Optimization of Laser Beam Oscillation Amplitude for Residual Stress Mitigation in TC4/TA18 Bottom-Locking Tube Joints

Laser beam oscillation has been recognized as an effective means to modify temperature gradients and relieve stress concentration in welded joints, yet its role in dissimilar titanium alloy bottom-locking configurations—where asymmetric thermo-mechanical responses arise from both material mismatch and structural constraints—remains unclear. This study investigates the influence of circular oscillation amplitude (0, 0.5, 1.0, and 1.5 mm) on residual stress distribution in laser-welded TC4/TA18 bottom-locking tube joints at a constant power of 1000 W. Unlike the monotonic effect of laser power, oscillation amplitude redistributes the heat source spatially. A three-dimensional thermo-elastic–plastic finite element model incorporating a moving oscillating Gaussian conical heat source was developed in ABAQUS and validated against weld macrographs, thermal cycles, and blind-hole residual stress measurements. The results reveal a non-monotonic dependence of residual stress on oscillation amplitude: as amplitude increases from 0 to 0.5 mm, the peak hoop residual stress rises, but further increasing amplitude to 1.5 mm substantially reduces both hoop and axial residual stresses while promoting a more uniform stress field. At 1.5 mm, the molten pool covers the entire bottom-locking step geometry, the region of high hoop tensile stress (>700 MPa) is minimized, and the peak axial tensile stress at the bottom-locking gap end (BLG End) is reduced by 41% (from 415 MPa to 243 MPa) compared with non-oscillation welding. An amplitude of 1.5 mm is therefore recommended as the optimal parameter for residual stress control of these joints at 1000 W within the investigated parameter range. This finding provides a practical guideline for mitigating residual stress-induced failure risks in aerospace hydraulic and fuel delivery systems.