**5. Discussion**

The previous methods for production of hybrid gears are generally represented by shrink fitting, friction welding [24] or bi-metal casting [25]. In the field of forging, there are only few investigations on the compound forging of straight bevel gears, where forming and the joining are combined in a single stage. The key challenge in joining by forming is the creation of the metallurgical bond between raw parts. Politics et al. studied the material flow behavior of different material combinations (steel-aluminum, steel-lead, steel-copper and copper-lead) depending on tooth ring thickness as well as friction properties in the interface zone and in the contact area between forging tool and workpiece [26,27]. Wu et al. carried out similar research for a steel-aluminum-combination focusing on gap size and height difference between ring and core [28]. In this context, the Tailored Forming technology using pre-joined workpieces represents an innovative approach for production of hybrid gears.

Each process step in the process chain of Tailored Forming contributes to the final quality of the bevel gear. The application of previously joined cladded workpieces ensures continuous joints and easy handling of multi-material workpieces throughout the whole process chain. The forging process after cladding facilitates a grain refinement of the coarse weld microstructure, which positively affects the mechanical properties of the parts. The heat treatment contributes to the functionality of the

bevel gears under operating conditions by increasing strength and hardness values of the tooth flanks. The micrographs in Figure 8 illustrate the microstructural evolution along the process chain.

As revealed by the hardness measurements, heat treatment has the most decisive influence on the strength of cladding and substrate. The hardness increase in the substrate can be attributed to the fact that the investigated area was located in the heat-affected zone (HAZ) during the heat treatment (Figure 10). Due to different outer diameters of the bevel gears, tooth sizes and axial temperature gradients, the depths of the HAZ at positions A and B differ. For instance, the higher heat input to the gear core at position B results in prolonged surface cooling. Therefore, a shallower HAZ is observed at this position. Independent of the initial substrate diameter, the HAZ is of a similar thickness at both positions A (Figure 10a,c) and B (Figure 10b,d) for the material combination 41Cr4/C22.8. In the case of material combination X45CrSi9-3/C22.8, the size of the HAZ is reduced in comparison with 41Cr4/C22.8 as a result of the different heat treatment strategy with higher self-tempering temperatures. At position B (Figure 10f), this effect is even more pronounced than at the position A due to the higher heat emanating from the bevel gear core.

**Figure 10.** Heat-affected zones after quenching and self-tempering in bevel gears of the material combinations 41Cr4/C22.8 (**<sup>a</sup>**–**d**) and X45CrSi9-3/C22.8 (**<sup>e</sup>**,**f**) at position A (**<sup>a</sup>**,**c**,**<sup>e</sup>**) and B (**b**,**d**,**f**).

Comparing the tensile strength values of the bevel gears of material combination 41Cr4/C22.8, the bevel gears with a substrate diameter of 28 mm show a higher strength after forging and heat treatment (Figure 9). Since the tensile specimens were prepared with the joining zone always located in the center of the specimens (Figure 7d), their extraction position from the bevel gears slightly differed due to a varying cladding layer thickness for Ø 27 mm and Ø 28 mm. The tensile specimens for workpieces with an initial substrate diameter of Ø 27 mm are slightly displaced in the direction of the tooth tip core. This results in a lower tensile strength compared to the Ø 28 mm specimens (Figure 9b,c). With respect to heat treatment, the larger initial substrate diameter and the corresponding thinner cladding layer resulted in an increased hardening depth in the part of the tensile test specimens that corresponded to the substrate. Thus, a slightly higher tensile strength for the Ø 28 mm specimens after quenching and self-tempering was observed (Figure 9d,e). The lower tensile strength of the X45CrSi9-3/C22.8 bevel gears compared to those consisting of 41Cr4/C22.8

results from the increased self-tempering temperatures, reducing strength in the C22.8 part of the tensile test specimens (cf. Figure 6). The heat in the component is therefore not dissipated as quickly as in the 41Cr4/C22.8 bevel gears. Here, the increased self-tempering temperatures are intended to foster a secondary hardening by the formation of chromium carbides [29]. Self-tempering at higher temperatures and the lower heat dissipation result in a decreased hardness of the substrate (188/268 HV 0.5 for X45CrSi9-3/C22.8 compared to 340/410 HV 0.5 for 41Cr4/C22.8); cf. Table 2. Accordingly, the tensile strength of such specimens is reduced.

In future studies, the functionality of the investigated bevel gears will be tested under operating conditions. This will require a grinding of the tooth flanks in order to achieve a high meshing of the forged pinion with the mating gear. These experiments will provide information on the service life behavior of hybrid bevel gears of different material combinations and with varying cladding thicknesses and heat-treatment conditions.
