**3.1 Tensile properties**

The survivability plot is an indication of the lower and upper limits of any variable which gives the reliability and reproducibility of the data. In **Figures 2**–**4**, the tensile property changes of Sc, Er, Y, and V added A357 in as-cast conditions are given. For the yield strength (**Figure 2**), 0.05 and 0.1 wt% Sc reveal the highest reproducible results where the strength value changes between 116 and 120 MPa. In terms of design parameters, the survivability plot gives information about the possible lowest and highest potential values that this parameter can provide. For

**Figure 2.** *Survivability plot of yield strength.*

*Characterization of Casting Properties of Rare-Earth Modified A356 DOI: http://dx.doi.org/10.5772/intechopen.101722*

**Figure 3.** *Survivability plot of ultimate tensile strength.*

**Figure 4.** *Survivability plot of elongation at fracture.*

example, 0.05 wt% Er castings reveal that they can exhibit a minimum of 109 MPa below which the material will always be in an elastic region. This particular test result also provides the information that this alloy can provide a yield strength of 140 MPa as well. Thus, it shows the potential of the alloy. However, since the scatter is too high (yield strength ranging between 109 and 140 MPa), the reliability of this parameter becomes very low. On the other hand, looking at 0.05 wt% Sc addition, it is very steep at values between 116 and 120 MPa which is quite narrow and thus

the reliability of this casting is too high. This states that the yield strength value will undoubtedly be between these values.

The highest scatter is observed with 0.05 wt% Er and 0.1 wt% Y additions. The values are in the range of 90–133 MPa for Y, and 107–140 MPa for Er. Although, Er shows high scatter, yet it had the potential that this addition has the highest yield strength value of 140 MPa. On the other hand, the lowest yield strength value was obtained when Y was used as a modifier to A357.

The ultimate tensile strength values for all additions were very close to each other (**Figure 3**). The highest reliable and most reproducible results were obtained with 0.05 wt% Sc samples which were 197 MPa with a standard deviation of ±3. Pramod [22] had found that Sc additions to A356 had increased the UTS 45% when Sc was added up to 0.4 wt%.

The highest scatter was observed 0.05 wt% Er castings. The lowest UTS values were found to be in Y modified alloy whereas the highest possible UTS values were obtained with 0.05 and 0.1 wt% Er added A357 with a potential of having 265 MPa. Colombo [14] reported that 0.3 wt% Er addition was the optimum ratio in their study to exhibit the highest tensile properties.

As can be seen in **Figure 4**, 0.1 wt% Y added A357 had the potential of having 9% elongation at fracture, however, this alloy has the highest scatter and thereby revealing the least reliable casting. The 0.05 wt% Y added A357 has the lowest elongation while 0.05 and 0.1 wt% Er show the highest reproducible results.

The change in the tensile properties after the samples were subjected to heat treatment is given in **Figures 4**–**9**. There is almost twice the change in yield strength after heat treatment with Sc showing the highest value of 260 MPa and Y showing the lowest of 216 MPa (**Figure 5**). The ultimate tensile strength increases approximately 25% where Sc and Er have the highest value of 300 MPa whereas Y additions show the lowest UTS of 240 MPa (**Figure 6**). There is an almost 70% decrease in elongation values for all additions, except Er additions. Therefore, the toughness change of Er with and with heat treatment is nearly too small and Er additions exhibit the highest toughness amongst the alloys studied in this work (**Figure 7**).

**Figure 5.** *Yield strength change with heat treatment.*

*Characterization of Casting Properties of Rare-Earth Modified A356 DOI: http://dx.doi.org/10.5772/intechopen.101722*

**Figure 6.**

*Ultimate tensile strength change with heat treatment.*

**Figure 7.**

*Elongation at fracture change with heat treatment.*

Hu [29] used the HPDC method to check the mechanical properties change of Al12Si alloy where Er content was changed between 0, 0.3, 0.06, and 0.9 wt%. It was reported that as Er content was increased, UTS was increased with decreased grain size, however, elongation at fracture values was decreased. This was attributed to the presence of Al3Er phases found on the SEM images of fracture surface analysis. Therefore, it was concluded that these precipitates acted as reinforcements to the matrix, thus increasing UTS but decreasing elongation. Gao [16] reported

