Multiphysical Analysis of Thermo-Elastic Effect in Laser-Material Interaction

Abstract

Thermo-elastic effect in laser-material interaction is of great interest for current industries because of its advantage of producing mechanical effect non-contactually. Phenomena induced by the thermo-elastic effect are cracking and thermo-elastic wave generation. Cracking can occur when the thermal stress is greater than the yield stress. A thermo-elastic wave can be generated by the fast variation of thermal stress when the irradiation time is very short (generally less than a microsecond). However, the thermo-elastic effect has not been sufficiently analyzed yet. In particular, laser-induced slipping, which is a kind of cracking phenomenon in non-metallic materials of crystal structure such as silicon, has been seldom studied. Furthermore, numerical model for laser-induced slipping has not yet been developed. Therefore, in this thesis, the multiphysical model to simulate laser-induced slipping was developed. In this model, volume absorption and temperature-dependent properties were considered. Directional shear stress and temperature-dependent yield stress were calculated to predict the slip occurrence with initiation time. In order to verify the model, the slip occurrence as a function of initiation time was experimentally measured using a laser scattering technique. In the experiments, a silicon wafer was irradiated by a continuous near infrared laser. In terms of thermo-elastic wave generation, the previous models are limited to the conditions of Gaussian spatial beam profile and constant material properties. However, when a slit mask is used to produce a line laser beam, the spatial beam profile irradiated on the surface is not Gaussian but almost square-like. Thus, it is necessary to consider a square-like spatial beam profile in this case. In addition, consideration of temperature-dependent material properties is required in order to match with real situations. Therefore, in this thesis, the multiphysical model was developed to simulate the thermo-elastic wave generation to take into account the square-like spatial beam profile and temperature-dependent material properties also. By using this model the thermo-elastic surface wave was simulated. In order to verify the model, the thermo-elastic surface wave was theoretically analyzed and experimentally measured. In the experiments, an aluminum alloy specimen was irradiated by a Nd:YAG pulsed laser and the thermo-elastic surface wave was detected using a piezoelectric transducer. Consequently, the multiphysical models were suitable for the analysis of laser-induced slipping and thermo-elastic wave generation induced by thermo-elastic effect in laser-material interaction. It is expected that the models can be used for laser-induced slipping prediction and thermo-elastic wave prediction and visualization.|레이저와 물질 상호작용에서의 열 탄성 효과는 비 접촉으로 기계적 효과를 발생시킬 수 있는 장점이 있어 현재 산업 분야에서 주목을 받고 있다. 열 탄성 효과에 의해 발생하는 현상은 크래킹과 열 탄성파 발생으로, 크래킹은 열 응력이 항복 응력보다 커질 경우에 발생하고, 열 탄성파는 펄스 레이저 조사에 의한 열 응력의 빠른 변화에 의해 발생한다. 그러나 열 탄성 효과에 대한 연구는 아직 미비한 실정으로, 특히 크리스탈 구조의 비금속 물질에서 발생하는 크래킹의 일종인 슬리핑에 대해서는 거의 연구되지 않았고 또한 해석 모델도 개발되지 않았다. 따라서 본 연구에서는 레이저 조사에 의한 슬리핑을 해석할 수 있는 다중물리 해석 모델을 개발하였다. 해석 모델에서는 볼륨 흡수와 온도의존적인 물성이 고려되었으며, 슬립 발생 및 발생 시간을 예측하기 위해 각 방향으로의 전단 응력과 온도의존적인 항복 응력이 계산되었다. 해석 모델을 검증하기 위해, 근적외선 레이저 조사에 의한 실리콘 웨이퍼에서의 슬립 발생 시간을 레이저 광 산란 기법을 이용하여 실험적으로 측정하였다. 열 탄성파 발생에 대해서는, 기존의 해석 모델들은 레이저 빔의 공간적인 세기 분포를 가우시안 그리고 고정된 물성으로 해석하는 모델들로 사각의 레이저 빔 분포 그리고 온도 의존적인 물성을 고려하지는 않았다. 그러나 라인 빔을 발생시키기 위해 슬릿 마스크가 이용될 경우에는 대상 재료에 조사되는 레이저 빔의 공간적인 세기분포가 사각이므로, 이 경우에는 사각의 레이저 빔 분포를 고려할 필요가 있다. 또한 보다 정확한 해석 결과를 얻기 위해 온도 의존적인 물성을 고려하는 것이 필요하다. 따라서 본 연구에서는 이러한 사각의 레이저 빔 분포 그리고 온도의존적인 물성을 고려한 열 탄성 파 발생 해석 모델 또한 개발하였으며, 해석 모델을 이용하여 대표적으로 열 탄성 표면파를 해석하였다. 해석 모델을 검증하기 위해, 펄스 레이저 조사에 의한 알루미늄에서의 열 탄성 표면파를 공진형 압전 트랜스듀서를 이용하여 실험적으로 측정하였다. 결과적으로, 본 연구에서 개발된 해석 모델들은 열 탄성 효과에 의한 슬리핑과 열 탄성파 발생을 분석함에 있어 적합하였으며, 슬립 발생 예측과 열 탄성파 예측 및 가시화에 활용이 가능할 것으로 예상된다.; Thermo-elastic effect in laser-material interaction is of great interest for current industries because of its advantage of producing mechanical effect non-contactually. Phenomena induced by the thermo-elastic effect are cracking and thermo-elastic wave generation. Cracking can occur when the thermal stress is greater than the yield stress. A thermo-elastic wave can be generated by the fast variation of thermal stress when the irradiation time is very short (generally less than a microsecond). However, the thermo-elastic effect has not been sufficiently analyzed yet. In particular, laser-induced slipping, which is a kind of cracking phenomenon in non-metallic materials of crystal structure such as silicon, has been seldom studied. Furthermore, numerical model for laser-induced slipping has not yet been developed. Therefore, in this thesis, the multiphysical model to simulate laser-induced slipping was developed. In this model, volume absorption and temperature-dependent properties were considered. Directional shear stress and temperature-dependent yield stress were calculated to predict the slip occurrence with initiation time. In order to verify the model, the slip occurrence as a function of initiation time was experimentally measured using a laser scattering technique. In the experiments, a silicon wafer was irradiated by a continuous near infrared laser. In terms of thermo-elastic wave generation, the previous models are limited to the conditions of Gaussian spatial beam profile and constant material properties. However, when a slit mask is used to produce a line laser beam, the spatial beam profile irradiated on the surface is not Gaussian but almost square-like. Thus, it is necessary to consider a square-like spatial beam profile in this case. In addition, consideration of temperature-dependent material properties is required in order to match with real situations. Therefore, in this thesis, the multiphysical model was developed to simulate the thermo-elastic wave generation to take into account the square-like spatial beam profile and temperature-dependent material properties also. By using this model the thermo-elastic surface wave was simulated. In order to verify the model, the thermo-elastic surface wave was theoretically analyzed and experimentally measured. In the experiments, an aluminum alloy specimen was irradiated by a Nd:YAG pulsed laser and the thermo-elastic surface wave was detected using a piezoelectric transducer. Consequently, the multiphysical models were suitable for the analysis of laser-induced slipping and thermo-elastic wave generation induced by thermo-elastic effect in laser-material interaction. It is expected that the models can be used for laser-induced slipping prediction and thermo-elastic wave prediction and visualization.