*3.1. Surface Measurement*

Surface roughness plays a decisive role when it comes to the later operating behavior of the hybrid component. Therefore, it is important to identify process parameters that produce a low surface roughness in both materials. Furthermore, it is important to reduce the shape deviation in the transition zone in order to maintain the dimension tolerances of the component.

In Figure 4, the influence of different feeds and microgeometries for turning and overlaps for deep rolling on surface roughness is presented. To investigate the effects of the varied parameters, the surfaces after friction welding and after pre-turning were measured and evaluated. It was ensured that the shafts were pre-turned in the same machining direction and with the same process parameters, in order to achieve a constant initial state before the finishing step of the shafts. The surface analysis after pre-turning showed throughout the investigations an unchanged step in the material transition zone in the range of 12–15 μm, whereby the material range SAE5140 was always higher than SAE1020. The roughness after pre-turning was in the amount of Ra = 0.5–0.6 μm in both material ranges. With decreasing feed, *f*, it is clearly visible that the roughness decreases, too. The reason for this is that, with the reduction of feed, consecutive roughness peaks are reduced. Consequently, the surface roughness values decrease. However, it can still be observed that the shape deviation in the material transition zone increases with decreasing feed rate. At a feed rate of *f* = 0.05 mm, the minimum chip thickness is not exceeded in the major part of the chip cross-section, especially in the case of large cutting-edge roundings. As a result, the material is accumulated in front of the cutting edge, and the material is diverted either under the cutting edge into the base material or over the rake face into the chip. This effect, which occurs especially when the chip thickness gets below the minimum chip thickness, is known as the ploughing effect. It is assumed that the properties of the microstructure in the joining zone area of the hybrid shafts are changed by the friction-welding process.

**Figure 4.** Surface topography and roughness, depending on the machining process (**a**) turning and (**b**) deep rolling.

At low feed rates, this leads to an amplification of the ploughing effect, resulting in a higher indentation depth of the transition zone during turning. In the harder material area of SAE5140, the passive forces increase and leads to a tool displacement. This again increases the workpiece height accordingly. This explains the strongly formed groove for *S* = 30 μm at a feed rate of *f* = 0.05 mm. With increasing feed, the "weaker" transition area during turning with the higher feed is mainly skipped, so that a step is created, but this is not strongly trough-shaped as with the smaller one. Concerning the influence of the cutting-edge microgeometry on surface roughness, a significant difference between both is not visible. Nearly the same surface roughness values are apparent for both. The shape deviation is smaller by using a cutting-edge rounding of *S* = 75 μm, compared to the cutting-edge rounding of *S* = 30 μm, at a feed of *f* = 0.05 mm. Here, the mechanical effect exceeds the thermal effect and leads to a smaller shape deviation. The results of the surface roughness measurements for deep rolling show that, with increasing overlap, the surface roughness remains nearly the same, but the shape deviation is significantly reduced. A small shape deviation is only visible in the transition zone. It is apparent that, at an overlap of u = 50%, a groove is formed in the material transition zone, and at an increase of the overlap to u = 85%, in contrast, a small hump is formed. It should be considered that the initial topography before the rolling tests always corresponds to a turned surface with *S* = 30 μm, *f* = 0.2 mm, and vc = 180 m/min. According to this, there is always a step in the material transition zone in the initial state before deep rolling. As in turning, deep rolling with an overlap of 50% also causes an indentation of the material due to a changed microstructure and microstructural properties in the material transition zone, as a result of the friction-welding process. If the overlap is increased, however, more material in the soft SAE1020 area is plastically deformed and pushed in front of the ball that the ball nearly fills the step and moreover starts to accumulate into a hump. This is compacted and hardened as the ball passes over it. In the harder SAE5140 area, the material is again densified. As a result, a small hump remains in the material transition zone. However, further, more detailed investigations are necessary to confirm this thesis.

