Another method used to detect internal defects is radiographic inspection. When done correctly, radiographic inspection is the best nondestructive method for detecting internal defects, such as shrinkage and inclusions.
In this method, a casting is exposed to radiation from an x-ray tube. The casting absorbs part of the radiation, and the remaining portion of the radiation exposes the radiographic film. Dense material withstands the radiation penetration, so the film is exposed to a lesser degree in those areas, giving the film a lighter appearance. Less dense materials allow more penetration and correlates to darker areas on the film. Any hole, crack or inclusion that is less dense than the casting alloy is revealed as a dark area.
When done correctly, radiographic inspection is the best nondestructive method for detecting internal defects, such as shrinkage and inclusions, and the radiograph serves as a permanent record of the casting quality that can be reviewed by multiple personnel. Casting thickness and density will limit the rang of inspection possible, depending on the energy level of the radiation.
Radiographic inspection also can be performed without film. Instead, the x-ray image is viewed on a video screen. Computerized axial tomography (CAT scanning) also is being used to develop 3-D computer imagery to inspect a casting’s soundness.