**1. Introduction**

The presence of porosity in aluminum castings is inevitable to a certain extent. Porosity in aluminum-silicon castings occurs when dissolved hydrogen liquid metal is rejected out during solidification through the interdendritic regions to compensate for the shrinkage associated with the solidification caused by the change in the casting volume. Hydrogen is the only gas that dissolves to a large extent in molten aluminum, leading to the formation of porosity [1, 2]. Majority of the studies of porosity formation in aluminum alloys have been carried out on aluminum silicon cast alloys [3–6]. According to Bian et al. [7], porosity can be classified into two categories: (i) microporosity (1–10 mm). This type of porosity is mainly shrinkage due to lack of molten metal feeding which is normally observed in alloys with long freezing range (e.g. 319 and 356 alloys), and (ii) microporosity (<500 μm) distributed uniformly throughout the matrix and caused by the rejection of the dissolved hydrogen gas.

Beside the hydrogen, modification with Sr or grain refining using Al-Ti-B master alloys would influence the formation of porosity. The porosity formed in Sr-modified castings has been related to a number of parameters i.e. hydrogen level of the melt, feedability (long mushy zone), and changes in the mechanism of eutectic nucleation [8]. Porosity characteristics in Sr-treated alloys depend on the amount of Sr oxides formed during solidification. Also, the presence of aluminum

oxide films would result in the formation of large pores that are linked together. The end result can be explained in terms of the difference in eutectic solidification in unmodified and Sr-modified alloys [9, 10]. Dinnis et al. [11] examined the amount, distribution, and morphology of porosity in sand-cast plates of Sr-free and Sr-containing Al-(0–9 wr%) Si alloys. The authors found that no apparent differences in the amount, distribution, and morphology of porosity were observed between Sr-free and Sr-containing alloys with no Si (i.e. pure Al). However, Sr modification significantly changed the amount, distribution, and morphology of porosity in alloys with high Si content.

Influence of oxides on porosity formation in Sr-treated alloys was investigated by Samuel et al. [12] in 319 and 356 alloys. Their findings show that porosity formation is frequently associated with strontium oxides (particles or films), as well as β-Al5FeSi phase platelets. The Al2SrO3 oxides are formed during melting by the high oxygen affinity of strontium. These oxides are difficult to be removed via degassing. Aluminum oxide films trapped in the molten metal result in the precipitation of coarser and deeper pores than those formed due to the strontium oxides [13].

According to Campbell and Tiryakioğlu [14], the beneficial effect of Sr as a modifying agent could be compromised by porosity development caused by the formation of oxide bifilms. The authors suggested an explanation in terms of the oxide population in the melt. A new multi-zone model is proposed by SkatTiedje et al. [15]. This model was developed by considering the effect of cooling rate on solidification and distribution of porosity in cast Al–Si alloys. The effect of Sr-modification on pore formation in reduced pressure and atmospheric conditions was studied by Miresmaeili et al. [16–18]. A new type of metallic cup with a riser was designed for use in a reduced pressure test as a mold. It was designed to improve directional solidification so that no macro shrinkage occurred in the test samples.

Another parameter to be considered is the presence of Fe-based intermetallics, in particular β-Al5FeSi. Taylor et al. [19] investigated the role of iron in the formation of porosity in Al-Si-Cu-based casting alloys. Their analysis pointed to inadequacies in the existing theories regarding the role of iron and suggests that a new theory is required to understand the observed behavior. Effect of alloy composition on the stability of the β-AlFeSi phase and the role of the latter in porosity formation in 319 and 356 alloys was studied by Khalifa et al. [20]. The β-phase particles are potential sites for porosity formation regardless of the alloy composition and the type or size of the β particles. SrO and Al2Si2 Sr particles were observed to have contact with the β-AlFeSi platelets in the microstructure. Thus, with the addition of Sr, finer, better distributed pores appear in the microstructure. The alloy tensile properties are greatly improved by: (a) Sr addition in the range 200–400 ppm, (b) increasing the Mn/Fe ratio to 0.7, (c) addition of 0.08%Be, (d) addition of 0.08% Be+0.02% Sr [21–23].

The present study was undertaken to emphasize the role of melt treatment parameters (stirring, degassing, Sr-modification, grain refining, hydrogen content, intermetallics) as well as alloy composition on porosity formation in Al-(6–11 wr%Si) cast alloys and their impact on the alloy hardness. The initial alloys were unmodified and contained 0.0075%Ti and less than 0.2%Fe.
