Formation mechanism of ring-like segregation and structure during directional solidification under axial static magnetic field

doi:10.1016/j.jmst.2021.05.050

Abstract

Abstract:

The effect of the axial static magnetic field on the macro-segregation and structure in the Al-Cu and Ni-Mn-Ga alloys during directional solidification is investigated experimentally and numerically. It is found that the ring-like segregation and structure in the above-mentioned two alloys form during directional solidification at a certain growth speed under a moderate magnetic field. For the Al-Cu and Ni-Mn-Ga alloys, the moderate values of the magnetic field under which the ring-like structure forms are about 0.5 T and 1.0 T at respective growth speed of 10 μm/s and 5 μm/s. Further, the distributions of the flow and solute in the Al-Cu alloy during directional solidification under the axial static magnetic field is numerically simulated. Numerical results reveal that the rotary thermoelectric (TE) magnetic convection forms in the mushy zone during directional solidification under an axial magnetic field. This flow will induce the formation of the ring-like macro-segregation and structure. Changes in structures under the magnetic field in the experimental results are in good agreement with the distributions of the TE magnetic convection and solute in the numerical results. Therefore, the formation of the ring-like structure and segregation under the magnetic field should be attributed to the solute redistribution induced by the TE magnetic convection.

Key words: Ring-like segregation and structure, Magnetic field, Directional solidification

Qingdong Zhong, Huaiyu Zhong, Hongbo Han, Mingyong Shu, Long Hou, Yanyan Zhu, Xi Li. Formation mechanism of ring-like segregation and structure during directional solidification under axial static magnetic field[J]. J. Mater. Sci. Technol., 2022, 99: 48-54.

Figures/Tables 7

Fig. 1. Transverse structures in the mushy zone at 3 mm from the quenched liquid-solid interface directionally solidified Al-Cu and Ni-Mn-Ga alloys without and with an axial magnetic field: (a1) and (a2) Structures in the Al-0.85 wt.%Cu alloy without and with the 0.5 T magnetic field at the growth speed of 10 μm/s, respectively; (b1) and (b2) structures in the Ni58Mn25Ga17 alloy without and with the 1.0 T magnetic field at the growth speed of 5 μm/s, respectively.

Fig. 2. Transverse structures in the mushy zone at 3 mm from the quenched liquid-solid interface in directionally solidified Al-0.85 wt.%Cu alloy at the growth speed of 10 μm/s under various magnetic fields: (a) 0.05 T; (b) 0.1 T; (c) 0.3 T; (d) 0.7 T.

Fig. 3. Comparison of the transverse structures in the mushy zone during directional solidification without and with an axial magnetic field: (a) EDS maps on transverse structures and the distribution of the component Cu in the Al-0.85 wt.%Cu alloy without and with a 0.5 T magnetic field; (b) gamma-phase and the distribution of the component Ni in the Ni58Mn25Ga17 alloy without and with a 1.0 T magnetic field.

Fig. 4. Development of the structure in the Ni-Mn-Ga and Al-Cu alloys with the directional solidification process at respective growth speed of 5 μm/s and 10 μm/s under an axial magnetic field: (a) schematic illustration of the directionally solidified samples and the selected three transverse sections; (b)-(d) corresponding microstructures of the Ni58Mn25Ga17 alloy on the selected transverse sections; (e)-(g) corresponding microstructures of the Al-0.85 wt.%Cu alloy on the selected transverse sections.

Table 1 Physical properties of Al-Cu alloys and initial condition used during the numerical simulation process [16], [17], [18], [19], [20].

Properties	Magnitude
Electrical conductivity of solid (σ_s, Ω^-1•m ^{- 1})	1.3 × 10⁷
Electrical conductivity of liquid (σ_L, Ω^-1•m ^{- 1})	3.6 × 10⁶
Absolute thermoelectric power of liquid (S_L, ν•K ^{- 1})	1 × 10^-7
Absolute thermoelectric power of solid (S_S, ν•K ^{- 1})	1.0 × 10^-6
Temperature gradient (G, K•m ^{- 1})	5 × 10³
Dynamic viscosity (μ, Pa•s)	2.4 × 10^-3
Density (ρ, kg•m ^{- 3})	3.0 × 10³

Fig. 5. Numerical simulation of the TE magnetic convection at the sample and dendrite scales in the Al-Cu alloy during directional solidification under an axial magnetic field: (a) and (c) 3D geometry used to perform the simulation of the TE magnetic convection at the sample and dendrite scales; (b) and (d) TE magnetic convection at the sample and dendrite scales corresponding to (a) and (b); (e) and (f) value of the TE magnetic convection at the sample and dendrite scales as a function of the magnetic field intensity, respectively.

Fig. 6. TE magnetic convection in the Al-Cu alloy and its effect on the solute distribution during directional solidification under an axial magnetic field: (a1)-(a4) computed TE magnetic convection under 0.1 T, 0.3 T, 0.5 T and 1.0 T axial magnetic fields, respectively; (b1)-(b4) distribution of the TE magnetic convection under various magnetic fields (i.e., 0.1 T, 0.3 T, 0.5 T and 1.0 T); (c1)-(c4) distribution of the solute Cu caused by the TE magnetic convection under various magnetic fields (i.e., 0.1 T, 0.3 T, 0.5 T and 1.0 T).

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