Acoustic emission studies of electron beam surface modification of aluminum
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INTRODUCTION
D I R E C T E D high energy sources (lasers and electron beams) are currently being evaluated for surface modification of materials, including surface heat treatment, t~j rapid surface melting and resolidification, t21 and surface alloying.t~'2J In surface melting and resolidification, the bulk substrate, in intimate contact with the molten surface layer, acts as the quenching medium, resulting in high liquid/solid interface velocities and short solidification times during cooling.t21 These rapidly solidified alloys exhibit enhanced wear and corrosion resistance, arising, it is believed, from improved chemical homogeneity and the fine scale of the microstructure. These materials offer promise for applications such as bearing surfaces, cutting tool faces, and corrosion resistant parts. The growing number of industrial applications has spurred interest in understanding the fundamental phenomena involved in surface modification. Particularly useful are recent applications of heat flow theory to solidification models which define the evolving temperature fields and melting and resolidification velocities under given heat flux conditions. [2,7-91These models are also used to predict the solidification microstructures, t21 Metallographics studies of surface-modified materials have begun to confirm the predictions of this modeling approach. I9'1~ Additional techniques, to provide in situ information on the sequence of events during surface modification, are being sought, c3j Such techniques would complement post solidification metallography in providing a critical experimental evaluation of the predictive models. If in situ characterization could be achieved, the possibility would also then exist of allying emerging process control techniques to electron beam surface modification.
ROGER B. CLOUGH and HAYDN N. G. WADLEY are with Institute for Materials Science and Engineering, National Bureau of Standards, Gaithersburg, MD 20899. ROBERT MEHRABIAN is Dean, College of Engineering, University of California of Santa Barbara, Santa Barbara, CA 93106. Manuscript submitted April 6, 1987.
METALLURGICAL TRANSACTIONS B
In considering the physical basis for an in situ chai'acterization technique for surface modification, we note that transient thermal stresses are created during these treatments. These could be a direct source of acoustic emission if they develop sufficiently rapid. If not, even relatively slowly varying stresses can be relaxed by the generation and propagation of high velocity dislocations or microfractures-events that are well-known sources of acoustic emission. These acoustic emissions can be analyzed in various ways to extract further information about the source, including its location, type, size, and orientation, which might shed useful light on the process. 14'51 Because of the complexity of wave propagation in bounded media, we do not at this point attempt a theoretical analysis of wave propagation in order to interpret the recorded signals. Instead, we digitally record a signal and then
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