## 3 Results
### Welding Process Simulation  
After qualifying the embedding of weld inserts in TPC, the aim was to determine suitable welding parameters for resistance element welding of the TPC-steel-joints. To ensure a resource-efficient determination of welding areas and reliable welding parameters for the joints, numeric parameter  
Figure 6: Analysis of the joining zone after embedding: (a) type A, (b) type B.  
Figure 7: Results of the push-out tests: (a) load-displacement curves, (b) typical failure behaviour. studies were carried out prior the experimental investigations. In order to validate the numeric simulation model, the results of a process simulation were compared to the results of corresponding experimentally welded joints with respect to both the geometry (height and diameter) of the weld nugget and the shape of the heat-affected zone. [PERSON] et al. [23] have reported that for REW of a TPC-steel combination acceptable joints can be provided using electrode caps of the type ISO 58211-A0-20-16-100 and an axial electrode force of 3.0 kN. For this reason, similar boundary conditions were selected for this study. For the validation of the simulation model, a weld current of 9.0 kA and a weld time of 60 ms were chosen. The geometric comparison of the experimental and simulative results as well as the evaluation of the weld nuggets are shown in Figure 8.  
By comparing the geometry of the welded area and the heat-affected zone, it can be seen that the process simulation has a good correlation with the experiment. The deviations of the nugget diameter and the nugget height are less than 10%. Therefore, it can be stated that the simulation model ensures a satisfying agreement with the experiments. It can be seen in Figure 8a that the chosen welding parameters lead to a slight deformation of the weld insert in the undercut area. A reason for this phenomena may be a too high thermal energy input during the welding process, which causes a softening of the material in the area of the undercut, where the cross sectional area is minimal. Due to the reduced cross-sectional area, the yield strength of the material is exceeded as a result of the contact pressure of the electrodes, causing the deformation of the weld insert. The thermal energy induced in the weld insert depends on several factors, such as the geometry of the insert, the weld time and weld current, or the electrode force, which has an influence on the contact resistance. Afterwards, using the optimisation tool available in SORPAS, weld growth curves were generated for discrete weld times in order to define suitable weld current ranges in which appropriate joints can be generated. For this purpose, the weld current (\(l\)) was increased in 0.5 kA steps for a given axial electrode force of 3.0 kN. Weld times (\(t_{w}\)) were increased in 10 ms steps in between 40 ms and 70 ms. For the welding areas, a nugget diameter (\(d_{n}\)) of 3.25 mm was defined as the lower limit and the occurrence of splashes as the upper limit. Figure 9 shows the weldability lobes for weld insert types A and B determined by this approach. According to the results determined by numeric simulation, weldability lobes extending over a range of approximately 2.5 kA (type A) to 3.5 kA (type B) can be expected. With the decreasing weld time, higher currents are required to produce sufficient weld nuggets. At the same time, an extension of the weldability lobes following a reduction of the weld time can be observed, which is consistent with the investigations from [23]. Based on the determined weldability lobes, suitable welding parameters were selected for the subsequent mechanical tests. As [PERSON] et al.