Effects of Treatment Duration and Cooling Rate on Pure Aluminum Solidification Upon Pulse Magneto-Oscillation Treatment
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refinement enhances the mechanical properties of casting products. Over the last decades, many structure refinement techniques were developed and successfully applied. These techniques include modification, inoculation,[1] and/or treatment by external fields, such as ultrasonic[2,3] or mechanical vibration[4] and electromagnetic.[5,6] In recent years, pulse magneto-oscillation (PMO) and electric current pulsing (ECP) have been extensively studied as novel techniques for solid structure refining. Pulses of electric current are transferred through a metal during its solidification by an external oscillating magnetic field (PMO) or by electrodes inserted into the melt (ECP). Several models have been proposed to explain the mechanisms underlying the effects of PMO or ECP treatment on solidification. One of these models suggests that current pulses lead to a decrease in the thermodynamic barrier for nucleation and to an increase in the number of nuclei, thus, refining the microstructure.[7–9] A ‘‘crystal rain’’ mechanism was proposed,[10–14] whereby nuclei detach from the mold surface by Lorentz forces, drift
ITZHAK EDRY, Ph.D. Student, is with the Department of Materials Engineering, Ben-Gurion University of the Negev, P. O. Box 653, 84105 Beer-Sheva, Israel, and is also with NRC-Negev, P. O. Box 9001, 84109, Beer-Sheva, Israel. Contact e-mail: hayuns@bgu. ac.il TOMER MORDECHAI, MSc. Student, NACHUM FRAGE, Head, and SHMUEL HAYUN, Assistant Professor, are with the Department of Materials Engineering, Ben-Gurion University of the Negev. Itzhak Edry and Tomer Mordechai have contributed equally to this work. Manuscript submitted July 26, 2015. Article published online January 11, 2016 METALLURGICAL AND MATERIALS TRANSACTIONS A
into the melt, and provide refinement of the structure. In other works,[15–18] structural refinement was attributed to convection in the melt activated by a pulse magnetic field, which led to the fragmentation of dendrites and, hence, to an increase in nuclei number. The previously mentioned models tried to explain the effects of PMO or ECP applied at temperatures below the melting point (Tm). None, however, considered the structural changes that occurred in cases when treatment was applied solely at temperatures above the melting point.[15,19–21] The effects of PMO and ECP treatment at different periods along the cooling curve were systematically investigated by Gong et al.[11] and Liao et al.,[12] respectively. Both reported no effect of the treatment when applied above Tm, with the main effect of the treatment being attributed to the incubation period. However, it should be pointed out that the treatment of the liquid metal was stopped at a value higher than the Tm (5 K[12] and 25 K[11]). Our previous work confirmed that when PMO treatment was stopped at a temperature closer to the Tm (1.3 K above Tm), noticeable structure refining was observed.[21] The cavities activation model was suggested to explain the effect of PMO treatment above Tm. According to this model, upon PMO treatment, subcritical atoms (also known as
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