Thermal-metallurgical-mechanical modeling and experimental validation for laser-directed energy deposition of TA15 titanium alloy

Residual stress (RS) and distortion significantly affect the performance and reliability of components fabricated by laser-directed energy deposition (LDED). Their evolution is governed by coupled effects of thermal history, mechanical constraints, and solid-state phase transformation (SSPT), which makes accurate prediction challenging. To address this issue, this study develops a thermo-metallurgical-mechanical (TMM) coupled model for LDED of thin-walled TA15 titanium-alloy components. The model is systematically validated via in-situ temperature and distortion monitoring, multi-scale phase transformation characterization, and RS measurements by X-ray diffraction and the contour method. The results show that spatial heterogeneity in thermal history induces a pronounced microstructural gradient along the build direction. The lower region forms martensite due to rapid cooling, which subsequently decomposes into α+β during thermal cycling, while the middle and upper regions undergo diffusion-type α transformation driven by thermal accumulation. Driven by alternating interlayer tensile and compressive stresses, the substrate undergoes periodic warping during deposition, evolving from an initial downward bend to an upward arch. Along the deposition direction, the cross-sectional RS of the thin-walled component exhibits a periodic alternation between tensile and compressive states. The incorporation of SSPT, which mitigates RS and distortion, enables the TMM model to achieve superior predictive accuracy over the thermo-mechanical coupled model. This study reveals the mechanisms of microstructure evolution and RS formation during LDED of TA15, and provides a theoretical basis for microstructure control and RS mitigation in high-performance components.