**3.1 Grain size**

Selected samples from three levels of grain refining were used for macrostructure analysis. The polished samples were etched in a solution composed of (66% HNO3, 33% HCl, 1% HF) and results are listed in **Table 4**. It is evident that with the increase in the amount of the added TiB2 there is a significant decrease in the alloy

**117**

**Code** **Si**

> B1

B3 B3 B5 B5 B6 B7 B8 B9

*\**

*B was not determined.*

**Table 3.**

*Actual chemical composition of A356.0 alloy melts prepared.*

6.18

0.1052

0.0887

0.0011

0.3102

0.0007

**0.1117**

<0.0002

**0.0001**

~0.25 ml/100 g

93.1

6.10

0.0990

0.1124

<0.0005

0.3116

<0.0005

**0.1094**

<0.0002

**0.0002**

~0.15 ml/100 g

93.2

5.96

0.1596

0.1992

0.0101

0.2716

<0.0005

**0.2307**

0.0360

**0.0172**

degassed

92.8

6.17

0.1781

0.2087

0.0107

0.2854

0.0007

**0.2104**

0.0360

**0.003**

degassed

92.6

6.09

0.0925

0.0066

<0.0005

0.3121

<0.0005

**0.1122**

<0.0002

**0.0193**

degassed

93.3

6.16

0.0943

0.0032

<0.0005

0.3149

<0.0005

**0.1082**

<0.0002

**0.0159**

degassed

93.3

6.29

0.0927

0.0067

<0.0005

0.3171

<0.0005

**0.1127**

<0.0002

**0.0001**

degassed

93.1

6.21

0.0991

0.0597

<0.0005

0.3143

<0.0005

**0.1075**

<0.0002

**0.0001**

degassed

6.24

0.0993

0.0574

0.0005

0.3137

0.0004

**0.0002**

<0.0002

**0.0001**

degassed

93.3

**Fe**

**Cu**

**Mn**

**Mg**

**Cr**

**Ti**

**B\***

**Sr**

**H2**

**Al**

*Melt Treatment-Porosity Formation Relationship in Al-Si Cast Alloys*

93.2

*DOI: http://dx.doi.org/10.5772/intechopen.94595*

**Element (wt%)**


*Melt Treatment-Porosity Formation Relationship in Al-Si Cast Alloys DOI: http://dx.doi.org/10.5772/intechopen.94595*

*Casting Processes and Modelling of Metallic Materials*

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.

**Metallurgical parameter A319.0 A356.0 A413.0** Without degassing A1 B1 C1 After degassing A2 B2 C2 WithTiB2 (0.11% Ti) A3 B3 D3 With Sr (0.02%) A4 B4 D4 With TiB2 (0.11% Ti) and Sr (0.02%) A5 B5 D5 H2 (0.15 ml/100 g) A6 B6 D6 H2 (0.15 ml/100 g) A7 B7 D7 With TiB2 (0.11% Ti) A8 B8 D8 With TiB2 (0.22% Ti) and Sr (0.02%) A9 B9 D9

*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.*

Selected samples from three levels of grain refining were used for macrostructure analysis. The polished samples were etched in a solution composed of (66% HNO3, 33% HCl, 1% HF) and results are listed in **Table 4**. It is evident that with the increase in the amount of the added TiB2 there is a significant decrease in the alloy

**116**

**3. Results and discussion**

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

**3.1 Grain size**

**Table 2.**

**Figure 2.**

**Table 3.**

*Actual chemical composition of A356.0 alloy melts prepared.*

grain size, reaching about 80% when the concentration of grain refiner is about 0.2%Ti. **Figure 4** reveals the significant reduction in the grain size of the A413.0 alloy refined with 0.22%Ti. EPMA examination indicated that the initial Al-Ti-B master alloy was decomposed into TiB2 (in the form of ultra-fine dispersed particles

