**1. Introduction**

Aluminum alloys are widely used in modern industry due to their combination of low density and high ductility [1–10]. Nowadays, aluminum alloys of the 6XXX series (Al–Mg–Si) are gaining popularity [11–18]. This series has an average strength that can be increased by heat treatment, namely by metastable β" (Mg5Si6) particles coherent to the aluminum matrix.

Further improvement of the properties of this alloy series is possible due to small additions of Sc and Zr. These elements are poorly soluble in the aluminum matrix and form coherent strengthening Al3(ScZr) nanoparticles [19–21]. In addition, these additives contribute to grain refinement during casting [19–21], and also contribute to the control of the recrystallization process [21] which also increases the mechanical properties. Alloying with zirconium and scandium is especially common for 5XXX group alloys, i.e., with a high

**Citation:** Aryshenskii, E.; Lapshov, M.; Hirsch, J.; Konovalov, S.; Bazhenov, V.; Drits, A.; Zaitsev, D. Influence of the Small Sc and Zr Additions on the As-Cast Microstructure of Al–Mg–Si Alloys with Excess Silicon. *Metals* **2021**, *11*, 1797. https://doi.org/10.3390/ met11111797

Academic Editor: Elisabetta Gariboldi

Received: 14 October 2021 Accepted: 5 November 2021 Published: 8 November 2021

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content of magnesium [22,23], however, it is effectively used also in other aluminum alloy systems [24]. In recent years, active research on the use of these additives in 6XXX series alloys has been started [25–28]. In the future, it is possible to achieve a significant increase in strength due to the combined effect of grain refinement, and the simultaneous usage of strengthening particles Al3(ScZr) and β" (Mg5Si6). At the same time, Sc alloying of this group is a rather difficult task. The reason is that silicon addition changes the kinetics and chemical composition of scandium-containing dispersoids. As a result it provokes the appearance of an undesirable, AlSc2Si2 phase [25]. Because of this, some sources have noted a negative effect of scandium on the properties of the Al–Mg–Si alloys [25,29]. In order to avoid AlSc2Si2, it is necessary to carefully select the chemical composition (mainly by changing the amount of zirconium) and control the casting and heat treatment modes. In this case, due to joint scandium-zirconium alloying, it is possible to improve the properties of the 6XXX group alloys even if the ratio of the wt. fractions is Mg/Si < 1.73 [25,30,31]. In alloys of the Al–Mg–Si system with ratio Mg/Si < 1.73, not all silicon combines with magnesium into the Mg2Si phase (or its metastable modifications such as β") and there is an excess of it. It should be noted that, on the one hand, the increased silicon content enhances the properties in this group of alloys, due to the more evenly distributed and more finely dispersed strengthening particles β" [17]. However, on the other hand, with the addition of scandium, excess silicon will increase the tendency to form AlSc2Si2. At the same time, the study of the processes occurring during casting and heat treatment of 6XXX group alloys with scandium-zirconium additives made it possible to avoid the undesirable phase and, in general, to improve the properties at a ratio Mg/Si > 1.73 in AA6082 [26,30,32]. The possibility of further decreasing this ratio is promising since it will allow scandium and zirconium to be added to alloys such as 6016 where the excess of silicon is extremely large. However, it requires a thorough study on the effect of casting and thermomechanical treatment on the microstructure [26,30]. This article is devoted to the study of microstructure formation during casting of the 6XXX group alloys with a high silicon content and complex scandium and zirconium additives, especially in the range of element concentrations for which this problem has not ye<sup>t</sup> be investigated.
