A new immersion testing method has been developed where the water of the conventional immersion method was replaced with powder. In this method, ultrasonic testing is carried out while compressing the powder couplant between probes and object surfaces. Even though the powder usually prevents the transmission of elastic waves, they can be transmitted through it under pressurized conditions. In addition, the acoustic impedance can be matched to that of the objects being tested by controlling the pressure applied to the powder couplant. In this paper, an ultrasonic Tungsten powder couplant with a high acoustic impedance was applied to testing magnesium alloy specimens with three different surface properties: a smooth surface, a surface with a man-made defect and a surface with V-shaped grooves. The results were that the reflections from the specimen surfaces were eliminated by suitably adjusting the pressure of the powder. The man-made defect located just below the surface could be successfully detected.
