Research On The Design Method Of Combined Die In Precision Forging Considering Thermal Effect

Precision forging is a forming process that the parts could be obtained to meet the final requirement with only forging process and minimum machining.In the precision forging,the combined die is widely used.In the practical forging process,the pressure caused by the deformation of billet makes the elastic deformation of combined die,which affects the precision of forgings,the tensile stress caused seriously affects the strength and life of combined die.Heat generation from the preheated billet and die,the energy dissipated by plastic deformation of the material and the sliding friction between the workpiece and die would partially transfer into the combined die,the rising temperature of the die could affect the precision of forgings.The thermal stress also affects the strength and life of combined die.Aimed at solving these problems,the design method of combined die in precision forging considering thermal effect is researched.The main research content is as follows:Considering the steady-state character of the practical forging process,the steadystate temperature fields of three-layer combined die with different thermal boundary conditions were determined.Under the steady-state temperature field,the expressions of thermo-elastic deformation of the combined die were derived according to thermo-elastic mechanics theories.The steady-state temperature distribution could be simplified as linear temperature distribution based on the equal area principle to compute the deformation of the combined die.The hypothesis uniform temperature distribution,which is always used as a boundary condition in the FE model of precision forging process,is not proper for the calculation of the deformation of the combined dieThe analytical solution for the autofrettage process of double-layer combined die based on the perfectly elastic-plastic and linear hardening material model was carried out.The influences of key parameters including the ratio of plastic area and yield strength of the die insert,the outer diameters of the die insert and stress ring on the autofrettage effect were investigated by a group of cases.Compared with the conventional combined die,the autofrettaged die could bear larger working pressure as expected.the increase of ratio of plastic area of die insert is more effective than change of the die material with higher yield strength.The autofrettaged die could save the expensive material of die insert and the working space of die set,which would contribute to the real forging process.As for the linear hardening material model,the effect of autofrettage is much better when the strain-hardening modulus is smaller.The analytical solution for the autofrettage process of double-layer combined die considering the thermal effect was carried out.The cases with different working pressures and different temperature distributions were analyzed.The thermal stress generated by the steady-state temperature distribution with the temperature gradient could offset the tensile stress caused by the working pressure.In the practical forging process,the steady-state temperature distribution with the temperature gradient may raise the bearing capacity of die.The effect of autofrettage is more obvious in the practical forging process compared with the working condition without considering thermal effect.