Effect of transverse static magnetic field on radial microstructure of hypereutectic aluminum alloy during directional solidification

doi:10.1016/j.jmst.2020.11.025

Abstract

Abstract:

The effect of different scales thermoelectric magnetic convection (TEMC) on the radial solidification microstructure of hypereutectic Al alloy has been investigated under transverse static magnetic field during directional solidification, focusing on the formation of freckle. Our experimental and numerical simulation results indicate that the TEMC circulation at sample scale under transverse static magnetic field leads to the enrichment of solute Al on one side of the sample. The TEMC and the solute enrichment degree increase with the increase of magnetic field when the magnetic field increases to 0.5 T. The enrichment degree of solute elements under magnetic field is affected by temperature gradient and growth rate. The non-uniform distribution of solute Al in the radial direction of the sample results in the non-uniform distribution of primary dendrite arm spacing (PDAS). Moreover, the applied magnetic field can lead to freckle formation and its number increases with the increase of magnetic field. The change of freckle is consistent with the anisotropy TEMC caused by the anisotropy of primary dendrite or primary dendrite network under magnetic field. Finally, the mechanism of synergism effect of the anisotropy TEMC, the distribution of solute Al and the PDAS on freckle formation and evolution is studied during directional solidification under magnetic field.

Key words: Static magnetic field, Directional solidification, Melt flow, Radial solidification microstructure, Freckle

Shaodong Hu, Long Hou, Kang Wang, Zhongmiao Liao, Wen Zhu, Aihua Yi, Wenfang Li, Yves Fautrelle, Xi Li. Effect of transverse static magnetic field on radial microstructure of hypereutectic aluminum alloy during directional solidification[J]. J. Mater. Sci. Technol., 2021, 76: 207-214.

Figures/Tables 10

Fig. 1. The microstructures near the liquid/solid interface in directionally solidified Al-40 wt% Cu alloy at 10 μm/s growth rate with various magnetic fields: (a) 0 T; (b) 0.1 T; (c) 0.3 T; (d) 0.5 T. (a1), (b1), (c1) and (d1) are the longitudinal solidification microstructures. (a2), (b2), (c2) and (d2) are the corresponding transverse solidification microstructures.

Fig. 2. Distribution of radial solute Al content of the sample in the Al-40 wt% Cu alloy during directional solidification under magnetic field.

Fig. 3. Distribution of radial PDAS of the sample in the Al-40 wt% Cu alloy during directional solidification under magnetic field.

Fig. 4. The microstructures near the liquid/solid interface in directionally solidified Al-40 wt% Cu alloy at various growth rates with and without magnetic field: (a) 0 T, 30 μm/s; (b) 0.5 T, 30 μm/s; (c) 0.5 T, 100 μm/s; (d) 0.5 T, 300 μm/s. (a1), (b1), (c1) and (d1) are the longitudinal solidification microstructures. (a2), (b2), (c2) and (d2) are the corresponding transverse solidification microstructures.

Fig. 5. The microstructures near the liquid/solid interface in directionally solidified Al-40 wt% Cu alloy at various temperature gradients with and without magnetic field: (a) 0 T, 60 K/cm; (b) 0.5 T, 60 K/cm; (c) 0.5 T, 90 K/cm; (d) 0.5 T, 120 K/cm. (a1), (b1), (c1) and (d1) are the longitudinal solidification microstructures. (a2), (b2), (c2) and (d2) are the corresponding transverse solidification microstructures.

Table 1 Physical properties of Al alloy in the numerical simulation [[20], [21], [22], [23]].

Name and symbol	Unit	Solid	Liquid
Absolute thermoelectric power, S	V/K	-1.5 × 10^-6	-2.25 × 10^-6
Electric conductivity, σ	(Ω m)^-1	7.9 × 10⁷	4.0 × 10⁶
Dynamic viscosity, μ	Pa s	-	2.9 × 10^-3
Density, ρ	Kg/m³	2.7 × 10³	2.4 × 10³
Thermal conductivity, λ	W/(m K)	150	90

Fig. 6. Numerical simulation for the thermoeletric magnetic effects near the sloping interface in the directionally solidified Al-based alloys under 0.1 T transverse magnetic field: (a) the geometry of computing domain; (b) the computed thermoelectric current (the unit in caption is A/m2); (c) the computed TEMC (the unit in caption is μm/s); (d) the computed TEMC in the X-Y plane (the unit in caption is μm/s). Red arrows show the direction of the TEMC and the colored surfaces present their magnitudes.

Fig. 7. Single dendrite scale numerical simulation for the thermoeletric magnetic effects in the directionally solidified Al-based alloys under 0.1 T transverse magnetic field: (a, d) the geometry of computing domain; (b, e) the computed TEMC (the unit in caption is μm/s); (c, f) the computed TEMC in the X-Y plane (the unit in caption is μm/s). The red arrows show the direction of the TEMC and the colored surfaces present their magnitudes.

Fig. 8. The schematic of TEMC under transverse magnetic field resulting in freckle formation in the mushy zone during directional solidification: (a) the TEMC circulation under transverse magnetic field; (b) the freckle formation caused by the anisotropic TEMC.

Fig. 9. The maximum value of the computed TEMC as a function of the magnetic field.

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