## 1 Introduction  
Hybrid parts consisting of continuous fiber reinforced thermoplastics (CFRTs) and high strength steel components offer interesting possibilities in use-cases with challenges that cannot be met with mono-material parts. One possible example is the combination of a demand for lightweight construction, where CFRTs can offer superior weight-related properties [1], with locally high thermal or abrasive stress, where CFRTs typically meet limitations [2]. This could be resolved with the combination of metal components to form a hybrid part. In this approach, the joining operation is the key challenge due to strongly diverging physical and chemical properties such as stiffness and thermal expansion coefficients. In the state of the art, bolted and riveted joints are commonly used; as for these joints, a comprehensive knowledge is available in the literature and it is possible to create strong and durable joints [3]. However, despite their widespread use, these form-fitting joining techniques come with relevant disadvantages. First, the need for a precise bolt hole [4] requires an additional process step, which increases processing costs and typically destroys load-bearing fibers. This can require additional reinforcements such as locally embedded titanium sheets or additional layers in the laminate. Furthermore, the need for additional auxiliary elements such as bolts, nuts and rivets leads to increased weight of the structure, which contradicts the idea of lightweight construction [5]. A fiber-friendly joining technology is the use of adhesives. While adhesive joining leads to an even introduction of force into the laminate, which reduce concentration of tension and typically does not require additional reinforcements [6], it also comes with distinct challenges, including the demand for extensive process quality control to ensure a reliable joint and especially with thermoplastic composites, joinability via adhesives is oftentimes limited. Poly nucleophilic polymers especially require surface treatment to be joinable via adhesives that add process steps and complexity to the joining process [7]. A comparably new joining process is the creation of form fitting joints via cold formed pin structures, which can embedded into the locally heated CFRT component [8]. In the field of thermoset-based composite (CFRP)/metal joining, the use of pin-reinforced adhesive joints is already widely described. However, in CFRP/metal joining, the pin needs to be embedded during the manufacturing process of the part [9]. This can either be into the dry fabric prior to the infusion process via vacuum assisted resin infusion [10], vacuum assisted resin transfer molding [11] or into pre-impregnated sheet during manufacturing before the curing stage of the matrix [12]. Thus, a separate, subsequent joining process is not possible. In the field of CFRT/metal joining, the direct pin pressing process into a locally heated CFRT component has been shown to be an uncomplicated joining process with promising mechanical properties [13]. In the current state of the art, rotational-symmetric pins have been primarily investigated [14], although in [PERSON] et al. [15], the uses of non-rotational-symmetric pins have shown potential to increase the maximum load of the joint in the field of metal to aluminum hybrid joining. Due to the good performance in metal/metal joining, the use of non-rotational-symmetric pin structures also appears promising in the use of CFRT/metal joining. However, the underlying mechanisms in the direct pin pressing of CFRT/metal parts are fundamentally diverse for steel/aluminum joining and consequently, a direct transfer of the knowledge from [PERSON] et al. [15] is not possible. In particular, the joints' dependency on the pin- and fiber orientation of the anisotropic CFRT material in relation to the load direction needs to be investigated. Considering the existing literature, the present study aims to create an understanding of the CFRT/metal joining process with non-rotational-symmetric pin structures.