Fig. 1: Round tensile specimen from a cast insulator.
Unusual results in tensile and Charpy impact tests prompted a detailed root-cause analysis at the MaterialtestCenter. Fractographic examinations using the scanning electron microscope (SEM) ultimately identified a classic shrinkage cavity as the cause of the markedly reduced mechanical properties.
Unusual tensile test results
A tensile test at the W.S. MaterialtestCenter clearly demonstrated the effect a shrinkage cavity can have on the mechanical and technological properties of a component. The technicians examined a B10×50 round tensile specimen taken from a cast insulator. Whereas an elongation after fracture of approximately A = 20% would normally be expected, the specimen achieved only A = 4%.
The strength values were also significantly below those expected, falling to around half as a result of the notch effect of the defect.
Fractography using the scanning electron microscope
The subsequent fractographic examination using the scanning electron microscope (SEM) clarified the cause of the unusual test results. A shrinkage cavity was discovered among the dimpled regions of the overload fracture (Fig. 2). It can be identified by numerous rounded structures that form when liquid metal solidifies freely within a cavity.
Dendritic structures form during solidification. They are clearly visible in the SEM and are considered a typical feature of a casting shrinkage cavity.
The effect of shrinkage cavities on component properties
Shrinkage cavities are volumetric defects in cast components and act as notches in the material microstructure. They can therefore significantly impair a component’s mechanical properties. Under static or dynamic loading in particular, such defects can act as initiation sites for cracks and premature component failure.
