# Online Deformation Measurement of Laser Repair Substrate Based on Orthogonal Sampling Moire
## 3 Online Measurement System of Substrate Deformation in Laser Repair  
In this paper, laser repair is carried out on the upper surface of a stainless-steel cantilever beam, as shown in Figure 3[17,18]. The laser wavelength is 1064 nm with a fixed power of 1100 W, and the beam diameter is 2-3 mm. During the repair process, nickel base alloy powder is introduced into the molten pool and finally forms a cladding layer. In order to realize deformation measurement, the high temperature resistant orthogonal grating is etched on the side of the cantilever beam by laser milling with the technique in Reference [19]. During laser repair, the substrate deforms together with the grating, images of which are recorded by a CCD camera at all times. Finally, the displacement fields in two directions are calculated with the orthogonal sampling moire method based on the grating images. However, the extreme environment during repair process brings many interferences, causing great challenges to optical measurement. In repair process, the power of laser can reach 800-1500 W, acting on the area with a diameter of 2-5 mm to produce ultra-high temperature metal powder splashing everywhere, which can influence the image quality. Moreover, relevant research shows that the local molten pool can reach a high temperature of nearly 3000 \({}^{\circ}\)C [20], which is also a challenge for optical measurement. As shown in Figure 4, if the traditional image acquisition method is adopted, then the CCD camera is used to take photos directly during the repair process and the image obtained is inferior, with the influencing factors of imaging quality including overexposure, high-temperature radiation, glare and splashing powder. By shortening the exposure time, changing to a smaller aperture and adjusting the position of lens, the interference caused by glare and splashing powder can be removed. However, the overexposure of the laser area and the high-temperature radiation still interfere with the imaging quality. Figure 3: Cantilever for online deformation measurement [17]. Referring to the combination of blue light illumination and blue light narrow-band filter used in high-temperature deformation measurements [21,22], two high-power blue LED lamps were used for illumination and a group of filter components for laser repair environment was designed. As shown in Figure 5, the filter group was composed of neutral attenuator, a narrow-band blue light filter and a 1064 nm cut-off filter, wherein a 1% neutral attenuator uniformly reduces the light intensity of all bands to 1%. The blue light filter only allows the light in the blue light band to pass through, and the 1064 nm cut-off filter rejects light above 950 nm. Figure 4: Interference factors in laser repair. Figure 5: Filter group: (**a**) filter assembly sequence and (**b**) transmittance of different wave bands of filters. Among the three filters, a 1% neutral attenuator can solve the problem of overexposure effectively; the blue light filter can filter out most of the interference light and only allow the light reflected by the grating, which is illuminated by the blue LED, to pass through; and the 1064 nm cut-off filter can reject laser light with wavelengths of 1064 nm. The effects of different filter combinations are shown in Figure 6. It is found in Figure 6b that, without the cut-off filter, the intensity of laser light is strong, resulting in overexposure around the molten pool. Thus, it is necessary to add a cut-off filter for the laser wavelength (1064 nm) to prevent the light in this band. In addition, it is found that, in some cases, the light is reflected between the three filters, resulting in ghosting. Through testing, the problem can be eliminated by slight rotation of the filters. Since the neutral filter will reduce the gray level of the image, the cantilever needs to be illuminated by the high-power blue light source.