Acoustic visualization of cold flakes in aluminum alloy die-casts and evaluation of die-casts strength

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Aluminum alloy die-casts ADC12 were subjected to acoustic microscopy to observe the distribution of cold flakes. The cold flake was observed in the acoustic image; the oxide layer formed on the initially solidified surface was observed as the bright region, and the body of the cold flake was observed as the dark region. By using the acoustic image, specimens containing the cold flake at the center were fabricated and then subjected to tensile testing and fatigue testing. With increasing size of the cold flake in the specimen, the tensile strength and the fatigue life decreased. Especially, when the cold flake was exposed on the specimen surface, the tensile strength and the fatigue life were largely reduced from the case of the embedded one. Then, the in-process ultrasonic measurement was carried out during tensile testing and fatigue testing by using a water bag. In the tensile testing, the ultrasonic wave was propagated in the specimen to obtain the echo reflected from the cold flake … More . During the tensile testing, the direction (the phase) of the reflection echo from the cold flake was reversed, which suggested detachment of the cold flake from the matrix to form a crack at the boundary between the cold flake and the matrix. And, in the fatigue testing, at required numbers of the fatigue cycles, the fatigue tester was paused and the in-process ultrasonic measurement was carried out. Then, the change in the direction of the reflection echo from the cold flake was observed in the fatigue process, which suggested the formation of the crack in the fatigue process. It was suggested that during the tensile deformation and the fatigue process, the cold flake was detached from the matrix to form the crack. Therefore, by using the final size of the crack, the fracture parameter was estimated. In the tensile testing, the critical stress intensity factor was estimated from the area of the cold flake, and a value near the fracture toughness was obtained. In the fatigue testing, from the final size of the crack, the critical stress intensity factor amplitude was estimated, and the maxim value of the critical stress intensity factor amplitude took a value similar to the critical stress intensity factor estimated from the tensile testing. Less