The relevance of forced melt flow to grain refinement in pure aluminum under a low-frequency alternating current pulse
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finement mechanism of alternating current pulse (ACP) on the solidification macrostructures of pure Al and the characterization of refining efficiency were investigated by embedding the wire mesh in the mold. The experiment results showed that ACP treatment during solidification led to the formation of fine equiaxed grain. There were remarkably differences with respect to the area of fine equiaxed zone inside and outside the tube. Lorentz force, induced melt flow and the rest of intrinsic effects of ACP inside and outside the tube were discussed in the present study. It demonstrated that the forced melt flow could lead to the columnar fragmentation and make the crystal nucleus on the mold wall fall off and drift in the liquid, leading to grain refinement. In addition, Reynolds number was suitable to characterize the refining efficiency of pure Al under ACP.
I. INTRODUCTION
Grain refinement has a strong influence on the improvement of mechanical properties in cast alloys, and thus is important for commercial application. Recently, the application of electric current pulse (ECP) during solidification has developed into a promising technique to achieve fine structures, and successfully applied in low-melting Pb–Sn alloys, middle-melting Al-based and Cu-based alloys, and even high-melting steel and iron.1–8 Despite the resulting morphology for the use of both pure Al and Al-based alloys in a number of industrial applications, a number of studies have been reported in the interrelation between microstructural effects such as the morphology of the resulting grains and/or microstructural array (e.g., interdendritic spacing formation) to the corresponding properties (e.g., mechanical or corrosion behavior).9–11 It is expected that the control of the resulting microstructural array would permit to prescribe guidelines with a view to preprogramming a required property.9–11 Although much attention has been attracted to the development of structures in Al-based alloys using ECP treatment, knowledge about the mechanism of ECP treatment leading to the decrease in grain size remains fragmentary. It is well known that ECP treatment usually affects the solidification process through a series of effects: electromigration, Joule heating, electromagnetic force, etc. Based on these effects, Contributing Editor: Jürgen Eckert Address all correspondence to these authors. a) e-mail: [email protected] b) e-mail: [email protected] DOI: 10.1557/jmr.2016.17 396
J. Mater. Res., Vol. 31, No. 3, Feb 15, 2016
various reasonable explanations have been suggested, such as the enhancement of nucleation rate resulting from more homogeneous liquid structure or an increase in the liquid–solid interfacial energy,1,12–14 the fragmentation of dendrites or the falling off of crystal nuclei from the wall of mold due to the forced convection,15,16 the suppression of grain growth due to Joule heating17,18 and strong local shear forces caused by magnetic pressure gradient.1 However, the dominant refinement mechanisms of ECP treatment are still controversial b
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