Laser Directed Energy Deposition (L-DED) has been widely employed in the repair of core components such as rolls, turbine blades, and bearings. However, when L-DED is directly applied to defect areas, geometric irregularities on the surface can lead to random and uneven distribution of laser energy, resulting in fluctuations in repair layer hardness, poor adhesion, and even secondary failures. Pre-machining suitably arc-shaped grooves in the defect areas and matching them with appropriate defocus parameters can regulate the laser energy distribution, serving as a key technical approach to enhance the stability of repair layer hardness and bonding quality. This study innovatively couples laser propagation, powder flow, and groove structure to establish a thermal-fluid-solid coupling model applicable to curved surfaces, quantifying the effects of positive/negative defocus modes on the powder flow field and heat transfer process in the molten pool. Furthermore, it reveals the influence mechanism of negative defocus on the temperature evolution of the molten pool in arc grooves and the deposition quality, establishing a quantitative matching relationship between arc grooves with different width-to-height ratio (D) and defocus distance (H). The results indicate that when the D = 2, the optimal H is −30 mm; when D = 2.5, it is −65 mm; and when D = 3 and 4, it is −85 mm. Through multi-physics simulations and experimental validation, it is demonstrated that the matching relationship between the D and the H of the arc groove can achieve a defect-free bonding interface after repair and stabilize the microhardness at 480–520 HV. This study provides a theoretical basis and process guidance for the L-DED repair of curved components, promoting the advancement of L-DED technology from planar/simple groove repairs to complex curved surface repairs.
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