# Clinching of High-Strength Steel Sheets with Local Preheating
## 3 Results and Discussion
### Analysis of Clinched Joints with Local Preheating  
After applying preheating by resistance spot welding to the No. 1.0941 and No. 1.0935 grade sheets and making the clinched joints, the fine martensitic region, which was marked in Figures 14a and 21a, was relocated to the neck region in the upper sheet and to the locking region in the lower sheet (Figures 28a and 29a). Zone \(C\), with a martensitic microstructure, was in the bottom region after the formation of the clinched joint. The narrow ferritic-martensitic and ferritic-perilite zone \(E\), which was formed at the point of contact of the electrodes with the sheets, was in the bottom region on the outer surfaces of the joined sheets after the formation of the clinched joint. Figure 27: Microhardness HV0.3, zones with gradient microstructure of steel No. 1.0935: (**a**) upper sheet (US), (**b**) lower sheet (LS). Figure 26: Microhardness measurement scheme (steel No. 1.0935): (**a**) upper sheet (US), (**b**) lower sheet (LS). Figure 30 shows a comparison of the load-bearing capacity Fmax [N] of the pure clinched joints (CL) and the preheated clinched joints (RSW + CL). Five groups of specimens were tested for each of the jointed steel sheets. Within one group, seven specimens were prepared. All of the samples shown in Figure 30 were preheated by resistance spot welding, with optimized values of the welding parameters: a welding current of I = 3.9 kA, a welding time of T = 36 periods (1 per. = 0.02 s), and a pressing force of the electrodes of Fz = 6 kN. For both types of tested steel sheets, a higher load-bearing capacity was obtained for the preheated clinched joints. The load-bearing capacity of the pure clinched joints for the 1.0941 (HCT600X+Z) sheets ranged from 5398 N to 5540 N. On the other hand, the load-bearing capacity of the preheated clinched joints for these steels ranged from 6227 N to 6525 N. The measured load-bearing capacity values for the preheated clinched joints of the 1.0941 steels were higher by an average of 17%. Regarding the joining of steel sheets 1.0935 (HX420 LAD+Z), the measured load-bearing capacity values of the pure clinched joints ranged from 3712 N to 3831 N. For the preheated clinched joints for these steels, the load-bearing capacity values ranged from 4224 N to 5200 N. On average, the load-bearing capacity of the preheated clinched joints for the 1.0935 steel was higher by 25%. Figure 28: Preheated clinched joint on steel No. 1.0941: (**a**) measurement scheme HV0.3, (**b**) HV0.3 microhardness measurements. Figure 29: Preheated clinched joint on steel No. 1.0935: (**a**) measurement scheme HV0.3, (**b**) HV0.3 microhardness measurements. The increase in the load-bearing capacity of the preheated clinched joints is related to the change in the microstructure of the observed parts of the joint (P1, P2, center), which was proven by measuring the microhardness (Figures 13, 28 and 29). The microhardness values in the observed parts of the joint were higher in the preheated clinched joints in both types of steel sheets. Figure 31 shows the failure mode of the clinched joints after the tensile test. It is obvious that, with the clinching method, the mode of failure \"crack in the neck with plastic deformation\" occurred in both examined steel sheets. This is typical when the strength of the interlock is slightly greater than that of the clinched neck (as was described by [PERSON] et al. [56]). This mode is also known as the hybrid neck fracture mode. After the application of local heating using resistance spot welding, the failure mode of the clinching joints was changed to a \"neck fracture\". An increase in the hardness of the neck zone of the preheated clinched joints resulted in an increase in the strength of the material, which affected the joint failure mode. Figure 31: Failure of joined samples after tensile test: (**a**) pure clinched joint (CL), steel No. 1.0941, with local plastic deformation (LPD), (**b**) preheated clinched joint (RSW + CL), steel No. 1.0941, (**c**) pure clinched joint (CL), steel No.