Study on Evaluation of Melting Conditions of Irradiation Part by YAG Laser (2nd Report, Relationships between Features of AE Signals and Melting Conditions by Irradiation Conditions)

The laser welding methods are widely used for manufacturing the products that the high quality welding conditions are required. Therefore, the establishment of the evaluation method for the melting conditions is desired. In the previous paper, the method by using the features of detected AE signals, when the irradiation energy was changed at the pulse width, was proposed. The results showed the following: (1) The frequency range of the signal of melting is near 300 kHz. (2) The correlation coefficients of the regression equations that evaluate the melting conditions from the AE energy of transversal wave components showed the high value. (3) The high correlations were recognized with the equations that evaluate the conditions by the led largest Lyapnov exponents from the wavelet transform results of the longitudinal wave component. However, it was proven that the frequency range of the thermal shock wave overlaps to the range of the melting, when the temperature rise rate lowers with the decrease of the lamp input voltage. Therefore, the evaluation method in the previous paper is not able to use in case of the evaluation of the melting conditions under the condition for changing the irradiation energy at the voltage. The method for evaluating the melting conditions, when the irradiation energy is changed by the voltage and the pulse width, is proposed in this paper. Concretely, the detected signals in the range (i.e. after 2 ms from the irradiation completion) that the influence of the thermal shock wave fades out, and the temperature of the irradiation part keeps the over melting point are analyzed with chaos and wavelet transform analysis method. The summaries of the result in this study are like the following. The correlation coefficients of the regression equations that evaluate the melting conditions from AE parameters showed the high value. One of the parameters is the correlation dimension Cd of transversal wave component, and the rest are the largest Lyapnov exponent LLy and the correlation dimension Cd of the time fluctuation waveform on the energy rate of the wavelet coefficient of longitudinal wave component. From the above results, it was shown to be possible that the melting conditions can be evaluated with the proposed method in this study, when the irradiation energy is changed by the lamp input voltage and the pulse width.