Titanium alloys are extensively used in the primary bearing structures of high-performance aircraft owing to their high specific strength, excellent corrosion resistance, and superior fatigue strength. Among these, the Ti-6Al-4V (TC4) titanium alloy is the most commonly used in aerospace applications. However, because of its low thermal conductivity and tendency to adhere to cutting tools, its machinability is relatively poor, resulting in high manufacturing costs. This severely limits the application of titanium alloys in the aerospace sector. Laser cutting offers several advantages over traditional methods, including a smaller heat-affected zone, reduced thermal deformation of the workpiece, and higher processing efficiency. However, the complexity of the cutting process parameters makes systematic testing essential for parameter optimization to achieve high cutting quality. Given the widespread use of medium-thickness titanium plates in aircraft primary bearing structures, conducting in-depth studies on the optimization of the laser cutting process is important.

Aiming at the laser cutting of 4 mm thick TC4 titanium alloy plates, a one-factor test is designed to select the key process parameters of laser power, defocusing amount, cutting speed, and auxiliary gas pressure, thereby studying the influence law and mechanism on cutting quality, such as kerf width, slag-hanging height, and abnormal tissue zone width. The kerf width and abnormal tissue zones are observed and measured using a metallurgical microscope, whereas the cut surface morphology and slag-hanging height are examined using a body microscope. Variations in hardness in the abnormal tissue zones are measured using a microhardness tester. Orthogonal tests are designed, and analysis of variance (ANOVA) and multiple regression methods are applied to analyze the effect of each process parameter. Prediction models for each quality evaluation index are developed; process parameters are optimized; and the accuracy of the prediction models is validated through experiments.

In studying the process parameter effects on cutting quality, when nitrogen is used as an assistive gas, the cut surface appears greyish-black (Fig. 8). From top to bottom, the surface is divided into three zones: smooth, rough, and slag-hanging (Fig. 9). The variation in the width of the smooth zone is primarily due to the combined effects of heat input and melt blowing (Fig. 11). After laser cutting, the remelted and heat-affected zones and the matrix show a significant decrease in microhardness, primarily because of the martensitic phase transformation at the slit edge (Fig. 12). The optimal process parameters for the narrowest cutting slit, as predicted by the regression equation fitted through response surface analysis (Table 6), are: laser cutting power of 2563 W, defocusing amount of -4 mm, cutting speed of 0.06 m/s, and gas pressure of 1.1 MPa. The optimal parameters for minimizing slag-hanging height are: laser power of 3060 W, defocusing amount of -5 mm, cutting speed of 0.12 m/s, and gas pressure of 1.8 MPa. The optimal parameters for minimizing the abnormal tissue zone are: laser power of 3091 W, defocusing amount of -5 mm, cutting speed of 0.12 m/s, and gas pressure of 1.7 MPa.

The test results indicate that the process parameters that most significantly affect the kerf width are laser power and defocusing amount, whereas gas pressure and laser power have the greatest influence on the slag hanging height. Cutting speed and defocusing amount are the factors most affecting the abnormal tissue zone. Based on the orthogonal test results, a regression model is developed for the kerf width, abnormal tissue zone width, and slag-hanging height, to accurately predict the optimal cutting quality process parameters. The resulting optimal parameter combinations lead to the narrowest kerf width of 0.214 mm, smallest slag-hanging height of 0.017 mm, and smallest abnormal tissue zone width of 180.88 μm. These findings provide valuable guidance for improving the laser cutting quality of medium-thickness titanium alloys.