Modern Ultrasonic Technique for Defect Detection in Cast Materials

Abstract

In casting flaw detection is exclusively concerned with manufacturing defects. The typical cast defects are (i) shrinkage cavities which form during solidification as a result of the reduction in volume when metal changes from the liquid to the solid state. Shrinkage cavities occur in situations where molten metal is not available to compensate for the volume decrease during solidi-fication,(ii) porosity and gas holes; porosity is small smooth-faced cavities, generally smaller than 1.5 mm diameter, usually caused by the release of gas from the molten metal as it cools. Gas holes are also smooth-faced cavities but greater than 1.5 mm diameter. Typical causes of gas holes are evolution of gas from molten metal during solidifications, gas trapped as the molten metal enters the mould etc. (iii) hot tears; these are jagged crack type defects resulting from stresses imposed on the cast metal when it is just below the solidification temperature and so is in a weak condition. The stresses usually arise when the casting is restrained during contraction by the mould, or by an already solid thinner section. The defect occurs mainly at or near a change of section and may or may not extend to the surface, (iv) cracks; these are dis-continuities due to the fracture of the metal during or after solidification, (v) inclusions; these are foreign non-metallic materials such as sand or slag trapped within the cast metal, (vi) cold shuts; these are basica-lly 'lack of fusion' defect caused by the failure of a stream of molten metal to form a continuous bond with a second stream, or solid metal such as an internal chill or splash. They are most prevalent in thin-walled castings.