Hence, in the present investigation, the influence of test surface roughness on ultrasonic echo amplitude in Al-Cu alloy cast specimens has been studied and the results are reported. Relative amplitude measurements of back-wall echoes in one-side roughness specimens of Al-4.5% Cu alloy casting were made by using immersion pulse-echo technique. A digital ultrasonic flaw detector and immersion probes, having central frequencies of 2, 5 and 10 MHz, respectively, were used to measure the back-wall echo amplitude of the test specimen was facing the transducer. The centre line average value of roughness of the test specimens ranged from 2.4 to 19.8 µm.

The roughness of one side of the each test specimen was varied by machining the surface with a sharper or by grinding the surface. The surface of the other side of all the test specimens was maintained smooth and at the same level of surface finish by mechanically polishing the specimen surface in successive steps with different grit size energy papers.

For each test specimen, the amplitude of the back-wall echo received from its rough surface was compared with the amplitude of the back-wall echo received from the smooth, polished surface (reference surface) of the standard specimen in order to determine the relative echo amplitude which is defined as the ratio of the amplitude of the back-wall echo received from the rough surface of the specimen under test divided by the amplitude of the back-wall echo received from the smooth, polished surface (reference surface) of the standard specimen. In the present study, both the sides of the standard specimen were maintained smooth and at the same level of surface finish of about 0.46µm by mechanically polishing the specimen surfaces in successive steps using different grit size emery papers.

The relative echo amplitude and also the percentage decrease in echo amplitude at three different probe frequencies were determined for each test specimen characterized by its surface roughness value.

The results of the present study indicate that as the value of surface roughness increases, the value of the relative back-wall echo amplitude at a given probe frequency decreases. Further, it is observed that as the probe frequency increases, the value of the relative echo amplitude decreases for a fixed value of the surface roughness. However, under the present set of experimental conditions, the decrease in backwall echo amplitude with the increase in surface roughness of the test specimen is found to be appreciable at 10 MHz probe frequency.

In general, about 24% decrease in the echo amplitude is observed when the value of surface roughness of the specimen is about 20µm and the probe frequency is 10 MHz. The results obtained during the current study are in conformity with the observations made in the published literature.

The possible reason for the observed decrease in the amplitude of ultrasonic back-wall echoes with the increase in surface roughness of the test specimen could be the interaction of impinging ultrasonic wave with the roughness (surface dilutions) on the specimen surface. Consequently, increased loss in wave energy due to the increase in reflection/scattering of the ultrasonic wave while transmitting and receiving (i. e. while entering and leaving the specimen) takes place and hence the observed decrease in echo-amplitude.