**1. Introduction**

The material choice for conventional monolithic components, commonly used in bulk metal forming at industrial scale, represents a compromise between material properties and economical requirements [1]. In parts with locally varying mechanical, thermal or chemical operating conditions, it is rather challenging to identify a single material, which fulfils the individual specifications. Increasing demands on technical components due to current trends towards resource and energy efficiency render this issue even more difficult and have triggered the development of alternative technological solutions to overcome the material-specific restrictions. In this context, multi-material (hybrid) designs combining the benefits of different materials in a single component offer a grea<sup>t</sup> potential for creating high-performance components with extended functionality and resource efficiency [2]. While this concept is well-established in the sheet metal industry (tailored blanks [3] or clad rolling [4,5]), further research is required in bulk metal forming, e.g., for hot forging processes. Specifically,

the development of novel technologies for manufacturing metallic multi-material parts is required. The Collaborative Research Centre 1153 investigates the innovative process chain of Tailored Forming to manufacture hybrid metallic high-performance components by hot or semi-hot forming of prior joined preforms. Combinations of different joining and forming processes are evaluated on the example of technical parts such as bearing washers [6], bearing bushings [7] and transmission shafts [8]. In the following, the hot forging of hybrid metal bevel gears from cladded preforms made of two different steel grades combined with a subsequent process-integrated heat treatment are presented. The investigations involve an analysis of the interface using metallographic examinations before and after forming as well as after the integrated heat treatment. For a quantitative characterization of the bond quality, tensile tests were conducted using micro tensile samples and hardness values were measured.

#### **2. State of the Art**

Forging is a key technology for manufacturing technical components with complex geometries and provides for continuous fiber flow and excellent mechanical properties. Depending on the initial state of the applied semi-finished parts, there are two methods of forming multi-material components: compound and hybrid forging. In compound forging, raw parts assembled without a metallurgical bond are used. The joining of these parts takes place during forming to the end geometry. The challenge in joining by forming is to create a metallurgical bond between the different materials. In compound forging, the focus is often on parameter case studies in order to achieve a sufficient bond quality, as the latter depends on specific process conditions such as temperature, contact pressures and relative displacement between the materials. For example, Sun et al. studied a hot isothermal compression bonding process of two different steel grades (Q235 and 316 L) using experimental and numerical investigations. They found that the element diffusion distance in the near interface zone grows with increasing deformation temperatures, effective strains and holding times. However, increasing effective strain rates have a negative impact on bond quality [9]. Investigations of Kong et al. on the forge welding of steel-aluminum compounds (AISI 316 L/6063 aluminum) reveal that the forming temperature has the highest influence on the resulting bond quality and the tensile strength of the joint [10]. Wohletz and Groche studied a joining process combining forward and cup extrusion for manufacturing steel-aluminum parts (AISI 1015/6082 T6 aluminum) [11]. They observed that an increased formation of oxide scale on the contact surfaces at elevated temperatures has a negative influence on the resulting bond quality. Due to the above-mentioned restrictions, which result in a narrow range of suited possible process parameters, it is difficult to ensure a homogeneous bond quality when forming hybrid parts with complex geometry, process-related variation in strains and non-uniform material flow. In such a case, the application of previously joined semi-finished parts featuring a metallurgical bond is advantageous for achieving uniform characteristics at the interface. This approach, called hybrid forging, aims at improving the microstructure and the mechanical properties of the material compound. For instance, Förster et al. investigated a two-step forging process of aluminum-enclosed magnesium work pieces, which had previously been joined by co-extrusion [12]. After forging, the magnesium core was crack-free and fully enclosed by aluminum, even at the front ends of the prior extruded, initially aluminum-free profile sections. Though in these regions no metallurgical bond was observed, the bonding achieved by extrusion was maintained. Domblesky et al. conducted axial and side compression tests of friction-welded workpieces with a serial arrangemen<sup>t</sup> made from the same materials or material combinations (copper, steel, aluminum) [13]. All material combinations demonstrated a good workability during the forming stage. For combinations of identical materials, the tensile tests showed uniform deformation and material flow similar to that of monolithic materials. In the case of dissimilar material combinations, most of the deformation and subsequent fracture occurred in the softer metal. Klotz et al. performed an isothermal forging of bi-metal gas turbine discs made of two different Ni-based superalloys from hot isostatically pressed billets [14]. They found that bond quality after forming depends on the initial state of the hot isostatically pressed

preforms. The formed specimens showed a refined microstructure due to recrystallization at hot forming temperatures.

If the hot-forged steel parts feature a near-net-shape geometry and the temperature after forming is in the austenite regime, a quenching and tempering can be carried out directly after hot forming [15]. Such an integration of the heat treatment into the hot-forming process is used to reduce process costs and times. By applying a controlled cooling with an air-water spray, locally adapted cooling rates can be achieved ranging from low cooling rates as in gas quenching up to high cooling rates as in immersion cooling in water [16]. In addition, the quenching process can be interrupted in order to self-temper the hardened surface using the residual heat remaining in the core of the component. By spray cooling, a continuous and gentle hardness transition is achievable, which can be favorable regarding stress distribution compared to parts featuring a steep hardness gradient [17]. Hence, the near-net shape Tailored Forming of bevel gears in combination with spray cooling o ffers the potential to manufacture parts with a high fatigue strength and extended service life.

#### **3. Materials and Methods**
