*4.1. Temperature Field*

The numerical simulation provided the time-dependent temperature field inside the metal sheets and inside the weld seam during the welding process, as well as during the post-welding cooling period. In particular, the evolution of the temperature field next to the weld butt at the top surfaces of the weld seam and of the two metal sheets is illustrated in Figure 6. Though the cooling conditions were equal for both sheets, the non-symmetric temperature field reveals that the majority of the welding heat flowed into the aluminum sheet. This is confirmed by the temperature curves illustrated in Figure 7, which show that the peak temperatures captured at equal distances from the weld butt were almost twice as high in the aluminum sheet as in the steel sheet. Obviously, comparing the temperature curves at particular positions reveals good qualitative and quantitative agreements between the welding experiments (Figure 7a,b) and the welding simulation, (Figure 7c).

**Figure 6.** Temperature field at the top surfaces of the aluminum sheet (Al), the steel sheet (St) and the weld seam at 5 s, 15 s, 25 s, and 35 s after starting the welding process.

**Figure 7.** Comparison between temperatures measured using thermocouples during CMT welding of (**a**) sample 62 and (**b**) sample 63, and (**c**) temperatures calculated in the finite element simulation.

The experimentally validated simulation model was used to calculate the temperature not only at the surface, but also at the interface between the weld seam and the steel sheet, which was not accessible for direct temperature measurements. Figure 10a shows that the weld seam contacted the steel sheet at three sections: (i) at the top section, (ii) at the side section, and (iii) at the bottom section. The idealized dimensions of both the top and the bottom sections were 250 mm × 5 mm (i.e., length of the weld seam × mean width of the weld seam contacting the steel sheet), but the dimensions of the side section were 250 mm × 0.8 mm (i.e., length of the weld seam × thickness of the steel sheet). Detailed information regarding the time-dependent temperature field occurring at the three sections of the bonding interface was required to calculate the time-dependent thickness of the IM layer formed during the welding process.
