PNNL

Abstract

Friction stir processing (FSP) is a promising solid-phase microstructure modification technique that repairs and enhances stainless steel components exposed to harsh environments. FSP involves complex thermomechanical and dynamic behaviors, along with numerous process and material parameters, thus posing challenges to understanding, controlling, and optimizing the process. In this study, correlations between process parameters, material flow, temperature, strain, strain rate, and recrystallized grain size in 316L stainless steel that had undergone FSP are established using high-fidelity multiphysics process modeling and electron backscatter diffraction (EBSD) imaging. We developed and validated a meshfree smoothed particle hydrodynamics model to accurately predict FSP field variables under different process parameters. Simulation results reveal that the tool-workpiece interface can achieve transient temperatures >300°C higher than those measured away from the tool pin tip. Moreover, the stir zone (SZ) temperature, strain rate, and grain size increase with increasing tool temperature and traverse speed. The model-predicted SZ temperature and strain rate are integrated with grain size measured from EBSD imaging to establish a robust relationship between the Zener-Hollomon parameter and recrystallized grain size. This calibrated relationship exhibits satisfactory accuracy when used to predict grain sizes at different locations in the SZ.