The typical features of the microstructure near the origin of the crack of cylinder #1 are presented in Figure 7. The frac- tographic analysis is presented in Figure 8.
The typical features of the microstructure near the origin of the crack of cylinder #3 of Cylinder Head B are presented in Figure 9. The fractographic analysis is presented in Figure 10.
The SDAS was around 30 µm in the combustion face and 40 µm in the region of the crack. The hydrogen level was around 0.15ml/100 g Al (specific gravity of 2,63 g/cc in RPT sam- ples). Since the microporosity level and the SDAS were con- sidered adequate, it was concluded that the premature failure of the casting was caused by the presence of new oxide film. The presence of new oxide films is normally associated with turbulent filling of the casting cavity.9
the source of the turbulence, a computer simulation of the fill- ing of the cylinder head was made using Magmasoft® 11 and 12 present results of the filling simulation.
In order to investigate . Figures
The original gating system is unpressurized from the sprue to the runner and pressurized from the runner to the in- gates. This design promotes at least two problems: incom- plete filling of the runner (opportunity for air aspiration and formation of back rolling wave – figure 11) and high velocity at the ingates (turbulence inside the cavity – figure 12). In order to reduce the metal flow turbulence and the formation and entrapment of the new oxide films, the gat- ing system was redesigned.
The new gating system was designed to avoid detachment of the metal flow from the mold wall.10,11
The sprue was
tapered to compensate for the gravity acceleration of the metal flow. The design of the runner is critical since the flow direction changes at the sprue/runner junction, creat- ing an opportunity for air aspiration. To counteract this ten- dency, the runner was designed with a shallow rectangular section (height limited to 6mm) and it was connected to the sprue by a fillet radius.
(a)
(b)
(c)
(d)
Figure 9. Cylinder Head B—typical features of the microstructure of cylinder #3 near the origin of the crack. The size of the microporosity is compatible with the fineness of the microstructure (a). The silicon particles are well spheroidized (c) and there are some undissolved Mg2
Si particles, in intercellular regions (d).
28
International Journal of Metalcasting/Fall 10
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