Self-Regulating Behavior of Ferromagnetic Granules in Inductive Sintering Process
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SELF-REGULATING BEHAVIOR OF FERROMAGNETIC GRANULES IN INDUCTIVE SINTERING PROCESS
L.S.BULATOVA AND A.V.DMITRIEV Inst. for High Frequency Currents, Dept. of Inductive Heating of Metals, Pargolovo, 194902, Leningrad, U.S.S.R. ABSTRACT A physical model is presented to describe solid-phase sintering of pressed ferromagnetic powders under conditions of self-organization.
Inductive heating can efficiently induce sintering of ferrous powders, and it does so without causing any appreciable diminution of the pores or shrinkage of the pressing 11,2]. An adequate explanatory model should therefore be based on a mechanism other than surface effects. Because of their substantial intrinsic (free) energy, metallic powder agglomerates are dynamically unstable, and thus capable of acting spontaneously. It follows that sintering, a transformation of objects consisting of a multitude of components involved in an interaction under thermodynamically nonequilibrium conditions, can proceed as a self-organized process. This suggests using the synergistic approach to
treating sintering [3,4.1.
In a self-organized system, the competition between thermal, magnetodynamic, and electrodynamic processes results in the dominance of one of them at any given time. Provided there is a match between the frequency of the source current and the initial level of the inductor's strength, this must be reflected in the orderly consumption of the energy stored inthe pressing. The related changes in the material's thermodynamic condition will then be governed by-average parametric time dependences that have an essentially graded character, and the alternation of the states as a result of operation of the selection mechanism will be revealed by the bifurcations in the corresponding curves. Figure 1, which shows the temporal behavior of two integral parameters, the mean temperature T(T) and the acting current I(J), clearly indicates that the sintering process in inductively heated, pressed ferromagnetic powders develops in three distinct steps. Of the longest duration is the incubation step; on the T(QE)curve it is represented by the portion extending from the origin to point b, with point a marking the end of the initial period of intense heating of the sample. The heating is caused by significant hysteresis losses coupled with the thermal losses due to inductive currents. These form the pattern of a "network", or a "maze", superimposed on a stochastic distribution of the interparticle contacts of differing resistivity [1l. Ductile remagnetization, which comes early on, upsets the orientation pattern typical for the magnetization vectors in a disperse medium, and alters the state of the system from ordered to "disordered". The resulting random distribution of the magnetization values leads, through an increase in the material's coercive force and magnetic ductility, to.a buildMat. Res. Soc. Symp. Proc. Vol. 195. 01990 Materials Research Society
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Fig. 1. Characteristic changes in the integral parameters of the mean temperture T(t) and t
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