*3.3. Hardness Distribution*

From the hardness evolution depending on the distance from the welding axis, it is possible to acquire information on the temperature profiles reached in the HAZ. The Al alloys, as objects of study, can be approximated to a pseudo binary Al-Mg2Si system. For this class of alloys, the hardening metastable phase is subjected to a precipitation process that is generally reported in the following steps: β" (hardening metastable phase), β' (intermediate metastable phase), and β (equilibrium phase Mg2Si). On the basis of previous work [56,57], the maximum hardness value was reached with the metastable phase β" having a needle-like structure. The hardness contribution of phase β', which had a rod morphology, was moderate, while that of the β equilibrium phase was almost negligible.

The allotropic transformations of Mg2Si correlate to their transformation temperatures, identifiable in about 240 ◦C as the upper limit for β" and approximatively 380◦ C for β' (see Figure 11 scheme). Considering that the local temperature peaks reached during the welding process decrease as distance from the fusion line increases, the following HAZ subzones can be identified:


**Figure 11.** Evolution of the micro-hardness HV (hardness Vickers) in the E-E sub-frame combination as a function of the position along a transverse section of the weld (MIG joint).
