The characterization of porosity in solids using the frequency dependence of the ultrasonic attenuation is discussed both from the theoretical and experimental viewpoint. The major thrust of our work is the determination of the volume fraction and size of the voids for the case of dilute porosity (<6%) in structural materials. An aluminum alloy (A357) was chosen for study due to its economic importance in large-scale casting and the particular suitability of aluminum for this type of study. Following recent papers the attenuation is described by an independent scatterer model for spherical voids. Numerical results are presented in a form suitable for use with a range of materials. A method for determining the volume fraction and pore size is given. Specific tabular results are given for stainless steel, IN-100, Ti, Si3N4, as well as aluminum. Figures of merit which partially describe those situations in which the method is usable are also presented. In the experimental work a digitized spectrum analysis system was used to measure the frequency dependence of the attenuation coefficient in A357 aluminum cast alloys. In the cast materials the average pore size was in the order of 100 μm and the pore concentration varied from essentially 0 to 6%. It was found that experimental measurement of the attenuation could be fit by the theoretical model. The resulting parameters yield a good estimate of the pore volume fraction.
The production of metallic fiber and filament directly from the melt has been practiced for some years. However, the application of these techniques has been limited due to materials-compatibility problems. Two new fiber-casting methods have been developed: Crucible Melt Extraction and Pendant Drop Melt Extraction. Both methods involve bringing the edge of a rotating disk into contact with a source of molten metal. Molten metal solidifies on the periphery of the disk, adheres there for a short time as it is removed from the melt source, and is then spontaneously released from the disk in the form of solid fiber. Both methods significantly reduce the technical difficulties heretofore associated with fiber casting and are amenable to operation in air, inert atmosphere, or vacuum. Metals as diverse as tin and niobium have been fiberized successfully.
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