Regulating thermal distortion and residual stress via an in-situ static magnetic field during laser additive manufacturing of Ti-6Al-4V alloy

Thermal distortion and residual stress are major challenges in laser-directed energy deposition (L-DED) due to large temperature gradients and rapid cooling, while effective mitigation strategies remain limited. In this study, an in-situ lateral static magnetic field was introduced to regulate thermal distortion and residual stress in a cantilevered thin-wall Ti-6Al-4V structure fabricated by L-DED. Temperature and distortion were monitored in situ using thermocouples, a laser displacement sensor, and an infrared thermal imager. Residual stress was systematically evaluated after distortion release using the contour method and XRD, supported by numerical simulations and microstructural characterization. The results indicate that the applied magnetic field leads to consistent reductions of 15 % in thermal distortion and 13.3 % in maximum tensile residual stress. The Lorentz force induced by electromagnetic damping (EMD) suppresses and modifies melt pool flow, reducing heat accumulation and local overheating while enhancing heat dissipation. Consequently, the maximum melt pool temperature, internal temperature gradient, and macroscopic temperature gradient between the substrate and melt pool are all reduced, yielding a more uniform temperature field and lower thermal distortion and tensile residual stress. Furthermore, the thermoelectric magnetic force (TEMF) induced by the thermoelectric magnetic effect refines the prior-β grain size from 851.89 μm to 787.88 μm and weakens the texture, as reflected by a 44.9 % reduction in maximum pole density from 13.86 to 7.63. This enhanced microstructural uniformity reduces stress concentration and anisotropy, thereby decreasing thermal distortion and tensile residual stress. Correlated reductions in distortion, temperature, tensile residual stress, and microstructural heterogeneity indicate a systematic magnetic field induced regulatory effect rather than measurement uncertainty. These findings demonstrate the potential of static magnetic-field-assisted L-DED to control thermal distortion and reduce post-release tensile residual stress in Ti-6Al-4V alloy.