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Clinching is the manufacturing process of joining two or more metal sheets under high plastic deformation by form and force closure without thermal support and auxiliary parts. Clinch connections are applicable to difficult-to-join hybrid material combinations, such as steel and aluminum. Therefore, this technology is interesting for the application of AISI 304 components, as this material is widely used as a highly formable sheet material. A characteristic feature of AISI 304 is its metastability, i.e., the face-centered cubic (fcc) γ-austenite can transform into a significantly stronger body-centered cubic (bcc) α’-martensite under plastic deformation. This work investigates the effect of heat treatment—a process that involves the formation of an oxidation layer on the sheet surface—on the forming process during joining and the resulting mechanical properties of clinch joints made from AISI 304. For this purpose, different joints made from non-heat treated and heat-treated sheets were examined using classical metallography and advanced SEM techniques, accompanied by further investigations, such as hardness and feritscope measurements. The shear tensile strength was determined, and the fracture behavior of the samples was investigated. Clear influences of heat-treatment-induced surface roughness on the joint geometry and strength were observed.

The very high cycle properties of Inconel 718 in two different heat treatment conditions were investigated at a test temperature of 500 °C. One condition was optimized for fatigue strength and displayed a finer-grained microstructure, while the second batch had a more coarse-grained microstructure. For the high-temperature ultrasonic fatigue testing, a new test concept was developed. The method is based on the principle of a hot-air furnace and thus differs from the conventionally used induction heaters. The concept could be successfully evaluated in the course of the investigations. The materials’ microstructure was analyzed before and after fatigue testing by means of metallographic and electron backscatter diffraction (EBSD)analysis. The results show a significant influence of the heat treatment on the fatigue strength caused by the specific microstructure. Further, a difference in crack propagation behavior due to microstructural influences and non-metallic precipitations was observed.