#### **Figure 3.** *Schematic distribution of the examined fields.*


### **Table 4.**

*Average grain size of the three alloys used.*

**119**

*Melt Treatment-Porosity Formation Relationship in Al-Si Cast Alloys*

(50–100 nm) causing the observed refining) and platelets of (AlSi)3Ti within the

*Decomposition of the added Al-5%Ti-1%B master alloy: (a) backscattered electron image, (b) X-ray image of* 

It is deemed important to present the solidification curves (0.8°C/s) of the three used alloys in order to arrive at a clear understanding of the parameters that control

It is well established that porosity is formed due to dissolved hydrogen in the liquid metal, shrinkage during solidification process, and oxide films. Severity of porosity depends on the amount of these three parameters [1–3]. **Tables 5–7** present the measurements of the porosity characteristics as a function of the applied melt treatment. Degassing for a sufficient amount of time would lead to reduction in the dissolved hydrogen in the liquid metal as well as removal of most of inclusions and oxide films [14–18]. Thus all alloy melts were degassed prior to casting except for the hydrogen–containing alloys (coded 6 and 7) where degassing was done prior to introduction of H2. It is inferred from these tables that hydrogen results in precipitation of large pores leading to a relatively high percentage of porosity with low pore density. Although Sr was added only 10 minutes before the end of degassing, however, the SrO formed during this period was enough to increase the amount of

The combined effect of oxides associated with the manufacturing of the grain refiner (mainly Al2O3) along with SrO contributed to the increase in the amount of porosity. It is noticed that the porosity in the A413.0 alloy is relatively less than in the other two alloys which may suggest that as-received alloy was previously degassed using chlorine, as inferred from its low Mg content (see **Table 1**). Also, it is should be taken into consideration that A413.0 alloy has a short freezing range compared to A319.0 and A356.0 alloys, see **Figure 5**. The average aspect ratio of the samples without degassing falls in the range of 2.0–2.3. After gassing, the average values of the aspect ratio lie in the range 1.2–1.32 indicating that the precipitated pores are more-or-less of spherical shape. However, in all cases, degassing with Ar

**Figure 7** presents a comparison of average pore area and porosity percentage parameters for the three used alloys, highlighting the effect of oxides (SrO, Al2O3 films and/or bifilms), and the effect of hydrogen content. As can be seen there is a direct proportionality between the two parameters, associated with low pore density indicating porosity agglomeration which would lead to poor mechanical properties.

*DOI: http://dx.doi.org/10.5772/intechopen.94595*

α-aluminum as demonstrated in **Figure 5** [25, 26].

porosity but not as high as that caused by H2.

could reduce the porosity by about 90%.

the porosity characteristics. These curves are shown in **Figure 6**.

**3.2 Porosity**

**Figure 5.**

*Ti distribution.*

**Figure 4.** *Reduction in the grain size of the A413.0 alloy: (a) 0.007%Ti, (b) 0.22%Ti.*

*Melt Treatment-Porosity Formation Relationship in Al-Si Cast Alloys DOI: http://dx.doi.org/10.5772/intechopen.94595*

**Figure 5.**

*Casting Processes and Modelling of Metallic Materials*

grain size, reaching about 80% when the concentration of grain refiner is about 0.2%Ti. **Figure 4** reveals the significant reduction in the grain size of the A413.0 alloy refined with 0.22%Ti. EPMA examination indicated that the initial Al-Ti-B master alloy was decomposed into TiB2 (in the form of ultra-fine dispersed particles

**Code Grains size (**μ**m) SD (**μ**m) %Refining** A1 678 29 base A3 544 248 20 A6 214 54 68 B1 811 178 base B3 692 293 15 B6 229 78 71 D1 2466 386 base D3 2156 269 12 D6 475 97 80

**118**

**Figure 4.**

**Figure 3.**

**Table 4.**

*Schematic distribution of the examined fields.*

*Average grain size of the three alloys used.*

*Reduction in the grain size of the A413.0 alloy: (a) 0.007%Ti, (b) 0.22%Ti.*

*Decomposition of the added Al-5%Ti-1%B master alloy: (a) backscattered electron image, (b) X-ray image of Ti distribution.*

(50–100 nm) causing the observed refining) and platelets of (AlSi)3Ti within the α-aluminum as demonstrated in **Figure 5** [25, 26].

