Detection of Metallic Inclusions in Metals by Thermoelectric Coupling
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Detection of Metallic Inclusions in Metals by Thermoelectric Coupling Hector Carreon1 1 Instituto de Investigaciones Metalúrgicas, Edif.”U” Ciudad Universitaria Morelia, MICH 58000-888, MEXICO ABSTRACT A comparison between reported analytical results with experimental data of the magnetic flux density on cylindrical tin inclusions of elliptical cross-section embedded in a copper matrix under external thermal excitation is presented. By changing the aspect ratios b/a designated by e of the elliptical inclusions, a wide range of real situation such as slender inclusions can be simulated. The experimental magnetic field distribution illustrated the potential for the non contacting thermoelectric technique to detect and characterize metallic inclusions of different geometries based on their magnetic signature. Preliminary results on a cylindrical hard alpha (TiN) inclusion embedded in Ti–6Al–4V matrix is also presented to demonstrate that the proposed non-destructive method might be applicable to a wide range of alloys including highstrength, high-temperature engine materials. INTRODUCTION It has been demonstrated that the thermoelectric coupling in metallic materials can be exploited as a viable means of characterizing all types of imperfections and material defects such as inclusions, inhomogeneity, residual stresses, texture, fretting and segregations [1-3]. This nondestructive detection is carried out in an entirely non-contact way by using various types of magnetometers to sense the weak thermoelectric currents around the affected region when the specimen is subjected to an external temperature gradient. A schematic diagram of the thermoelectric measurement process in the presence of material imperfections is shown in Figure 1. For this case, an external heating or cooling is applied to the specimen to produce a temperature gradient in the region to be tested. This creates a situation in which different points of the boundary between the host material and the imperfection are at different temperatures, therefore also at different thermoelectric potentials. This will produce opposite thermoelectric currents inside and outside the imperfection. The thermoelectric currents form local loops that run in opposite directions on the opposite sides of the imperfection relative to the prevailing heat flux. When the specimen is scanned with a highly sensitive magnetometer, the magnetic field of these thermoelectric currents can be detected even when the imperfection is buried below the surface few milimeters and the sensor is as far as a couple of centimeters from the specimen [4,5]. In a reviewed article, Faidi and Nayfeh demonstrated the existence of the underlying physical phenomena by presenting a theoretical model capable of predicting the magnetic field produced by thermoelectric currents around cylindrical inclusions of elliptical cross-section under external excitation [6]. They investigated the shape and magnitude of the resulting thermoelectric signal with respect to the inclusion geometry. The effect of the orienta
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