A modified Johnson-Cook model to determine plastic flow behavior of Fe-30Mn-9Al-0.8 C low-density steel during warm multiaxial forging

Abstract

The microstructural changes in Fe-30Mn-9Al-0.8 C low-density steel upto equivalent strain 2.3 during multiaxial forging (MAF) and their impact on tensile strengthening have been thoroughly investigated. The processed sample at 0 pass, 1 pass, 3 pass, and 5 pass with 33% normal strain in each pass are characterized by X-ray diffraction (XRD) and electron backscattered diffraction (EBSD). The analysis of XRD confirms a noticeable increase in dislocation density progressing from 0.79 × 1015 m−2 to 8.98 × 1015 m−2, attributed to a decrease in crystallite size and an increase in microstrain from 3.66 × 10−3 to 6.8 × 10−3. The grain size reduces from 50 μm to 13 μm. The yield strength increases from 380 MPa to 1528 MPa, while the ultimate strength increases from 762 MPa to 1548 MPa after 5 passes. It is observed here that both dislocation density and grain boundaries contribute to strengthening the material. However, up to three passes, dislocation strengthening is significant. Additionally, a numerical framework is developed to model the evolution of dislocation density, grain size and yield strength at each stage of MAF. To emphasize the role of strengthening parameters in predicting the flow stresses in every MAF pass accurately, a material VUMAT subroutine in ABAQUS has been created for a modified Johnson-Cook (J-C) constitutive model that accounts for the role of increased dislocation density and grain size. The developed model demonstrates its capability to accurately capture essential material features, including equivalent strain, dislocation density, and grain size. Furthermore, it exhibits the ability to predict the yield strength approximately following each MAF pass.