Residual stress modeling considering microstructure evolution in metal additive manufacturing

Abstract

Metal additive manufacturing (AM) process induces residual stress which can hinder the applicability of AM. Residual stress build-up causes part failure due to the crack initiation and growth, and also distortion during or after fabrication. Consequently, it is of great importance to accurately and rapidly predict residual stress within the AM parts. During the thermal loading, the grain size is altered at the subsurface through dynamic recrystallization (DRx) and subsequent recovery. The yield strength of the alloys is largely determined by the size of nucleated grains, and it has a substantial influence on residual stress build-up. In this work, a physics-based analytical model is proposed to predict the residual stress considering the microstructure of the additively manufactured part. The thermal signature of this process is predicted using a transient moving point heat source. Due to the high-temperature gradient innate in this process, material may experience high thermal stress which often exceeds the yield strength. The thermal stress is obtained from Green’s functions of stresses due to the point body load. The modified Johnson-Cook flow stress model is used to predict the yield surface. In this flow stress model, the yield strength parameter is modified to incorporate the effect of grain size using Hall-Petch equation. The dynamic recrystallization and the resultant grain size are predicted by utilizing the Johnson–Mehl–Avrami–Kolmogorov (JMAK) model for IN718 alloy. Moreover, a grain refinement model is used to include the effect of the rapid solidification on grain size. Then, as a result of the cyclic heating and cooling and the fact that the material is yielded, the residual stress build-up is precited from incremental plasticity and kinematic hardening behavior of the metal according to the property of volume invariance in plastic deformation in coupling with equilibrium and compatibility conditions. Results from the analytical residual stress model showed good agreement with X-ray diffraction measurements used to determine the residual stresses in the IN718 specimens. Moreover, the phases present in the IN178 samples built via direct metal disposition (DMD) are evaluated using X-ray diffraction.