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

对ADC12铝合金压铸件进行了声学显微镜观察，观察了冷裂纹的分布情况。在声像图上观察到冷片，在初始凝固表面形成的氧化层是亮区，冷片的本体是暗区。利用声像技术制作了中心含有冷片的试件，并对试件进行了拉伸和疲劳试验。随着试件中冷片尺寸的增大，抗拉强度和疲劳寿命降低。尤其是当冷片暴露在试件表面时，试件的抗拉强度和疲劳寿命大大低于埋片的情况。然后，在拉伸试验和疲劳试验过程中，利用水囊进行了在线超声测量。在拉伸测试中，超声波在试件中传播，以获得从冷片…反射的回波更多。在拉伸试验过程中，冷片反射回波的方向(相位)发生了反转，表明冷片从母材中剥离，在冷片与母材的交界处形成裂纹。在疲劳试验中，在所需的疲劳循环次数时，暂停疲劳试验机并进行过程中的超声波测量。然后，在疲劳过程中观察到冷片反射回波方向的变化，说明疲劳过程中裂纹的形成。结果表明，在拉伸变形和疲劳过程中，冷剥落从母材中剥离形成裂纹。因此，利用最终的裂纹尺寸来估计断裂参数。在拉伸试验中，根据冷剥落的面积估算了临界应力强度因子，得到了接近断裂韧性的值。在疲劳试验中，从最终的裂纹尺寸估计出临界应力强度因子幅值，并且临界应力强度因子幅值的最大值与从拉伸试验中估计的临界应力强度因子的值相似。较少

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