Jianbo Yu

Effects of a high-gradient magnetic field on the migratory behavior of primary crystal silicon in hypereutectic Al?Si alloy

Abstract The migration of primary Si grains during the solidification of Al–18 wt%Si alloy under a high-gradient magnetic field has been investigated experimentally. It was found that under a gradient magnetic field, the primary Si grains migrated toward one end of the specimen, forming a Si-rich layer, and the thickness of the Si-rich layer increased with increasing magnetic flux density. No movement of Si grains was apparent under a magnetic field below 2.3 T. For magnetic fields above 6.6 T, however, the thickness of the Si-rich layer was almost constant. It was shown that the static field also played a role in impeding the movement of the grains. The primary Si grains were refined in the Si layer, even though the primary silicon grains were very dense. The effect of the magnetic flux density on the migratory behavior is discussed.

Fabrication and Characterization of Porous Alumina-Based Ceramics Using Silicone Resin as Binder

ABSTRACT Porous alumina based ceramics were fabricated by injection method, where alumina powders were used as raw materials and solvent silicone resin was used as binder. During the heat treatment process, an organic-inorganic transformation occurred in silicone resin. The increase of heating temperature promoted the formation of mullite phase when the temperature was higher than 1500°C. The weight loss of porous alumina based ceramics was maintained at 5.7∼5.9%. When the heating temsperature was 1500°C, porous alumina based ceramics showed the biggest shrinkage rate, lowest apparent porosity and highest bending strength. GRAPHICAL ABSTRACT

Columnar-to-Equiaxed Transition and Equiaxed Grain Alignment in Directionally Solidified Ni3Al Alloy Under an Axial Magnetic Field

The effect of an axial magnetic field on the solidification structure in directionally solidified Ni-21.5Al-0.4Zr-0.1B (at. pct) alloy was investigated. The experimental results indicated that the application of a high magnetic field caused the deformation of dendrites and the occurrence of columnar-to-equiaxed transition (CET). The magnetic field tended to orient the 〈001〉 crystal direction of the equiaxed grains along the magnetic field direction. The bulk solidification experiment under a high magnetic field showed that the crystal exhibited magnetic crystalline anisotropy. Further, the thermoelectric (TE) magnetic force and TE magnetic convention were analyzed by three-dimensional (3-D) numerical simulations. The results showed that the maximum value of TE magnetic force localized in the vicinity of the secondary dendrite arm root, which should be responsible for the dendrite break and CET. Based on the high-temperature creep mechanism, a simple model was proposed to describe the magnetic field intensity needed for CET:

Effect of silicone resin on the properties of silica ceramic cores by heating treatment

B \ge kG^{ - 1.5} R^{1.25}

Improvement in creep life of a nickel-based single-crystal superalloy via composition homogeneity on the multiscales by magnetic-field-assisted directional solidification

B≥kG-1.5R1.25. The model is in good agreement with the experiment results. The experimental results should be attributed to the combined action of TE magnetic effects and the magnetic moment.

Effect of nitrogen content on the microstructure and mechanical properties of a cast nickel-base superalloy

We report the magnetic field dependence of the critical solidification rate for the stability of liquid-solid interfaces. For a certain temperature gradient, the critical solidification rate first increases, then decreases, and subsequently increases with increasing magnetic field. The effect of the magnetic field on the critical solidification rate is more pronounced at low than at high temperature gradients. The numerical simulations show that the magnetic-field dependent changes of convection velocity and contour at the interface agree with the experimental results. The convection velocity first increases, then decreases, and finally increases again with increasing the magnetic field intensity. The variation of the convection contour at the interface first decreases, then increases slightly, and finally increases remarkably with increasing the magnetic field intensity. Thermoelectromagnetic convection (TEMC) plays the role of micro-stirring the melt and is responsible for the increase of interface stability within the initially increasing range of magnetic field intensity. The weak and significant extents of the magneto-hydrodynamic damping (MHD)-dependent solute build-up at the interface front result, respectively, in the gradual decrease and increase of interfacial stability with increasing the magnetic field intensity. The variation of the liquid-side concentration at the liquid-solid interface with the magnetic field supports the proposed mechanism.

Mechanism of Desulfurization from Liquid Iron by Hydrogen Plasma Arc Melting

Abstract Silica ceramic cores prepared by heat-press molding were strengthened by impregnating silicone resin. The effect of heating treatment conditions on the properties of silica ceramic cores was analyzed. Results showed that the ambient bending strength increased from 9.3 ± 2.0 MPa to 24.8 ± 1.5 MPa by curing process at low temperature of 250 °C. However, further heating treatment at high temperature ranging from 1150 to 1300 °C made the strength of the samples lower than that of the cured samples owing to the decomposition of silicone resin. But the strength of the samples was still higher than that of raw samples. The increasing heating treatment temperature promoted an increase in the strength by a densification process.

The mechanism of inclusion removal from molten steel by dissolved gas flotation

Abstract Reliable Si3N4/nickel-based superalloy joints were obtained by partial transient liquid phase bonding with a Ni/Cu/Ti multi-interlayer. The flexural strengths of the joints were investigated at elevated temperatures. The maximum high-temperature bending strength of 115 MPa was achieved when the joint was tested at 1073 K. Microstructures and compositions of the joints after high-temperature testing were analyzed by scanning electron microscopy, energy dispersive spectroscopy, and transmission electron microscopy (TEM). A reaction layer with a thickness of about 3 μm was developed at the interface. The TEM results revealed a fine grain microstructure in the reaction layer of the joint after undergoing bending test at 1073 K. Cu-rich precipitates are observed in the fine-grained reaction layer by TEM. In addition, the high-temperature fracture mechanism of the joints was discussed. The fractures of the joints at room temperature exhibited a brittle fracture characteristic, while plastic deformation of the interlayers in the joint occurred at elevated temperatures.