In spite of these improvements, Cylinder Head B failed after only 180 hours in the thermo-cycle durability test. The general aspect of Cylinder Head B after a longitu- dinal cut parallel to the combustion face is presented in


Figure 6, which also shows the position of the cracks in cylinders #1 and #3. Both cracks initiated in the water jacket and propagated in the direction of the combus- tion chamber.


Table 4. Chemical Analysis for Cylinder Head B (357 Alloy)


(a)


(b)


(c) intercellular regions (d).


(d)


Figure 4. Cylinder Head A—typical features of the microstructure of cylinder #4 near the origin of the crack. Presence of microporosity (a) and (b), quite well spheroidized silicon particles (c) and some undissolved Mg2


Si particles in


24


International Journal of Metalcasting/Fall 10

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