Objective High-speed laser cleaning mainly uses two independent and controlled orthogonal motors to realize the spiral filling and ablation in a two-dimensional area. A galvanometer motor is used as the positioning motor. In the fixed trajectory mode, the ablation points of spiral filling are distributed in a network. The intersection points of the network are overlapped and ablated, and there are non-ablated grid gaps. In the laser-cleaning process, the scanning speed of an edge area is slower than that of a centre area, the thermal action time becomes longer and the ablation is too heavy. The above mentioned problems can easily lead to two aspects of undesired results. First, the laser-cleaning surface left marks and large format edge area exhibits severe overlap ablation, even high-power laser-cleaning edge cutting phenomenon. Second, repeated and excessive ablation in the local area for a long time causes severe heat accumulation, easily damaging the substrate. Therefore, this study aims to solve the abovementioned problems during the laser-cleaning process.

Methods The double-motors spiral-scanning mode was adopted to separate the motor drive board from the cleaning gun head. A random yaw factor was used to regulate the noise voltage introduced by the wire group between the motor driving plate and the cleaning gun head. The introduced factor was adjusted to make the galvanometer motor spiral and yawed randomly to ensure that each coordinate position in the plane was unique and not repeated, which greatly reduced the probability of excessive ablation. In the edge area, the simple opening and closing method of controlling the laser was changed. Then, the laser energy was controlled in the edge area by modulating the laser frequency, reducing the degree of accumulation of thermal effects. Therefore, separating the machine and card of the galvanometer motor and introducing a random factor into the spiral-cleaning path was designed for achieving more uniform cleaning effect and more stable control system and thermal environment.

Results and Discussions The uniformity problem and excessive edge ablation are destructive for the two-dimensional laser cleaning. Aiming at a series of problems exposed by the traditional progressive ablation-cleaning method, a random spiral-filling path is designed to control the independent movement of X/Y motors (Fig.4). The initial angles of the two motors differ when setting the swing, i.e., the initial phases are different. Dual motors swing at full speed according to the designed trajectory, and the dynamic phase difference forms a network distribution to fill the entire surface area (Fig.5). The scanning speed of the galvanometer in the non-edge area is the combined speed of the two motors due to their independent movement, and the cleaning efficiency is improved. A certain motor is always maintained in the edge area to reduce the dwell time of the laser in the non-edge area (Fig.6) and weaken the edge over-burning. Similar to progressive ablation, there are still gaps in the mesh distribution under fixed trajectory scanning, which can be controlled by adjusting the subdivision degree of phase difference. For a large mesh gap (larger than the laser spot), there are still gullies between the gaps after the ablation process (Fig.8). By introducing a random yaw factor (Fig.7), the position of the motor is not required to be fixed each time when scanning back and forth, thereby avoiding the formation of a fixed net-like distribution. the removal amount is uniform under random scanning and the removal amount of fixed trajectory scanning is unevenly distributed (Fig.9). Uniform cleaning is difficult to achieve via regular spiral scanning. It is more likely to produce cracks in the transition zone and cause damage to the substrate. Under the random spiral trajectory, the probability of the occurrence of sample’s subsurface cracks is reduced (Fig.10).

Conclusions The two motors are linked to fill a two-dimensional area during the scanning process of a high-speed laser-cleaning system using a spiral path. The scanning efficiency is improved compared with that in the case of using only a single motor during line-by-line cleaning. However, spiral filling appears at a mesh-crossing position and repeated ablation occurs. Although line-by-line cleaning has no ablation and intersection area, the cleaning efficiency is low. When the area overlaps, the spiral or progressive scanning has the problem of excessive area overlap and ablation, resulting in area overlap marks during laser cleaning. Therefore, this study separates the motor drive board from the cleaning head and actively introduces the random superimposed noise factor into the control system to improve laser-cleaning efficiency. The random yaw factor is introduced into the cleaning path planning of the galvanometer spiral surface filling, which helps to prevent repeated ablation with the reticular cross for the fixed track scanning. Meanwhile, the laser frequency is modulated in the scanning period to reduce the laser energy near the cleaning edge and avoid excessive ablation during the motor deceleration process. The results show that the random spiral scanning in the non-edge region is useful for making the random distribution of laser ablation points and avoiding repeated ablation at a single point during laser cleaning. The laser scanning speed at the edge is slowed down, and the ablation energy is reduced. Thus, considerably reducing the probability of crack formations in the transition zone of the substrate. Therefore, the cleaning system with position modulation of the laser frequency under the spiral random scanning mode can effectively address the problem of uneven distribution of laser ablation degree and vulnerability of substrate after cleaning.