Structural transition and macrosegregation of Al-Cu eutectic alloy solidified in the electromagnetic centrifugal casting
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W.Q. ZHANG, Associate Professor and Head of the Department, formerly Doctorate Student, Institute of Metal Research, Chinese Academy of Science, is with the Department of Materials Science and Engineering, Liaoning University of Engineering and Technology, Fuxin 123000, Liaoning, People’s Republic of China. Y.S. YANG, Professor and Head, and Q.M. LIU, Associate Professor, Department of Superalloys and Special Casting, Y.F. ZHU, Postdoctoral Researcher, and Z.Q. HU, Professor, Academician of Chinese Academy of Engineering, and Head, State Key Lab of Rapidly Solidified Nonequilibrium Alloys, are with the Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110015, People’s Republic of China. Manuscript submitted June 20, 1996. 404—VOLUME 29A, JANUARY 1998
(a)
(b)
(c) Fig. 1—Macrostructures of the alloy solidified from superheat 150 K with the mold rotating at 1200 rpm and the external magnetic field remaining: (a) B 5 0 T, (b) B 5 0.10 T, and (c) B 5 0.23 T. METALLURGICAL AND MATERIALS TRANSACTIONS A
transformed into the equiaxed. Meanwhile, fine columnar structure forms in the outer region. An increase of magnetic field intensity results in refinement of the generated equiaxed structure. It is worth noting that the thickness of the generated equiaxed zone is not uniform, which possibly results from the nonuniform flow in solidification,[6] and that employing a magnetic field with higher magnetic intensity leads to higher porosity in the inner regions of the samples. The macrostructural features are related to melt flow, which is determined by the body forces, including centrifugal force (fc), gravity ( fg), and Lorentz force (fe). These forces are depicted in Figure 2. According to Maxwell equations and Ohm’s law, the Lorentz force is written as fe 5 j 3 B 5 s (2¹ Fe 1 V 3 B) 3 B
[1]
Here, the gradient of electrostatic potential can be assumed as zero.[7] In estimation of the Lorentz force, the radial and axial components of flow velocity should be negligibly small compared with the tangential component because of the EMBR effect of the DC magnetic field. Hence, the volumic body forces in component forms can be expressed as 1 ˆ fe 5 2( sn B 2 sin2 u rˆ 1 s n B 2 cos2 u u) 2 fc 5
r n2 rˆ r
ˆ fg 5 2(r g cos u rˆ 1 r g sin u u)
[2] [3] [4]
where r is the melt density; s is the electric conductivity; g is the gravitational acceleration; j is the induction current; V and v are the velocity vector and its tangential component; B and B are the magnetic flux density in vector and scalar; Fe is the electric potential; and rˆ and uˆ are the radial and azimuthal unit vectors. From the preceding equations, the conclusions can be drawn that the force on the melt is dependent on both radial and azimuthal coordinates; the radial and azimuthal flows are expected to form due to the corresponding force components. Since the Lorentz force and gravity vary with azimuthal position, melt flow should also be dependent upon it. Furthermore, the negative feature of the tangential component of the Lorentz force implies
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