Chenyin Ni

Numerical simulation of heat transfer and fluid flow in laser drilling of metals

Laser processing as laser drilling, laser welding and laser cutting, etc. is rather important in modern manufacture, and the interaction of laser and matter is a complex phenomenon which should be detailed studied in order to increase the manufacture efficiency and quality. In this paper, a two-dimensional transient numerical model was developed to study the temperature field and molten pool size during pulsed laser keyhole drilling. The volume-of-fluid method was employed to track free surfaces, and melting and evaporation enthalpy, recoil pressure, surface tension, and energy loss due to evaporating materials were considered in this model. Besides, the enthalpy-porosity technique was also applied to account for the latent heat during melting and solidification. Temperature fields and melt pool size were numerically simulated via finite element method. Moreover, the effectiveness of the developed computational procedure had been confirmed by experiments.

Study of Dual-Laser-Generated Surface Acoustic Waves Interacting with Multiangled Surface Breaking Cracks by Finite Element Method

In this paper we study the situation of a laser line source directly irradiating a crack modeled by a slot, which is an essential process in the scanning laser line source (SLLS) technique. The finite element method (FEM) was used to calculate the scattering of an ultrasonic wave. The propagation paths of ultrasonic waves were analyzed and corresponding waves were identified accordingly. The displacements for various slot orientations were then calculated and analyzed, and it was shown that the arrival times of some ultrasonic wave modes can be used for detecting the slot orientation. As a result, the relationship between the slot orientation and the ultrasound diffraction pattern was clarified.

Numerical simulation of evaluation of surface breaking cracks by array-lasers generated narrow-band SAW

Most of the factors limiting the extensive application of laser-based ultrasonic for nondestructive evaluation of surface breaking crack are its poor sensitivity, low efficiency relative to conventional contact ultrasonic methods and limit on the dimension of the cracks. For this reason, a new technique that multiplepulse narrow-band ultrasound generated by laser arrays has been proposed. It is found that crack detection dependent on spectrum of narrow-band ultrasound generated by laser arrays can be operated with low amplitude requirements. In this paper, the narrow-band ultrasound generated by pulse laser arrays interacting with surface breaking cracks has been simulated in detail by the finite element method (FEM) according to the thermoelastic theory. The pulsed array lasers were assumed to be transient heat source, and the surface acoustic wave (SAW) which propagating on the top of the plate was computed based on thermoelastic theory. Then the frequency spectrums of both reflected waves by crack and transmission ones through crack were compared with the direct waves. Results demonstrate that multiple-frequency components of the narrow-band ultrasound were varied with change of the depth of surface breaking cracks significantly, which provides the possibility for precise evaluation of surface breaking cracks.

Measurement of velocity distribution of Laser-generated Rayleigh wave on welded structure

A new method based on laser-generated ultrasound and piezoelectric transducer (PZT) is proposed to measure the velocity distribution on welded metal structure. High-frequency Rayleigh waves are excited by the Nd: YAG pulsed laser and probed by self-made transducer. A serial of ultrasonic pulses can be detected on the surface of the sample by the transducer through the scan of the line source with translation stage. The waveform cross-correlation technique is applied to compute the propagation velocity of Rayleigh waves. Then analogically, a series of wave velocities at different positions are detected, by which the distribution of velocities is obtained. It is found that high frequency wave signals excited by laser line pulse can be probed effectively using the PZT, and results indicate that this method can provide the basis for precision detection with quick scanning and the reliable measurement of velocity distribution.