*4.2. Thickness of the Intermetallic Layer*

Figure 8 shows the calculated temperature, *T*, (left column) and the corresponding thickness of the IM layer, *xIM*, (right column) at the top section of the bonding interface between the steel sheet and the weld seam at the welding times *tw* = 10 s, 20 s, 30 s, and 40 s.

**Figure 8.** Temperature and thickness of the IM layer, calculated at the top section of the bonding interface between the steel sheet and the weld seam at (**a**) 10 s, (**b**) 20 s, (**c**) 30 s, and (**d**) 40 s after starting the welding process.

As illustrated in Figure 8, the temperature wave related to the movement of the welding torch propagated in welding direction. The peak of this wave was located close to the actual position of the torch. The growth of the IM layer was initiated when the temperature at the wave front exceeded 400 ◦C, and it proceeded as long as the temperature stayed above this limit. The thickness of the IM layer in welding direction was constant from *z* ≈ 50 mm to *z* ≈ 230 mm, but it varied distinctly perpendicular to the welding direction. Obviously, the thickness did not decrease when the weld cooled down. However, note that the thickness of the IM layer was overestimated at the end of the weld line, since the thermal weld penetration observed in the experiment (i.e., excessive melting of the sheets due to overheating at the end of the weld line) was not considered in the numerical model. Figure 9 illustrates the time-dependent evolution of both temperature and IM layer thickness at ten different positions (*z*-coordinates) along the welding direction, and at three different distances (*x*-coordinates) from the weld butt at the top section of the bonding interface.

**Figure 9.** Time-dependent evolution of temperature and IM layer thickness at the top section of the bonding interface (**a**) directly at the weld butt, (**b**) at the center of the interface, and (**c**) at the base corner of the weld seam.

The grey horizontal line shown in the diagrams of the upper row marks the temperature *T*<sup>0</sup> at which the growth of the IM layer was assumed to start in the present numerical model. The diagrams show (a) peak temperatures of approximately 800 ◦C directly at the weld butt, and (b) peak temperatures of approximately 600 ◦C at the base corner of the weld seam. Nevertheless, at all positions the temperature decreased about 90% within 60 s. However, the peak temperature strongly influenced the thickness of the IM layer, as shown in the diagrams of the lower row. The calculated thickness was about 17 μm directly at the weld butt, but it was less than 2 μm at the base corner of the weld seam.