## **3.2 Porosity**

It is deemed important to present the solidification curves (0.8°C/s) of the three used alloys in order to arrive at a clear understanding of the parameters that control the porosity characteristics. These curves are shown in **Figure 6**.

It is well established that porosity is formed due to dissolved hydrogen in the liquid metal, shrinkage during solidification process, and oxide films. Severity of porosity depends on the amount of these three parameters [1–3]. **Tables 5–7** present the measurements of the porosity characteristics as a function of the applied melt treatment. Degassing for a sufficient amount of time would lead to reduction in the dissolved hydrogen in the liquid metal as well as removal of most of inclusions and oxide films [14–18]. Thus all alloy melts were degassed prior to casting except for the hydrogen–containing alloys (coded 6 and 7) where degassing was done prior to introduction of H2. It is inferred from these tables that hydrogen results in precipitation of large pores leading to a relatively high percentage of porosity with low pore density. Although Sr was added only 10 minutes before the end of degassing, however, the SrO formed during this period was enough to increase the amount of porosity but not as high as that caused by H2.

The combined effect of oxides associated with the manufacturing of the grain refiner (mainly Al2O3) along with SrO contributed to the increase in the amount of porosity. It is noticed that the porosity in the A413.0 alloy is relatively less than in the other two alloys which may suggest that as-received alloy was previously degassed using chlorine, as inferred from its low Mg content (see **Table 1**). Also, it is should be taken into consideration that A413.0 alloy has a short freezing range compared to A319.0 and A356.0 alloys, see **Figure 5**. The average aspect ratio of the samples without degassing falls in the range of 2.0–2.3. After gassing, the average values of the aspect ratio lie in the range 1.2–1.32 indicating that the precipitated pores are more-or-less of spherical shape. However, in all cases, degassing with Ar could reduce the porosity by about 90%.

**Figure 7** presents a comparison of average pore area and porosity percentage parameters for the three used alloys, highlighting the effect of oxides (SrO, Al2O3 films and/or bifilms), and the effect of hydrogen content. As can be seen there is a direct proportionality between the two parameters, associated with low pore density indicating porosity agglomeration which would lead to poor mechanical properties.



#### **Table 5.**

*Characteristics of porosity in A319.0 alloy.*

In general, the formation of pores is governed by the relation: ΔP = 2σ/r, where σ is the surface tension, and ΔP is the critical pressure needed to create a nuclei of radius r [27]. Thus, introducing a high level of H2 in the liquid

**121**

*Melt Treatment-Porosity Formation Relationship in Al-Si Cast Alloys*

**Average pore length (**μ**m)**

> **Average pore length (**μ**m)**

**Aspect ratio Density** 

**Aspect ratio Density** 

**Ave. SD Ave. SD Ave Ave. Ave SD**

D1 157 244.9 15 08 2.12 0.38 0.17 0. 20 0.004 D2 82 71.44 15 09 1.76 0.62 0.19 0.01 0.002 D3 454 462 30 14 1.56 0.65 0.83 0.134 0.038 D4 832 236 40 16 1.56 0.55 2.19 0.357 0.222 D5 935 296 61 21 1.87 0.60 1.14 0.592 0.148 D6 1490 341 17 21 1.31 0.53 1.72 0.643 0.157 D7 1840 802 46 21 1.29 0.41 2.08 0.857 0.186 D8 785 122 19 07 1.67 0.75 5.97 0.305 0.066 D9 965 181 35 05 1.70 0.66 6.58 0.552 0.182

