Effect of Planar Flow Melt Spinning Parameters on Ribbon Formation in Soft Magnetic Fe 68.5 Si 18.5 B 9 Nb 3 Cu 1 Alloy
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INTRODUCTION
OF the rapid solidification techniques developed over the past few decades, the planar flow melt spinning (PFMS) process[1] has gained much importance commercially because of its capability of producing thin, wide, and continuous ribbons. The process is used extensively for producing Fe-based soft magnetic ribbons for transformer applications, as most of the soft magnetic materials are used in the form of ring cores or torroid, which can be fabricated directly from the ribbons. Because the permeability and lamination factor of a transformer core depend on the structure and surface quality of the ribbons,[2] it is essential to produce wide and continuous amorphous ribbons with good surface finish. The formation of quality ribbon by the PFMS process depends on the shape and stability of the melt puddle formed between the nozzle and the rotating copper wheel.[3] A continuous ribbon is obtained only when a dynamic equilibrium is established in the puddle. Several process parameters such as nozzle wheel gap, wheel speed, initial melt temperature, ejection pressure, etc. and material properties such as density, surface tension, viscosity, thermal conductivity, etc. influence the quality and soft magnetic properties of the as-spun ribbons. The severe conditions in the PFMS process (high wheel speed, large thermal gradients, small dimensions of the active zone, and small durations of the process) M. SRINIVAS, Scientist B, B. MAJUMDAR, Scientist E, and D. AKHTAR, Scientist G, are with the Advanced Magnetics Group, Defence Metallurgical Research Laboratory, Hyderabad 500 058, India. Contact e-mail: [email protected] G. PHANIKUMAR, Associate Professor, is with the Department of Metallurgical & Materials Engineering, Indian Institute of Technology Madras, Chennai 600 036, India. Manuscript submitted August 14, 2010. Article published online February 4, 2011. 370—VOLUME 42B, APRIL 2011
impose considerable restrictions on experimental investigations and therefore require numerical modeling to describe the real-time situation. Attempts have been made to predict numerically the thickness obtained under a given set of parameters.[4–7] Although numerical simulation of the PFMS process has been reported by several authors[8–10] to predict the fluid flow and temperature distributions in the puddle, experimental evidence of the variations in the puddle geometry with change in process parameters is limited. Few investigations using a high-speed camera have been carried out to study the transient shape of the puddle as a function of processing parameters.[4,11,12] However, these studies have been carried out on Al-based alloys using a backlight technique, and not many reports are available on puddle dimension of Fe-based alloys. Moreover, a window of processing parameters for these commercially important alloys is not available in the literature. Fe73.5Si13.5B9Nb3Cu1 alloy (FINEMET) ribbons produced by the PFMS process have attracted considerable interest as they possess a unique combination of soft magnetic properties that a
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