Metallurgical and Materials Transactions B最新文献

The present study speaks of development of a two-dimensional phase field (PF) model to simulate the cooling slope rheoprocessing of the novel Al-15Mg₂Si-4.5Si composite, in view of process optimization and investigation of physics of microstructure formation. In case of cooling slope rheoprocessing, the composite melt starts losing its superheat once it impinges over the slope and transforms into semi-solid slurry during its length of travel over the slope. After experiencing shear flow over the slope, the melt fills an isothermal slurry holding furnace where it undergoes coarsening for a certain length of time. The present PF model simulates how heterogeneous nucleation of solid grains is supposed to happen within the melt, during cooling slope processing, adopting a seed undercooling-based nucleation model. Moreover, the PF model implements a grain coarsening model to simulate the isothermal globularization process of the evolving solid grains of primary Mg₂Si and primary Al phases. The interfacial free energy of Al–melt interface is taken from literature, whereas a molecular dynamics (MD) model is employed to estimate the interfacial energy value of the Mg₂Si–melt interface. The cooling rate values employed in the present PF model for different melt pouring temperatures are determined experimentally from initial trial experiments, whereas the validation experiments are performed to collect the slurry samples from chosen locations of the melt flow front over the slope and from isothermally kept slurry holding furnace. Micrographs obtained from the above samples confirm the accuracy of the developed 2D PF model to capture microstructural morphology of the composite slurry. Moreover, the model predictions of quantitative parameters such as grain diameter, shape factor/sphericity, and solid fraction are found to be close to the experimental measurements. For example, a representative simulated value of grain size and sphericity of primary Mg₂Si grains, after 8 minutes of slurry holding, are as follows: 24.01 and 0.834 μm, whereas the corresponding experimental values are 29.0 and 0.885 μm, respectively.

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Hot tears are typically classified as intergranular fractures, but recently we have found evidence of dendritic main stem fracture during the hot tearing in steel. This study conducted mechanical property tests on the mushy zone of the steel with stress directions parallel or perpendicular to the primary dendritic arms. The fracture strength and brittle toughness under the two conditions were clearly contrasted. Hot tearing prediction should consider the effects of dendrite morphology and stress direction.

Graphical Abstract

Compared to traditional manufacturing processes, the layer-by-layer deposition process of wire arc additive manufacturing brings significant differences in microstructure, resulting in distinct deformation behaviors. This study focuses on developing an appropriate constitutive model to characterize the flow behavior of wire arc additive manufactured (WAAMed) steel. To analyze the deformation behavior of WAAMed steel, the hot compression tests at the temperature range of 850 °C–1150 °C and strain rate range of 0.01–10 s⁻¹ were conducted by Gleeble thermomechanical simulator. The strain-compensated Arrhenius model and modified Johnson–Cook model have been proposed to predict the flow stress under different temperatures and strain rates, as well as the genetic algorithm-back propagation method (GA-BP). The prediction capability of these models has been compared with experimental data using various statistical measures. It can be concluded that all three constitutive models are capable of accurately predicting the flow stress of WAAMed steel. The predictive capability and stability of back propagation artificial neural network were significantly improved by incorporating a genetic algorithm. Compared to the other models, GA-BP model demonstrates the highest accuracy and stability, achieving a relative coefficient of 0.99669 and an average absolute relative error of 3.39 pct.

In the temperature range of 1773 K to 1873 K, nitrogen solubility and TiN formation in molten nickel were studied using melt-gas and melt-gas-nitride equilibrium at atmospheric nitrogen pressure. The nitrogen solubility increased with the increase of Ti content in molten nickel. TiN was formed when titanium and nitrogen in molten nickel reach critical contents. The critical titanium and nitrogen contents for TiN formation were significantly increased with the increase of melt temperature. Thermodynamic analysis of the experimental results was carried out in the form of Wagner interaction parameter. The standard Gibbs free energy change of nitrogen dissolution reaction in molten nickel was obtained as (Delta G_{{text{N}}}^{0} = 53803 + 27.76T{text{ (J/mol)}}). Based on the study of nitrogen solubility in molten Ni-Ti-N alloys, the temperature dependence of interaction parameters between N and Ti can be expressed as (e_{{text{N}}}^{{{text{Ti}}}} = - 800/T + 0.25) and (e_{{{text{Ti}}}}^{{text{N}}} = - 2728/T + 0.85). According to the thermodynamic study of TiN formation in molten Ni-Ti-N alloys, the standard Gibbs free energy change of TiN formation was (Delta G_{{{text{TiN}}}}^{0} = - 203553 + 65.29T , ({text{J/mol}})), and the interaction parameter of Ti on itself was obtained as (e_{{{text{Ti}}}}^{{{text{Ti}}}} = 0.046 , (1773{text{ to }}1873{text{ K}})).

As a typical metallurgical defect, macrosegregation seriously affects the internal quality of the continuous casting billet, and it cannot be solved by processes such as high-temperature diffusion and rolling. For continuous casting billet, the solidification shrinkage and thermal shrinkage of the microstructure directly affect the macrosegregation defect. In order to reveal the effects of solidification shrinkage and thermal shrinkage on the melt flow, microstructure distribution, and solute segregation, a multiphase solidification model based on the Eulerian–Eulerian approach was established in this work. The growth behaviors of the columnar dendrite trunk and the columnar dendrite tip were fully considered, as well as the nucleation, growth, free migration of equiaxed grains, and the columnar-to-equiaxed transition (CET). Besides, the corresponding relationship between the secondary dendrite arm spacing (SDAS) and the cooling rate has also been taken into account in the model, which makes the net mass transport source term of the mass conservation equations more accurate. The calculation results show that when no any shrinkage behavior is considered in the model, the melt flow velocity in front of the solidification end will gradually decrease until it is the same as the casting speed, and the segregation index at the billet center will gradually increase until it reaches the maximum value at the solidification end. Both thermal shrinkage and solidification shrinkage can generate a negative pressure zone in the billet center, sucking the poor-solute melt located the upstream of continuous casting strand flows towards the solidification end, and mixing with the enriched-solute melt before the solidification end, thereby inhibiting macrosegregation. However, compared with the solidification shrinkage, the effect of thermal shrinkage on reducing the positive segregation index in the billet center is limited.

Abstract

In this work, the random pore model (RPM) is utilized for the kinetic study of hematite reduction to Iron with CO. This can significantly contribute to the more effective design of reduction reactors in Iron production plants. Indeed, the developed RPM in this work employs a real pore size distribution (PSD) of the solid reactant, resulting in more realistic and accurate kinetic parameters. Accordingly, the kinetic parameters were calculated via RPM using the data from the reduction experiments of a highly porous pure hematite pellet. Validation of such kinetic parameters by different pure hematite and industrial pellets with various porous structures demonstrated RPM as the most comprehensive non-catalytic gas–solid reactions model. The activation energy obtained for the mentioned reaction was calculated at 25.5 kJ/mol. In addition, oxygen ions showed a mean diffusion coefficient of 1.18 × 10⁻¹⁶ m²/s for the industrial pellets through the Iron product layer. Furthermore, the importance of adjusting the CO–CO₂ ratio on the conversion in the reduction reactor was discussed. The results of this work could help reduce the amount of required CO and CO₂ product during the reduction of hematite to Iron.

Graphical Abstract