# Clinching of High-Strength Steel Sheets with Local Preheating
## 2 Materials and Methods
### Local Preheating  
For the investigation of the clinching method with local preheating, overlapped samples with the same material combination were prepared: HX420 LAD+Z with HX420 LAD+Z and HCT600X+Z with HCT600X+Z.  
Local intense heating (RSW) was applied to a pair of overlapping hot-dip galvanized sheets, each 1.5 mm thick. The local heating occurred at the location of the future clinched joint (Figure 3a). The local heating was performed with a BPK 20 spot pneumatic welder (VTS Elektro s.r.o. Bratislava, Slovakia) with adjustable welding parameters (electrode pressure, welding time, welding current), and with 30 different pre-set welding programs. The heating parameters were selected on the basis of the experience gained in the field of welding and then optimized for the implementation of local heating of the material. For local heating, Cu-Cr welding electrodes were used. The working surfaces of the welding electrodes were \(\mathcal{O}\)8 mm with a conical finish.  
The diameter of the welding electrode was selected based on a die diameter of \(\mathcal{O}\)8 mm for clinching joining. The resistance spot welding parameters used in the preheat optimization are shown in Table 4. Preheating optimization started from the lowest of the listed values. The aim was to achieve a change in the microstructure that reached the neck region of the joint (t\({}_{\text{N}}\)) during the clinching process. However, a spot weld with a typical weld nugget was not allowed to occur. After the preheating phase ended, the samples were joined by the clinching method in place of the local change in the microstructure.  
Figure 3: Procedure of making samples for experiments: (**a**) preheating by welding electrodes and (**b**) clinching preheated area.  
Based on the optimization of the preheating process, the following welding parameters were selected for further research:  
* Welding current of I = 3.9 kA;
* Welding time of T = 36 periods (1 per. = 0.02 s);
* Pressing force of electrodes of Fz = 6 kN.  
Preheating with the higher values already led to the formation of a welding lens, which was undesirable with respect to the subsequent clinching joining. The weld nugget prevented formation at the bottom of the clinched joint. After the local preheating using spot welding electrodes, the samples were subsequently joined at the preheating area by the clinching method (Figure 3b). A punch with a diameter of \(\mathcal{O}\)5 mm and a die with a diameter of \(\mathcal{O}\)8 mm were used. The force applied to the die was set to 80 kN.  
A flow chart illustrating the research procedure applied in this study is shown in Figure 4.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Sample Series**} & **Welding Current** & **Welding Time** & **Pressing Force** \\  & **I (mm)** & **T (per)** & **Fz (kN)** \\ \hline I & 3.5 & 12 & 6 \\ II & 3.5 & 18 & 6 \\ III & 3.5 & 24 & 6 \\ IV & 3.5 & 30 & 6 \\ V & 3.5 & 36 & 6 \\ VI & 3.9 & 12 & 6 \\ VII & 3.9 & 18 & 6 \\ VIII & 3.9 & 24 & 6 \\ IX & 3.9 & 30 & 6 \\ X & 3.9 & 36 & 6 \\ XI & 4.3 & 12 & 6 \\ XII & 4.3 & 18 & 6 \\ XIII & 4.3 & 24 & 6 \\ XIV & 4.3 & 30 & 6 \\ XV & 4.3 & 36 & 6 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Welding parameters for optimization of preheating process for both investigated materials.  
Figure 4: Flow chart of research procedure.  
Local preheated samples prepared by resistance spot welding and joined by clinching with local preheating are shown in Figure 5.