**3.3 Mechanical tests**

### *3.3.1 Microhardness*

Microhardness measured in alloys with and without Zn additions is shown in the bar chart of **Figure 9**. As FGMs regard, the microhardness matrixes measured through the interfaces are shown in **Figure 10**.

Average interface hardness appears similar in FGM and in FGM with Zn addition in alloy EN AC 47000, respectively 72 ± 16 HV0.15 and 69 ± 15. On the other hand, in the FGM with Zn addition in both alloys, the interface was characterized by a higher average microhardness, of almost 81 ± 16.

Overall, the standard deviations in FGM specimens are similar (16, 15 and 16). The same behaviours were observed in alloy EN AC 47000, with and without Zn additions. Microhardness measured in EN AC 47000 was 95 ± 12 HV0.15 without Zn addition and 80 ± 12 HV0.15 with Zn addition.

*Development and Characterization of New Functionally Graded Aluminium Alloys DOI: http://dx.doi.org/10.5772/intechopen.101022*

#### **Figure 8.**

*EDS maps analysis in FGM (with Zn addition in both the alloys) interface.*

#### **Figure 9.**

*Average microhardness measured in bulk alloys EN AC 47000 + Zn, EN AC 51100 + Zn, EN AC 47000, EN AC 51100 (bars 4, 5, 6, 7) and average microhardness measured for FGM, FGM with Zn addition in EN AC 47000 and FGM with Zn addition in both alloys (bars 1, 2, 3).*

#### **Figure 10.**

*Microhardness maps for FGMS. A: FGM; B: FGM with Zn addition in EN AC 47000; C: FGM with Zn addition in both alloys.*

The lower microhardness in alloy EN AC 47000 after Zn addition in respect to the same alloy without Zn (bars #4 and #7, respectively) can be explained with the higher inhomogeneity detected in the alloy with Zn addition. In fact, such behaviour was not noticed in the FGMs interfaces with Zn, where microhardness resulted in improvement after Zn additions.

Considering the portion of EN AC 47000 in FGMs samples, the average microhardness measured was 78 ± 14, 74 ± 14 and 90 ± 20, respectively for FGM, FGM with Zn in EN AC 47000 and FGM with Zn in both compositions. These results seem to suggest that 1.5 w.t.% of Zn in EN AC 47000 does not significantly affect the alloy microhardness; on the other hand, Zn addition in both alloys causes an increase in microhardness of EN AC 47000 near the interface. This behaviour may be attributable to the higher Zn content detected near the interface between EN AC 51100 and EN AC 47000. The mixing of the compositions during the casting did not cause a depletion in Zn, because this constituent is present in both alloys.

Alloy EN AC 51100 has few alloying elements; thus, Zn addition may easily affect the average microhardness in this alloy, which increased from 60 ± 7 HV0.15 to 76 ± 3 HV0.15. Considering the portion of EN AC 51100 in FGM samples, the average microhardness measured were 71 ± 18, 67 ± 16 and 76 ± 11, respectively for simple FGM, FGM with Zn in EN AC 47000 and FGM with Zn in both compositions. These results suggest that Zn and the other alloying elements that characterize the alloy EN AC 47000 affect the microhardness of the alloy EN AC 51100. On the other hand, Zn addition only in alloy EN AC 47000 does not cause a further increase in EN AC 51100 microhardness, despite the composition mixing during the casting. Only the Zn addition in both alloys leads to an increase in microhardness.

The slight depletion of Mg and Zn at the interface of FGMs, detected in SEM-EDS maps in **Figures 4**, **6** and **8**, may explain the difference in microhardness measured respectively in the bulk alloy and near the FGM interfaces.

#### *3.3.2 Tensile tests and fracture surfaces*

Mechanical tensile tests were performed on FGMs specimens as well as on single alloy specimens.

