Temperature Rise, Stress, Fluctuation Of Stress And Evolution Of Plastic Zone During Laser Heating Of Metal Plate

Calculation models of temperature rise, stresses and deformation are established to analyze and calculate some important problems during laser heating of metal plate by using finite element method (FEM).Based on heat transfer theory and FEM, a calculation model of temperature rise during laser heating of a metal plate is established to compute the temperature rise during the metal plate heated by the laser beam of which the energy has different spatial and temporal distributions. The reasons for the difference of temperature rise are analyzed. The advantages of the heating efficiency of long-pulsed laser with high energy are indicated. The absorptivity of the metal material is determined by solving inverse heat transfer problem. The temperature rise results with different absorptivity models are compared. The results shows that when both solid state and liquid state are induced by laser heating the assembled constant absorptivity model has high precision and is more convenience and efficient to solve the temperature rise problem.A spacial axisymmetric finite element calculational model is established to calculate temperature rise and thermal stresses during static laser heating of a metal plate by using thermal elasto-plastic constitutive relation and incremental FEM. The influence of the combined action of thermal expansion and thermal softening on thermal stresses is analyzed. "Cyclo-hoop effect" is put forward to explain distribution characteristics of thermal stresses induced by Gaussian laser beam. A three-dimensional calculational model is established to calculate temperature rise, thermal stresses and deformation during moving laser scanning heating of a metal plate by using thermal elasto-plastic constitutive relation and incremental FEM. The reasons for the distributions and variation of stress field are represented by using the theory of "cyclo-hoop effect".Stresses appear fluctuation during static laser heating of the metal plate. It is found that the fluctuation is induced by the gradual aggravation of thermal expansion and thermal softening, and is closely related to the evolution of plastic zone in the metal material. According to the process that the plastic zone comes forth, outspreads gradually and crosses the depth of the metal plate, plastic penetration is put forward to describe this process. Such as plastic penetration time, plastic affected zone, depth and width of plastic affected zone are defined to quantitatively describe the affected range and the process of development and evolution of plastic zone during laser heating of a metal plate.The reasons for the stress fluctuation during laser scanning heating of the steel plate under the temperature gradient mechanism are analyzed and investigated. It is found that the stress fluctuation is due to the continuous changes of the "cool zone" and the "hot zone" induced by the moving of laser spot and cooling contraction of the material after scanning. Stress fluctuation and the development and evolution of plastic zone appear simultaneously. Therefore the distribution characteristics of plastic strain and its evolution behavior with the moving of laser heat source are obtained by our simulation calculation.These research results in this paper will provide the theoretical basis for the analysis and prediction of temperature rise, stress and deformation during laser heating of metal plate and, furthermore, deepen the understanding and recognition of the thermal behavior and the mechanical phenomenon during laser heating of a metal plate, which enhance application efficiency and improve the laser machining accuracy.