Figure 21 shows an example of the staircase plot generated for the W319 alloy made in the component engine block bulkhead. Figure 22 shows the mean value of the fatigue staircase data for the W319 with added Sr, W319 and the U328 alloy. The mean value is determined as indicated in the experimental procedure, along with a -3σ calculation indicated in Figure 22. The result of lower porosity, par- ticularly the largest pore measured, and higher fatigue per- formance, is expected. Note that a 40% increase in fatigue performance from the W319 alloy to the U328 alloy is observed. This difference in performance was dif- ficult to see from the fatigue performance comparison of the wedge castings due to the number of test sam- ples available, and due to the much lower porosity levels seen in the wedge castings.
Confirmation that pores were responsible for initiat- ing fatigue failures in test samples was performed us- ing Scanning Electron Microscopy in the Secondary Electron (SEM/SE) mode. SEM/SE images of one of the failed fatigue test samples are shown in Figure 23 where the dashed white line marking the transition between the slow fracture region includes a 229 µm pore near the surface of the fatigue test sample, and a second pore measured at 575 µm. Note that the pore seen on the SEM/SE fracture surface is larger than the largest pore diameter measured from the metal- lographic analysis in Figure 19. Both the portion of fatigue test samples from the component casting that did not run-out, and all of the fatigue test samples from the wedge casting, were confirmed to have initi- ated from micro-shrinkage pores.
Discussion
The objective of this work was to establish whether the wedge cast- ing alone was adequate to answer which alloy composition investi- gated would provide the best op- tion for high durability in an en- gine block type casting, or would it be better to cast new alloys directly into a component engine block de- sign for this determination, or both.
The wedge castings were effective at indicating a ranking in porosity severity from the chill end to the riser end with 2-D metallographic analysis. However, the fatigue testing performed was not able to provide a clear improvement in terms of the number of cycles until failure, except that lower Cu alloys seem to have long lives before fail- ure. This was due to the fact that
International Journal of Metalcasting/Fall 10
. This very observation in porosity in the com- ponent engine block castings has also equated to a 40% increase in mean fatigue stress attained using the staircase
the number of test samples used may have been too low, and that the differences in porosity generation between the alloys became minimized with progressive solidifica- tion conditions. However, the component engine block casting exhibited a larger difference between the W319 and the U328 alloys in porosity for the same measured value of λ2
Figure 17. The plot of fatigue test sample lives, ranked from longest (sample #1) to shortest (sample #7) from the U319, W319, U328 & W319 alloys made from the wedge casting. Fatigue conditions are R = -1, 103.4 MPa, 45 Hz. The λ2
of the
seven (7) fatigue test samples range from 50 to 80 µm, which is a PAF porosity below 0.10%.
Figure 18. The percent area porosity as measured for both the wedge & cast components having similar λ2
as indicated above. 45
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