3.2.1. Frame Samples

The 6082–T6 and 6181–T6 alloys, which are quite similar in composition and consequently also in mechanical behavior, have similar metallographic structures. In Figure 6, it is possible to observe the microstructure of the extruded 6181–T6 where the deformation direction and the consequent anisotropy are clearly visible. The grains are rather small thanks to the manganese fining properties.

**Figure 6.** (**a**) Microstructure of the 6082-T6 alloy, BM. Keller etch, 500×; (**b**) Microstructure of 6181-T6 alloy, BM. Keller etch, 100×.

The EN AC 42100-T6 alloy microstructure is a typical example of an Al–Si alloy from a foundry. Cast alloys usually contain more alloying elements, therefore cooling can be quite long and a coarse-grained dendritic structure can be achieved. Like all foundry materials, it suffers from the presence of microand macro-segregation. Figure 7a shows the dendritic microstructure of the EN AC 42100-T6 BM, the large white dendrites (α-Al), and dark Al–Si eutectic in the space between the grains. Additionally, in this figure, it is possible to note some large shrinkage porosity, a frequent defect in the observed cast samples. In Figure 7b, the clear difference between the HAZ and FZ of the cast component side is highlighted. The PMZ was about 200–300 μm long. It should be also noted that during solidification, the material tends to also maintain a crystallographic continuity in the interface zone; in fact, the dendrites in the PMZ are developed starting from those already present in the cast. The microstructures at the interface between E-E and E-C are visible in Figures 3 and 4, respectively. Figure 3 reports the particular structure observed at the interface between the extruded material and the FZ. Starting from the extruded grains, the following changes occurred in the metallurgical structure: first planar, then cellular, and finally dendritic. These microstructural changes are a consequence of the growth rate effect at the solidification front. Moreover, the Keller etch highlighted two remarkably different structures on the extruded parts. In the upper one, the structure had coarse equiassic grains close to the outer side of the extrusion and was gradually thinner toward the center; the lower part had a completely different microstructure, probably due to the different reduction ratios and extrusion parameters. It is important to note that in both laminas, there was a skin zone on the outer faces.

**Figure 7.** (**a**) Microstructure of the EN AC 42100-T6 alloy, 50×; (**b**) Microstructure present at the fusion zone (FZ)/cast component interface, 50×.
