The use of ultrasonic thickness gauge casting material, the casting material due to its special nature, relatively large grains, the organization is not close enough, coupled with the often in the matte state on the measurement, so the measurement encountered greater difficulties. 

The first is the coarse grain and tissue intimacy caused by the great attenuation of acoustic energy, attenuation is caused by the material scattering and absorption of acoustic energy. The degree of attenuation is closely related to the grain size and the ultrasonic frequency. The attenuation increases with the increase of the grain diameter at the same frequency, but there is a highest point beyond which the grain diameter increases again and the attenuation basically increases At a fixed value. The attenuation for different frequencies increases with increasing frequency. 

Second, due to coarse grains and coarse out-of-phase structure in the casting, abnormal reflections, ie grass-like echoes or tree-like echoes will occur, causing erroneous readings in the thickness measurement, resulting in misjudgment. In addition, with the coarse grains, the elastic anisotropy in the direction of metal crystallization becomes more pronounced, resulting in differences in sound velocities in different directions, with a maximum difference of up to 5.5%. Moreover, the compactness of the tissue at different locations within the workpiece is also inconsistent, which will also result in differences in the speed of sound. All of these will result in inaccurate measurements. 

Therefore, castings should be especially careful. Casting measurements should be noted: the casting surface in the measurement of non-processed, you must use a larger viscosity of oil, butter and water glass as a coupling agent. It is best to use the same material with the object to be measured, and the direction of the object to be measured is the same as that of the standard test block to calibrate the speed of sound of the material. Two-point calibration if necessary.

Previous：Material hardness testing in the ceramic industry