**1. Introduction**

The last few decades were characterized by significant growth of the aluminium market. This growing interest in aluminium alloys is attributable to their excellent properties such as good or excellent specific mechanical strength, lightweight and generally good corrosion resistance [1]. During previous years, in the automotive market, these properties have allowed replacing some heavy components made in steel with the same components made in aluminium, leading to cost savings, structural lightening and CO2 emission reduction. However, if, on the one hand, aluminium alloys have good properties that permit the production of components with homogenous characteristics providing high performances, on the other hand, some applications may require specific features as graded structures. Functionally graded materials (FGMs) have graded structures characterized by different compositions or microstructures, as deeply investigated in [2]. In particular, gradient types were classified as chemical composition gradients, porosity gradients or microstructural gradients [3], while microstructural gradients are further subdivided

into fraction gradient, shape gradient, orientation gradient and size gradient [4]. Various production methods such as powder metallurgy, centrifugal casting, vapour deposition [5] or additive manufacturing [6] can be used to produce FGMs. FGMs usage involves various markets, such as biomedical, chemical, aerospace, electronics, nuclear or energy; for instance, in [7], the powder metallurgy method was used, and the authors successfully fabricated a porosity gradient Ti-Zr FGM for biomedical application.

As aluminium-FGMs regard, various publications concern the functionally graded metal-ceramic composites rather than metal-metal ones. In [8], authors realized a layered Al-SiC FGM by powder metallurgy technique. Each layer was composed of different SiC content, and the consolidation between each layer was assured by cold compaction. Similarly, in [9] a layered Al-Al2O3 FGM was suggested. Other manuscripts suggest the adoption of Al alloys instead of the commercially pure aluminium powder. In particular, in [10], FGM produced with aluminium alloy A7075 was suggested for gears or brake drums. In addition to powder metallurgy, FGMs may be prepared in other ways: in [11], authors proposed the centrifugal casting technique to realize A356-SiC FGMs. The mould rotation permits a radial distribution of SiC particles, and in the end, the samples could be distinguished into three different zones—reinforced zone (outer zone), transitional zone (middle zone) and the unreinforced alloy (inner zone). Centrifugal casting was also successfully adopted in [12] to produce automotive pistons by adopting two aluminium alloys—A336 and A242. As centrifugal casting regards, different production parameters must be considered, as pointed up in the review article [13].

On the other hand, gravity casting is a newish casting method used to produce FGMs. In [14], the authors used gravity casting to realize two metal-metal FGM, adopting the aluminium alloys A390-A319 (both casting alloys) and A390-A6061 (casting and wrought alloys). In particular, to obtain a good metallurgical bonding between alloys, some casting parameters were carefully monitored, as the time gap during sequential pouring and the pouring temperature. In this work, metal-metal FGMs were realized by controlling the mould filling using gravity casting [15]. Alloys EN AC 47000 and EN AC 51100 were used, with and without Zn addition, to assess the mechanical properties and the metallurgical bonding in this new kind of FGM. FGMs properties were then compared to the properties of the single alloys.
