8 research outputs found
Tailoring magnetic hysteresis of Fe-Ni permalloy by additive manufacturing: Multiphysics-multiscale simulations of process-property relationships
Designing the microstructure of Fe-Ni permalloy by additive manufacturing
(AM) opens new avenues to tailor the materials' magnetic properties. Yet,
AM-produced parts suffer from spatially inhomogeneous thermal-mechanical and
magnetic responses, which are less investigated in terms of process simulation
and modeling schemes. Here we present a powder-resolved multiphysics-multiscale
simulation scheme for describing magnetic hysteresis in materials produced via
AM. The underlying physical processes are explicitly considered, including the
coupled thermal-structural evolution, chemical order-disorder transitions, and
associated thermo-elasto-plastic behaviors. The residual stress is identified
as the key thread in connecting the physical processes and in-process phenomena
across scales. By employing this scheme, we investigate the dependence of the
fusion zone size, the residual stress and plastic strain, and the magnetic
hysteresis of AM-produced Fe21.5Ni78.5 permalloy on beam power and scan speed.
Simulation results also suggest a phenomenological relation between magnetic
coercivity and average residual stress, which can guide the magnetic hysteresis
design of soft magnetic materials by choosing appropriate AM-process
parameters
Variational quantitative phase-field modeling of non-isothermal sintering process
Phase-field modeling has become a powerful tool in describing the complex pore-structure evolution and the intricate multiphysics in nonisothermal sintering processes. However, the quantitative validity of conventional variational phase-field models involving diffusive processes is a challenge. Artificial interface effects, like the trapping effects, may originate at the interface when the kinetic properties of two opposing phases are different. On the other hand, models with prescribed antitrapping terms do not necessarily guarantee the thermodynamics variational nature of the model. This issue has been solved for liquid-solid interfaces via the development of the variational quantitative solidification phase-field model. However, there is no related work addressing the interfaces in nonisothermal sintering, where the free surfaces between the solid phase and surrounding pore regions exhibit strong asymmetry of mass and thermal properties. Also, additional challenges arise due to the conserved order parameter describing the free surfaces. In this work, we present a variational and quantitative phase-field model for nonisothermal sintering processes. The model is derived via an extended nondiagonal phase-field model. The model evolution equations have naturally cross-coupling terms between the conserved kinetics (i.e., mass and thermal transfer) and the nonconserved one (grain growth). These terms are shown via asymptotic analysis to be instrumental in ensuring the elimination of interface artifacts, while also examined to not modify the thermodynamic equilibrium condition (characterized by a dihedral angle). Moreover, we demonstrate that the trapping effects and the existence of surface diffusion in conservation laws are direction-dependent. An anisotropic interpolation scheme of the kinetic mobilities that differentiates between the normal and tangential directions along the interface is discussed. Numerically, we demonstrate the importance of the cross-couplings and the anisotropic interpolation by presenting thermal-microstructural evolutions
Nanoparticle Tracing during Laser Powder Bed Fusion of Oxide Dispersion Strengthened Steels
The control of nanoparticle agglomeration during the fabrication of oxide dispersion
strengthened steels is a key factor in maximizing their mechanical and high temperature reinforcement
properties. However, the characterization of the nanoparticle evolution during processing
represents a challenge due to the lack of experimental methodologies that allow in situ evaluation
during laser powder bed fusion (LPBF) of nanoparticle-additivated steel powders. To address this
problem, a simulation scheme is proposed to trace the drift and the interactions of the nanoparticles
in the melt pool by joint heat-melt-microstructure–coupled phase-field simulation with nanoparticle
kinematics. Van derWaals attraction and electrostatic repulsion with screened-Coulomb potential are
explicitly employed to model the interactions with assumptions made based on reported experimental
evidence. Numerical simulations have been conducted for LPBF of oxide nanoparticle-additivated
PM2000 powder considering various factors, including the nanoparticle composition and size distribution.
The obtained results provide a statistical and graphical demonstration of the temporal
and spatial variations of the traced nanoparticles, showing ~55% of the nanoparticles within the
generated grains, and a smaller fraction of ~30% in the pores, ~13% on the surface, and ~2% on
the grain boundaries. To prove the methodology and compare it with experimental observations,
the simulations are performed for LPBF of a 0.005 wt % yttrium oxide nanoparticle-additivated
PM2000 powder and the final degree of nanoparticle agglomeration and distribution are analyzed
with respect to a series of geometric and material parameters
Influence of silver nanoparticle additivation on Nd-Fe-B permanent magnets produced by Laser Powder Bed Fusion
Powder bed fusion of metals using a laser beam (PBF-LB/M) is an established additive
manufacturing (AM) method that can be used to fabricate geometrically complex Nd Fe-B magnets. However, the magnetic properties of Nd-Fe-B magnets manufactured
by PBF-LB/M are typically inferior to conventionally produced magnets. To overcome
this drawback, we modified the surface of the permanent magnet feedstock powder
with 1 wt.% surfactant-free Ag nanoparticles (NPs) supporting the formation of relevant
phases required for permanent magnetic performance to achieve a suitable micro- and
nanostructure after AM. Our study is accompanied by finite element simulations,
revealing the impact and dependency of process parameters during PBF-LB/M: a wide
temperature field with a high-gradient profile in the front and on the bottom of an
overheated region, implying a vast local heating/cooling rate and in-process high
thermal stress. We found experimentally that the as-built part density can be affected
by both the laser power and scan speed, causing a reduction in density as both
parameters increase. The functionality and microstructural properties are also
investigated via VSM, HR-SEM, EDX, EBSD, and exemplarily with HR-TEM-EDX and
APT. Our study found that modifying MQP-S with Ag NPs increases the coercivity by
approximately 20%, which we correlate to a decreased grain size. Additionally, we
identified three distinct phases in the modified and unmodified samples, where Ag is
primarily found in the intergranular and Nd-rich phases of the as-built parts. Overall,
the study\u27s findings contribute to the understanding of the factors that affect the quality
and magnetic properties of Nd-Fe-B magnets fabricated through PBF-LB/M and
provide valuable insights for further research in this area