Thermal distortion evolution and residual stress characteristics of Ti-6Al-4 V alloy by laser-directed energy deposition: In-situ monitoring, contour method, and XRD

Abstract

Thermal distortion and residual stress are major issues affecting dimensional accuracy and mechanical properties in laser additive manufacturing (LAM). This study investigates the evolution of thermal distortion and the formation mechanism of residual stress in Ti-6Al-4V alloy during laser-directed energy deposition (L-DED). An in-situ monitoring system recorded the distortion and temperature histories of the sample during deposition. Residual stress in various regions and directions of the final thin-wall was analyzed using the contour method and XRD. The results show that during each layer deposition, the interaction between tensile and compressive stresses causes the free end of the substrate to initially bend downward and subsequently upward. Different linear energy densities (E_l) lead to varying distortion modes of the substrate. For E_l values between 75 and 150 J/mm, substrate distortion height increases with deposition layers. For E_l values between 150 and 300 J/mm, it first increases and then decreases. By combining in-situ monitoring data with residual stress results, this study clarifies the evolution of thermal distortion, the mechanism of residual stress formation, and their relationship during l-DED. Samples with greater distortion tend to exhibit lower residual stress. The distortion of E_high samples is 0.73 mm greater than that of E_low samples, yet their maximum residual stress is 82.8 MPa lower. The higher cooling rate in the deposition region creates local tensile stress, while compressive stress forms in the surrounding region. As deposition progresses, the region of maximum tensile stress shifts upward along the building direction until complete. This study provides new insights into the evolution of thermal distortion and the mechanisms of residual stress formation in LAM, contributing to the control and reduction of residual stress.