A study was conducted to investigate the effect of solidification rate on the growth behavior of small fatigue cracks in a 319-type aluminum alloy, a common Al-Si-Cu alloy used in automotive castings. Fatigue specimens were taken from cast material that underwent a hot isostatic pressing (HIP) process in order to eliminate shrinkage pores and to facilitate the observation of surface-initiated cracks by replication. Naturally initiated surface cracks ranging in length from 17 m to 2 mm were measured using a replication technique. Growth rates of the small cracks were calculated as a function of the elastic stress-intensity-factor range (⌬K ). Long-crack growth-rate data (10 mm Յ length Յ 25 mm) were obtained from compact-tension (CT) specimens, and comparison to the small-crack data indicates the existence of a significant small-crack effect in this alloy. The solidification rate is shown to have a significant influence on small-crack growth behavior, with faster solidification rates resulting in slower growth rates at equivalent ⌬K levels. A stress-level effect is also observed for both solidification rates, with faster growth rates occurring at higher applied-stress amplitudes at a given ⌬K. A crackgrowth relation proposed by Nisitani and others is modified to give reasonable correlation of smallcrack growth data to different solidification rates and stress levels.

the form of acicular needles, blocky plates, or a refined fibrous structure (Figure 1). Also present are intermetallic phases and porosity, both of which can be deleterious to the fatigue resistance of cast 319 Al alloys. These features can act as stress raisers for crack nucleation or as low-energy paths for crack propagation. In general, increasing the solidification rate tends to reduce the average size of these features as well as to promote a finer distribution of gas and microshrinkage porosity.The effect of solidification time and heat treatment on the tensile properties of 300-series aluminum alloys has been studied and documented. [1][2][3][4][5][6][7][8] However, the effect of solidification time and heat treatment on the fatigue properties of 319-type alloys is less well documented. Several studies [9,10,11] in 319-type aluminum alloys found that fatigue cracks initiated from casting pores that were almost always located either close to or at the specimen surface. These studies found that, as the solidification time increased, the fatigue-initiating defect size increased, and the fatigue life decreased. Further, these studies found that the initiating pore diameters measured metallographically were up to an order of magnitude smaller compared to initiating pore diameters on the fracture surfaces. Similar results have been found for 356-type aluminum alloys. [4,6,[12][13][14][15][16] In general, it can be concluded that the pores reduce the time to crack initiation by creating a high stress concentration in the material adjacent to the pores. Thus, the crack initiation phase is negligible and most of the fatigue life is spent in crack growth.The absence of porosity has a dramatic effect on the fatigue life in cast aluminum alloys. Several studies [6,17,18] in 356-type alloys show that the elimination of porosity can increase the average fatigue life by up to a factor of 10. Examination of the failure surfaces indicated that fatigue crack initiation occurred on microstructural features such as intermetallics, the closed and welded remains of pores, and on persistent slip bands.Pore size appears to be the dominant factor in fatigue life compared to area or volume percentage porosity. A number of studies have found that the fatigue life is sensitive to the maximum pore size, but that no simple relationship existed between metallographic pore size and fatigue life.Solidification time and heat treatment are known to have a large effect on the microstructure of cast aluminum alloys. This study was conducted to quantify how the fatigue properties of a 319-type aluminum alloy are affected by solidification time and heat treatment. Both porosity-containing (non-hot isostatically pressed (HIP)) and porosity-free (HIP) samples in the T6 ("peak aged") or T7 ("overaged") heattreated conditions were tested. As the solidification time increased, the average initiating pore diameter increased and stress-controlled fatigue life decreased. Heat treatment was observed to have a large effect on fatigue properties of the HI...

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