Metallographic observations of β -AlFeSi phase and its role in porosity formation in Al–7%Si alloys

The present study was carried out to investigate by metallographic observation, the effect of alloy composition on the stability of the β-AlFeSi phase and the role of the latter in porosity formation in Al–7%Si alloys. The results showed that Mn is more efficient in stabilising the Chinese-script morphology phases (i.e. α-AlFeSi phases) in the 356 alloys than in 319 alloys. The number of β-AlFeSi particles increases at the expense of the α-AlFeSi particles with increasing Mg level, indicating that Mg counters the effect of Mn. Sr is effective in reducing the number and fraction of β-AlFeSi phase through fragmentation and dissolution of the latter, counteracting the effect of Mg on the same. SrO and Al2Si2 Sr particles were observed to have frequent physical contact with the β-AlFeSi platelets in the microstructure. The effect of P in promoting the β-AlFeSi platelets is believed to be in part due to the role of (Al, P)O2 in the nucleation of β-phase. Stirring for long time, addition of Sr, and melt superheating all favour the formation of co-eutectic β-AlFeSi particles and reduce their volume fraction. The β-phase particles are potential sites for porosity formation regardless of the alloy composition and the type or size of the β particles, whereas the α-AlFeSi particles do not seem to be frequently associated with pores under the same conditions. With the addition of Sr, the porosity profile changes so that finer, better distributed pores appear in the microstructure. The SrO particles play a role in pore formation, and this is probably a reason for the appearance of widely distributed porosity in the Sr-modified Al–Si alloys.