The effect of solidification time and heat treatment on the fatigue properties of a cast 319 aluminum alloy

Abstract

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...

Mechanisms of fatigue-crack initiation and their impact on fatigue life of AlSi7 die-cast components

“…1 (right) the fatigue strength S aD50% , is plotted against the mean pore size d equ . The data, taken from literature [1][2][3][4] as well as from own fatigue tests, include AlSi7-alloys with varying Mg-and Cu-content as well as different heat treatment conditions (T6, T7, F). All data lie within the same scatter band and can be described using a Kitagawa-model.…”

“…The corresponding threshold value K th is in the range of 2.0 to 2.8 MPa √ m. For slip band induced fracture origin it was found that fatigue strength can be estimated based on the materials yield strength, since the yield strength correlates with the resistance toward dislocation movement necessary for the formation of fatigue slip bands. This result was based on data from literature [1,3,5] of different Sr-refined AlSi7-alloys with varying Mg-and Cu-content in T6 and T7 heat treatment conditions in addition to own fatigue results using specimens from HIPed components.…”

Effet des additifs sur la microstructure et les propriétés mécaniques des alliages d'aluminium-silicium /

“…The third source of porosity develops from the natural volume contraction which occurs when a liquid solidifies; this is often referred to as microshrinkage porosity. 40 The presence of porosity is crucial in that it lowers the soundness of a casting and thereby its mechanical properties.…”

Effet des imperfections de la coulée sur les propriétés en fatigue des alliages de fonderie aluminium silicium /

“…Savelli et al 61 studied two AlSiMgO.3 alloys with different porosity contents with regard to fatigue properties. It was observed that quantitative parameters characterizing size or shape of internal pores could be measured on three dimensional reconstructed images and correlated to the fatigue properties.…”