VOID EVOLUTION BEHAVIOR AND CLOSURE CRITERION INSIDE LARGE SHAFT FORGINGS DURING A FORGING PROCESS

In this study, the effects of different void positions, void shapes and sizes on the evolution of voids were discussed in detail using experiments and simulations. The results show that the influence of the void size on the void closure can be ignored, while the void position and void shape have a great influence on the closure of a void. Considering the complexity of the void-shape change in a forging process, we proposed a quantitative expression of the void-shape coefficient, which is affected by the effective stress and effective strain. Meanwhile, the void-shape evaluation parameter, defined as a function of the stress deviator, effective strain and effective stress, was proposed to describe the changes in the void aspect ratio. Finally, WHF (wide die heavy blow) forging experiments were conducted using a 5MN hydraulic press to verify the numerical-simulation results. Based on the experimental and simulation results, a new mathematical model for void-closure determination was established during a forging process of large shaft forgings. The experimental results were consistent with the simulation results, showing that the void-closure model can accurately determine whether a void is closed or not.

Numerical and experimental simulation of shrinkage porosity closure during hot rolling of bars

Hot rolling of bars issued from continuous-casting aims at refining the material structure and guaranteeing the central soundness of the metallurgical product. The rolling route must be designed to achieve the complete closure of the shrinkage porosity inherent in the continuous casting process. To predict the void evolution, many models exist that can be implemented in the finite element simulation of the process. Nevertheless, these models need parameter adjustments to be adapted to the forming process, the formed material, and the real geometry of the void. Real scale tests being very expensive in the long product rolling mill, an improved representativeness experimental configuration was designed to reproduce at the laboratory scale the key characteristics of the thermomechanical path driving the void closure phenomenon. This testing consists of successive forming stages with shaped anvils applied to samples containing a shrinkage cavity. The shaped anvils and the forming conditions are calibrated to reproduce the levels of strain and the stress triaxiality of rolling stands, and the alternation of the forming direction of the industrial process. The geometry of the voids before and after the forming paths are measured by tomography. The simulation of the test with an explicit modelling of the void is developed parallel to the experiments. The simulation/experiment comparison allows the validation of the numerical model. The obtained model will be used in future works to perform a more extended design of experiments to characterise void closure during hot rolling of bars.

Establishment and Application of the Void Closure Prediction Model of 316LN

The presence of voids in the ingot affects the mechanical properties of the final products of the forging process. It is essential to establish a void closure model to predict cavity closure in the forging process to optimize the forging process and improve forging quality. The main purpose of this study is to obtain an accurate prediction model of void closure for 316LN stainless steel. Using the FEM simulation method to study the closure of spherical voids during forging compression of 316LN materials, we can accurately characterize the state of void closure. The void closure ratio K under different deformations at 1,200°C was counted, and the relationship between K and the effective strain was established to obtain the void closure prediction model of 316LN stainless steel. The void closure prediction model is implanted into DEFORM software through the secondary development method to generate the void closure ratio K. In the postprocessing module of DEFORM software, the void closure status of each part during the forming process can be directly observed. Comparing the results of large-scale upsetting experiments and simulation results, the closure error of each part was only 3%, which indicates that the void closure model established in this paper has higher accuracy, which is helpful for the optimization of the forging process and the control of forging quality.