# Clinching of High-Strength Steel Sheets with Local Preheating
## 1 Introduction
The study examined the impact of heat treatment, which involves the formation of an oxidation layer on the sheet surface, on the forming process and resulting mechanical properties of the clinch joints. The findings demonstrated clear influences of heat treatment-induced surface roughness on the joint geometry and strength. [PERSON] et al. [37] studied a new method to enhance the strength of clinch joints, which involved heating the sheet from the side of the die prior to the joining process. The results demonstrated that increasing the temperature of the bottom component before deformation led to an increase in the shear strength as well as in the cross-tensile strength. [PERSON] et al. [38] evaluated a novel method of joining for aluminum alloy 5052. The method combined principles of mechanical clinching and resistance spot welding. However, the resulting joint shape was not a clinched joint with its characteristic shape. The results demonstrated that the resistance spot clinched joints exhibited superior load-bearing capacity compared to traditional resistance spot welding joints. [PERSON] et al. [39] introduced a novel method called incremental laser shock clinching, which is used to create a round-end rectangular joint with a total length much larger than the diameter of the laser spot. The research verified the feasibility of this method to join copper, aluminum, and stainless steel. The development of the hot stamping clinching tool was investigated by [PERSON] [40]. The stamping clinching tool incorporates a forming system, a heating system, and a cooling system. They found that the high cooling rate employed in the experiments increased the tensile strength of the sheet at the clinching point by 3 to 4 times. A novel joining method called electro-hydraulic clinching was introduced by [PERSON] et al. [41]. By utilizing electrical energy discharge to generate a shock wave in the fluid, a clinching joint is formed. The resulting joint efficiency was approximately twice as high as that achieved through the traditional clinching process. [PERSON] et al. [42] studied the rectangular shear clinching process of ultra-high strength steel sheets. The study indicated that the rectangular shear clinching process, even without an airtight joint, was effective for joining ultra-high-strength steel sheets with low ductility and high flow stress. The same author [43] presented a clinching technique with the application of counter-pressure from a rubber ring for joining galvanized ultra-high-strength steel sheets. The interlock between the sheets was enhanced by promoting metal flow using the counter-pressure exerted by the rubber ring in the die cavity. It was observed that the appropriate shape of the rubber ring significantly improved the joinability, enabling the successful joining of the sheets with the aid of counter-pressure. Quite extensive research has been devoted to mechanical joining methods using the rivet as a fastener. The methods using this fastener are mainly clinch-riveting and self-piercing riveting [44]. These techniques are employed in the joining of sheet materials, establishing a mechanical interlock between the sheets. This method has gained significant attention in the automotive industry due to its suitability for joining lightweight, high-strength, and dissimilar materials. The biggest shortcoming of the clinching method is the low load-bearing capacity of the joints compared to resistance spot welding and other methods of mechanical joining, such as clinch-riveting and self-piercing riveting. This study deals with increasing the load-bearing capacity of clinched joints by locally modifying the microstructure of the joined materials HCT600X+Z and HX420 LAD+Z, especially in the area of the neck of the joint. A local change in the microstructure of the clinched joint was achieved by using resistance spot welding. Numerical simulation software was used to predict the material flow of the preheated microstructure of the joined materials during the clinching process.