*3.1. Geometry and Mesh*

As shown in Figure 2, the model included three parts: the aluminum sheet, the steel sheet, and the weld seam, with dimensions according to the experimental welding configuration. The complete model consisted of approximately 504,000 nodes and 425,000 elements. The interface between the weld seam and the steel sheet was meshed with comparatively fine elements, since the temperature at this interface was of primary interest for calculating the thickness of the IM layer. Figure 2 also shows that the mesh was coarser with increasing distance to the weld seam, in order to limit the required calculation time. A cross-section of the meshed model is shown in Figure 3.

**Figure 2.** Meshed three-dimensional finite element model.

**Figure 3.** Cross-section of the meshed model based on (**a**) the micrograph of sample 63 and on (**b**) the joint surface reconstruction of sample 62.

According to the results of the welding experiments, the maximum thickness of the weld seam was about 3.8 mm, and the width between the base corners of the weld seam was about 8 mm. These dimensions were estimated from micrographs of the weld cross-section, as exemplarily shown in Figure 3a. The micrographs were captured using a Zeiss Axio Observer.Z1m (Jena, Germany) optical microscope equipped with a Zeiss Axio-Cam MRc5 camera. The dimensions of the weld seam were validated with a three-dimensional reconstruction of the joint surface as shown in Figure 3b, which was captured using the optical 3D scanning system GOM ATOS III Triple Scan (Braunschweig, Germany). Neither small variations of the actual weld seam geometry nor thermal distortion of the sheets were considered in the model, since their effects on the growth and on the thickness of the IM layer are negligible.
