Ultrasonic Detection of Crack Opening at Cold Flake Interface of Aluminum Alloy Die-Cast

Ultrasonic measurement was carried out with specimens made by aluminum alloy die-cast plates (ADC12) having a cold flake to observe echoes reflected from the surface, bottom and cold flake. The echo reflected from the cold flake had the same phase as that of the surface echo, and it was found that the ultrasonic wave was reflected from the oxide layer of the cold flake. This means that the cold flake and the matrix are not separated but stick together or join together. Next, ultrasonic measurement was carried out with specimens containing cold flake transversely to the tensile direction during tensile testing with a water bag to examine the change in the echo reflected from the cold flake. In the case of the specimen containing coarse cold fake, the phase of the echo reflected from the cold flake was reversed in the tensile testing, and it was found that debonding occurred between the cold flake and matrix to open the crack at the cold flake. And in some cases, the echo reflected from the cold flake approached to the surface 0.1 mm or less with increasing tensile deformation. Similar results were obtained with specimens containing rather smaller cold flake. These results show that a crack initiates at the cold flake in the tensile deformation of the aluminum alloy die-casts, and also suggests that a crack may grow shortly before final failure.