3.2.2. Bar Samples

The microstructural features of the interface between HAZ, PMZ, and FZ in all combinations for the three weld techniques are reported in Figures 8–10. In particular, the FZ of the MIG, CMT, and fiber laser joints was constituted by a dendritic structure of aluminum solid solution (α-Al) and Al–Si eutectic for all of the cases examined, with the dendrite arm spacing slightly wider in the MIG and CMT joints with respect to that of the fiber laser. Looking at the HAZ, it is possible to observe that the extruded portion of the bar contained few elongated grains. Regarding the cast bar, in the HAZ, the white dendritic Al primary phase that resulted extended from the PMZ zone and was longer than that in in the base metal; a grey spheroidized Al–Si eutectic was also present at the grain boundary.

For the C-C combination (Figure 8) the PMZ for the MIG and CMT techniques had a regular evolution and covered about 200–300 μm and 500 μm of the interface area, respectively. It should also be noted that the PMZ for the laser weld had an irregular profile and this area was smaller but had longer dendrites.

**Figure 8.** Macrostructures of the interface between the HAZ (heat-affected zone), PMZ (partially molten zone), and FZ (fusion zone) in the Cast-Cast combination (C-C), Keller etch, 50×.

For the E-E combination, it is possible to observe the PMZ, which was characterized by the presence of intermetallics visible even at low magnification (Figure 9). In this type of joint, the PMZ was slightly smaller than that in the C-C case, especially for the laser technique.

**Figure 9.** Macrostructures of the interface between the HAZ, PMZ, and FZ in the Extruded-Extruded (E-E) combination, Keller etch, 50×.

Figure 10 reports the E-C coupling; the extrusion–weld and the casting–weld interfaces are reported in the left and right side of the picture, respectively. The FZ of the joint consisted of a fine-grained dendrite structure formed by α-Al and Al–Si eutectic. Regarding the FZ/cast bar interface, the PMZ was due to the re-melting of the eutectic compound [55]. In this case, the grain size in PMZ was larger than that in the FZ, but the microstructure had a dendritic aspect with dimensions increasing from FZ to HAZ and a crystallographic continuity through the section due to the partial remelting. Looking at the extrusion–FZ interface, the change of structure was clearly less evident. The FZ microstructure in this part was similar to the cast alloy side. In the PMZ, the low melting point segregation phase was etched severely, was not uniform, and agglomerated near the FZ. In addition, the extrusion grains were more elongated in this configuration when compared to E-E, most likely because the casting side of the coupling implies an increase in local heat, due to the reduced thermal conductivity.

**Figure 10.** Macrostructures of the interface between the HAZ, PMZ, and FZ in the Extruded-Cast combination (E-C), Keller etch, 50×.
