7,350 research outputs found
Thermo-mechanical analysis of additively manufactured hybrid extrusion dies with conformal cooling channels
Profile overheating and surface defects during hot aluminum extrusion can occur when seeking higher productivity rates at increased ram speed velocities. The incorporation of cooling channels in the die-design allows overcoming this process limitation by keeping the profile temperature below the melting point of the alloy used [1]. Selective laser melting (SLM) of conformal cooling channels provides, in contrast to conventional manufacturing techniques, the opportunity to place the cooling circuit inside the mandrel of a porthole-die in a well-defined position to the critical bearing region [2]. In the framework of this study, a preliminary numerical investigation on the extrusion process under the assumption of liquid nitrogen cooling is analysed. The results show, that by combining conformal cooling channels with liquid nitrogen as a cooling media high cooling rates, which are well beyond the state of the art of conventional dies, can be achieved. In a hybrid extrusion die set-up, a part of the mandrel, that is additively manufactured, is either joined [3] or directly selective leaser melted onto the conventionally manufactured parts [4]. For a proper implementation in the extrusion process, material testing of the welded joint are needed. Thus, in the current study, tensile tests performed at room temperature for hybrid specimens, partially consisting of conventionally processed tool steel 1.2343 and partially additively manufactured 1.2709, will be presented. Moreover, four different heat treatment sequences of the hybrid specimens will be discussed. In addition, for each configuration, micro-structural images are taken to investigate failure at the bonding region. Finally, an optimal manufacturing sequence for a hybrid die with the described material combination is proposed
On the ground state of solids with strong electron correlations
We formulate the calculation of the ground-state wavefunction and energy of a
system of strongly correlated electrons in terms of scattering matrices. A
hierarchy of approximations is introduced which results in an incremental
expansion of the energy. The present approach generalizes previous work
designed for weakly correlated electronic systems.Comment: 17 pages, Latex(revtex
Closed-shell interaction in silver and gold chlorides
Hartree-Fock and coupled-cluster calculations have been performed for cubic
AgCl and for AuCl having a cubic or the observed structure with space group
I4_1/amd. Cohesive energies and lattice constants are in excellent agreement
with experiment for AgCl; for AuCl we find good agreement, and the experimental
structure is correctly predicted to be lower in energy than the cubic one.
Electron-correlation effects on lattice constants are very large, of up to 0.8
\AA for cubic AuCl. We especially discuss the strength of the closed-shell
interactions, and for the first time a quantitative analysis of the so-called
"aurophilic" Au(I)-Au(I) interaction is presented in solids.Comment: accepted by J. Chem. Phy
Correlation-induced corrections to the band structure of boron nitride: a wave-function-based approach
We present a systematic study of the correlation-induced corrections to the
electronic band structure of zinc-blende BN. Our investigation employs an ab
initio wave-function-based local Hamiltonian formalism which offers a rigorous
approach to the calculation of the polarization and local charge redistribution
effects around an extra electron or hole placed into the conduction or valence
bands of semiconducting and insulating materials. Moreover, electron
correlations beyond relaxation and polarization can be readily incorporated.
The electron correlation treatment is performed on finite clusters. In
conducting our study, we make use of localized Wannier functions and embedding
potentials derived explicitly from prior periodic Hartree-Fock calculations.
The on-site and nearest-neighbor charge relaxation bring corrections of several
eV to the Hartree-Fock band gap. Additional corrections are caused by
long-range polarization effects. In contrast, the dispersion of the
Hartree-Fock bands is marginally affected by electron correlations. Our final
result for the fundamental gap of zinc-blende BN compares well with that
derived from soft x-ray experiments at the B and N K-edges.Comment: 18 pages, 8 figures; the following article has been submitted to J.
Chem. Phy
First-Principles Calculation of Electric Field Gradients and Hyperfine Couplings in YBa2Cu3O7
The local electronic structure of YBa2Cu3O7 has been calculated using
first-principles cluster methods. Several clusters embedded in an appropriate
background potential have been investigated. The electric field gradients at
the copper and oxygen sites are determined and compared to previous theoretical
calculations and experiments. Spin polarized calculations with different spin
multiplicities have enabled a detailed study of the spin density distribution
to be made and a simultaneous determination of magnetic hyperfine coupling
parameters. The contributions from on-site and transferred hyperfine fields
have been disentangled with the conclusion that the transferred spin densities
essentially are due to nearest neighbour copper ions only with marginal
influence of ions further away. This implies that the variant temperature
dependencies of the planar copper and oxygen NMR spin-lattice relaxation rates
are only compatible with commensurate antiferromagnetic correlations. The
theoretical hyperfine parameters are compared with those derived from
experimental data.Comment: 14 pages, 12 figures, accepted to appear in EPJ
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