Dafan Du

EBSD Study on the Effect of a Strong Axial Magnetic Field on the Microstructure and Crystallography of Al-Ni Alloys During Solidification

The effect of a strong magnetic field on the microstructure and crystallography of the primary and eutectic Al3Ni phases in Al-Ni alloys was investigated by using EBSD. The results show that the magnetic field significantly affected the microstructures and crystallography during both volume and directional solidification. As a result, the Al3Ni primary phases were aligned with the 〈001〉 crystal direction along the magnetic field and formed a layer-like structure. The magnetic field intensity, solidification temperature, growth speed, and alloy composition played important roles during the alignment process of the Al3Ni primary phase. Indeed, the alignment degree increased with the magnetic field and the solidification temperature during normal solidification. Moreover, the effect of the magnetic field on the crystallography of the Al-Al3Ni eutectic in the Al-Ni alloys was also studied. The applied magnetic field modified the orientation of the preferred growth direction of the Al3Ni eutectic fiber and the crystallographic orientation relationship of the Al-Al3Ni eutectic. The orientation of the preferred growth direction of the Al3Ni eutectic fiber depended mainly on the solidification direction and the alignment of the Al3Ni primary phase. Furthermore, a method for controlling the crystallization process by adjusting the angle between the solidification direction and the magnetic field was proposed.

Investigation on macrosegregation and dendrite morphology during directional solidification of Al-Cu hypereutectic alloys under a strong magnetic field

The effect of a strong magnetic field (up to 12 T) on the macrosegregation and dendrite morphology during directional solidification of the Al-22at.%Cu alloy has been investigated. Experimental results show that the application of the magnetic field caused the freckle macrosegregation and the fracture of the Al2Cu dendrites during directional solidification. With the increase of the magnetic field, the size of the freckle and dendrite decreases. Moreover, the electron back-scatter diffraction (EBSD) was applied to study the effect of the magnetic field on the morphology and orientation of the Al2Cu dendrite. The EBSD results revealed that although the dendrites were destroyed under the magnetic field, the magnetic field did not yet change the orientation of the Al2Cu crystal. The formation of the freckles and the fracture of the dendrites under the magnetic field may be attributed to the TE magnetic effects.

High Magnetic Field Processing of Metal Alloys

Recently, Direct Current (DC) magnetic field processing of materials has found widespread applications in metallurgy, especially in metals and semiconductor industries. The main goal is to control the behavior of melts during solidification so as to improve process performance and achieve better quality products. DC magnetic fields are effective in introducing some special magnetohydrodynamic effects, e.g., flow damping, which are commonly used in continuous casting of steels or crystal growth control. In parallel, the development of super conducting technology, which is able to produce high magnetic fields in a large space, has open many new possibilities in control of the processing of materials in solid and liquid state. The novelty comes from the creation of magnetization forces on non-magnetic or feeble magnetic materials due to high magnetic fields. Morerover, it has been realized quite recently that the thermo-electric phenomena under high DC magnetic field can produce strong electromagnetic forces in solid and liquid metals, leading to a phenomenon called Thermo-Electric-Magnetic Convection (TEMC). The forces are able to generate significant liquid motion especially when temperature gradients are present, and therefore strongly influence the solidification of metallic alloys. This chapter reviews the major progresses and applications related to the uses of strong/intense DC magnetic fields in processing of materials (mainly metallic alloys) in solidification processes. In the first section, we review the underlying principles in magnetohydrodynamics and magnetic effects. In the second section, we discuss the phenomena induced by DC magnetic fields in materials processing. We deal in particular with flow damping effects on liquid metals, and control of structure of materials during solidification, including texturing, phase separation and thermoelectric effect. Finally we give two examples of successful industrial applications.

Control of dendrite growth by a magnetic field during directional solidification

In this work, the alignment behavior of three kinds of dendrites (Al3Ni, alpha-Al and Al2Cu dendrites) with a remarkable crystalline anisotropy during directional solidification under an axial magnetic field is studied by the EBSD technology. Experimental results reveal that the magnetic field is capable of tailoring the dendrite alignment during directional solidification. Further, based on the crystalline anisotropy, a method to control the dendrite alignment by adjusting the angle between the magnetic field and the solidification direction is proposed. Copyright (C) EPLA, 2016

High-magnetic-field-induced formation of aligned equiaxed grains during directional solidification

The application of a high magnetic field is capable of inducing the formation of aligned equiaxed grains in alloys during directional solidification. The alignment and refinement of the grains is enhanced as the magnetic field intensity increases. The thermoelectric power difference at the liquid/solid interface in four alloys has been measured in situ during directional solidification and it is concluded that the formation of aligned equiaxed grains in a magnetic field should be attributed to the combined action of a thermoelectric magnetic force and a magnetization force.

Effect of a transverse magnetic field on solidification structure in directionally solidified Al-Cu-Ag ternary alloys

The effect of a transverse magnetic field on solidification structure in directionally solidified Al-Cu-Ag ternary alloys was investigated experimentally. The results show that the application of the transverse magnetic field significantly modified the solidification structures. Indeed, the magnetic field caused the formation of macrosegregation and the transformation of the liquid/solid interface from cellular to planar. Moreover, it was found that the magnetic field refined the eutectic cell and decreased the mushy zone length. This may be attributed to the thermoelectric magnetic convection between eutectic cells.