Full length articleResearch on the interaction between surface laser-pit of Ni-based single crystal alloy and lamb wave under micro-conditions
Introduction
In the field of military and civilian engines, it often occurs that the Ni-based single crystal alloy turbine blades suddenly break during the flight. The main reason is that the research on remaining service life of Ni-based single crystal alloy is not deep enough, which is mainly concentrated on the late stage of macroscopic defects at present. In the late service stage of Ni-based single crystal alloy, the sudden change of its mechanical properties show a strong nonlinearity. The late stage is very short and difficult to define. Even if it is too late to evaluate the remaining service life, the turbine blades have broken. In the future, it is highly necessary to study how to accurately predict the remaining service life of Ni-based single crystal alloy at an earlier stage, which is important for improving flight safety and reducing flight accidents. The damage status of Ni-based single crystal alloy determines its remaining service life. However, there have been no reports on the damage status of Ni-based single crystal alloy at the early and mid-term service stages. Therefore, this study will establish a fast and non-destructive nonlinear quantitative evaluation method to promote the damage evaluation of Ni-based single crystal alloy from the late macro-defect stage to the early micro-defect stage.
The nonlinear ultrasonic Lamb wave have high propagation efficiency and can penetrate deeply into the material, which provides a new method for effectively evaluating the damage status of Ni-based single crystal alloy. However, the nonlinear harmonic components [[1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]] generated by nonlinear ultrasonic Lamb wave [12] during propagation is very complicated, which makes the theoretical research extremely difficult. Reference [13] proposes that only when the phase velocity of the fundamental frequency Lamb wave is equal to that of the second harmonic Lamb wave, and the energy flow is transmitted from the fundamental frequency mode to the harmonic mode, can the Lamb wave with accumulation effect be excited. There are a few studies on the propagation characteristics of Lamb wave in materials. The propagation characteristics of Lamb wave in HP40Nb alloys have been reported in [13,14], which also has been less studied in other metals [[15], [16], [17], [18], [19], [20], [21]]. However, the propagation characteristics of Lamb wave in Ni-based single crystal alloy have not been reported yet.
Based on the existing research results [13], this study will establish a theoretical relationship that can reflect the interaction between the microstructure evolution of the Ni-based single crystal alloy and the Lamb waves. Microscopic defects of different specifications and sizes are actively produced on the surface of Ni-based single crystal alloy by means of laser cratering. The Ni-based single crystal alloy samples with different damage levels are experimentally measured, and the damage statuses of different samples are quantitatively characterized by nonlinear parameters. Finally, the molecular dynamics method is used to quantitatively calculate the dislocation density, dislocation chord length and stress of Ni-based single crystal alloy, of which the change law can assist to analyze the variation law and cause of nonlinear parameters.
Section snippets
Experiment and simulation
The mechanical properties of different nickel-based alloys have been extensively studied [[22], [23], [24], [25], [26], [27], [28], [29]], the sample used in the experiment was DD6 Ni-based single crystal alloy, and a femtosecond laser was used to make micropits on the surface of the sample. There were five samples with pore diameters of 0 μm, 10 μm, 23 μm, 37 μm, 50 μm, respectively, and each sample has 100 micropits. The elastic modulus (E) of DD6 Ni-based single crystal alloy is [30,31]
Results and discussion
The phase velocity dispersion curve of the Ni-based single crystal alloy is shown in Fig. 1a, and the group velocity dispersion curve is shown in Fig. 1b. The number of modes of the dispersion curve is 6, A is an antisymmetric mode, and S is a symmetric mode. It can be observed that when the excitation frequency is 2.05 MHz, the A1 mode phase velocity is equal to the A2 mode phase velocity, and the A1 mode group velocity is equal to the A2 mode group velocity. The corresponding phase velocity
Conclusions
In this paper, a theoretical relationship that reflects the interaction between the microstructure evolution of Ni-based single crystal alloy and the Lamb wave is established. Through experimental measurements, it is found that in the early damage stage of Ni-based single crystal alloy, the second harmonic amplitude and nonlinear characteristic parameter enlarge with the increase of micropit diameter, indicating that the second harmonic amplitude and nonlinear characteristic parameter increase
Acknowledgements
This work was supported by the National Natural Science Foundation of China (No. 51210008), the Fundamental Research Funds for the Central Universities (WUT: 2018IVA023) and the Open Foundation of State Key Laboratory of Materials Processing and Die & Mould Technology, Huazhong University of Science and Technology (No. P2019-020).
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