3.1.2. Bar Samples

Figure 5 shows an overview of the microstructure for each weld seam analyzed. These images were obtained from a collage of numerous local 50× microstructures of the weld cross section in order to guarantee high quality of the analysis. In Figure 5, different geometries of the weld seams, depending on the welding technology, can be noted: v and/or u groove and width from 6 to 8 mm; u shaped butt joint and width from 9 to 10 mm; and v groove and width from 2 to 4 mm for MIG, CMT and fiber laser welding, respectively.

**Figure 5.** Weld geometry and defects of MIG, CMT (cold metal transfer), and fiber laser-MIG hybrid joints.

From a quality point of view, in the C-C combination, the CMT samples had the highest and most homogeneous diffusion of porosity in the fusion zone (FZ); MIG welds had a slightly smaller porosity in the FZ and the fiber laser-MIG hybrid welded samples only had some porosities. E-E and E-C were almost porosity free, thanks to the use of a wobbling head.

The average porosity diameter ranged from 21 μm to 145 μm and 18 μm to 120 μm for CMT and MIG, respectively. The fiber laser-MIG hybrid only had one significant porosity with a diameter of 270 μm, while a few others had dimensions similar to that in the CMT case.

It is remarkable to note that the overall highest level of porosity was observed in the joints with cast bars, especially in C-C. Indeed, castings have a high amount of Si, which reduces the thermal conductivity and increases the local heat near the welding seam. In addition, castings have more porosities than extrusions due to the lower solubility of hydrogen in solid aluminum than in liquid aluminum, which results in a diffusion of the entrapped hydrogen from the casting to the FZ during welding. This defect could be reduced by using vacuum casting in welding structures. It should also be noted that another defect was present, the incomplete penetration of the joint, mainly in the C-C combination of MIG welding. For the sake of clarity, incomplete joint penetration is defined as a condition of a weld where the filler metal does not extend through the entire joint thickness.
