**2. Materials and methods**

#### **2.1 Materials**

Aluminium alloys adopted to realize FGMs are EN AC 47000 and EN AC 51100. Details about alloys chemical composition are reported in **Table 1**. Alloy EN AC 51100 (AlMg3) has a wide range of applications in the automotive industry; Mg increases corrosion resistance and solution hardening [16].

EN AC 47000 (AlSi12(Cu)) is a eutectic alloy having Si as the primary alloying element. Si and Cu increase mechanical properties, in particular after heat treatment.

Commercially pure Zn ingot was used as an alloying element in a quantity of 1.5 w.t.% in some specific samples.

#### **2.2 FGM production**

FGMs were realized by controlling the mould filling during the gravity casting [17, 18]. The alloys are melted in two different crucibles while the mould is painted **EN AC 47000** w.t.% Cu Mg Si Fe Mn Zn Al 0.62 0.09 12.98 0.54 0.26 0.47 Bal. **EN AC 51100** w.t.% Cu Mg Si Fe Mn Zn Al 0.042 2.786 0.432 0.285 0.188 0.049 Bal.

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

#### **Table 1.**

*FGM alloys composition.*

with BN-based stop off-paint and preheated at 400°C. The mould is a C40 steel mould having dimensions 85 mm (depth) × 200 mm (height) × 15 mm (width). The alloys were poured in a precise amount to fill half of the mould figures (see dotted lines in **Figure 1A** and **B**). Approximately 150 g of EN AC 51100 alloy was poured at 710°C into the mould, as indicated by the orange arrow in **Figure 1A**, filling half of it. After the first casting, 185 g of alloy EN AC 47000 was poured, at 710°C, over EN AC 51100 to fill the second half of the mould, as shown in **Figure 1B**. The pouring of alloy EN AC 47000 was performed without any time delay after casting the first composition. Castings are manually extracted from the mould in the fastest way possible by using pullers and then quenched in water at 25°C; the manual extraction method may cause little change in cooling rate (that was not specifically measured) between subsequent castings. Because of the mould shape, there is no specific pouring channel for the second alloy, EN AC 47000, which must be poured into two different parts of the mould, as indicated in **Figure 1B**. This mould configuration may affect the junction tightness inside castings, causing the presence of gas porosities

#### **Figure 1.**

*Filling operations. A: Step 1, half-mould image; the casting of alloy EN AC 51000 inside the mould carried out as indicated by the orange arrow. B: Step 2, casting alloy EN AC 47000 inside the mould carried out as indicated by orange arrows. C: Step 3, casting obtained. D: Step 1 of casting, nucleation of* α*-Al dendrites near nucleation site and mould walls after the casting of composition EN AC 51100. E: Step 2 of casting, nucleation of* α*-Al dendrites of alloy EN AC 47000 into the interdendritic channels of alloy EN AC 51100 previous poured. F: EN AC 47000 alloy into the interdendritic channels of alloy EN AC 51100 after the pouring.*


**Table 2.**

*Experimental conditions.*

and oxides. An example of the casting obtained is shown in **Figure 1C**. Each FGM bar measures 25 mm (depth) × 125 mm (height) × 15 mm (width), and the interface between alloys is approximately placed in the middle.

As mechanical properties are relevant in automotive FGM, Zn addition was performed to increase mechanical strength. For this reason, three different types of FGM were realized—FGM without Zn, FGM with Zn addition in alloy EN AC 47000 and finally FGM with Zn addition in both the alloys. Moreover, in order to evaluate the alloys' mechanical properties, single-alloy specimens have been cast, with and without Zn addition. The different experimental conditions are summarized in **Table 2**.

#### **2.3 Mechanical and microstructural tests**

With the aim to evaluate the mechanical properties of the produced FGMs and compare them with those of the single alloys, tensile tests, three-point bending tests and microhardness measures were made. Tensile tests were performed following the norm ASTM B557–15. Specimens were machined in a plate dog-bone shape from the rectangular castings; in FGM tensile specimens, the interface between the alloys was placed in the middle of the samples. Since the properties of the FGM without Zn addition are the work's focus, a higher number of the tensile test specimens were produced. Overall, these were realized six specimens for this type of FGMs without Zn, while the other types of casting were machined with two samples.

Three-point bending tests were performed on bar measures 10 mm × 10 mm × 60 mm, adopting a support span of 40 mm, test speed 0.004 1/s and a preload of 5 N. During the tests, the load is applied in the middle of the specimens, while on the opposite side the sample is supported by two wedges. In FGM bending specimens, the interface between the alloys was placed in the middle of the samples. Overall, there were six specimens for FGMs without Zn, while the other FGMs were machined in two samples. Four specimens were tested for the single alloys (castings with and without Zn).

Microhardness Vickers tests were performed on each specimen. FGMs interfaces were subjected to a microhardness matrix 8 × 8; the distance between each indentation was 150 μm, the applied load was 15 gf for 15 s and the diagonal was measured after the indentation varies almost from 25 to 35 μm. Single alloys were also tested, performing five indentations for each sample. Microhardness was chosen instead of Vicker hardness because it may be more sensitive to the slight hardness variations near the interface of the FGM samples. In fact, the junction areas of alloys are pretty low, of a few hundred microns.

Microstructures were evaluated through SEM microscope, and EDS semiquantitative analyses were also carried out. Castings were cut and polished by using SiC papers *Development and Characterization of New Functionally Graded Aluminium Alloys DOI: http://dx.doi.org/10.5772/intechopen.101022*

from 180 to 2500 grit, and then, colloidal silica having a granulometry of 0.03 μm was used for mirror-polishing. Finally, specimens were etched with Keller's reagent. Fracture surfaces after tensile tests were also investigated through SEM analysis.