**Table 3** shows the tensile properties measured. FGMs are characterized by an average UTS (ultimate tensile strength) of almost 164 MPa, similar to FGMs with Zn addition. After adding Zn only in alloy EN AC 47000, the average UTS decreased to almost 151 MPa, while the elongation to fracture A% increased. In both cases, Zn addition caused a decrease of Rp0,2. In effect, in [20], the authors noticed that after adding 1.5–2% Zn, the mechanical UTS of Al-Si alloys decreases. Alloy EN AC 47000 affect the mechanical properties of all FGMs, regardless of the presence or absence of Zn. The alloy EN AC 47000 with Zn was characterized by almost 170 MPa of UTS, while the elastic module resulted very similar to the module of FGM with the addition of Zn only in EN AC 47000. Similarly, alloy EN AC 51100 with Zn has a UTS of about 148 MPa, as the UTS of FGM with the addition of Zn only in EN AC 47000. Zn addition in alloy EN AC 47000 seems not to affect UTS, resulting in slightly higher in the alloy without Zn; the same behaviour was previously noticed for the microhardness values.

EN AC 47000 has the higher average UTS, followed by EN AC 47000 with Zn and FGM without Zn. Not considering the average data, the higher UTS was detected for FGM specimen without Zn (184 MPa) while the lower was detected for alloy EN AC 51100 (122 MPa). As attended, alloy EN AC 51100 has shown the lowest mechanical properties and significant elongation to fracture A%.

In FGM specimens, it seems that Rp0,2 decreases with the addition of Zn. This behaviour goes against the solid solution hardening expected. In fact, Zn has a

*Development and Characterization of New Functionally Graded Aluminium Alloys DOI: http://dx.doi.org/10.5772/intechopen.101022*


**Table 3.**

*Average tensile tests results.*

high solubility in the aluminium matrix. On the other hand, the Rp0,2 decreasing was only detected in FGMs specimens, while in alloy EN AC 47000 Rp0,2 remains almost constant and in alloy EN AC 51100 increases. In this sense, there are two possible explanations. First, the Rp0,2 decrease may be associated with a higher defect population in the FGM specimens that affect the yielding of the casting. The second hypothesis is that during the tensile test dislocations cannot pass through the Zn-straightened matrix and the primary Si particles, inducing concentrations of efforts that cause crack nucleation sites. The lower UTS after Zn addition may be similarly explained considering the additional presence of a certain defect population that affect the positive effect of Zn.

Particularly, the high standard deviation in the UTS of FGM without Zn addition was due to the presence of two specimens that fractured near casting defects. Such defects were primarily shrinkage porosities along the junction and in the proximity of the junction near the EN AC 51100. These porosities were caused by an incorrect elapsing time between the casting of the alloy EN AC 51100 and the subsequent pouring of EN AC 47000, causing air entrainment and mixing in the junction. Another important consideration must be done; notwithstanding FGM without Zn fractured preferentially in alloy EN AC 51100, the mechanical properties of the FGM without Zn still remain higher than the properties of the single alloy EN AC 51100 without Zn. This behaviour can be attributed to two reasons. The first one is that the presence of the alloy EN AC 47000, which is able to assure a high strength resistance, positively affects the FGM resistance, while the second is due to a good quality of the junction realized that provides a favourable stress distribution between the alloys.

Surface fractures were observed through SEM. In general, FGM specimens' rupture has occurred on the EN AC 51100 side; in one specimen, it occurs at the interface while, in one sample, the rupture was detected in alloy EN AC 47000 close to a defect. **Figure 11a** and **b** show fracture surfaces of two FGM without Zn. Mainly, **Figure 11a** shows the surface fracture of the FGM sample with the higher UTS, 188 Mpa. This specimen is characterized by a brittle fracture that occurred on alloy EN AC 47000. The fracture reveals a vast number of dendrites. **Figure 11b** displays an

#### **Figure 11.**

*Surface fractures for a few selected specimens. a, b: FGMs without Zn addition. c: FGM with Zn addition in EN AC 47000. d: FGM with Zn in both alloys. e,h: Alloys EN AC 47000 with and without Zn. f, g: Alloy EN AC 51100 with and without Zn addition.*

FGM without Zn fractured along the interface between the alloys. A brittle fracture and dendrites characterize the fracture; this behaviour is symptomatic of a weak interface bonding between the two alloys. Overall, FGMs without Zn specimens are broken into the less resistant alloy, apart from specimens affected by casting issues.

**Figure 11c** and **d** show the fracture surfaces for both FGMs with Zn in EN AC 47000 and FGM with Zn in both compositions. Particularly, FGM with Zn only in EN AC 47000 fractured at the interface with pretty high elongation. On the other hand, FGM with Zn in both the alloys fractured with very low mechanical strength at the interface (147 MPa).