**(pores/**μ**m2**

**)**

**Ave. SD Ave. SD Ave. SD — Ave. SD**

B1 201 72 74 43 2.01 0.52 3.22 0.570 0.455 B2 59 81 14 11 1.83 0.43 2.08 0.026 0.049 B3 178 325 19 19 1.67 0.84 2.58 0.168 0.052 B4 1113 208 48 24 1.35 0.65 1.30 0.424 0.078 B5 1014 129 123 46 1.24 0.85 4.61 0.604 0.170 B6 1230 612 54 22 1.29 0.39 1.17 1.451 0.283 B7 1893 189 98 34 1.15 0.40 2.89 1.952 0.196 B8 484 274 26 11 1.75 0.68 2.75 0.388 0.060 B9 1488 452 29.76 24.77 1.70 0.69 5.78 0.556 0.126

**(pores/**μ**m2**

**)**

**Ave. area percentage (%)**

> **Ave area percentage (%)**

aluminum would increase ΔP, decreasing r, and hence increasing the porosity percentage. **Figure 8(a)** shows an example of the pore shape observed in gassed A413.0 alloy (coded D6) where the pore is almost round. **Figure 8(b)** represents a pore caused by the shrinkage of the casting on going from liquid to solid state. Due to volume change associated with this process, the final casting shrinks by about 7%, which is accommodated within the final cast in the form of irregular pores passing through the interdendritic structure (**Figure 8(b)**). In certain cases, the gas pore could later on change into a shrinkage pore as seen in

Formation of oxide films during the course of melting due to insufficient degassing could lead to formation of massive pores as in the case of Sr-modified alloys where Sr was exposed to the outer atmosphere for a long period of time, as examplified in **Figure 8(d)**. Li et al. [28] determined the nature of this oxide as Al2.3SrO3.3 using WDS analysis. Another type of oxide films associated with the pores is displayed in **Figure 8(e)** where the molten metal was partially stirred. According to Mohanty et al. [29] this type of oxide can act as nucleation sites for pore formation

**Figure 8(c)**, depending on the level of dissolved gas.

*DOI: http://dx.doi.org/10.5772/intechopen.94595*

**Code Average pore area (**μ**m2 )**

**Table 6.**

**Table 7.**

*Characteristics of porosity in A356.0 alloy.*

*Characteristics of porosity in A413.0 alloy.*

**Code Average pore area (**μ**m2 )**


### *Melt Treatment-Porosity Formation Relationship in Al-Si Cast Alloys DOI: http://dx.doi.org/10.5772/intechopen.94595*

#### **Table 6.**

*Casting Processes and Modelling of Metallic Materials*

**120**

**Table 5.**

**Code Average pore area (**μ**m2 )**

**Figure 6.**

*Characteristics of porosity in A319.0 alloy.*

**Average pore length (**μ**m)**

In general, the formation of pores is governed by the relation: ΔP = 2σ/r, where σ is the surface tension, and ΔP is the critical pressure needed to create a nuclei of radius r [27]. Thus, introducing a high level of H2 in the liquid

**Aspect ratio Density** 

**Ave. SD Ave. SD Ave. SD — Ave. SD**

A1 682 231 41 16 2.3 0.70 5.86 0.356 0.207 A2 99 123 12 08 1.63 0.69 2.80 0.029 0.018 A3 129 61 25 17 1.53 0.76 5.89 0.125 0.108 A4 844 119 30 17 1.60 0.56 2.78 0.780 0.041 A5 762 371 75 25 1.86 0.69 9.89 0.488 0.135 A6 975 325 40 12 1.32 0.62 6.64 1.845 0.317 A7 1505 368 106 23 1.29 0.53 5.61 2.414 0.039 A8 465 100 36 21 1.78 0.61 9.66 0.418 0.215 A9 1023 448 43 14 1.78 0.68 7.05 0.843 0.267

*Solidification curves and their first derivatives obtained for: (a) A319.0, and (b) A356.0, (c) A413.0 alloys.*

**(pores/**μ**m2**

**)**

**Ave area percentage (%)** *Characteristics of porosity in A356.0 alloy.*


#### **Table 7.**

*Characteristics of porosity in A413.0 alloy.*

aluminum would increase ΔP, decreasing r, and hence increasing the porosity percentage. **Figure 8(a)** shows an example of the pore shape observed in gassed A413.0 alloy (coded D6) where the pore is almost round. **Figure 8(b)** represents a pore caused by the shrinkage of the casting on going from liquid to solid state. Due to volume change associated with this process, the final casting shrinks by about 7%, which is accommodated within the final cast in the form of irregular pores passing through the interdendritic structure (**Figure 8(b)**). In certain cases, the gas pore could later on change into a shrinkage pore as seen in **Figure 8(c)**, depending on the level of dissolved gas.

