Thermal And Mechanical Effect During Millisecond Laser Heating Of Metals

Series of thermal and mechanical effect will occur during millisecond laser heating of metals. In this dissertation, the plastic damage, melting and hole formation processes are investigated by theoretical calculation, finite element simulation and experiment.By means of experimental and numerical simulation, the plastic yielding process of metals induced by millisecond laser is studied. The reverse bulging in a thin aluminum plate is detected. According to which, this deformation is simulated by the finite element method (FEM) based on the heat conduction equation and thermal elasto-plastic constitutive relation. And the numerical algorithm is verified by comparing the experimental and the calculated results. Furthermore, a physical model of the millisecond laser interaction with an aluminum alloy slab is established. By using of FEM, the distributions of the temperature field and the stress field are studied numerically. In particular, the yielding time, the range of plastic damage region and the magnitude of residual stress are obtained. And the plastic damage for Gaussian and Top hat laser are compared according to these three results.A semi-infinite axisymmetric model is established for millisecond laser melting of a metal slab. The analytical solutions of the entire temperature filed and the melting depth are obtained based on the heat conduction theory and the distribution of temperature field. The molten pool obtained in calculation agrees with the experiment results. Moreover, the melting depth versus the laser pulse width is investigated. It is found that with the increasing of the pulsed width, the depth increases at first and reaches the maximum value, then turn to decrease. This phenomenon is discussed.An upward drilling method is proposed. In the experiment, the laser beam is designed to transmit along the opposite direction of the gravity and drill hole at the bottom of an aluminum slab. Based on the experiment and the temperature solution, the analytical solution of hole depth is obtained. For further verifying the gravity action, the downward (along the gravity direction) and the upward drilling cases are carried out in experiment. Meanwhile, the removed mass of molten material, the adherent dross and the hole volume are compared. The results show that more molten material is expelled with the assistance of the gravity, and more laser energy is used to melt the aluminum slab in the upward drilling. In a word, it is more efficient to drill hole upwardly. Thereafter, the hole depth versus laser energy is studied. It is found that the depth with high energy is deep and the defocusing effect should be considered in the calculation. In addition, the modified solution is used to calculate the drilling speed of millisecond laser for different metals.The model of light propagation in a hole during laser drilling is established. The total absorptance and absorbed intensity distribution inside the hole are calculated with the ray-tracing and physical optics methods respectively. The comparison of the results show that the ray-tracing method is not applicable if the diffraction at the hole entrance is important, or the hole diameter is smaller than the wavelength of the incident beam. Particularly, the influence of diffraction depends on the polarization state, the incident angle, the beam waist and the relative intensity on the hole entrance.The research consequences may offer theoretical and experimental references to the further study of the temperature rising, the thermal stress, the plastic damage, the melting and the formation of hole, and accelerate the application of millisecond laser in manufacturing and military.