Fitting-Free Diagnosis of Conduction-Model Breakdown in Laser Powder Bed Fusion.

The Al-Zn-Mg-Cu alloy 7A65, with a high Zn/Mg ratio, offers a favorable combination of strength, toughness and corrosion resistance, making it a promising material for thick-section aerospace components, such as fuselage frames and bulkheads [,,]. For joining plates thicker than 10 mm, double-sided friction stir welding (DS-FSW) has been developed to overcome the limitations of single-pass FSW, including excessive heat input, insufficient penetration, and through-thickness microstructural gradients [,,,,,]. In DS-FSW, two welding passes are applied from opposite sides of the plate. The conventional symmetric DS-FSW (S-DS-FSW) uses identical tools for both passes, whereas asymmetric DS-FSW (A-DS-FSW) employs a smaller or differently designed tool for the second pass, aiming to reduce the thermal impact on the first weld [,,]. Crucially, the tool size, specifically the shoulder diameter and pin length, directly determines the frictional heat generation and plastic strain imposed on the material. Tool 1 (shoulder: Φ30 mm; pin length: 16 mm) generates significantly higher peak temperatures and a wider thermomechanically affected zone compared to Tool 2 (shoulder: Φ24 mm; pin length: 12 mm). Thus, the S-DS-FSW joint (hereafter referred to as the S-joint) imposes two high-heat thermal cycles, while the A-DS-FSW joint (A-joint) reduces the total heat input, leading to distinct microstructural evolutions and corrosion responses. Although previous studies have compared the microstructures and mechanical properties of S- and A-DS-FSW joints [], their corrosion behaviors have received little attention. For high-strength aluminum alloys in chloride-containing service environments (e.g., marine or coastal atmospheric conditions), corrosion resistance is as critical as mechanical integrity [,,]. In DS-FSW joints, the dual-pass thermal cycle inevitably introduces complex microstructural heterogeneity along both the transverse and thickness directions. Key microstructural zones include the weld nugget, thermomechanically affected zone, heat-affected zone (HAZ), and double-stirred zone (DSZ), where the merging of the upper and lower welds is unique to DS-FSW. These regions differ significantly in terms of their precipitate sizes/distributions, grain structures, dislocation densities, and residual stresses, leading to spatially varying electrochemical activity [,,]. Under immersion in NaCl solution, such heterogeneity often drives localized corrosion, where certain zones become preferential anodic sites while others act as cathodes. Furthermore, the coupling between different zones across the weld cross section can result in differential corrosion behavior that evolves with the immersion time [,,]. Despite the practical importance, the current understanding of the corrosion behavior of thick-plate DS-FSW joints remains very limited. To the best of our knowledge, no systematic study has addressed the local electrochemical corrosion behavior of the bond zone and HAZ in DS-FSW 7A65 joints, nor has the overall corrosion evolution of the entire weld cross section been characterized under prolonged immersion. In particular, the influence of welding asymmetry (S- vs. A-DS-FSW) on the localized corrosion susceptibility and time-dependent corrosion morphology across the joint has not been explored. Such understanding is essential for assessing the long-term durability of DS-FSW components and for optimizing welding parameters to balance mechanical and corrosion performance.