Assessment of circumferential cracks in hypereutectic Al-Si clutch housings

- Differences in morphology between two sides of the crack are indicative of different solidification cooling rates.
- Fast solidification leads to hot tearing involving crashing of eutectic silicon as a side effect.
- Differenced in solidification rates are probably resulted from non-uniform cooling.
- Brittle silicon and iron-rich intermetallic phases act as easy crack paths.

As in situ natural composites with silicon phase acting as the reinforcing phase, Al-Si alloys are among most commonly used aluminum alloys in automotive applications (i.e. engine component). Silicon contributes to the strength of Al-Si alloys through load transfer from the Al matrix to the hard (rigid) Si phase in the microstructure (load-carrying capacity). Casting parameters (i.e. solidification rate, elemental segregation, secondary dendrite spacing…) as well as the size and distribution of the microstructural constituents in Al-Si alloys (i.e. morphology of Si particles, intermetallic compounds, secondary dendrite spacing) contribute directly to the mechanical response and failure (or fracture) behavior of the alloy within the service. In hyper-eutectic Al-Si alloys (i.e. B390.0), distribution of coarse pre-eutectic Si particle mainly contribute to stress concentration, crack initiation and propagation during the actual service condition. In the present paper, the parameters contribution to the formation of the circumferential cracks in clutch housings made of die cast hyper-eutectics B390.0 Al-Si alloys are assessed through optical microscopy and scanning electron microscopy. Casting variable, cooling rate, their effect on the cracks as well some of the possible causes are also discussed in detail.