*3.4. SEM Analysis*

The PMZ and HAZ are crucial areas in weld alloys that are hardened by artificial aging. The transformation phase along the welding profiles was explained in the previous section. In addition, in the literature [55,56], it is documented that the microstructure of the welded Al–Si alloys is

characterized by the presence of elongated particles (dimensions of about 1–10 μm) and extremely fine particles (size approximately of 0.1–0.5 μm) dispersed in a matrix with uniformly distributed micropores. This is remarkable, since it has been reported that Si-rich precipitates are found due to the excess silicon in the alloys. The larger particles were identified as the (Fe, Mn)3SiAl12 compound, while the finer ones were Mg2Si, the hardening phase obtained during thermal treatment (T6).

The structures of both EN AC 42100-T6 and 6000 series joints were revealed by the SEM/EDS investigations, as reported in Figure 16. The fusion zone had a chemical composition that was affected by the AA 4043 filler metal in addition to the welded materials. The chemical analysis of the white particles confirmed that these could be the compound (Fe, Mn)3SiAl12, with an average size about 3–5 μm. These particles were observed in both the extrusion and casting components, where the concentration was higher. The analysis of the PMZ highlights that the fiber laser-MIG hybrid samples had the highest presence of these particles with a finer morphology, while for the MIG and CMT techniques, the amount and dimensions were very similar.

**Figure 16.** Some examples of the precipitate types in the PMZ: chemical composition in wt%.
