Acoustic visualization of cold flakes and crack propagation in aluminum alloy die-cast plate

Acoustic images received at different depths of aluminum alloy (ADC12) die-cast plates containing coarse cold flakes were compared with microstructures at the same depth. The bright and dark regions in the acoustic images coincided with the oxide layer on the initially solidified surface of the cold flake and the body of the cold flake, respectively. These results show the ability of nondestructive detection of cold flakes by the ultrasonic microscopy. The specimens were then fabricated to contain the cold flake at a center, and subjected to three-point bending test. When the coarse cold flake existed apart from the tensile side surface of the specimen, a crack initiated at a small cold flake on the side surface and propagated in the specimen. In this case the bending strength was almost the same as the tensile strength (285 MPa) of the die-cast plate. When the cold flake existed on the tensile side surface or at a position less than 0.5 mm inside the surface, the crack propagated along the oxide layer of the cold flake. In this case, the bending strength decreased from the tensile strength when the cold flake approached the side surface. A relation between the bending strength and the position of the cold flake in the specimen was discussed from the standpoint of the fracture mechanics.