3.1.2. Anisotropy

As already mentioned, the special microstructure of additively manufactured Al-Si alloys is the result of the exceptionally high cooling rates and direction of the thermal gradients [36]. The particular directional distribution of thermal gradients during the fusion and solidification of the different layers of the material produces an intrinsically anisotropic microstructure [36,42]. The small α-Al cells surrounded by the Si network are rather rounded in the plane parallel to the building platform, while in the plane perpendicular to the building platform these cells have an elongated shape [36,42]. Additionally, in the surfaces parallel to the building platform (XY plane), elongated laser tracks are easily identified, while in the surfaces perpendicular to the building platform (XZ plane) a scale-like feature of melt pool borders is generally seen [32,33,44–48].

Concerning the influence of this anisotropy on the corrosion behaviour/resistance of these materials, various and contradictory results have been reported in the literature. For AlSi10Mg prepared by SLM, Cabrini et al. [48,50] concluded from potentiodynamic polarization experiments in diluted Harrison's solution that the surface of the XZ plane presents a slightly lower pitting corrosion resistance than the XY plane. The researchers associated this behaviour with the higher density of melt pool borders found in the XZ plane compared to that of the XY plane. However, in a later work, the same researchers concluded through a statistical approach that the building direction does not significantly influence the corrosion resistance of the analysed surface [56]. Moreover, Revilla et al. [32,33,54] found no difference in the electrochemical behaviour of the different planes during potentiodynamic polarization tests in aerated NaCl solution.

On the other hand, Cabrini et al. [46] demonstrated in another work by conducting an intergranular corrosion test that the corrosion in as-built specimens propagates mainly along the MPBs. Therefore, the penetration depth of the corrosion attack is highly influenced by the anisotropy of the melt pools. A more penetrative corrosion attack was seen on the surface parallel to the building platform compared to the surface perpendicular to the building platform. This could demonstrate that even if similar electrochemical behaviour is seen during potentiodynamic polarization tests, grea<sup>t</sup> anisotropy can still exist on the morphology of the corrosion attack and corrosion penetration depth.

For SLM AlSi7Mg0.6 and AlSi12, no considerable di fference was reported between the behaviours of the two di fferent planes during potentiodynamic polarization tests in NaCl solution [33]. Nevertheless, a clear distinction in the corrosion resistance of the di fferent planes was shown by Chen et al. [42] for SLM Al-Si12. The researchers presented experimental data (open circuit potential, potentiodynamic polarization, and electrochemical impedance spectroscopy measurements) supporting a better corrosion resistance of the XZ plane compared to the XY plane. This is the opposite trend as that reported for AlSi10Mg by Cabrini et al. [48,50]. Chen et al. [42] associated this behaviour with the depth of the Al/Si cells in each plane. The small and round cells were seen in the XY plane are deep since they are elongated in the perpendicular direction. According to the researchers, this could lead to the growth and even deposition of corrosion products, which could extrude and crack the Si shells, exposing the underlying Al substrate to further corrosion attack. On the other hand, the cells seen in the XZ plane are shallow, which could limit/preven<sup>t</sup> the deposition of corrosion products. Moreover, Chen et al. [42] reported a clear di fference in the weight loss rate during corrosion in NaCl solution for the di fferent planes. These results are shown in Figure 7. A reduced weight loss rate is seen for the XZ plane compared to the XY plane, possibly due to the same reason as that given for the di fference in the electrochemical behaviour.

**Figure 7.** The weight loss rate for the different surface planes (parallel to the building platform—XY—and perpendicular to the building platform—XZ) of as-built and mechanically polished SLM AlSi12 after immersion in 3.5 wt.% NaCl solution at room temperature [42].

Due to the limited and contradictory results found in literature concerning the e ffect of the anisotropic microstructure on the corrosion behaviour, further research is needed to better understand this issue. It is also important to keep in mind that di fferent alloys (even from the same alloy family) might display intrinsically di fferent behaviours.

#### 3.1.3. Comparison with Conventional Alloys

While the microstructure of as-built AM Al-Si alloys presents a very fine distribution of alloying elements into a three-dimensional network, the traditional cast alloy is characterized by the presence

of large Si crystals [32,44] and for the case of cast AlSi10Mg, other intermetallic particles such as AlFeSi and Mg2Si, among others, are also present [44]. These grea<sup>t</sup> di fferences in microstructure could result in di fferences in the corrosion behaviour of these materials. The corrosion resistance of AM Al-Si alloys has also been compared to that of conventionally manufactured alloys [32,44,51,59–63]. The results obtained for AlSi10Mg and AlSi12 are somewhat varied and in some cases contradictory.
