As a new pattern engraving method of chemical milling parts, laser engraving is one of the important processes in chemical milling for aeroengine casing. This technique can effectively improve the precision and the efficiency of chemical milling. Moreover, it is greatly significant in improving the thrust-weight ratio and the manufacturing efficiency of the aeroengine. In the laser engraving process, according to the numerical control (NC) machining program based on the geometric pattern information and the process parameters of chemical milling, the geometric pattern is engraved on the protective adhesive layer by laser ablation under the control of the optical electromechanical cooperative control system. Laser engraving combines laser processing with the NC technology and a digital manufacturing process that has high precision and efficiency, digitization, and flexibility. The method can also be used for primary/secondary engraving on complex surfaces to solve the engraving bottleneck problem of aerospace complex thin-walled structures. The laser engraving research in China is still in its initial stage and mainly focuses on investigating the primary laser engraving process parameters and the engineering application of foreign laser engraving machines. Less research has been conducted on the key technologies and equipment used for the laser engraving of three-dimensional (3D) complex structure parts, and many technical difficulties have not yet been overcome. This work investigates the key technologies of the engraving process parameters, including laser engraving trajectory planning, optical electromechanical collaborative optimization model, and adaptive matching mechanism. The six-axis, five-linkage NC laser engraving machine tool is developed to provide a new solution to the bottleneck problem of engraving in the chemical milling of the complex thin-walled structures of the aerospace.

First, based on the laser multiple engraving process, a laser engraving trajectory planning algorithm considering the chemical milling evolution is proposed herein to solve the laser engraving problem of the complex surface on aeroengine casing. The basic processes of trajectory planning and automatic programming of the pattern features for multiple laser engraving are given. The multi-axis motion trajectory of the laser engraving position and direction is fitted by a complete B-spline curve and a segmented double B-spline curve. The number of control points and the fitting error of the curve are then analyzed. Second, an opto-mechatronics collaborative optimization model is established aiming at the minimum processing time and the minimum width of the heat-affected zone while the adhesive layer is etched through. In this model, the bow height error of the trajectory curve, speed, acceleration, and jerk of the feed axis are considered. Furthermore, the minimum processing time is equivalent to the maximum feed speed. Third, an adaptive matching optimization algorithm for the engraving process parameters is established to solve the optimization problem of the motion and laser process parameters. The laser process parameters that satisfy the constraints under different speed conditions are simulated and calculated, providing theoretical parameters for the optical electromechanical cooperative control of laser engraving. Finally, the structure of the six-axis, five-linkage NC laser engraving machine tool, the high-precision optical path flexible transmission and positioning, and the optical electromechanical cooperative control system are implemented. The six-axis, five-linkage NC laser engraving machine tool is developed to realize the application of primary/secondary laser engraving.

First, for the trajectory planning of the laser engraving position points, a complete B-spline curve and a segmented B-spline curve are used to generate the trajectory that meets the accuracy requirements. The fitting accuracy of each curve is less than 0.008 mm (Fig. 5). To ensure the fitting accuracy, the complete B-spline curve needs more control points, while the segmented B-spline curve needs less control points (Table 1). The segmented double B-spline curve is used to generate the trajectory for the engraving position and direction. The fitting accuracy of the segmented double B-spline curve of the laser engraving position and direction can reach 0.005 mm (Fig. 5). The maximum error of the direction vector angle by the segmented double B-spline curve is 0.0061 rad, which effectively meets the laser engraving process requirements. Second, the simulation results of the opto-mechatronics collaborative optimization model illustrate that the energy in the heat-affected zone exceeding the threshold is mainly considered in the low-speed movement section. In addition, the engraving speed is increased to ensure the engraving quality (Fig. 8). The kinematic constraints of the equipment are mainly considered to complete the engraving processing with the highest efficiency in the high-speed movement section. The comprehensive balance between the engraving quality and efficiency is realized in this model. Third, to optimize the motion and process parameters in the engraving process, the comprehensive optimization results under different weight conditions are given, and the corresponding process parameters of the laser energy density and the duty ratio under different speeds are calculated (Fig. 9). Different laser motion and laser parameters can be quickly selected through different weight settings. Fourth, the primary engraving/secondary engraving of the annular thin-walled milling cylinder parts of an aeroengine casing is realized. The accuracy error of the secondary laser engraving can reach 0.034 mm, meeting the process requirements of the secondary laser engraving accuracy that should be less than 0.05 mm.

This study investigates the key technologies of the laser engraving process, including laser engraving feature trajectory planning and automatic programming, collaborative optimization control of the laser engraving process, high-precision optical path flexible transmission and positioning, and optical electromechanical collaborative control system. The principle and engineering prototypes of the six-axis, five-linkage NC laser engraving machine tool are successfully developed, consequently providing the key technologies and the equipment support for solving the laser engraving problem of aerospace chemical milling structural parts. The key technologies of the laser engraving process and the six-axis, five-linkage NC laser engraving machine tool will not only solve the manufacturing problem of aerospace chemical milling parts, they can also be widely used in the fine manufacturing of 3D complex surfaces, which will effectively improve the performance and the manufacturing efficiency of major instruments and equipment.