3.3.1. Frame Samples

The micro-hardness results obtained for the E-E and E-C samples are summarized in Figures 11 and 12, respectively. Regarding the E-E sample, it can be observed that the FZ maintained a fairly constant hardness, around 60 HV, while there was first a hardness increase (about 90 HV) in the PMZ, followed by a decrease to 60 HV in the HAZ. Finally, the BM hardness was restored at about 20–25 mm from the fusion line for both the extrusion and casting interfaces. For the E-C sample, the HV profile of the extruded component showed a qualitative trend similar to the previous E-E analysis. In this case, however, the local minimum was approximately 80 HV; this suggests that during the welding process, the critical temperature of 380 ◦C was not reached, or not maintained long enough to complete the transformation of Mg2Si in the equilibrium phase β. The HAZ was only about 12 mm from the fusion line, confirming the possibility that in this case, the welding process was faster than in the previous one. For the cast component, the micro-hardness profile was similar to that of the extrusion, but showed that the plastic deformation aluminum alloy was more sensitive to the thermal cycle than the cast one.

**Figure 12.** Evolution of the micro-hardness HV in the E-C sub-frame combination as a function of the position along a transverse section of the weld (MIG joint).
