Experimental Determination Of Key Heat Transfer Parameters And Research Of Temperature Field For Sheet Extrusion Conditions

Sheet extrusion has great advantages in forming parts with functional features and variable thickness features.It has attracted much attention with the promotion of automobile lightweight and energy saving and emission reduction.However,it has problems such as large load and high temperature in local area,which have a significant impact on the dimensional accuracy of forming parts and the life of die.As a common method for forming temperature field analysis,the finite element method provides an important reference for process optimization and die design,but its accuracy has always been restraint by key heat transfer parameters,such as thermal conductivity,convective heat transfer coefficient,and thermal contact conductance(TCC).In sheet extrusion,heat transfer parameters change significantly with fluctuations of contact stress and temperature.It is difficult to measure heat transfer parameters,for they are sensitive to influence factors.Existing studies often choose fixed values to simulate the forming process,and the accuracy needs to be improved.Therefore,it is of great significance to improve the reliability of the finite element analysis by setting up a device to determine key heat transfer parameters and establish a mapping relationship between the parameters and influencing factorsIn order to measure key heat transfer parameters independently,a set of measuring device is established in this paper,which can determine the thermal conductivity,convective heat transfer coefficient and thermal contact conductance.The evaluation of heat dissipation through side surface and measurement result of parameters proves that the device have good reliability.On this basis,the relationship between thermal contact conductance and temperature,contact stress,contact surface roughness was studied.The results show that the increase in contact stress and temperature will cause the thermal contact conductance to increase significantly.To obtain the thermal contact conductance suitable for high stress state of sheet extrusion,the mapping relationship between thermal contact conductance and multiple variables for elastoplastic deformation is constructed.Through the device to obtain the thermal contact conductance within the elastic deformation range,it is assumed that the thermal contact conductance,contact stress,and contact temperature meet a polynomial relationship,referring to the elastoplastic empirical formula,and a mapping relationship applicable to the whole domain is obtained.Based on the mapping relationship,the dynamic update of contact thermal conductivity is realized in the temperature field simulation of sheet extrusion,according to the changes in temperature and contact stress.Compared with temperature field obtained by simulation with constant empirical parameters,the peak temperature of die in simulation using the whole domain dynamic update parameters is higher,accompanied by larger temperature fluctuation.Comparing experiment with simulation,it is found that the temperature field based on the whole domain dynamic update parameters matches the experimental results well,which proves that the assumed mapping relationship can provide more reliable temperature data and is suitable for sheet extrusion conditions.In summary,using the method of the whole domain dynamic update parameters can more effectively analyze the change of temperature during sheet extrusion process,and further study the dimensional accuracy of the formed parts and the service life of the die.