| Femtosecond lasers have received wide attentions in the high-end equipment manufacturing industry due to their excellent processing performance.The high-precision micro-hole machining including film holes on turbine blades of aero-engine and the nozzle hole on the automotive engine can be realized by femtosecond lasers.However,in the actual hole drilling process of femtosecond lasers,there still are problems,e.g.,low efficiency,unstable processing quality,and the lack of model guidance for the selection of processing parameters,which restricts its application in engineering.In this paper,the simulation and experimental research of femtosecond laser helical drilling are carried out for the problems mentioned above.A numerical simulation model for describing the helical drilling process and a mapping relationship model between the main machining parameters and the ablation depth of the material are established,and the rules of the micro-hole forming during the helical drilling process are revealed.The research results provide a theoretical basis and model guidance for the femtosecond laser helical drilling process,and have an important theoretical significance and engineering application values for the high-precision and high-efficiency machining of micro-holes.The main research contents and innovations of this paper include:1_The beam characteristics of the Gaussian femtosecond laser and its main ablation mechanism during the interaction with metal materials are analyzed.The two-temperature model describing the interaction process between the femtosecond laser and metal materials and its shortcomings in describing the helical drilling process are discussed.Based on the existing two-temperature model,the helical motion equation with focus on the plane and the velocity control term in the direction of beam propagation are added to the laser source term,which helps build an improved two-temperature model to describe the femtosecond laser helical drilling process.The optimized model improves the shortcomings of the existing numerical model in describing the femtosecond laser helical drilling process.2.Based on the proposed improved two-termperature model and the critical point phase separation model,copper is used as the processing material,and there are three parameters simulated,i.e.,the single pulse energy,the repetition rate,and the rate of focus down in the drilling process.The ablation depth curves of different single-pulse energies,different repetition rates,and different rates of focus down reveal the influence of various parameters on the ablation depth.Moreover,the single-pulse ablation depth at different energy densities and the results shown on the literature are compared to verify the rationality of the proposed simulation model.3.We use the 304 stainless steel as the target material,select main parameters of helical drilling by the femtosecond laser,and design orthogonal experiments with 5 factors and at 5 levels.The significance levels of ablation depth affected by 5 main parameters,i.e.,the single pulse energy,the repetition rate,the rotation rate,the rate of focus down,and the air blowing pressure,are analyzed.Based on the back propagation(BP)neural network,we establish the relationship model between the 5 main parameters and the ablation depth of the laser drilling.The test results show that the prediction error of the relationship model is less than 3%.Moreover,the single factor experiment is designed to analyze the influence of the rate of focus down during the helical drilling.The effect of different rates of focus down on the hole-forming during the femtosecond laser helical drilling process is presented.4.The nickel-based single crystal superalloy DD6 is used as the target and the single factor experiment is designed with the single pulse energy,the rate of focus down,and the spot scanning trajectory as variables.The effects of various parameters on the drilling process and micro-hole morphology parameters are revealed. |
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