Objective Many thin-walled cavity parts in the industrial and aerospace fields need to be processed with many microholes. As a non-contact processing method, laser processing has unique advantages in precision and low-damage processing of such thin-walled cavity structure; however, it causes a back strike problem. First, the materials near the back strike area will be damaged in the working state, leading to a chain reaction and reducing the comprehensive performance of thin-walled cavity parts, such as the turbine blades. It makes the severe back strike area become the key reason for the failure and fracture of multiple parts. Thus, reducing or even eliminating the back strike is of great significance to laser machining of thin-walled cavity parts. The back strike protection of laser machining of the semiclosed thin-walled cavity microhole, represented by a turbine blade, is a worldwide problem, and most current ones are limited to public patents, lacking in detailed process parameters and quantitative indicators. Laser processing back strike protection technology is the core technology of all countries. Thus, systematic experimental researches are urgently needed to investigate the formation mechanism and protection methods of the back strike in thin-walled cavity microhole laser machining, to identify the main factors affecting the degree of the back strike in laser machining and effectively reduce or even avoid back strike and improve the reliability of laser machining.
Methods Nanosecond laser drilling with the scanning filling method was used to process microholes in 7075 aluminum alloy. The microscopic morphology of the back strike pits was observed by a three-dimensional (3D) confocal laser scanning microscope. First, the effects of the machining parameters, such as the laser energy density, scanning speed of the galvanometer, and defocusing amount on the back strike, were investigated using a single factor control variable method. Afterward, several feasible back strike protection materials were selected through the CTQ verification tests, including polyurethane (PUR), polyacrylamide (PAM), paraffin wax, graphite, silicon dioxide (SiO2), and tungsten carbide (WC). Then, PUR, PAM, and paraffin were filled into the thin-walled cavity of 7075 aluminum alloy for the back strike protection test of the laser processing of the microholes.
Results and Discussions At the same energy density, the diameter of the back strike pit decreases with the increase in the cavity thickness. With the same cavity thickness, the pit's diameter increases with increasing energy density (Fig.8). The change trend of the pit depth was consistent with that of the diameter (Fig.9). The scanning speed has little effect on the entrance diameter of the pit. Under the same cavity thickness, the back strike depth decreases with the increasing scanning speed; however, the fluctuation range was not large. When the scanning speed increases, the spot overlap rate and laser removal ability will decrease, so as the depth of the pit caused by the back strike. Although the taper of the microholes is small, the low laser scanning speed may cause severe thermal damage (Table 6). The diameter of the back strike pit increases with the defocusing distance, which first decreases and then increases (Fig.11). The pit depth is mainly affected by the cavity thickness, and the negative defocusing distance is equivalent to reducing the cavity thickness. The pit depth increases as the defocusing distance changes from negative to positive (Fig.12). Through CTQ verification tests, PUR, PAM, and paraffin can be used as protective materials of the back strike of microhole laser processing (Table 7). Based on the back strike protection test of three protective materials of 7075 aluminum alloy triangular prism cavity filled with paraffin, PUR, and PAM, it is found that the protective materials play a significant role in the back strike inhibition compared with cavity processing without protective materials. The protective effect of flowing PAM was better than that of PUR, which in turn was better than that of paraffin. The protective effect of flowing PAM on the back strike is almost one order of magnitude higher than that of PUR and paraffin (Fig.14). Further back strike protection tests on 7075 aluminum alloy PAM with quadrilateral prism cavities showed that when the flow rate of PAM increased to 3.0 m/s, all cavities of 0.5--3 mm thickness had no back strike for laser processing within 50 s (Fig.16).
Conclusions At the same energy density, the diameter and depth of the back strike pit decrease with an increase in the cavity thickness. With the same empty cavity thickness, the diameter and depth of the back strike pit increase with an increase in the energy density. The scanning speed has little influence on the entrance diameter of the back strike pit. With the same cavity thickness, the back strike depth gradually decreases with an increase in the scanning speed; however, the fluctuation range is not large. The defocusing distance has a significant influence on the diameter of the back strike pit. When the focus is near the workpiece surface, the back strike is more severe. Proper processing under the state of forward defocusing can reduce the back strike. For 7075 aluminum alloy cavities, the depth of the back strike pit increases with the extension of processing time after filling paraffin, PUR, and flow PAM with 2.0 GW/cm 2 laser intensity and 0.5--6 mm thickness triangular or quadrangular prism cavity. The back strike protection effect of PUR is better than that of paraffin. Besides, the back strike protection effect of the flowing PAM is better than that of PUR and paraffin. At the same cavity thickness or processing time, the pit's depth decreases with the increase in the flow rate of PAM. When the flow rate of PAM reached 3.0 m/s, all cavities with a range of 0.5--3 mm had no back strike for 50 s. These studies are expected to provide a basis for the follow-up back strike protection methods by real-time monitoring of laser drilling status and turning off the laser on time. Besides, it will lay a foundation for the complete processing of microholes on the thin-walled cavity, such as the turbine blades without back strike.
