Laser Pulse Heating of Surfaces and Thermal Stress Analysis

Laser heating offers considerable advantages over the conventional methods such as precision of operation, local treatment, and low cost. Laser at high intensity when interacts with the solid surface, the absorption takes place. This in turn causes internal energy gain of the substrate material and heat release from the irradiated region. Since the process, in general, is fast, temperature gradients remain high in the irradiated region. This results in high thermal strain and thermally induced stresses in this region. Moreover, in laser treatment process, the end product is important from the application point view. The high stress levels formed in the irradiated region may cause failure of the surface through stress induced cracking. Consequently, a care needs to be taken during the laser treatment process. This chapter provides the information about the importance and limitations of the laser treatment process in terms of the thermal stresses formed in the irradiated region.

When the heating duration becomes greater than the thermalization time of the substrate material, equilibrium heating takes place in the laser irradiated region. In this case, the classical Fourier heating law governs the energy transport. Although the heating process is complicated, some useful assumptions enable to obtain the closed form solution for temperature and stress fields. Since the analytical solution provides the functional relation between the dependent variable and the independent parameters, it provides better physical insight into the heating problem than that of the numerical analysis. In this chapter, equilibrium heating of solid surfaces heated by a laser beam is considered. The closed form solution for the resulting temperature and stress fields are presented for various heating situations. The study also covers the phase change taking place at the irradiated region during the laser treatment process.

Laser short pulse heating of metallic surfaces initiates non-equilibrium energy transport in the irradiated region. In this case, thermal separation of electron and lattice sub-systems takes place. The thermal communication of these sub-systems occurs through the collisional process and the electrons transfer some of their excess energy during this process. Although electron temperature attains significantly high values due to the energy gain from the irradiated field through absorption, lattice site temperature remains low. Since the heated region is limited within a small volume, temperature gradients remain high across the irradiated region despite the attainment of low temperature field. Consequently, high temperature gradients cause the development of high thermal stress field in the small region. This limits the practical applications of the laser treatment process at microscopic scales. In this chapter, heat transfer at micro-scale is formulated and temperature field is presented analytically. The closed for solutions for the temperature and stress fields are obtained for various heating situations.

Laser ultra-short pulse heating of metallic surfaces causes the hyperbolic behavior of energy transport in the heated region. The consideration of the parabolic nature of the non-equilibrium heating situation fails to formulate the correct heating process. Although heating duration is ultra-short, material response to the heating pulse is not limited to only heat transfer and the mechanical response of the heated surface also becomes important. Consequently, mechanical response of the surface under ultra-short thermal loading becomes critical in terms of the generation of the high stress levels. In this chapter, hyperbolic behavior of heat transfer is introduced in the laser heated region. The closed for solutions for the temperature and stress fields are obtained for various heating situations. Two-dimensional effect of heating on temperature rise is also considered for nano-scale applications.

Thermal stress developed in the laser irradiated material is governed by laser and workpiece materials. The main laser parameters include the pulse length and the laser power intensity while the important material properties are the thermal conductivity, absorption depth, elastic modules, thermal expansion coefficient, and Poisson’s ratio. Depending upon the duration of the laser pulse, the heating process can be classified into two categories, which are equilibrium and non-equilibrium heating situations. In the case of equilibrium heating, duration is longer than the thermalization time of the substrate material and Fourier heating law governs the heating process. However, laser pulse durations comparable and shorter than the thermalization time of the substrate material, wave behavior of stress field takes place and the hyperbolic nature of the governing equations are used to account for the wave behavior. Consequently, care must be taken to formulate the heating and thermal stress problems in line with the physical aspects and scale of the problem. The conclusions derived from the body of this book are presented according to the following sub-headings and in line with the previous studies [1–18].