A novel two-dimensional reflective coherent gradient sensor and its applications in residual deformation measurement of laser repaired metal components

Abstract

Directed Energy Deposition (DED), known as an important branch of additive manufacturing, are becoming increasingly crucial in different industrial fields. However, this extreme manufacturing process may bring about quality issues for the DED product, and in-situ monitoring for characterizing mechanical parameters of thick DED products still needs to be improved. In this manuscript, for the convenient in-situ bidirectional residual deformation measurement of thick DED metal components, a novel two-dimensional coherent gradient sensor (2D-CGS) is established based on crossed Ronchi gratings. Firstly, the diffraction characteristic of the crossed Ronchi grating used in 2D-CGS is analyzed, and then the principles of 2D-CGS are analyzed in detail for bidirectional out-of-plane displacement gradient measurement. Furthermore, two temporal phase-shifting techniques: plane-parallel plate rotating (PPR) method and prism translation (PT) method, are extended from one dimension to 2D for improving the accuracy of 2D-CGS post-processing, named as 2D-PPR method and 2D-PT method. Verification experiment shows that the 2D-CGS system can synchronously measure the bidirectional curvature fields of the standard spherical reflector with the maximum relative error of 3.80%, which is equivalent to the accuracy of typical CGS. Finally, in the application of zig-zag deposited DED process, the bidirectional residual deformation fields of repaired components are in-situ monitored by portable 2D-CGS system, and the relationship of dwell time between adjacent tracks and the residual deformation is analyzed based on bidirectional centerline curvature on the bottom surface of substrate. A new parameter named as the orthotropic symmetry coefficient is defined based on curvature to analyze the full-field in-plane orthogonal anisotropy. Results show that both the degree of in-plane orthogonal anisotropy and value of residual deformation decreases with the increase of dwell time from 0 to 5 s, while varying a little with the dwell time exceeding 5 s, which also indicates the proposed parameter could well evaluate the degree of full-field orthotropic anisotropy. Meanwhile, the position of maximum curvature also gradually moves from the end toward the beginning in deposition direction. In summary, the proposed 2D-CGS method can measure the full-field bidirectional deformation in high efficiency, which may be promising in future in-situ monitoring applications.