number of cycles till failure. For example, all of the sam-
ples with shortest number of cycles till failure have a λ2 ~ 75 µm, while the samples with the longest number of cycles till failure were closest to the chill.
Engine Block Component Casting Analysis
From the overall analysis of the wedge castings, the only consistent metric (metallographic feature) that provides the possibility for enhanced fatigue performance over the W319 alloy in the U328 alloy is the porosity measurements. The fatigue testing used did show that there was considerable improvement for the lower Cu alloys, but the metallographic analysis indicates that the U328 alloy has the lowest intrinsic porosity. The U328 al- loy did not provide the best tensile prop- erties; however, the heat treatment used was not tailored to the Si or Cu concen- trations specific to the U328 alloy. It has been reported that porosity dominates over the type of heat treatment used as far as fatigue performance is concerned,12
thus it
is presumed that the focus should continue on the U328 alloy cast in the component casting compared to the production W319 version of the same alloy.
65µm. The first observation is that the porosity levels mea- sured from the component cast- ing are much larger when com- pared to the same alloys used for the wedge casting. This re- flects the effect of the progres- sive solidification conditions encountered in the wedge cast- ing. Boileau et al.,12
which used
the same wedge casting design, did not report such a large drop in porosity, as seen in Figures 18 and 19, except for fracture surface SEM observations, not metallographic analysis on the cross-sections. This is due to the elevated Sr concentrations
44
Figures 18 and 19 show the measured val- ues of PAF Porosity and largest pore di- ameter measured from a single metallographic sample from bulkhead section of the W319 alloy, W319 + Sr and the U328 alloy component engine blocks plotted along side the same results from the wedge cast- ings, all with λ2
between 57 and
Figure 15. The effect of chemistry on the value of the percent area fraction porosity (%).
used in Boileau’s wedge and component engine block cast- ings, while in this work no Sr additions were used.
Figure 20 shows the tensile properties of the bulkhead sec- tion of the component engine block casting with the W319 alloy, W319 with 70 ppm Sr added to the melt, and the U328 alloy. As seen in Figure 16 the ductility drops for the U328 alloy, again due to a larger volume fraction of Al-Si eutectic with lower α-Al primary dendrites; when the W319 alloy has added Sr the ductility also drops. With high Sr concentrations, the PAF porosity is higher reducing the effective cross-section of the tensile bar during loading in the tensile test.
Figure 16. The monotonic properties from the U319, W319, U328 & W319 alloys made from the wedge casting. λ2
in the legend. International Journal of Metalcasting/Fall 10
of the tensile test bars are indicated
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