**Figure 11e** and **h** display the EN AC 47000, with and without Zn addition. Both fractures are brittle, and the elongation to fracture was approximately 2%. Cleavage plains were observed.

#### *Development and Characterization of New Functionally Graded Aluminium Alloys DOI: http://dx.doi.org/10.5772/intechopen.101022*

Alloy EN AC 51100 shows the highest elongation to fracture, of about 8%, while the elongation drops to about 2% after Zn addition. Despite the good mechanical properties, the elongation to fracture, in **Figure 11g**, is clearly visible extended dendrites. In **Figure 11f**, are visible the eutectic phases and a small amount of dimples.

Overall, intermetallic rod-like shapes or plate shapes cause brittle fractures for the concentration of efforts, as noticeable in **Figure 12**. Defects detected in surface fractures are mainly dendrites, but in FGM with Zn in both alloys were also noticed gas porosities. From the fracture analysis, it seems a certain grade of microporosity into the casting was realised, especially into alloy EN AC 51100. This behaviour was not previously highlighted in SEM analysis.

Near the rod-like intermetallics, are noticed cleavage plains. The very thin acicular microstructure of the eutectic regions α-Al/Al3Mg2 seems does not affect the rupture mode, resulting in an excellent elongation to fracture (**Figures 11f** and **12**).

### *3.3.3 Three-point bending test*

**Table 4** reports the results of bending tests for each specimen. As maximum values regard, FGM without Zn has the highest ultimate force Fmax (394 Mpa). FGMs' deformation at rupture appears most influenced by alloy EN AC 47000 except for one sample, where the elongation resulted in about 10%.

FGM with Zn addition only in EN AC 47000 has shown the lower deformation at fracture and the lower Fmax; particularly, in sample #2, the Rp0,2 was so low that was not recorded. In FGM with Zn only in EN AC 47000, deformation at rupture is similar to deformation at rupture of alloy EN AC 51100, while in FGM with Zn in both alloys, it is similar to the deformation of alloy EN AC 47000.

Alloy EN AC 51100, as attended, has the highest deformation at rupture.

#### **Figure 12.**

*Fracture details. FGM type 1 is the FGM without Zn; FGM type 3 indicates the FGM with Zn in both compositions; types 6 and 7 are alloys EN AC 51100 and EN AC 47000 without Zn.*


#### **Table 4.**

*Three-point bending tests average results. \* a brittle fracture affects both Rp0,2 (absence of yielding) and high standard deviation.*

As the three-point bending test regard, results are comparable with tensile tests. In fact, for example, FGMs in tensile conditions present similar UTS as the Fmax measured in FGM without Zn and with Zn in EN AC 47000. FGMs with Zn in both alloys resulted not comparable because of defects inside castings. Alloy EN AC 47000 has shown similar UTS with and without Zn, while in the bending test, the absence of Zn causes an increase in the specimen deformability, thanks to the lower solid solution straightening. Finally, EN AC 51100 bending behaviour is similar with or without Zn in terms of Fmax, despite the deformability decrease with Zn, as noticed for tensile test results.

### *3.3.4 Mechanical property connections*

Average mechanical properties were compared to each other, and results are shown in graphs of **Figure 13**. **Figure 13a** shows a graph between the average ultimate tensile strength and the average elongation to fractures for each batch of samples. Graph a shows that alloy EN AC 47000 (type 4 with Zn and type 7 without Zn) has the highest Rm but low elongation to fracture. Conversely, alloy EN AC 51100 (type 5 with Zn and type 6 without Zn) have the lowest Rm, while A% changes as a function of Zn addition. Without Zn, EN AC 51100 castings reach the maximum average elongation.

As FGMs regard, Zn addition does not influence the Rm that remains almost constant, while dL% increases. The increase in dL% may be caused to the presence of nanoparticle intermetallic compounds Mg-Zn that positively affect the fracture mode, thanks to their rounded shapes [21]. Despite that, the intermetallic phases Mg-Zn are nanometric and, thus, are not easily observable at SEM. These intermetallic phases nucleate during the solidification; as the rate of Zn increases, the rate of Mg dissolved in α-Al decreases to form Mg-Zn intermetallic phases.

