Fatigue behavior in an aluminum casting alloy (A356.2): Effects of some defects, SDAS, Hipping and strontium modification

Effects of pore, secondary dendrite arm spacings (SDAS ), hot isostatic pressing (Hipping), and strontium-modification on fatigue behavior were studied in an aluminum casting alloy (A356.2). Microstructures were revealed by X-ray radiography, light microscopy and scanning electron microscopy. Small-cracks were monitored by taking replicas of the surfaces with which the cracks intersected.; As the SDAS increases from 15 to 55 μm, fatigue life decreases by a factor of 3 in low-cycle fatigue, and 100 in high-cycle fatigue. When SDAS is less than 30 μm, the pore size is below a critical size of ∼80 μm and large eutectic constituents initiate cracks; and the initiation life is as high as 70% of the fatigue life. As the SDAS increases beyond 30 μm, pores are the main crack-initiation sites; the initiation life is as low as 5% of the fatigue life. Near-surface oxides initiate the fatigue crack regardless of SDAS. When crack initiated at pore and oxides, fatigue life is well correlated with the size of the initiation site and the effect of SDAS is overshadowed by the effect of pore.; Non-hipped A356.2 without Sr shows better fatigue life and the deleterious effect of pores overshadowed the beneficial effect that Sr-modification might have had. Hipping significantly increased the initiation life and small-crack propagation life of A356.2 with Sr as a result of the elimination of the porosity. However, hipping did not significantly improve the fatigue life of A356.2 without Sr. After hipping, Sr-modification is beneficial in improving the crack initiation life, and increasing both small-crack and long-crack propagation lives.; Fracture mechanics models (Newman-Raju, and Trantina-Barishpolsky models) yielded similar results on the crack-propagation rate against the effective stress-intensity factor range. In the micro-mechanics model, the theory of continuously distributed dislocations was applied to represent crack and crack-tip plastic zone, and the propagation rate was related to the length of the crack-tip plastic zone. When the grain size is used as the characteristic length of the microstructures, the model predicts the oscillations of the propagation rates and the predicted rates agreed reasonably well with those from experiments.