(squeeze and SSM) components having the smallest size of pores. The fracture surface of all specimens were examined under a microscope, to determine the size of the defect ini- tiating fatigue crack growth. The measured fatigue life, Nf the stress amplitude (maximum applied stress), σa defect area, Ai


, , and the , were related by the equation:17, 18, 19 Equation 11


where m is equal to 4.2 and log B is 16, when one uses units of MPa, µm2


life, respectively.


The relationship given in equation [11] appears to have a wide range of validity. For example, in the study by Couper et al.,20


several different heat treatments were applied to vary


the strength. This had no significant effect on the fatigue life. The size of defects controlled fatigue life in all con- ditions. Similar results were found when the solution time was changed.21


grade automotive castings.22


There are also recent studies of commercial In these materials the largest


defect was almost always a pore; which controlled the rate of fatigue failure.


Above we considered the effect of solidification rate on the tensile properties. It will now be useful to consider the effect of freezing rate on pore size and fatigue strength. Looking at the data from the study by Fang and Granger,7


we find


the results in Figure 12. Pore diameter is reduced by rapid solidification. Also, by comparing curves (1) and (3), we see that grain refinement reduces the pore size. Hence, grain re- finement and rapid solidification contribute to smaller pores and an improved fatigue life.


These considerations have been confirmed experimentally. Figure 13 gives an example of how fatigue life relates to pore size and to solidification rate in a casting.


Equation 11 and the results plotted in Figure 13 were obtained from failed fatigue specimens. That is, the size of the pore initiating fatigue failure was measured on the fracture surface. Un- fortunately, determining the size of the ‘largest pore’ by standard metallo- graphic examination is not straightfor- ward. As noted above, the distribution of porosity in commercial castings is non uniform. This problem was consid- ered by Wang and Jones, who offered a solution provided by the use of extreme value statistics.23


From a practical point of view, howev- er, the implications are clear. Control of porosity is the single most important


International Journal of Metalcasting/Winter 11 Cooling Rate, ºC/s Figure 12. Pore size in A356 alloy castings.7 15 and cycles for stress, defect area and fatigue


• good degassing and melt treatment • effective grain refinement • proper modification practice, and • rapid solidification


Also, some new casting processes apply pressure to solidify- ing castings, to reduce the amount of porosity and the size of the resulting pores.


It is also possible to reduce porosity in castings by the use of Hot Isostatic Pressure treatment (HIPing). Numerous studies have shown that fatigue life can be improved in this way.


It will now be instructive to consider the sources of casting defects.


Sources of Casting Defects


found. These act as defects. Because of the stochiometry of the compound, and the results shown in Figure 9, even small amounts of iron have a significant effect: At 0.3% Fe one- half of the elongation is lost. Thus, iron contents must be held to low levels for best quality.


In Figure 4 we saw that iron in Al-7Si-0.3Mg alloy cast- ings resulted in a significant loss in elongation and tensile strength. Iron has low solubility in solid aluminum, so most of the iron in the liquid metal forms brittle interme- tallic compounds. In Al-Si casting alloys Al5


FeSi plates are


factor in obtaining good fatigue life in net-shaped castings. Compared to porosity, the strength level of the alloy (as de- termined by heat treatment or alloy composition) is less im- portant. This means that best results are obtained by:


Silicon may also form large, brittle flakes in Al-Si-based casting alloys. This is especially true in large, slowly cooled


Equivalent Average Pore Diameter (dp), µm


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