Title: The effect of the grain size of divertor’s cooling pipe on the capability of high-frequency ultrasonic tests to evaluate the quality of the bond between divertor’s cooling pipe and armor
Abstract: This study evaluated the effect of the grain size of the divertor’s cooling pipe on the capability of high-frequency ultrasonic tests to evaluate the quality of the bonded interface between the divertor’s cooling pipe and armor. First, simple oxygen-free copper and copper-chromium-zirconium block samples with different grain sizes were prepared and measured by an ultrasonic microscope with a 35 MHz probe. The results of the measurements confirmed that the non-uniformity of backwall echoes increased with the grain size of the samples. Samples with large grains provided distinctive signals that can be clearly confirmed on the ultrasonic C-scan images. Subsequently, two bonded samples consisting of 2.5 mm oxygen-free copper bonded with a block of pure tungsten that meets the material specifications of tungsten for ITER components which mimicked the basic design of a divertor’s cooling pipe and a monoblock, were measured to evaluate their bonded interfaces. One of the bonded samples bonded at a high temperature provided distinctive signals due to the enlargement of the grain of the oxygen-free copper. Results confirmed that the grain enlargement is the reason for reduced defect detection capability of the high-frequency ultrasonic tests as was suggested previously. This study also revealed that the enlargement of grain caused by improper manufacturing would be non-destructively detectable by high-frequency ultrasonic tests.