*3.5. Damage Analysis*

For each tailored forming production strategy, the sample lying in the middle of the Weibull distribution was examined for damage analysis. Figure 9a shows each spalling damage on the surface which led to the stop of the test with use of laser scanning microscopy. Rolling direction is from bottom to top. The spalls have a size of around 1000 × 500 μm<sup>2</sup> and a maximum depth of −90 to −140 μm. Subsequently, the samples were cut in the longitudinal direction for damage analysis. Figure 9b depicts micrographs of the joining zone and the microstructure in the rolling contact area after fatigue testing. For this, the samples were etched with 2% alcoholic nitric acid.

**Figure 9.** Damage analysis: (**a**) surface laser scanning microscopy of each event-relevant spalling damage, and (**b**) microstructural details in axial section plane.

The depth of the damage lies within the area of maximum orthogonal shear stress induced by the roller bearing. Together with the subsurface crack propagation parallel to the raceway in Figure 9b, this indicates rolling-contact fatigue as the damage mechanism. However, micropittings are visible to the right side of each spall in Figure 9a. Due to the di fferent material properties of the hybrid component, a slightly unevenly shaped deflection of the shaft could cause skewing in the bearing, which, in turn, would result in edge loading. The conventional rolling element features a higher hardness of > 850 HV0.5, which can lead to surface disruption on the shaft. The martensitic microstructure in the hardened area and the ferritic–pearlitic microstructure in the base material are clearly visible in Figure 9b. In the transition zone of these two microstructure areas, some sporadic ferrite grains are visible in the martensitic area. This could be a possible reason for the slightly lower hardness of the martensitic structure. However, structural failure within the joining zone or in the transition zone from the surface-hardened area to the base material did not occur in any of the samples. The test bench-induced alternating bending stress in the joining zone is therefore not critical for the operating parameters presented here.

#### **4. Conclusions and Outlook**

The machining and operating behavior of friction welded and extruded hybrid shafts of SAE1020-SAE5140 were investigated. Turning and deep rolling experiments were carried out for manufacturing of the shafts. Subsequently, the influence of di fferent process control variables on the subsurface properties (residual stresses, microstructure, and hardness) was analyzed. For bearing raceways, surface quality is very important in order to exclude failure of the component due to surface defects. Due to this demand, the influence of process parameters on surface roughness and topography were examined first. The feed for turning and overlap for deep rolling have a significant influence on surface roughness. In particular, certain e ffects occur in the material transition zone, depending on these variables. Depending on the feed rate during turning and overlap during deep rolling, a step, groove, or hump can be observed in the transition area. These are mainly caused by changes in the material properties in the transition area and enhanced by e ffects such as tool deflection and plastic material deformation. To exclude component failure caused by insu fficient surface quality, for further investigations, process parameters which produce the lowest possible surface roughness were selected. In turning experiments, a feed of 0.05 mm, and in deep-rolling experiments an overlap of 85%, were chosen.

The cutting-edge rounding of the tool has a significant influence on the subsurface properties. With increasing cutting-edge rounding, compressive residual stresses are induced. However, deep rolling significantly increases compressive residual stresses regarding induction depth and in terms of residual stress values. In order to investigate the influence of residual stresses on the operating behavior, three di fferent process parameters were used. The hybrid shafts were produced by variation of the cutting-edge rounding in case of turning and by deep rolling. Di fferent cutting-edge roundings led to di fferent sizes of the influenced microstructure area and hardness values in the subsurface. The reason for this is that, with increasing cutting-edge rounding, more material is deformed under the cutting-edge rounding. Deep rolling leads to a further increase of the influenced microstructure area and of surface hardness. The stronger compression of the material results in work hardening and increased hardness. In deep rolling, the observed e ffects increase even further, since there is hardly any thermal load in this process and the component is thus almost exclusively subjected to mechanical load.

In this first preliminary study to evaluate finishing strategies for tailored forming components, three samples per production strategy were manufactured. Due to the complex production process, a larger sample number, of more than three samples per finishing strategy, was currently not achievable. With the produced samples, fatigue tests were carried out on a bearing fatigue test bench. The aim of the tests was to investigate the fatigue behavior of the hybrid shaft compared to mono-material shafts and, in particular, to highlight the influence of manufacturing processes on the fatigue life

regarding rolling-contact fatigue. Based on the sample series manufactured with sharp cutting-edge geometry, it could be shown that tailored forming machine elements can have a comparable fatigue life to conventional components. An early failure of a sample manufactured with a rounded cutting edge did not allow an evaluation of this series. It seems that an increase in service life can be achieved by additional deep rolling. The reason for this can be improved surface and subsurface properties, such as roughness and hardness. Furthermore, the compressive residual stresses induced seem to have a positive e ffect on the material microstructure with regard to its fatigue strength behavior. However, it is absolutely necessary to repeat the examinations to confirm this thesis.

In order to further refine the results and, in particular, to increase the empirical significance, the fatigue tests will be continued in the further course of the project. This includes further tests with the same load parameters, as well as other load levels. Thus, di fferent mechanisms of fatigue damage can be provoked, and the manufacturing strategy can be adapted to the later load case. In addition, the production parameters are to be transferred to other tailored forming components with di fferent geometries. This will ensure optimum operating and performance characteristics for future applications of these new high-performance components.

**Author Contributions:** Designed the machining process, performed the turning and deep rolling experiments, and carried out the analysis of surface and subsurface properties, V.P.; supervised the work, B.D., B.B. and A.K.; designed the samples and carried out the fatigue tests and accompanying analyses, T.C.; supervised the work, G.P. and F.P. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation)—CRC 1153—grant number 252662854, in the subprojects B4 and C3.

**Acknowledgments:** The results presented in this paper were obtained within the Collaborative Research Centre 1153 "Process chain to produce hybrid high-performance components by tailored forming" in subproject B4 and C3. The authors would like to thank the German Research Foundation (DFG) for the financial support of this project. The authors would also like to thank the subprojects B2, B3, A2, and A4 for their support in producing samples through the use of friction welding, impact extrusion, and hardening, as well as in metallographic analysis.

**Conflicts of Interest:** The authors declare no conflict of interest.
