# Case Study of a Functionally Graded Aluminum Part
## 2 Materials and Methods
### Methods  
An aluminum FGM was obtained with the sequential gravity casting of the composition A (AlSi7 Mg0.3) followed by the composition B (AlSi12 CuNiMg). Overall, five types of manual castings were performed using a mould (made in heat treated steel) that permitted to obtain square bars of 25 mm \(\times\) 125 mm \(\times\) 15 mm. Optimization of the casting process included: (i) selection of the pouring order for the two compositions; (ii) casting temperatures of the alloys; (iii) waiting time between the sequential castings; and (iv) mould temperature.  
The pouring order depends on the alloy-gap between solidus and liquidus temperatures. As previously demonstrated by DSC analysis (Differential Scanning Calorimetry) and detailed in [10; 11], the hypoeutectic alloy (AlSi7 Mg0.3) was the first poured composition for its larger gap of solidification (Figure 1).  
The DSC curve of the hypoeutectic alloy presents two peaks: the eutectic temperature and the liquidus temperature (as can be seen in Figure 1). This gap of solidification permits to obtain a barrier effect for the second alloy (with eutectic composition) when poured, due to the solidification of the alpha-phase of the first alloy that avoids the mixing of the composition. At the same time, it is important to perform the second casting during the solidification temperature gap of the first alloy, in order to allow a metallurgical bond due to the mixing of the Si-rich phase of the second poured alloy inside the interdendritic channels of the first one.  
Casting temperatures of the alloys must ensure the bonding of the two compositions avoiding the premature solidification of the first alloy poured. The waiting time between the sequential castings depends on both the temperature gap liquidus-solidus for the two alloys and the casting temperature of the alloys; in fact, this time must permit the bond of the alloys at the interfaces avoiding the mixing of the compositions. The mould temperature influences the degassing process: the higher the temperature of the mould, the faster the elimination of the humidity is.  
For all these reasons, alloy temperatures were set at 710 \({}^{\circ}\)C (AlSi7 Mg0.3) and 750 \({}^{\circ}\)C (AlSi12 CuNiMg), with a mould temperature of 400 \({}^{\circ}\)C and several waiting time in the range of 10-50 s, to investigate their effect on the bonding region. No modification or refining was performed during the melting.  
After the casting process, qualitatively RX analysis (X-ray radioscopic inspection) was carried out to evaluate the bonding rate, the porosity and the inclusion grade into the casted parts, especially at the interfaces between the two compositions. These observations were conducted by a Bosello High Technology Industrial X-Ray system on the casted parts.  
Figure 1: Differential Scanning Calorimetry (DSC) analysis [10].  
Tensile tests were done by a ZWICK ROELL machine with the aim to evaluate the resistance of the joint between the alloys for different elapsing time. Each casting specimen was machined into the typical dog-bone shape. Considering the composite nature of the FGMs and the possibility to obtain two specimens for each FGM casted, which lead to perform a better statistical analysis of the results, it was decided to realize rectangular-shape specimens, following the standard ASTM B557-15 for samples preparation and the ASTM E8 for testing methods. After tensile tests, the fracture surfaces were observed with the scanning electron microscope (SEM), Leo 1450 VP.