Formation of oxide films during the course of melting due to insufficient degassing could lead to formation of massive pores as in the case of Sr-modified alloys where Sr was exposed to the outer atmosphere for a long period of time, as examplified in **Figure 8(d)**. Li et al. [28] determined the nature of this oxide as Al2.3SrO3.3 using WDS analysis. Another type of oxide films associated with the pores is displayed in **Figure 8(e)** where the molten metal was partially stirred. According to Mohanty et al. [29] this type of oxide can act as nucleation sites for pore formation

**Figure 7.**

*Comparison of oxides and hydrogen level on the alloy pore size and porosity percentage: (a, b) effect of oxides as a function of the total amount of added Sr and grain refiner, (c, d) effect of hydrogen level.*

**123**

**Figure 9.**

**Figure 8.**

*Melt Treatment-Porosity Formation Relationship in Al-Si Cast Alloys*

*Effect of melt treatment on the pore morphology in A413 alloy: (a) D6, (b) D9, (c) D7, (d) D3, (e) D1, (f) D2.*

*(a) Backscattered electron image of D9 alloy, (b) X-ray map showning distribution of O2 in (a).*

*DOI: http://dx.doi.org/10.5772/intechopen.94595*

*Melt Treatment-Porosity Formation Relationship in Al-Si Cast Alloys DOI: http://dx.doi.org/10.5772/intechopen.94595*

*Casting Processes and Modelling of Metallic Materials*

**122**

**Figure 7.**

*Comparison of oxides and hydrogen level on the alloy pore size and porosity percentage: (a, b) effect of oxides* 

*as a function of the total amount of added Sr and grain refiner, (c, d) effect of hydrogen level.*

**Figure 8.** *Effect of melt treatment on the pore morphology in A413 alloy: (a) D6, (b) D9, (c) D7, (d) D3, (e) D1, (f) D2.*

#### **Figure 11.**

*(a) High magnification backscattered electron image of the white area in Figure 7 (broken line), (b, c) X-ray maps revealing the distribution of Ti and B, respectively, in (a).*

as shown by the black arrows in **Figure 7(e)**. **Figure 8(f )** exhibits the microstructure of a well degassed sample. **Figure 9(a)** is a backscattered electron image of D9 alloy (high Ti content) revealing the oxide films or bifilms [14] associated with the

**125**

**Figure 12.**

**Figure 13.**

*particles situated inside the pore [30, 31].*

*Melt Treatment-Porosity Formation Relationship in Al-Si Cast Alloys*

manufacturing of the Al-Ti-B master alloy (white arrows) and present in the sample due to insufficient degassing or mechanical stirring. The black arrows highlights the

*(a) SrO films and particles observed within a pore in 319 alloy; (b) backscattered electron image showing SrO* 

*(a) Backscattered electron image of TiB2 particles mixed with SrO particles/films in D9 alloy, (b and c) X-ray* 

*maps of Ti and O2, respectively in (a). The bright spots are fragments of SrO films.*

**Figure 10** is a backscattered electron image of **Figure 8(c)** displaying the change in the nature of the precipitated pore from round (due to H2) into irregular (shrinkage-white arrow) pore during the coarse of solidification leading to the formation of a massive pore in D9 that contains high amounts of Ti and Sr. The presence of utlra fine particles was noticed within the gas pore (white broken lines) in **Figure 10**. A high magnification image of this part is displayed in **Figure 11(a)** revealing the presence of

presence of (Al.Si)3Ti platelets in the vicinity of the oxide films.

