*1.3. Aluminum Alloys for Automotive Field*

Against this background, the Al–Si–Mg alloy class is one of the most widely used for the production of aluminum casting components. In particular, the cast Al−7Si−Mg (EN AC 42100, also defined as A 356) alloy is widely used in automotive applications thanks to its high specific strength. Its microstructure consists of primary α (Al) grains and eutectic (Al–Si) structures. T6 heat treatment is normally used to obtain the desired mechanical properties. The solution treatment dissolves the β phase (Mg2Si particles) in the Al matrix, homogenizes the alloying elements in the casting, and modifies the morphology of the eutectic structures [13–16]. Wrought aluminum is a widely used alloy in the automotive field, especially the heat-treatable 6xxx series, which is characterized by high strength and good corrosion resistance [17]. Two of the most commonly used are the wrought EN AW-6181-T6 and EN AW-6082-T6 aluminum alloys. These are age-hardenable alloys, thus their mechanical properties are mainly controlled by the hardening precipitates contained in the material. When the material is subjected to a solution heat treatment followed by a quenching and a tempering treatment, their mechanical properties reach their highest level. According to the literature [18–20], the T6 temper of the 6xxx alloys involves very thin precipitates, namely β"needle shaped precipitates, with a nanometric size and is partially coherent with the matrix. One of the most interesting characteristics of these alloys is the good weldability that, along with other properties, makes them very attractive in transport for complex structures assembled by welding [21,22]. Several works have studied the welding of aluminum and other alloys such as magnesium, steel, or titanium [23–26]. Only a limited number of scientific papers [27–32] have investigated the welding of dissimilar aluminum alloys together. These papers mainly deal with friction stir welding (FSW) [33,34], which, despite its potential, still has a high cost that needs to be improved in order to be used in industrial high volume applications. Wang et al. [35] studied the tensile properties and microstructure of a joined wrought EN AW-6181 aluminum alloy and vacuum high pressure die cast A356 aluminum alloy by using the metal inert gas (MIG) technique. The results showed that the low strengths of the A356-T6 alloy should be attributed to the absence of Mg-based intermetallic phase, coarse grain, and porosity, but the effect of the microstructure of the two base metals (BM) on the mechanical properties was not reported. An interesting study by Nie et al. [36] examined the microstructure, distribution of alloying elements, and mechanical properties of the wrought aluminum alloy 6061-T6 and cast aluminum alloy A356-T6 joined using a pulse MIG welding process. Additionally, the influence of welding speed on the microstructure and mechanical properties of the joints was investigated. They observed brittle Fe-rich phases in the partially melted zone and minimum hardness in the A356 aluminum alloy side. Some authors have recently tried to apply hybrid laser-arc welding to Fe–Al dissimilar joints [37], but in that case, the process was instable because of the significant difference in the thermal- and fluid-dynamic properties

of the two metals. On the other hand, for this configuration, full penetration and low defectiveness were obtained by laser offset welding. Wang et al. applied laser welding with different beam oscillating modes on 5A06 aluminum alloy sheets and found that welding defects such as welding porosity could be improved by laser beam oscillation [38].