**Figure 8.** *Toughness change with heat treatment.*

**Figure 9.**

*Quality index change with heat treatment.*

similar results claiming that nano-sized Al3Er, Al3Zr, and Al3(Zr,Er) precipitates show resistance to dislocations and increase the strength of pure aluminum significantly. Shi [15] proposed that Al3(RE) intermetallic phases do not have the grain refinement effect but rather act as barriers to dislocation movement and thereby increase the tensile properties of Al-Si alloys. Shi [15] also reported that Er has more physicochemical activity than that of other modifiers which have the potential to eliminate bifilms [27, 30–36] and increase the melt quality, therefore exhibiting

*Characterization of Casting Properties of Rare-Earth Modified A356 DOI: http://dx.doi.org/10.5772/intechopen.101722*

higher mechanical properties. In the trials reported by Pramod [22], 0.4 wt% Sc addition to A356 had resulted in achieving 300 MPa levels with the lowest SDAS (10 μm) and refined Si morphology. In this work, 30 μm SDAS was good enough to achieve 300 MPa.

Pourbahari [21] studied the effect of La addition and they reported that the presence of intermetallic like Al4La was more spherical and distributed in the microstructure (at low levels of La addition), and therefore they did not act as stress rising locations that would decrease the ductility of the alloy. However, when La content was higher than 0.1 wt%, new intermetallic phases such as AlSiLa were formed which were flakey and needle-like, thereby reducing the toughness of the matrix. The fracture surfaces were quasi-cleavage facets and not dimple like as in low La levels.

Drouzy's quality index [37] assessment was also used in this study and the results are given in **Figure 9**. It can be seen that Er and Sc both have the highest index values. Additionally, those elements are the only ones that show an increase in the quality index after heat treatment.

### **3.2 Microstructure**

Although, the mechanical properties of each different grain refiner addition show a wide range of values and there is a quite difference between the samples, yet the microstructural analysis had revealed that the DAS and SDAS of all castings were very close to each other (**Figures 10** and **11**). Only 0.05 wt% Y addition shows the highest DAS value compare to the others, but all SDAS measurements of castings were recorded to be between 30 and 40 μm as seen in **Figure 12**. It is interesting to note that only in Y additions Si morphology was altered to fibrous type (**Figures 10b** and **11b**). Colombo [14] added 0.2 and 0.4 wt% Er and found that Er acted as a modifier for Si

**Figure 10.** *Microstructures of 0.05 wt% (a) V, (b) Sc, (c) Er and (d) Y added A356.*

### *Aluminium Alloys - Design and Development of Innovative Alloys, Manufacturing Processes...*

**Figure 11.** *Microstructures of 0.1 wt% (a) V, (b) Sc, (c) Er and (d) Y added A356.*

*Characterization of Casting Properties of Rare-Earth Modified A356 DOI: http://dx.doi.org/10.5772/intechopen.101722*

**Figure 13.** *Fluidity change of different grain refiner (a) 0.05 wt%, (b) 0.1 wt% additions.*

together with a 30% reduction in SDAS. However, in this work, 0.05 and 0.1 wt% Er were added and no modification of Si was observed (**Figures 10c** and **11c**).

Pramod [22] reported that 0.4 wt% Sc had a reduction capability of SDAS by 50%. Compare to 0.2 wt% Sc addition, the silicon size was also significantly decreased which resulted in having the highest UTS values of 300 MPa. Pandee [38] added three different levels of Sc to A356; namely 0.24, 0.40, and 0.65 wt%. Two different cooling rates were studied. At both cooling rate levels (0.1 and 3°C/s), as Sc content was increased, Si morphology was modified to a fibrous structure. At 3°C/s, the modification of Si was found to be the optimum for 0.4 wt% Sc modified A356. In a similar kind of work, W. Zhang [4] also reported similar findings that higher than 0.15 wt %Sc addition to A356, Si morphology was completely modified.

On the other hand, Xu [25] found almost a linear decrease in grain size with increased Sc content varying from 0.2 towards 0.8 wt% Sc additions to F357 where the modification of Si was started at 0.2 wt% additions. With UTS values as high as 350 MPa.

### **3.3 Fluidity**

The fluidity test results were analyzed under the wt% additions as seen in **Figure 13**. When 0.05 wt% addition levels are compared, it can be seen that Er and Y appear to have higher fluidity lengths compared to the other additions. On the other hand, when 0.1 wt% additions are analyzed, Sc has significantly higher fluidity compared to the other additions. In each of the cases, V reveals the lowest fluidity results. Prukkanon [39] reported that up to 0.2 wt% Sc addition, the fluidity of A356 was increased, but a higher amount of Sc addition had not affected the fluidity and remained unchanged at 660, 690, and 720°C.
