3.3.2. Bar Samples

In Figures 13–15, the macro and micro hardness results for the bar welded specimens are reported. Concerning the analysis of all samples, the HV profile in the MIG welded samples showed the strongest reduction of mechanical properties; the fiber laser-MIG hybrid welded samples showed the best behavior, while the CMT welded samples had intermediate characteristics. The use of UNI ER 4043 filler material, softer than EN AW 6082, certainly contributed to the mechanical properties, especially in the FZ. Regarding the E-E samples, the microhardness profiles confirmed the results and the interpretations obtained in the E-E frame case. In addition, a different behavior could be observed looking the macro-hardness HRF profiles. In this case, MIG and CMT had a very similar trend, with about 62 HRF in the FZ and little loss of hardness up to the HAZ, which was wider in the MIG technique case. For the laser technique, the weld area was the softest, there was a slight decrease at 5 mm, and the hardness finally tended to increase. These differences revealed in the FZ and PMZ are probably due to the diverse heat input from the MIG, CMT, and laser welding process on the external surface of the samples. Furthermore, the specimens were thinner compared to the frame sections. These considerations clarify why the heat exchange was greater in the bar surfaces, both with respect to the cross section of the same samples and to the joints of the frame. For the C-C joints, the micro-hardness profiles showed a clear definition of the BM, HAZ, and FZ. The trend was very similar

for all techniques, but the hardness in the FZ center was very different, probably due to the presence of typical defects of the cast alloy used. Similar considerations can be advanced for hardness variations also observed in other areas of the samples. In the PMZ zone, the HV reached a hardness value of about 78–80 for MIG and CMT and 86 HV for the laser technique. In the HAZ, there was a slight drop of HV and afterward, the area far from the melt zone around 20 mm tended toward the stabilization in the HV value (50–55 HV for MIG and CMT, 80 HV for laser). The evolution of the macro-hardness HRF followed the trend already achieved in the case of the E-E combination. Noteworthy is the trend difference between CMT and MIG. Although the evolution was similar, the CMT welding showed a softer passage between the various zones. Once again, this fact is due to the different heat input during welding. It should be noted that MIG welding had drastic effects on the hardness, which reached values typical of an annealing. CMT welding had similar, but less pronounced effects. This could be caused by the high thermal input on thin samples, which caused an effect equivalent to an annealing treatment. This is probably also related to the high percentage of silicon (~7%) in this cast alloy, which entails a low material conductivity and a consequent difficult dissipation of the welding heat. On the other hand, during the laser technique, the heat input was highly localized in the FZ, thus this behavior was not observed.

Indeed, for the E-C coupling, the HV values confirmed the results of the tests conducted on the other two combinations and the E-C frame case studied. In particular, it was confirmed that the extruded component (Si~1%) better conducted the heat produced during welding, resulting in a reduced change in hardness [58–60]. Looking at the hardness profiles, considerations similar to E-E and E-C can be advanced for extrusion-welding and casting-welding interfaces, respectively. With regard to the HRF hardness test, the typical evolution was obtained, with an initial HRF peak in the PMZ, a subsequent decrease, and a gradual restoration of the properties as distance increased. It is worthwhile also noting that the HRF profile demonstrated the difference in heat exchange between the cast and extruded component during welding.

**Figure 13.** Evolution of the macro-hardness HRF and micro-hardness HV in the E-E combination of the weld for laser, CMT, and MIG.

**Figure 14.** Evolution of the macro-hardness HRF and micro-hardness HV in the C-C combination of the weld for laser, CMT, and MIG.

**Figure 15.** Evolution of the macro-hardness HRF and micro-hardness HV in the E-C combination of the weld for laser, CMT, and MIG.
