In situ measurement of full-field deformation for arc-based directed energy deposition via digital image correlation technology

Deformation measurement and control is a critical challenge in metal additive manufacturing. The metal deposition and solidification occur under the condition of high temperature gradient and large restriction accompany with complex metallurgical phenomena and stress evolutions. High stresses may result in deformation over yield strength of material, or even crack and fracture over the ultimate strength. Thus, it is necessary to monitor the deformation during additive manufacturing process. This paper focus on an in situ measurement of full-field deformation for arc-based directed energy deposition process via digital image correlation (DIC) technology. The measurement qualities were evaluated from the following aspects including system error, speckle pattern, influence of arc light and comparison in 2D-DIC and 3D-DIC. Results indicated that the relative system error of the measured deformation is no more than 0.12 % within a total deformation of 12 mm. Both the artificial speckle and natural speckle can meet the requirement of 2D-DIC system. The image qualities for artificial speckle is higher than that of natural speckle. For 3D-DIC, however, artificial speckle must be applied. The moving arc light will weaken the image qualities, which must be shielded in order to prevent the interference of intense light and the damage of speckle. An L-type shield was proposed and proved to be practical. Finally, DIC measuring was applied in both single-wall and cylindrical AM parts, comparing with numerical simulation. Results indicated that the evolution of the measured deformations showed good agreement with that of numerical simulation. Deformation evolution mechanism was revealed through numerical simulation and the evolution process was divided into three stages. For the cylindrical part, the final outer contour measured by DIC was consistent with that of 3D laser scanning and numerical simulation. As a conclusion, DIC technology was proved to be a feasible approach for in situ monitoring of full-field deformation of arc-based directed energy deposition process.