SONG Yuxiang, DONG Lan, XU Liandi, BIE Qingfeng, YIN Xianxin, XIN Haiyuan, CHEN Shouhai, LI Guanqun, LIU Dewei, XU Wenhao, WANG Xiaoming, YANG Min, LI Changhe
The significant increase in global vehicle fleet annually calls for solutions to address petroleum resource depletion and environmental pollution caused by automobiles. Lightweight technology is crucial for achieving decarbonization and emission reduction. Thin-walled components, which are characterized by their low weight, ease of formation, high aerodynamic efficiency, and resource utilization, demonstrate broad application potential in the aerospace, automotive, and related industries. As a lightweight yet high-strength metallic material, 6061 aluminum alloy has been widely adopted in manufacturing complex thin-walled structures because of its exceptional advantages in terms of weight reduction, ductility, formability, weldability, corrosion resistance, machinability, and cost-effectiveness. Milling, as a core forming process in the manufacturing of thin-walled lightweight components, directly determines the assembly accuracy and service reliability of key components. The low elastic modulus and geometric characteristics of thin-walled structures synergistically endow them with significantly reduced dynamic stiffness, thus rendering them highly susceptible to regenerative chatter during machining. This issue results in dimensional deviations and surface-morphology degradation, thereby severely constraining the lightweight progress and service safety of critical components. Ultrasonic-assisted milling based on interrupted cutting effects and tool-geometry optimization is an effective solution for machining stability control. However, compared with ferrous alloys such as steel, aluminum alloys feature high plasticity, which suppresses the chip-breaking capability of conventional milling tools under high-frequency ultrasonic vibration, thus severely constraining the lightweight advancement and in-service reliability of critical components. The implementation of variable helix angle milling cutters for achieving chip fracture through multi-edge cutting is promising for overcoming this technical bottleneck. However, the universal principles governing instantaneous milling forces under the coupled effect of ultrasonic vibrations and tool structure remain insufficiently understood, thus hindering the efficient theoretical prediction of milling forces. Hence, an instantaneous milling force model was developed for thin-walled workpiece machining using two distinct milling cutters with ultrasonic assistance. Machining experiments were conducted to compare the cutting-force performance of conventional milling, conventional cutters with ultrasonic vibration, and unequal helix angle milling cutters with ultrasonic vibration under identical machining parameters, which validated the accuracy of the milling force model. First, using a discretization approach, an instantaneous milling force model was established by incorporating thin-walled workpiece deformation, stiffness enhancement from ultrasonic effects, interrupted cutting mechanisms, and the structural features of an unequal helix milling cutter. Subsequently, numerical analysis was performed, which revealed the influence mechanisms of variable helix angle ultrasonic milling on the cutting forces. Next, milling experiments were conducted under diverse conditions to investigate the performance of conventional and variable helix angle cutters at different ultrasonic frequencies. Additionally, considering the unique structural design of the unequal helix angle milling cutter, the instantaneous milling force model was further optimized. The cutting-force coefficients of the unequal helix angle milling cutter were determined using the milling slope transformation method. The results showed that the unequal helix angle milling cutter did not significantly reduce the cutting forces as the ultrasonic frequency increased further. Even without ultrasonic assistance, the milling forces reduced significantly and the surface quality improved. Compared with conventional milling cutters, the unequal helix angle milling cutter reduced Fx, Fy, and Fz forces by 11.92%, 9.31%, and 6.53%, respectively, with a total force reduction of 10.22%. Ultrasonic-assisted milling performed using an unequal helix angle milling cutter significantly reduced the cutting forces and improved the surface quality. Compared with conventional milling, at an ultrasonic frequency of 40 kHz, reductions of 45.63%, 42.13%, and 32.87% in Fx, Fy, and Fz forces were achieved, respectively. Based on the instantaneous milling force models of both tools under ultrasonic assistance, numerical analysis was performed using Matlab. Comparative results with experimental data showed prediction errors of 7.32% , 16.54%, and 11.46% for Fx, Fy, and Fz, respectively, relative to conventional ultrasonic-assisted milling, whereas the unequal helix angle milling cutter and ultrasonic-assisted milling exhibited errors of 6.67% for Fx. The strong agreement between the experimental and predicted results validates the effectiveness of the model, thus providing theoretical and technical support for the actual machining and production of lightweight thin-walled automotive components.
