Kang Deng

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.

Design and application of differential thermal analysis apparatus in high magnetic fields.

The differential thermal analysis (DTA) apparatus has been developed for a commercial superconducting magnet. The DTA apparatus could detect the kinetics and thermodynamics of phase transformation with and without a magnetic field. Preliminary results for Al-Al2Cu eutectics are presented. The DTA curves indicate the similarity at several rates regardless of a magnetic field; however, at the same rate, melting transformation seems not to be influenced by a magnetic field, while solidification could be delayed via suppressing nucleation and crystal growth in a magnetic field. It will be believed that the DTA apparatus can be used to investigate the phase transformation of substances of interest in a magnetic field.

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

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

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.

Effects of electromagnetic vibration on the structure and mechanical properties of Al-6%Si alloy during directional solidification

The effects of electromagnetic vibration on the grain refinement in directional solidification were investigated. It was found that the electromagnetic vibration applied in the melt not only can refine grains remarkably but also can enhance both tensile strength and ductility values of Al-6%Si alloy. SEM graphs show that coarse dendrite structure was broken up into a somewhat globular structure, and the morphology of eutectic silicon was changed from flaky to fibrous under electromagnetic vibration treatment. The refine mechanism under electromagnetic vibration was discussed.

Solidification structure of eutectic Al-Si alloy under a high magnetic field-aid-electromagnetic vibration

An electromagnetic vibration was generated by simultaneously imposing a strong static magnetic field (up to 10 T) and an alternative electricity current to the metal. Its effects on the solidification structure of eutectic Al-Si alloy have been investigated experimentally. It is found that the eutectic structure has been refined by solely imposing high magnetic field while it is coarsened under the electromagnetic vibration. Furthermore, polyhedral Si grains and non-dendritic α-Al appeared when the electromagnetic vibration strength was strong enough. The refining of eutectic structure is attributed to the decrease of diffusion coefficient caused by the strong magnetic field. The coarseness of eutectic structure may be attributed to the convection caused by electromagnetic vibration. Strong convection may break co-operative growth of eutectic phases to form polyhedral Si grains and non-dendritic α-Al.

Investigation on Properties of the Silica Ceramic Cores for the Hollow Blades Prepared by the Conversion of the Silicon Resin

The working conditions of the ceramic cores are extremely harsh with the development of the preparation technology and improvement of the use conditions in the hollow blades, which brings forward high request for the preparation and properties of the ceramic cores. In this paper, silica ceramic powders were firstly coated by the silicon resin. The obtained composite powders were used to prepare the ceramic cores by the dry pressing method. And the phase compositions, microstructure and properties of the ceramic cores with the sintering temperatures were investigated. The results showed that the composite powders prepared by the coating showed good formability. The sintering temperature promoted the formation of the cristobalite. With the increase of the sintering temperature, the porosity of ceramic cores gradually decreased, the shrinkage rate and bulk-density increased. The prepared ceramic cores at 1250°C had highest bend strength of 19.25Mpa.

Solidification of horizontally continuous casting of super-thin slab in stable magnetic field and alternating current

The solidified structures of horizontally continuous casting (HCC) of super-thin slab and its relations with the current were studied under the electromagnetic vibration (EMV). The results show that, under the action of the periodical forces from EMV, the solidified structures of the super-thin slab of pure tin is greatly refined, and the extent of grain refinement is increased with the magnitude of alternating current. For the Sn-10%Pb alloy, it is shown that the EMV promotes the growth of equiaxed grains in the center of super-thin slab, and the grains are refined with the alternating current increasing. This is useful to prevent some solidification defects in the horizontally continuous casting of super-thin slab, such as columnar grains butting, porosity, inclusions and gases gathering, and composition segregation in the centre of slab.

Effect of a transverse magnetic field on primary-Si distribution during directional solidification in hypereutectic Al-Si alloy

The effect of a transverse magnetic field on the distribution of the primary Si in a directionally solidified Al-21 wt.% Si alloy is investigated. The results reveal that the application of the magnetic field leads to the appearance of banded structures of primary Si. Furthermore, the inclination of the banded structure decreases with the increase of magnetic-field intensity. The in situ measurement results of the Seebeck signal confirm the existence of a thermoelectric power difference between the solid phase and the liquid phase at the solid/liquid interface in the directionally solidified Al-21 wt.% Si alloy. Thus, the formation of the banded structures should be attributed to the thermoelectric magnetic convection (TEMC) and the resultant force of the primary Si, i.e., gravity force and thermoelectric magnetic force (TEMF). The migration of the primary Si toward the lower left side of the sample is induced by the resultant force, which leads to the formation of banded structures. Moreover, the increase of magnetic-field intensity increases the resultant force of the primary Si, resulting in a decrease of the inclination of banded structure.

Influence of Compaction Pressure on the Properties of Silica Ceramic Cores

Based on the application background of the ceramic cores for the hollow blades, silica ceramic cores were prepared by dry-pressing and subsequent pressureless sintering. The silica powders coated by silicone resin were used as the raw materials. The influence of compaction pressure on the properties of silica ceramic cores was analyzed. The results showed that with the increasing of compaction pressure, both the shrinkage rate and apparent porosity of the samples decreased. On the contrary, the increasing compaction pressure promoted the increasing of bulk-density and bending strength of the samples. The phase composition of the sample was composed of amorphous and crystallized silica. A number of cracks formed at the crystallization front of the grain owing to the volume change of crystallized silica during the cooling.