Finally, comparing the surface roughness and topography of the different machining processes of turning and deep rolling, the results are evident. It can be clearly seen that deep rolling produces significantly better surface roughnesses and topographies than turning of hybrid components.

#### *3.2. Microstructure and Hardness*

Figure 5 shows the microstructure of the hybrid shafts produced with different machining processes for the fatigue-life investigations. The area influenced by the machining process is significantly larger in the deep rolled specimens than in the turned specimens. The joining zone is not directly located in the bearing raceway but in the area of the chamfer, about 3 mm from the rolling element. The characteristic microstructure of the joining zone due to the extrusion process is also clearly visible. Deep rolling induces higher mechanical stress in a greater material depth, *z*, which modifies the microstructure to a higher depth. Consequently, in addition to the residual stresses, structural properties are also influenced by different machining processes and must be considered in fatigue tests, too.

**Figure 5.** Influence of different manufacturing processes (1) turning with sharp cutting edge, (2) turning with rounded cutting edge and (3) deep rolling on the microstructure of hybrid components.

Hardness measurements were carried out in order to investigate the influence of different processes and their parameters on hardness. In Figure 6, the results are presented. Differences in the hardness values, depending on the machining process, are obvious. The transition to basic hardness depends on the depth of the heat-affected zone during induction heating and lies within the usual process tolerance, cf. Figure 5.

Deep rolling leads to an increase in hardness of the subsurface material. During deep rolling, the material is compacted due to the contact pressure of the rolling tool. This plastic deformation causes an increase in hardness. An influence of different cutting-edge roundings on surface hardness can also be seen. A larger cutting-edge rounding leads to a slight increase in the hardness of the component. This is also due to greater plastic deformation or compaction of the material with larger cutting-edge rounding. Consequently, the investigations show that the machining process influences surface hardness, which can also lead to a different operating behavior of the hybrid component.

**Figure 6.** Influence of different manufacturing processes on surface hardness.

## *3.3. Residual Stresses*

In Figure 7, residual stress depth profiles of hybrid components for lifetime investigations, adjusted by turning with different cutting-edge microgeometries and by deep rolling, are displayed. The residual stress measurements were carried out on the bearing running surface, in order to be able to investigate the influence of the subsurface properties on the operating behavior.

The residual stresses were measured both parallel and transverse to the feed direction. For the residual-stress measurements, the energy-dispersive method, in combination with electrolytic-material removal, was used. The procedure is shown schematically on the right side in Figure 7. By first looking at the different machining processes of turning and deep rolling, it is noticeable that, parallel to the feed direction, different courses of the residual stress graphs can be seen. In turned components, on the one hand, maximum compressive residual stresses can be observed near the surface. With increasing depth, *z*, the residual stress profile has a positive gradient. In deep-rolled components, on the other hand, residual compressive stresses also occur near the surface, but these increase further in the direction of compression with increasing depth. The maximum compressive residual stresses are not surface near. In deep rolling, the maximum elongation is below the surface of the workpiece. Due to the mechanical coupling of the plastically expanded subsurface of the workpiece with the plastically undeformed workpiece environment, compensating strains occur which cause a surface-parallel compression of the plastically deformed subsurface and thus cause residual compressive stresses. Since the maximum strain is below the surface of the workpiece, the maximum plastic deformation also occurs here.

Therefore, the residual compressive stress maximum is also localized in this area. For deep rolling, the mechanical load is much higher than for turning processes. Therefore, higher values for compressive residual stresses are obtained by deep rolling. The residual-stress depth profiles obtained by the variation of cutting-edge roundings (*S* = 30 μm and *S* = 75 μm), show no significant differences. The influence of these different residual-stress depth profiles on the application behavior, especially between turned and deep-rolled samples, was further investigated during fatigue testing.

**Figure 7.** Influence of different manufacturing processes on the residual-stress depth profile measured with the energy dispersive method combined with electrolytic polishing (**<sup>a</sup>**–**<sup>c</sup>**).
