The Experimental And Theoretical Investigations Of Laser Hardening Treatment Temperature Field

Since high power laser was appeared in 70 years of twenty century, laser surface modification is widely used in industry. After twenty year development, except laser surface modification, laser cutting, drilling, welding, keyhole welding, fast molding technologies etc, are widely applied in industry manufacture. For the laser industry applications, no-melting laser surface modification is a facility-controlled technology. As laser heat treatment is a very complex procedure, there are too many physical phenomena (heat absorption, thermal transfer, phase transformation) take place during the procedure, and it is the result of too many factors co-effects. For laser heat treatment, there are four main group parameters which will influence the treatment results: the first sets physical properties: such as the phase transformation temperature, thermal conductivity, and volumetric specific heat, melting point etc. The second set parameters used to describes the performance of the laser: wavelength, power density distribution, and polarization. The third set parameters used to describes the interaction mode of laser material: different frequency and different pulse wide pulse radiation and different scanning mode; The fourth set parameters used to describes laser material interaction parameters: such as material surface energy absorption coefficient for the given wavelength and given polarization of laser beam; the laser induced position, workpiece shape and the thermal conductive condition which determined by material thermal physical characters. Normally, many parameters discussed above not only are the function of temperature, but are determined difficulty for a given practical thermal treatment condition Furthermore, although gained these parameters, it is a very complicated question to describe the practical boundary condition mathematically and precisely. So up to now, an optimization laser machine process or laser heat treatment process is fulfilled by summing-up the experiment rule. It is negative to the further application of this technology.In fact, the essential of control of laser heat treatment is to control the heat effect time of treated material. For a long time, many researchers never stopped to study the optimization control of laser heat treatment. In the research of laser heat treatment, the continuous medium heat conductive equation which derived from Fourier law is the basic theory to be adopted usually to calculate the laser heat effect temperature field by many researchers. Whereas, as the laser material surface treatment is too complicated, in principle, a precise calculation of temperature field practically reflecting the laser heat treatment results can but can be finished by the pure numerical calculation methods such as FEM(Finite Element Method), FDM(Finite Difference Method) etc. Without doubt, the pure numerical calculations are important and helpful to disclose the laser material interaction rule; however they are difficult to be used in practical application due to their lengthiness calculation time and high calculation cost.In order to simplify the research, in the solution of heat conductive equation, the laser beam is usually being regarded as basic Gauss beam or other form beam that is convenient for analysis; and the laser beam irradiation object is looked as half infinite body with constant physical properties. Based on above hypothesis, some easy calculation expressions are derived. As the simplified model is too far from the practical situation, it cannot be used in practical application. So the method that can be practically used to predicate and optimize the experiment result still is a very important subject for the researchers.Kunming University of science and technology of has engaged in cooperation research in the field of laser material interaction with several famous universities of France for nearly 20 years, and has gained many fruitful results. In the cooperation research, from the analytical solution of thermal conductive equation in infinite continuous medium, strictly considerate the practical power density distribution and practical workpiece shape, we have obtained a complete set of semi-analytic calculation method which fulfill the practical precision need. Especially, after FFT is applied in the analytical solution calculation of thermal conductive equation, the laser heat effect temperature process of calculation is tremendously improved. The success engaged the attention of internal and abroad researcher as its practical calculation ability. In the thesis, the semi-analytic calculation is compared with the equivalence precise pure numerical calculation method; and the main results of semi-analytic calculation method of laser heat effect are concisely summarized; the practical example of laser heat treatment, which is calculated by semi-analytic calculation method, is given out.During the thesis research work, I have finished many laser heat treatment experiments with the experiment condition of LTM Laboratory of Creusot of France and laser equipments of Kunming university of science and technology, at the same time have done the laser heat treatment temperature field numerical simulation with rapid calculation method, semi-analytic calculation method and FEM with Fastflo software. In order to ensure the reliability of some important parameters used in calculation, the material surface energy absorption coefficient was measured by the calorimeter that developed by LTM, and a prediction method of laser material surface energy absorption coefficient is proposed; for the experiment in Kunming University of science and technology of, the power density distribution was measured by thermal paper which is strictly collimated. In the research of using different methods to calculate temperature field, in order to let the research work is mostly approach to the complex practical laser power density distribution, we have used the scalar diffraction theory to calculate intensity distributions in different spatial planes behind an optics system. The temperature field calculation induced by arbitrary laser beam energy distribution which has complicate interference and refraction character, was calculated by the semi-analytic calculation formula.The comparison research shows that for no-melting laser heat treatment, the rapid calculation method and semi-analytic calculation method not only gain a good agreement with experiment, but also both methods have high efficiency. For example: we have used the same PC with different calculation methods to finish a laser hardening treatment temperature calculation (chapter 5), the calculation time of FEM is 24 hours, rapid-calculation method of semi-analytic calculation method needs 54 seconds. For a given heat treatment question, as just calculate the hardening depth and middle width is sufficient to predict the heat treatment result, so for semi-analytic calculation method just one minutes; even to calculate whole phase transformation belt, the semi-analytic calculation method just needs 12 minutes. Compared with pure numerical temperature field calculation, the advantage of rapid-calculation method and semi-analytic calculation method are extremely obvious in practical calculation.However, as laser material interaction is so complicated, at the present research, the thermal properties of material are considered to be constant independent of temperature in the derivation process of heat conductive equation, and therefore the analytic solution or the semi-analytic solution just is the approximate expression to describe quenching. For the laser remelting treatment , although some relevant mathematic models were built up, analytical solution cannot be gained up to now as the complicated thermal conduction and heat transfer are involved in the procedure. So finite element method (FEM), boundary element method (BEM), finite difference method (FDM) are the absolutely necessary methods to be used to solve numerical solution of mathematical model with real boundary conditions in the deep investigation of laser material interaction. It should be pointed out that accepting suitable mathematical model with the necessary parameters, space and time intervals are dispersed and then physical metallurgy processes in the laser heat treatment can be traced using of the pure numerical calculation methods. This is important for disclose natural rule of laser heat treatment interaction. Furthermore, accompany with the development of computer technology, some commercial FEM calculation software provide a very friendly GUI , and gives a convenient method to the deeply research of laser heat treatment interaction.So the analytical solution and pure numerical solution of laser heat treatment mathematical model are complement each other, we can choose which method to do the laser heat treatment temperature field simulation according to the practical needs. Otherwise, from the point of theory, according to the practical physical question to build correctly mathematical model, to gain correctly analytical solution without doubt is most efficient express, we will continue to do research in this direction.