When comparing Rm to average microhardness values, as in **Figure 13c**, it becomes evident that EN AC 51100 castings have shown the higher Rm and higher microhardness, followed by FGM with Zn in both alloys and alloy EN AC 47000

*Development and Characterization of New Functionally Graded Aluminium Alloys DOI: http://dx.doi.org/10.5772/intechopen.101022*

#### **Figure 13.**

*Average values from* **Tables 3** *and* **4***. a: Average values Rm and A% obtained after tensile tests. b: Average values Fmax and dL% obtained after bending tests. c: Average values HV0.15 and Rm obtained after tensile tests and microhardness measurements. d: Average values HV0.15 and Fmax obtained after bending tests and microhardness measurements. FGM, FGM with Zn addition in EN AC 47000 and FGM with Zn addition in both alloys indicated as #1, #2, #3, while EN AC 47000 + Zn, EN AC 51100 + Zn, EN AC 47000, EN AC 51100 indicated as #4, #5, #6, #7.*

with Zn addition. On the other hand, as expected, EN AC 51100 castings present both the lowest Rm and microhardness. FGM with Zn only in EN AC 47000 presents a relatively low average Rm and microhardness; this was caused by a defective specimen that affects the batch's average values.

**Figure 13b** shows a graph between the maximum strength and the average deformation at ruptures dL% for each batch of samples. In both tensile and bending tests, EN AC 51100 has higher elongation/deformation at rupture (green points in **Figure 13a** and **b**), while EN AC 47000 specimens have the higher ultimate tensile strength/ maximum force (violet points). FGM with Zn in both the alloys shows the lowest deformation at rupture. **Figure 13d** highlights the relation between the microhardness and the maximum force of bending tests; EN AC 47000 castings have the higher Fmax and higher microhardness, followed by alloy EN AC 47000 with Zn addition.

### **4. Conclusions**

In this work, new kinds of functionally graded materials realized by controlling the mould filling were produced. The alloy Al-Si EN AC 47000 was adopted along with the Al-Mg composition EN AC 51100, obtaining castings with good-quality and low retained defects. The Zn addition was taken into account to increase the mechanical properties of the as-cast FGM, especially by nucleating the Mg-Zn intermetallic phases in composition EN AC 51100. Functionally graded materials were therefore produced in three variants—the first type was the simple casting of the two compositions; the second type involved the addition of commercially pure Zn into the Al-Si alloy, while the third type involved the Zn addition into both the

alloys composing the FGM. In order to better understand the FGM properties and compare the results, samples made in purely alloys Al-Si and Al-Mg were cast, with and without Zn addition, using the same production process of FGMs.

All the castings produced were mechanically tested, and their microstructures were observed at the scanning electron microscope SEM-EDS.

FGMs, realized by alloys Al-Si and Al-Mg, presented good bonding, which the mechanical testings have highlighted. Moreover, SEM observations of the FGMs interfaces highlight, in general, the absence of extended defects.

In alloy EN AC 51100 (**Figure 2**), intermetallic phases such as Al-Fe-Mn-Si phases, the eutectic α-Al/Al3Mg2 phase and Mg2Si phase were found. In alloy EN AC 47000 (**Figure 3**), intermetallic Fe-Cu-Mn phase, Q phase, polygonal eutectic silicon and Chinese script α-AlFeSiCuMg were detected.

FGM interface is shown in **Figure 4**; intermetallic phases are coherent with those observed in **Figures 2** and **3**.

The silicon diffusion into Mg-based alloy was clearly noticeable, especially in the eutectic regions α-Al/Al3Mg2. Moreover, Mg was detected in both the alloys with a slight depletion at the interface, despite the single alloy EN AC 47000 was not containing Mg (see **Figure 7**). In the FGM with Zn addition in both alloys, either Zn adn Mg depletion at the interface were observed. The slight depletion of Mg and Zn at the interface of FGMs may explain the difference in microhardness measured respectively in the bulk alloy and near the FGM interfaces—in bulk alloys, microhardness appears higher with respect to microhardness near the interface.

Overall, the tensile strength and the maximum bending force seems to decrease with the addition of Zn. During the tensile test, probably, dislocations cannot pass through the Zn-straightened matrix, straightening intermetallic phases and primary Si particles, inducing concentrations of efforts that cause crack nucleation sites. The lower UTS after Zn addition may be explained considering the additional presence of a certain defect population that affect the mechanical properties.
