6 research outputs found
The anisotropic grain size effect on the mechanical response of polycrystals: The role of columnar grain morphology in additively manufactured metals
Additively manufactured (AM) metals exhibit highly complex microstructures,
particularly with respect to grain morphology which typically features
heterogeneous grain size distribution, anomalous and anisotropic grain shapes,
and the so-called columnar grains. In general, the conventional morphological
descriptors are not suitable to represent complex and anisotropic grain
morphology of AM microstructures. The principal aspect of microstructural grain
morphology is the state of grain boundary spacing or grain size whose effect on
the mechanical response is known to be crucial. In this paper, we formally
introduce the notions of axial grain size and grain size anisotropy as robust
morphological descriptors which can concisely represent highly complex grain
morphologies. We instantiated a discrete sample of polycrystalline aggregate as
a representative volume element (RVE) which has random crystallographic
orientation and misorientation distributions. However, the instantiated RVE
incorporates the typical morphological features of AM microstructures including
distinctive grain size heterogeneity and anisotropic grain size owing to its
pronounced columnar grain morphology. We ensured that any anisotropy arising in
the macroscopic mechanical response of the instantiated sample is mainly
associated with its underlying anisotropic grain size. The RVE was then used
for meso-scale full-field crystal plasticity simulations corresponding to
uniaxial tensile deformation along different axes via a spectral solver and a
physics-based crystal plasticity constitutive model. Through the numerical
analyses, we were able to isolate the contribution of anisotropic grain size to
the anisotropy in the mechanical response of polycrystalline aggregates,
particularly those with the characteristic complex grain morphology of AM
metals. Such a contribution can be described by an inverse square relation
Thermo-micro-mechanical simulation of bulk metal forming processes
The newly proposed microstructural constitutive model for polycrystal
viscoplasticity in cold and warm regimes (Motaman and Prahl, 2019), is
implemented as a microstructural solver via user-defined material subroutine in
a finite element (FE) software. Addition of the microstructural solver to the
default thermal and mechanical solvers of a standard FE package enabled coupled
thermo-micro-mechanical or thermal-microstructural-mechanical (TMM) simulation
of cold and warm bulk metal forming processes. The microstructural solver,
which incrementally calculates the evolution of microstructural state variables
(MSVs) and their correlation to the thermal and mechanical variables, is
implemented based on the constitutive theory of isotropic
hypoelasto-viscoplastic (HEVP) finite (large) strain/deformation. The numerical
integration and algorithmic procedure of the FE implementation are explained in
detail. Then, the viability of this approach is shown for (TMM-) FE simulation
of an industrial multistep warm forging