*DOI: http://dx.doi.org/10.5772/intechopen.94595*

*Melt Treatment-Porosity Formation Relationship in Al-Si Cast Alloys DOI: http://dx.doi.org/10.5772/intechopen.94595*

#### **Figure 12.**

*Casting Processes and Modelling of Metallic Materials*

*Backscattered electron image revealing a massive pore composed of gas and shrinkage.*

**124**

**Figure 11.**

**Figure 10.**

as shown by the black arrows in **Figure 7(e)**. **Figure 8(f )** exhibits the microstructure of a well degassed sample. **Figure 9(a)** is a backscattered electron image of D9 alloy (high Ti content) revealing the oxide films or bifilms [14] associated with the

*maps revealing the distribution of Ti and B, respectively, in (a).*

*(a) High magnification backscattered electron image of the white area in Figure 7 (broken line), (b, c) X-ray* 

*(a) Backscattered electron image of TiB2 particles mixed with SrO particles/films in D9 alloy, (b and c) X-ray maps of Ti and O2, respectively in (a). The bright spots are fragments of SrO films.*

#### **Figure 13.**

*(a) SrO films and particles observed within a pore in 319 alloy; (b) backscattered electron image showing SrO particles situated inside the pore [30, 31].*

manufacturing of the Al-Ti-B master alloy (white arrows) and present in the sample due to insufficient degassing or mechanical stirring. The black arrows highlights the presence of (Al.Si)3Ti platelets in the vicinity of the oxide films.

**Figure 10** is a backscattered electron image of **Figure 8(c)** displaying the change in the nature of the precipitated pore from round (due to H2) into irregular (shrinkage-white arrow) pore during the coarse of solidification leading to the formation of a massive pore in D9 that contains high amounts of Ti and Sr. The presence of utlra fine particles was noticed within the gas pore (white broken lines) in **Figure 10**. A high magnification image of this part is displayed in **Figure 11(a)** revealing the presence of

#### **Figure 14.** *The formation of porosity in: (a) unmodified alloys, (b) Sr-modified alloys [32].*

#### **Figure 15.**

*(a) Backscattered electron image of oxide films mixed with* β*-Al5FeSi platelets in D9 alloy, (b and c) X-ray maps of Fe and O2, respectively in (a).*

**127**

**4. Conclusions**

may be drawn:

*Melt Treatment-Porosity Formation Relationship in Al-Si Cast Alloys*

TiB2 particles withing the gas pore as idendified by the X-ray electron maps presented in **Figure 11(b)** and **(c)**. As can be seen the X-ray map of Ti is well defined in the form

*Variation of the hardness of A319.0 alloy as a function of aging temperature and the applied melt treatment.*

**Figure 12** reveals another feature observed in D9 alloy the co-existence of TiB2 (white arrows) alongwith SrO oxide films (or bifilms - bright spots) resulting in the formation of microporosity (black arrows). In earlier studies [30, 31], the authors' group showed the presence of microporosity in Sr-modified A319.0 alloy sur-

The above-mentioned discussion contradicts the theory proposed by Argo and Gruzleski [32], who suggested that in unmodified alloys, the eutectic is characterized by its irregular solid/liquid interface, leading to entrapping of small pockets of liquid between advancing solidification fronts, causing the formation of microporosity, as shown in **Figure 14**. In Sr-modified alloys rather, a regular or planar

**Figure 15(a)** illustrates another source of porosity formation in D9 alloy. In this case, both the oxide films (gray arrows) and β-Al5FeSi platelets (black arrows) participated in the nucleation of irrgular pores [33–36]. Apparently the β-Al5FeSi platelets also act as a barrier blocking the propagation of the pore through the matrix. **Figure 15(b)** and **(c)** confirm the presence of the oxide films interacted with the β-Al5FeSi platelets. The white arrow in **Figure 15(a)** indicates the presence

