Effect of a transverse magnetic field on solidification structure in directionally solidified Al-40 wt% Cu alloys
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gnoud, Yves Fautrelle, and Rene Moreau SIMAP-EPM-Madylam/G-INP/CNRS, PHELMA, 38402 St Martin d’Heres Cedex,France
Xi Lia) State Key Laboratory Advanced Special Steel, Shanghai University, Shanghai 200072, People’s Republic of China; and SIMAP-EPM-Madylam/G-INP/CNRS, PHELMA, 38402 St Martin d’Heres Cedex,France (Received 14 August 2015; accepted 15 December 2015)
Effect of a transverse magnetic field on macrosegregation and growth of primary Al2Cu dendrites in directionally solidified Al–40 wt% Cu alloys was investigated experimentally. The experimental results indicated that the magnetic field caused the formation of channel-like and freckle segregations. It was also found that the application of the magnetic field benefited the growth of primary Al2Cu dendrites and the axial segregation. Moreover, the magnetic field decreased the primary dendrite spacing and the mushy zone length; however these effects weakened with the increase of the magnetic field intensity. The above experimental results should be attributed to the formation of the thermoelectric magnetic convection during directional solidification under the transverse magnetic field. I. INTRODUCTION
Convection is well known to play a key role in affecting the solidification structure in solidification processing. Hence, the effective control of convection is required to achieve high-quality castings. A great deal of research has showed that convection due to thermal and compositional variation can cause the formation of channel-like and freckle segregation defects1,2 and variation in the dendrite spacing.3,4 The impact of the convection forced by external conditions during solidification has also been investigated.5–7 Particularly, the effect of magnetic fields on convection during directional solidification has been shown quite dramatically.8–10 Indeed, Youdelis and Dorward8 found that the value of the effective distribution coefficient (the ratio of the actual solute concentration in the solid to that in the liquid near liquid/solid interface) in directionally solidified Al–Cu alloys decreased with a 3.4 T transverse magnetic field, suggesting the magnetic field enhanced mass transport in the liquid. Moreau9 et al. showed that the application of a magnetic field during the dendritic solidification of metallic alloys produces strong flows around dendrites, and large freckles were observed. Tewari10 et al. observed that the application of a 0.45 T transverse magnetic field caused distortion in a cellular array, and the freckles formed during the directional Contributing Editor: Michael E. McHenry a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2015.396 J. Mater. Res., Vol. 31, No. 2, Jan 28, 2016
solidification of Pb–Sn alloys. Subsequently, the quantitative experiments of the thermoelectric magnetic convection (TEMC) effect were studied. It was found that a local current due to the Seebeck effect appeared in the dendritic network, and its intensity depended on the electrical conductivity, the difference of thermoelectric (TE) power between
