Influence of incident angle of ultrasonic waves on thickness measurement of remelted zone formed on aluminum alloy castings

The influence of the incident angle of the ultrasonic wave on the thickness measurement of a remelted zone formed on aluminum alloy castings was examined. When ultrasonic waves generated from a focused probe were incident on a specimen surface with oblique angles, the focusing of the waves in the specimen worsened with the increasing angle of incidence. When the incident angle was less than 10 degrees, the remelted zone thickness was tested from a distribution of backscattering wave intensity. However, tested values of the remelted zone thickness became larger with the increasing incident angle. When the incident angle was larger than 10 degrees, the intensity of the backscattering wave at a boundary between the remelted zone and the matrix became so small that the position of the boundary was not detected distinctly. The travelling time of the ultrasonic wave from the probe to the boundary was calculated numerically to estimate the remelted zone thickness. When the attenuation coefficient was assumed to be 1 to 2 dB/mm, the calculated thickness was in agreement with that tested by the ultrasonic measurement.