A novel coherent gradient sensor for quadric surface and its application in laser repair process

In-situ deformation measurement of laser directed energy deposition (LDED) repaired product is conducive to the process monitoring and troubleshooting of LDED, laying experimental foundations for further LDED process parameter optimization as well as quality control of the product. However, the diversity of repair structures and process parameters may form complex deformation on curved surface, demanding for wider measurement range of in-situ monitoring method. Considering the relevant requirements, this paper puts forward a novel theoretical model of coherent gradient sensor for curved surface (curve-CGS), and the accurate deformation measurement of curved surface has been achieved by CGS for the first time. Then, an integrated curve-CGS system with translation compensation system (TCS) is designed and established independently for accurate quadric measurement based on the theoretical model, and two curved surface measurement modes: the optimal compensation for minimum full-field systematic error and the confocal compensation, are proposed and analyzed through simulation. The accuracy of curve-CGS system is then experimentally studied through measurement of different spherical mirrors with standard curvature. Results demonstrate that the proposed curve-CGS achieves curvature measurement accuracy$\Delta \kappa /\kappa$ < 4 % within the extended measurement range of 0–2 $m^{-1}$, while conventional CGS shares the same accuracy magnitude within 0–0.2 $m^{-1}$. Finally, the curve-CGS system is applied in zig-zag LDED multi-layer repair process to in-situ monitor the full-field residual deformation (RD) of curved metal substrate. Based on curvature data, the spherical and aspherical RD accumulation of each deposition layer in LDED repair is analyzed. Results show the spherical curvature of repaired components remains basically the same while the aspherical curvature continues to increase and accumulate after each deposition layer, which is fundamental to the anisotropic distribution of RD. The curve-CGS proposed in this study expands the measurement range of CGS, which has a good potential in future applications of curved surface deformation measurement at real working sites.