*4.3. Experimental Validation*

In order to validate the results of the calculations, a sample was extracted from the center of the welded blanks at *z* ≈ 125 mm. This sample was embedded, ground, polished, and prepared for metallographic investigations by means of an optical microscope. The merged micrograph of the entire weld cross-section (both unetched and etched using Barker's reagent) is shown in Figure 10. As marked in Figure 10a, three sections of the bonding interface between the weld seam and the steel sheet can be distinguished: (i) the top section, (ii) the side section, and (iii) the bottom section. The surrounding micrographs illustrate the varying thickness of the IM layer (dark grey seam between the

light grey aluminum weld and the medium grey steel sheet) at different positions on the three sections of the bonding interface.

**Figure 10.** Weld seam cross-section of sample 63, (**a**) unetched and (**b**) etched with Barker's reagent. The surrounding micrographs illustrate the thickness variations of the IM layer.

According to the micrographs shown in Figure 10, the mean thickness of the intermetallic layer was 10.9 μm (*SD* = 4.2 μm) at the top section, 4.8 μm (*SD* = 2.1 μm) at the side section, and 7.8 μm (*SD* = 2.2 μm) at the bottom section. The maximum thickness of approximately 20 μm was found close to the weld butt, at the position where the peak temperatures occurred and where the majority of the filler was deposited. From this point, the thickness of the IM layer decreased bidirectionally: just slightly toward the cutting edge of the steel sheet, but markedly toward the base corner of the weld seam. This was also predicted by the numerical model, as visualized in Figures 8 and 9. However, lack of fusion—which was not included in the present numerical model—occurred at the base corners of the weld seam. The etched micrograph shown in Figure 10b allowed identification of the width of the fusion zone (i.e., the width of the weld pool) between the aluminum sheet and the steel sheet. This width was about 2 mm.
