Study on process method and forming quality of large-overhang thin-walled curved tubes fabricated by annular laser metal deposition

Current laser metal deposition (LMD) techniques for fabricating overhanging structures predominantly rely on substrate rotation to circumvent geometric overhangs, which inherently constrain the freeform fabrication angles. However, adopting dynamic nozzle reorientation strategies significantly exacerbates the molten pool flow deflection and laser-powder coupling inefficiencies owing to the gravitational effects. To achieve high-quality fabrication of large-overhang thin-walled curved tubes, this study proposes a variable-posture non-uniform slicing (VP-NUS) method based on annular laser metal deposition (ALMD). The process is initiated using a hybrid geometric segmentation strategy that combines interlayer slicing and intralayer discretization. Subsequently, the adaptive matching of discrete unit dimensions with laser process parameters enables the deposition of constant-width variable-height beads, ensuring dimensional consistency across complex topologies. To regulate the molten pool morphology at various inclination angles, an asymmetric annular gas flow field was directionally applied along the gravitational vectors, with pressure differential modulation compensating for the inclination-induced pool deflection. The research team successfully implemented continuous printing of curved tubes ranging from −45° to 135° using a self-developed ALMD system. A comprehensive characterization of a 90°curved tube was conducted, encompassing geometric accuracy, surface topography, and microstructural evolution. Geometric analysis revealed minor deviations in the average wall thickness between the inner and outer contours (1.48 ± 0.08 mm and 1.50 ± 0.09 mm), with negligible roundness errors across the cross-sections. Roughness measurements indicated consistently low values ( S _a 8.77 µm and S _a 9.46 µm) at both minimal and maximal layer heights. Microstructural and tensile property studies revealed that the regions fabricated with elevated scanning velocities featured refined grain sizes and enhanced tensile strengths. Fractographic examination revealed ubiquitous dimples on all tensile specimen fracture surfaces, exhibiting quintessential ductile fracture characteristics. The VP-NUS method significantly enhances LMD’s freeform capabilities and forming quality for complex overhangs, confirming its potential for industrial applications that require high-precision curved components.