**Table 1.**

*Casting Processes and Modelling of Metallic Materials*

porosity in alloys with high Si content.

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

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

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%

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

**Table 1** lists the chemical composition of the as received ingots. The ingots of each alloy were cut, cleaned and melted in an electrical heated furnace using 6 kg SiC crucibles. The melting temperature was about 750°C ± 5 °C. Degassing was

unmodified and contained 0.0075%Ti and less than 0.2%Fe.

**114**

samples.

Be+0.02% Sr [21–23].

**2. Experimental procedure**

*Chemical composition of the as-received alloys (wt.%).*

carried out using a perforated graphite impeller turning at 135 rpm using pure Ar gas (0.492 cubic meter/hr) for about 30–40 minutes. The same impeller was also used as stirrer without gas. Strontium was added in the form of Al-10%Sr master alloy (waffles) whereas grain refiner was introduced as Al-5%Ti-15B master alloy in the form of short rods. It should be mentioned here that the manufacturing of the grain refiner produces Al2O3 which will increase in the amount of porosity. For this reason, the grain refiner in this study was added in the form of short rods (cold rolled) to minimize the presence of such oxides.

Due to the high affinity of Sr to react with oxygen to form SrO oxides, Sr was added during the last 10 minutes of degassing. As for varying the hydrogen level, small pieces of raw potato were added at the end of degassing, (in this case, Sr was not used) followed by AlScan™ measurement. In addition, reduced pressure test (RPT) samplings were taken from the melts. **Figure 1** shows examples of RPT samples sectioned in half to examine the porosity observed under different melt conditions. In all cases, samples for chemical analysis were also obtained from each melt prior to pouring.

Following melt treatment, the molten metal was poured in L-shaped metallic molds (75 mm × 35 mm × 30 mm). The molds were coated with fine grained boron nitride and were preheated at 450°C **Figure 2**. **Table 2** summarizes the sample codes and their melt treatment whereas **Table 3** shows complete analysis of A356.0 alloy as an example.

Samples for metallographic examination were sectioned from the middle of the cast bars (25 mm × 25 mm) and polished following standard procedures [24]. Porosity characteristics were evaluated using a Clemex image analyzer in conjunction with an optical microscope. Measurements were carried out at 100× as shown in **Figure 3**. For each sample at least 100 fields were scanned to enhance the accuracy of measurements. Selected samples were examined using an electron probe microanalyzer (EPMA) equipped with EDS and WDS systems, operating at 20 kV.

In order to quantify the effect of melt treatment parameters, hardness measuremnts were carried out on the sample surface shown in **Figure 2** using Brinell hardness testing machine using a 10 mm diameter steel ball and 500 kgf force. For each condition, an average of 8 readings was reported. The samples of A319.0

**Figure 1.** *RPT test taken from: (a) as received alloy, (b) after degassing, (c) 0.25 ml/100 g H2.*

#### **Figure 2.**

*L-Shape castings: (a) L-shape mold, (b) geometry of the L-shape casting, (c) cutting sequence of L-shaped casting to produce smaller rectangular bars, and (d) hardness test bars.*


#### **Table 2.**

*Codes of the used alloy s and their melt treatment.*

alloy were solution heat treated at 510°C for 8 h followed by water quenching. The samples thereafter were aged at various temperatures for 5 h followed by air cooling.
