Research On Inverse Estimation Of Thermal Boundary Condition And Reconstruction Of Temperature Field In Laser Melting Process

Laser melting is a kind of material surface strengthening technology.In the process of machining,the surface of the workpiece has a certain thickness of material which can melt and solidify rapidly.The quality of the laser melting process is determined by the properties of the new structure formed after the melting.According to the theory of solidification,the temperature field is the key factor that influences the formation,residual stress and thermal deformation of the new structure.And the shape of molten pool formed by the process also represents the quality of processing to a certain extent.Numerical simulation is an effective method to obtain the whole temperature field and is widely used in the simulation of laser melting process.But the interaction of laser and material is very complicated,there are many factors which can affect the laser absorption rate,such as laser wavelength,the condition of material surface,surface temperature,etc.,combined with the actual process parameters in the process of changing and the diversification of processing materials,led to the actual absorption of laser surface heat as the boundary conditions of numerical simulation is difficult to grasp.In this paper,the thermal flux boundary conditions of the process are deduced based on the measured temperature inversion,and the temperature field is reconstructed and the shape of the molten pool is obtained.For the research present situation and existing main problems laser melting process of thermal boundary conditions inversion and the temperature field reconstruction,this paper combined with the finite volume method(enthalpy model)and the inversion algorithm based on dynamic matrix control(DMC)to study the process of heat flux boundary inversion and temperature field reconstruction problem.The main work includes:(1)A mathematical model of laser melting heat transfer process is established,the enthalpy method is used to handle phase change latent heat of the process,and the finite volume method is used to solve the heat conduction differential equation in order to obtain the temperature field distribution;The simulation results of finite element simulation software ANSYS are used to verify the applicability of the program.The effects of laser heat source form and material on the temperature field and the shape of molten pool are discussed.The results show that both heat source and laser absorptivity have great influence on temperature field.(2)Based on the solving of the positive problem with the finite volume method and enthalpy method,the inversion model of thermal flux boundary based on the DMC inversion algorithm is established.For the change of the relation matrix in the nonlinear case,the relational matrix in the inversion process is updated by the approximate substitution method,which avoids the iterative calculation of the nonlinear inverse heat transfer problem.(3)Simulation experiments are carried out on the inverse estimation of thermal flux boundary condition in the laser melting process,and the correctness and applicability of the inversion model established in this paper are verified.The influence of future time step,regularization parameter,temperature measurement error,time varying heat source,pulse heat source and temperature measuring location on inversion results is discussed.The simulation results show that the selection of time step has little influence on the inversion results.The selection of regularization parameters has great influence on the inversion results,and the proper regularization parameters can be selected by using the principle of temperature residuals.The temperature measurement error has a great influence on the inversion results.The larger the measurement error is,the worse the inversion result is,and the satisfactory inversion results can be obtained in a certain error range.The inversion model established in this paper can effectively inversely estimates the constant heat source,the time varying heat source and the pulsed heat source,and can obtain the satisfactory inversion results under certain temperature measurement error conditions.The location of temperature measurement points has a great influence on the inversion results.The closer the location is to the heat source,the better the inversion results will be.The validity of the inversion model is proved by the discussion of various influencing factors.