To meet the extreme corrosion resistance requirements of deep-sea API 5 L X65 carbon steel pipelines, this study utilizes the Box-Behnken response surface methodology to optimize the laser cladding of Inconel 625. Unlike previous studies focused on single-performance metrics, this research provides a novel coordinated regulation of multi-factor interactions to control interface martensite thickness. By establishing a quadratic regression model (R-squared = 0.9484), we systematically explored the effects of laser power, scanning speed, and powder feeding rate. Results indicate the powder feeding rate most significantly impacts cladding thickness (P < 0.0001), while its interaction with scanning speed (P = 0.0422) is critical for geometric regulation. The optimized parameters—1910 W laser power, 1365 mm/min scanning speed, and 0.47 r/min powder feeding rate—yielded an ideal 1 mm cladding thickness. This precise control window produces refined dendrites and a stable 50–60 μm martensite layer, offering highly reliable process parameters for pipeline remanufacturing.

Aluminum alloy, due to its low melting point and high thermal conductivity, deforms and contracts significantly during welding. To mitigate this and achieve full penetration in a single pass, this study uses GTAW (Gas Tungsten Arc Welding) additive manufacturing and optimizes welding groove parameters via the Box-Behnken Response Surface Methodology. The focus is on improving tensile strength and penetration depth by analyzing the effects of groove angle, root face width, and root gap. The results show that groove angle most significantly affects tensile strength and penetration depth. Hardness profiles exhibit a W-shape, with base material hardness decreasing and weld zone hardness increasing as groove angle rises. Root face width reduces hardness fluctuation in the weld zone, and an appropriate root gap compensates for thermal expansion, enhancing joint performance. The interaction between root face width and root gap most impacts tensile strength, while groove angle and root face width interaction most affects penetration depth. The optimal welding parameters for 7xxx aluminum alloy GTAW are a groove angle of 70.8°, root face width of 1.38 mm, and root gap of 0 mm. This results in a tensile strength of 297.95 MPa and penetration depth of 5 mm, a 90.38% increase in tensile strength compared to the RSM experimental worst group. Microstructural analysis reveals the presence of β-Mg₂Si and η-MgZn₂ strengthening phases, which contribute to the material’s enhanced mechanical properties. Fracture surface examination exhibits characteristic ductile fracture features, including dimples and shear lips, confirming the material’s high ductility. The coexistence of these strengthening phases and ductile fracture behavior indicates excellent overall mechanical performance, balancing strength and plasticity.

Direct fusion of corrosion-resistant copper–nickel (CuNi) cladding to X65 steel can cause Fe–Cu intermixing, segregation, and interface-associated cracking. This study comparatively evaluates whether an ERNiCu-7 transition layer regulates the interface more effectively than preheating/slow cooling under otherwise comparable pulsed gas tungsten arc welding conditions. Four cladding schemes were characterized by optical microscopy, SEM-EDS, EBSD, Vickers hardness, and Charpy impact testing. In direct ERCuNi cladding, preheating/slow cooling widened the measured Fe–Cu–Ni interdiffusion zone from approximately 10 to 30 μm. This change coincided with coarse Fe-rich bcc-indexed segregation, a maximum interfacial hardness of approximately 280 HV, severe fusion-line cracking, and a decrease in single-specimen impact energy from 35 J to 18 J. In contrast, the ERNiCu-7 transition-layer schemes exhibited continuous interfaces, high-angle grain-boundary fractions above 82.9%, smoother hardness transitions, and impact energies of 86 J and 68 J. Because one impact specimen and one complete hardness traverse were available per scheme, these mechanical results are interpreted as comparative screening data. Overall, the converging microstructural and mechanical evidence shows that the Ni-Cu transition layer reduces direct Fe–Cu interaction and improves interfacial continuity. Transition-layer design is therefore more reliable than preheating/slow cooling alone for suppressing cracking in CuNi-clad X65 components.