Research On Evolution Of Void Defects In Heavy Forgings During Upsetting Process

As key parts of large complete sets of equipment, heavy forgings have played important roles in national economic constructions and defense development. In addition to producing dimensional satisfactory forgings, the research on upsetting process pays greater attention on modification of inner defects, improvement of material tissue as well as creating favorable mechanical conditions which is able to prevent new fractures or inclusion-induced cracks. As basic forging techniques, upsetting can eliminate void defects so that the mechanical performance, inner quality and carrying ability of materials will be greatly improved. The law of void evolution, criterion of void closure and factors which affect void closure are revealed in this paper. The main work is as follows:1) The law of void evolution and factors which affect void closure were investigated based on Gurson model which comes from Meso-mechanics. The criterion of void closure was also put forward. 2) The constitutive relationship of Gurson model and the update algorithm were studied and implemented through subroutine Hypela which defines the material property of porous material and facilitates the numerical simulation.3) The simulation results of artificial model and Gurson model were compared. It is found that infinite hydraulic stress or equivalent strain is required when Gurson Model is employed to realize void closure of materials. However, such condition does not conform to reality. This paper obtained modified parameters of Gurson Model by Orthogonal experimental design and simulation under different sampling parameters through embedded subroutine. The modified Gurson model can be applied to simulate void volume variation in heavy forgings. The results demonstrate that the void volume change ratios are quite close to those obtained by artificial models, which implies that the predictions of void closure are greatly improved. It is also found that void closure in the center is the easiest one to achieve while the difficulty increases as it approaches the surface. The harder the matrix material, the worse the void closure degree will be.4) By setting tiny artificial voids inside lead samples whose recrystallization temperature equates to room temperature, the criterion of void closure is verified by upsetting experiment. The void volume change ratios obtained from physical experiments are in agreement with those from numerical simulations.