**Figure 16** illustrates the variation in the hardness of the A319.0 alloy as a function of aging temperature and applied melt treatment. As can be seen, regardless of the applied treatment, all curves follow the same pattern since they are controlled by the precipitation of Al2Cu phase particles. Apparently, the age hardening mechanism is completely independent of the melt treatment. In contrast, the hardness level revealed a clear response to the melt treatment used. When the alloy was not degassed, the hardness values were the lowest compared to other treatments, whereas the situation is inversed after proper degassing. Due to oxides associated with the grain refiner as well as the presence of SrO, the hardness dropped markedly. Increasing the hydrogen content to 0.25 ml/100 g (and hence a significant amount of porosity) led to hardness levels close to those obtained for mechanically

Based on the results documented in the present work, the following conclusions

of round spots whereas that of B is almost covering the entire area.

rounded by SrO oxides as demonstrated in **Figure 13**.

interface results in widely dispersed larger porosity.

stirred alloy (i.e., with no degassing).

of a coarse Al2Cu phase particle with no pores associated with it.

*DOI: http://dx.doi.org/10.5772/intechopen.94595*

**Figure 16.**

*Melt Treatment-Porosity Formation Relationship in Al-Si Cast Alloys DOI: http://dx.doi.org/10.5772/intechopen.94595*

*Casting Processes and Modelling of Metallic Materials*

*The formation of porosity in: (a) unmodified alloys, (b) Sr-modified alloys [32].*

*(a) Backscattered electron image of oxide films mixed with* β*-Al5FeSi platelets in D9 alloy, (b and c) X-ray* 

**126**

**Figure 15.**

*maps of Fe and O2, respectively in (a).*

**Figure 14.**

**Figure 16.** *Variation of the hardness of A319.0 alloy as a function of aging temperature and the applied melt treatment.*

TiB2 particles withing the gas pore as idendified by the X-ray electron maps presented in **Figure 11(b)** and **(c)**. As can be seen the X-ray map of Ti is well defined in the form of round spots whereas that of B is almost covering the entire area.

**Figure 12** reveals another feature observed in D9 alloy the co-existence of TiB2 (white arrows) alongwith SrO oxide films (or bifilms - bright spots) resulting in the formation of microporosity (black arrows). In earlier studies [30, 31], the authors' group showed the presence of microporosity in Sr-modified A319.0 alloy surrounded by SrO oxides as demonstrated in **Figure 13**.

The above-mentioned discussion contradicts the theory proposed by Argo and Gruzleski [32], who suggested that in unmodified alloys, the eutectic is characterized by its irregular solid/liquid interface, leading to entrapping of small pockets of liquid between advancing solidification fronts, causing the formation of microporosity, as shown in **Figure 14**. In Sr-modified alloys rather, a regular or planar interface results in widely dispersed larger porosity.

**Figure 15(a)** illustrates another source of porosity formation in D9 alloy. In this case, both the oxide films (gray arrows) and β-Al5FeSi platelets (black arrows) participated in the nucleation of irrgular pores [33–36]. Apparently the β-Al5FeSi platelets also act as a barrier blocking the propagation of the pore through the matrix. **Figure 15(b)** and **(c)** confirm the presence of the oxide films interacted with the β-Al5FeSi platelets. The white arrow in **Figure 15(a)** indicates the presence of a coarse Al2Cu phase particle with no pores associated with it.

**Figure 16** illustrates the variation in the hardness of the A319.0 alloy as a function of aging temperature and applied melt treatment. As can be seen, regardless of the applied treatment, all curves follow the same pattern since they are controlled by the precipitation of Al2Cu phase particles. Apparently, the age hardening mechanism is completely independent of the melt treatment. In contrast, the hardness level revealed a clear response to the melt treatment used. When the alloy was not degassed, the hardness values were the lowest compared to other treatments, whereas the situation is inversed after proper degassing. Due to oxides associated with the grain refiner as well as the presence of SrO, the hardness dropped markedly. Increasing the hydrogen content to 0.25 ml/100 g (and hence a significant amount of porosity) led to hardness levels close to those obtained for mechanically stirred alloy (i.e., with no degassing).
