Study on the microstructure evolution during radial-axial ring rolling of IN718 using a unified internal state variable material model

Microstructure control during radial-axial ring rolling (RARR) of IN718 is important for increasing the performance in service of IN718 rings. However, RARR is an extremely complex dynamic rolling process with non-uniform local deformation and non-uniform temperature distribution, making the microstructure control difficult. This paper presented an internal state variable (ISV) material model which enables the unified prediction of flow behavior and microstructure evolution during dynamic and post dynamic regime. Based on user defined subroutine, a multiscale finite element (FE) model with adaptive motion control of rolls was established to study the evolution of dislocation density, recrystallized fraction and grain size during RARR of IN718. RARR experiment was conducted to verify the multiscale FE model. The predicted outer diameter of the ring and the radial rolling force as well as the microstructure distribution on the ring cross section were in good agreement with the measured results. The evolution and distribution of ISVs were discussed, and the effect of mandrel diameter, main roll diameter, initial temperature, and rolling ratio on the microstructure evolution of the ring was analyzed. Sensitivity analysis of rolling parameters was conducted. The initial temperature is the most sensitive parameter and the initial temperature should be as high as possible with the pinning of δ phase particle to promote the recrystallization process. It should be careful to increase the rolling ratio when the rolling ratio is higher than 1.667. Because the refine effect of grain structure decreases and the rolling force may increase dramatically due to the low temperature.