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

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

The effects of annealing temperature on the in-field Jc and surface pinning in silicone oil doped MgB2 bulks and wires

The effects of sintering temperature on the lattice parameters, full width at half maximum (FWHM), strain, critical temperature (T-c), critical current density (J(c)), irreversibility field (H-irr), upper critical field (H-c2), and resistivity (rho) of 10 wt.% silicone oil doped MgB2 bulk and wire samples are investigated in state of the art by this article. The a-lattice parameter of the silicone oil doped samples which were sintered at different temperatures was drastically reduced from 3.0864 angstrom to 3.0745 angstrom, compared to the un-doped samples, which indicates the substitution of the carbon (C) into the boron sites. It was found that sintered samples at the low temperature of 600 degrees C shows more lattice distortion by more C-substitution and higher strain, lower T-c, higher impurity scattering, and enhancement of both magnetic J(c) and H-c2, compared to those sintered samples at high temperatures. The flux pinning mechanism has been analyzed based on the extended normalized pinning force density f(p) = F-p/F-p,F-max scaled with b = B/B-max. Results show that surface pinning is the dominant pinning mechanism for the doped sample sintered at the low temperature of 600 degrees C, while point pinning is dominant for the un-doped sample. The powder in tube (PIT) MgB2 wire was also fabricated by using of this liquid doping and found that both transport J(c) and n-factor increased which proves this cheap and abundant silicone oil doping can be a good candidate for industrial application. (C) 2012 Elsevier Ltd. All rights reserved

Advances in catalysis and related subjects

In this paper, we report the doping effects of succinic acid, CHO (from 0 to 30 wt%) on the lattice parameter, critical temperature (T), critical current density (J), upper critical field (H), and irreversibility field (Hirr) in MgB2 superconductor. It was found that MgB2 doped with 10 wt% C4H6O4 and sintered at 900 °C exhibited excellent J above 104 Acm−2 at 5 K and 8 T. Impurity scattering due to C substitution, improved crystallinity and the least amount of MgO in 10 wt% doped sample improves Jc very significantly. The MgO amount is rapidly increased in 20 and 30 wt% doped samples which causes a strong depression of J, H, H due to poor inter and intra-grain connectivity

Development of high current capacity mono- and 18-filament in situ MgB2 cables by varying the twist pitch

Undoped and carbon doped magnesium diboride (MgB ) cables have been assembled by braiding six Nb/Monel and Nb/Cu/stainless steel (SS) sheathed mono-and multifilament strands with a central copper stabilizer for improving the operational environment. This paper presents the fabrication and characterization of two types of in situ powder-in-tube processed mono (pure) and multi-filament (carbon doped) MgB cables with different twist pitch lengths; thereby making them possible candidates for industrial AC applications. Critical current is not influenced by the cabling that results in various twist lengths. The total critical current of the braided cables is obtained by multiplying the critical current of six single wires without any dissipation. The critical current density (Jc) of pure (mono) and carbon doped (18-filament) six stranded cable reached 10 000 A/cm at 5.5 and 10 T, respectively; without any observable deleterious effect caused by varying the twist pitch. The engineering current density (Je) of both cables reached the same value of 10 000 A/cm at 3.5 and 6.5 T, respectively. Compared to the literature, this work reports some of the highest Jc and Je values for carbon doped multifilament cables that remain unaffected upon varying the twist pitch. The present results are promising in terms of scaling up these cables to industrial lengths for transformers, fault-current limiters-based applications and paves the way for the development of optimal protocols for practical functionality

Schildkröten im Fokus

A binary magnesium diboride (MgB2) cable has been assembled by braiding six Nb/Monel sheathed monofilament strands around a central copper stabilizer for improving the operational environment. The total critical current (I-c) of the braided cable is obtained by multiplying the I-c of six single wires, without any dissipation. In this work, various mechanical deformations, i.e., swaging, two-axial rolling, groove rolling, and cold high-pressure densification (CHPD) at 1.8 GPa have been applied to the 6-stranded cable to obtain additional densification. The highest critical current density at both 4.2 and 20 K has been achieved in this work through the CHPD treated cable due to higher filament mass density. The present results are promising in view of the cable, particularly in power applications at industrial lengths that pave the way to seeking an optimal protocol to meet a practical functionality