2.2.2. Deep Rolling

Additionally, deep rolling experiments were conducted on a CTX420 linear Gildemeister machine tool. For this, the indexable insert of the turning tool was replaced by a hydrostatic rolling tool HG6 with a ball diameter of *d*b = 6.35 mm, manufactured by the company ECOROLL AG. Analogy studies were also initially carried out, and the findings were then transferred to the final machining. Before deep rolling, the shafts were turned with the process parameters of the final component, as shown in Table 1, with a cutting-edge geometry of *S*α/*S*γ = 30 μm. During the deep rolling process, the rolling pressure and the deep rolling speed were kept constant at *p*r = 40 MPa and *v*w = 180 m/min. The rolling overlap, *u*, was varied in three steps. Just as for turning, the parameters for fatigue life investigations for deep rolling experiments were also selected according to the best achieved surface quality. The deep rolling direction did not show any significant influence on the surface results. Therefore, the machining direction was chosen analogous to turning. The process parameters for deep rolling are summarized in Table 2.

**Table 2.** Process parameters of the deep-rolling operation.


#### *2.3. Residual Stress Measurement*

Residual stress measurements were carried out via X-ray diffraction (Agfa NDT Pantak Seifert GmbH & Co KG, Ahrensburg, Germany), using the sin<sup>2</sup>ψ-method described by Macherauch and Müller [33]. In order to ge<sup>t</sup> a non-destructive and depth-resolved measurement of residual stresses, the energy-dispersive measurement method is applied. Here, the determination of lattice strain is also based upon the well-established Bragg's law:

$$n \cdot \lambda = 2 \cdot d\_{\text{lkl}} \cdot \sin \theta\_{\text{lkl}} \tag{1}$$

where *n* is a natural number indicating the diffraction order, λ is the X-ray wavelength, *dhkl* is the interatomic lattice spacing, and θ*hkl* is the diffraction angle (Bragg angle). For the energy-dispersive measurement, white X-radiation from a tungsten anode tube on a Seifert XRD Space Universal diffractometer is used (Figure 2, left). High depth resolution is achieved at an acceleration voltage of *U*a = 50 kV and an anode current of *I*a = 60 mA. The Bragg angle, θ, remains constant throughout the measurement, at 20◦. Thus, all interference lines are simultaneously determined in one diffraction spectrum (Figure 2, right). The peaks determined by this method represent a function of the wavelength, λ, or photon energy, *Eph.* Since the different interference lines are distinguished by different energy levels in the spectrum, they can be referred to different depth information. The more peaks in the direction of increasing photon energy can be evaluated, the more depth information about residual stresses is obtained (Figure 2, right). The collimator used has a diameter of 2 mm. Peak position was analyzed by the center of gravity method. The main difference between the well-established angle dispersive and the energy dispersive methods is that, with the energy-dispersive method, the wavelength or photon energy, λ, is varied at a constant Bragg angle, θ, and in the angle-dispersive method, the Bragg angle, θ, is varied at a constant wavelength, λ. The attainment of residual stress depth information up to 35 μm in the hybrid transition zone of steel is non-destructively possible with a single measurement [34]. In order to ge<sup>t</sup> a higher information density in the depth direction, electrolytic removal of material was additionally used. Measurements were done parallel and transverse to the feed direction.

**Figure 2.** Energy-dispersive residual-stress measurement.

#### *2.4. Fatigue Life Testing*

After machining, bearing-fatigue tests were carried out on a test bench, according to Figure 3. The shaft with material transition in axial direction was later used to transmit torque and additional rolling contact stresses of a cylindrical roller bearing with an inner ring integrated into the shaft. Here, the specimen is mounted with two conventional deep groove ball bearings of type 6305 as supporting bearings. The SAE5140-part of the shaft acts as an inner ring for a cylindrical roller bearing (CRB) type RNU204. On the CRB, a radial preload of *F* = 2 kN is applied through a disc spring assembly. The resulting Hertzian contact stress at the inner race contact is *p*H = 1.9 GPa. The operating conditions with approximately 2 GPa pressure correspond to medium to high loads for a roller bearing and thus represent the usual operating conditions for shortened life tests. The bearings were lubricated by a temperature-controlled circulating oil lubrication system with a constant volumetric flow rate of . *V* = 0.3 L/min per bearing. Additional test parameters are shown in Figure 3, on the right. The radial loading leads to a superposition of rolling-contact stresses and rotating-bending stresses in the contacting surface between the shaft and the rolling element. Within the path of load, measurements using piezoelectric vibration transducers were carried out. In this way, bearing failure during over-rolling of surface spalls was detected by a self-designed condition monitoring system. The tests were automatically stopped if a critical vibration threshold of 150% of the steady state signal was exceeded. A statistical evaluation of the probability of failure of the series was carried out with these lifetimes. After completion of each test, the damage resulting from a surface chipping in the middle of the raceway on the hybrid shaft was documented. The damage and the microstructure were further analyzed by micrographs.

**Figure 3.** Fatigue life test in three-bearing arrangemen<sup>t</sup> (**left**), and test parameters (**right**).

#### **3. Results and Discussion**

In this section, the surface and subsurface properties of the hybrid shafts and the results of the bearing fatigue tests are presented. In turning investigations, only the cutting-edge microgeometry was varied, because it has a decisive influence on the subsurface properties. The cutting-edge microgeometry is defined by the parameters S<sup>γ</sup>, S<sup>α</sup>, and the form factor κ. At this, Sγ and Sα are the distance between the separation point of the cutting-edge rounding and the tool tip of an ideal sharp cutting at rake face and flank face, respectively. In the presented study, symmetrical cutting-edge roundings, as described in Section 2.2.1, were used. Machining parameters were selected which provide a surface finish with Ra < 0.14 μm as minimum requirement. The knowledge of this was drawn from the analogy studies described above.
