Ultrasonic method to determine gas porosity in aluminum alloy castings: Theory and experiment

Abstract

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.

Surface Roughness Effects in Porosity Assessment by Ultrasonic Attenuation Spectroscopy

“…Porosity assessment of flat smooth surface cast aluminum samples by ultrasonic attenuation spectroscopy were found to be in good agreement with other porosity assessment methods, e.g., weight-density, and optical techniques [1]. Ultrasonic results were also obtained for samples with their as-cast rough surface intact.…”

“…The total attenuation for the smooth surface sample, L, is a combination of many factors, and will be considered first: L LIMP+ LDIFF + LGRAIN + ··· + Lp (1) AF and AB are the front and back-wall amplitudes of the sample. LIMP and LDIFF are due to the impedance mismatch between solid-liquid interfaces and diffraction loss due to beam spread, respectively.…”

Computer Aided Ultrasonic Porosity Assessment for Cast Aluminum Alloy

“…4 C a(k) = -(--) r(ka p ) 3 a p (3) The calculated value for the reduced cross section of the scatterers, r(ka~), where ka p = 1.05 was substituted into Eq. 3 [1]. The results obta1ned from the porosity assessment program are shown graphically in Fig.…”

“…Af L = Ln Ab = LIMP + LDIFF + LGRAIN + LSURF + Lp (1) Attenuation losses from impedance mismatch, LIMP' and diffraction, LDIFF' caused by the liquid-solid interfaces and beam spread of the transducer, respectively, were calculated from known parameters and eliminated. Because of the effective operating frequency range and the material, the attenuation losses from grain scattering, LGRAIN' were neglected.…”

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Successful determination of the volume fraction and pore size of porosity in cast aluminum by ultrasonic attenuation spectroscopy has previously been reported [1]. Cast aluminum samples having a dilute distribution of pores ranging from 1% to 5% volume fraction and an average pore radius from 50 ~m to 500 ~m were considered.

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