The typical feature of the microstructure near the origin of the crack in cylinder #3 is presented in Figure 2. The fracto- graphic analysis is presented in Figure 3.


The typical features of the microstructure near the origin of the crack in cylinder #4 are presented in Figure 4. The frac- tographic analysis is presented in Figure 5.


The main evidence obtained from the failure analysis was the large incidence of microporosity and the large size of the pores in the fracture surface as well as the fatigue crack


nucleation at the microporosity region in cylinders #3 and #4. The typical hydrogen content of the melts in the hold- ing furnace was evaluated by measurements of specific gravity in Reduced Pressure Test (RPT) samples measured at 2,56 to 2,58 g/cc (between 0.20 to 0.25 ml/100 g Al). The hydrogen level was considered excessive and probably the main reason for the high level of microporosity.


In addition, the SDAS in the combustion face was around 40 µm and in the region of the crack was 50 µm, both high- er than the recommended 35 µm maximum.


(a)


(b)


(c) intercellular regions (d).


(d)


Figure 2. Cylinder Head A—typical features of the microstructure of cylinder #3 near the origin of the crack. Presence of microporosity (a) and (b), quite well spheroidized silicon particles (c) and some undissolved Mg2


Si particle in


22


International Journal of Metalcasting/Fall 10


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