present significant challenges for conventional welding techniques. This research focuses on the impact of relatively low rotational speeds, specifically within a range of 700 to 1300 rpm, on the mechanical and microstructural properties of the welded joints. By employing a short dwell time of 3 s, this study aims to enhance productivity in the automotive and aerospace industries. The experimental work evaluated the joints’ thermal cycles, macrostructure, microstructure, hardness and load-carrying capacity. Results indicated a linear relationship between rotational speed and heat input. Although all welds exhibited a significant grain size reduction in the stir zone (SZ) compared to the base material (29.7 ± 6.1 μm), the SZ grain size increased with rotational speed, ranging from 4.7 ± 1.4 to 8.3 ± 1.3 μm. This study identified 900 rpm as the optimal parameter, achieving the highest load-carrying capacity (7.35 ± 0.4 kN) and a high SZ hardness (99 ± 1.5 HV). These findings confirm that joint strength is a balance between grain refinement and thermal softening. The presence of precipitates and the fractography of the tensile–shear tested specimens were also investigated and discussed.

1. 1.1. Recent studies focus on developing flexible pipe designs for hydrogen storage, transport, and marine uses through fiber reinforced composites like carbon and glass (Pöllinger et al. 2023; F...

This study examines the effects of optimized stacking sequences and NaOH surface treatment on the mechanical and dielectric performance of hybrid polyester composites reinforced with natural fibers (flax, sisal) and synthetic fibers (carbon, glass). Twelve laminates were fabricated using hand lay-up technique to assess hybridization effects on strength, insulation, and cost efficiency. Results revealed that the all-carbon six-layer laminate achieved the highest tensile strength (86 MPa), while the optimized hybrid [2 Carbon/2 Flax/2 Carbon] retained 85% of this strength at 40% lower material cost. Alkali-treated flax laminates exhibited a 93% improvement over untreated samples, confirming enhanced fiber–matrix interfacial bonding. A semi-empirical model based on a modified rule of mixtures predicted tensile strength within an 8% error margin, validating its reliability for hybrid systems. Natural fiber composites showed elevated dielectric constants (ε′ = 9.5 at 0.5 kHz), indicating excellent insulation capability. The integrated experimental, theoretical, and microstructural analyses propose an economical pathway toward developing lightweight, multifunctional composites for aerospace, marine, and electrical insulation applications.