establish the quality of each casting process. Also, with the Weibull modulus it is easier to predict the probability of fail- ure, at some reduced level of stress. The modulus is also a way to compare components made by different processes. For example, the best casting design shown in Figure 16 had a Weibull modulus of 50, which is close to values reported for aerospace forgings.32


As noted above, and shown in Figures 8 and 9, the elonga- tion to fracture is a more sensitive indicator of casting qual- ity. So, calculating the Weibull modulus for the elongation to fracture is probably the best way to determine material quality for safety critical applications.


Possible Improvements in Fatigue Life


In the above we found that good melt treatment (especially degassing), pouring practices and optimum mold design can produce castings having a tensile properties close to a theo- retically predicted ‘maximum’ value. But what about fatigue life? Are we reasonably close to a ‘maximum’ theoretical value? Or is significant progress still possible?


Earlier we saw that fatigue life in commercial castings is usually controlled by pore size (equation 11). But the amount of porosity and the size of the pores can be reduced by rapid solidification and good degassing. It is also pos- sible to HIP a casting after solidification. In practice, how- ever, it may be sufficient to reduce the maximum pore size to a reasonably small value, less than about 30-50 microns equivalent diameter.39


Now, once pores have been removed as a source of fatigue cracks, we are left with the seemingly ubiquitous oxide films. These take over as the largest flaw in the metal. Can these be removed? The answer is: “Yes.” Careful filtration of the metal is possible. Certain fluxing practices are also efficient at removing oxides. So fatigue life can be improved still further. What happens when we do this? Fatigue cracks then form on slip bands, which accumulate at the surface of the metal dur- ing application of cyclic stresses. This case presumably repre- sents the ‘ultimate’ fatigue strength of cast aluminum.


A recent study40 measured how different defects in the metal


determined fatigue life in Sr-modified A356-T6 alloy cast- ings. Figure 17 shows a Weibull plot of the measured fatigue life for three cases: crack initiation on pores, crack initiation on oxides, and crack formation on slip bands. It can be seen that removing porosity in a casting increases the fatigue life by about six times. Removing the oxides increased it further by almost ten times. In other words, the fatigue life of ‘aver- age’ commercial castings may be improved by as much as 50 times.41


Another example of the improvement possible with bet- ter casting practices is given in the study by Nyahumwa, Green and Campbell,42


who tested the fatigue properties of


the bottom filled (non-modified) bar castings, whose tensile strengths were plotted in Figure 16. Their results are shown in Figure 18 using the Weibull format. As expected, filter- ing the metal gave a significant improvement in fatigue life (cycles to failure, Nf


). They also HIPed unfiltered castings, which gave the best fatigue resistance.


Strength/% Figure 16. Weibull plot of normalized tensile data.32 International Journal of Metalcasting/Winter 11 19


Percent Failed


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