*2.1. Materials*

In the presented work, two different steel alloys, SAE5140 and SAE1020, were combined by friction welding. These materials were chosen in order to combine a high-strength steel for highly loaded part zones with a cheaper material for supporting structures. A shaft with material transition in axial direction serves as specimen geometry (see Figure 1a).

For the friction welding process, shafts with a diameter of *d* = 30 mm, a length of *l* = 220 mm for SAE1020, and *l* = 150 mm for SAE5140 were used. As process parameters, a friction speed of *n* = 2000 min−1, a friction pressure of *p* = 60 MPa, and a friction path of *s* = 4 mm were applied. Finally, a compression of *p* = 150 MPa and a compression time of *t* = 6 s created the final bond. In the next step, the friction weld bead was removed. Afterward, the friction-welded shafts were impact extruded at a temperature of *T* = 900 ◦C, in order to change the bonding zone geometry. Hereafter the hybrid shafts were sawn to a total length of *l* = 146.5 mm and centered on both sides. The machining process used is divided in two steps. First, the hybrid shafts were roughly turned to an oversized diameter. Then the bearing surface of the pre-turned hybrid shafts was locally hardened at the largest diameter, at a temperature of 860 ◦C, by an induction hardening process. The heating time was *t* = 0.5 s, with *P* = 90 kW. Air and water with a pressure of *p* = 3 bar each were used for cooling by means of a spray nozzle. The cooling time was *t* = 15 s. The properties of the materials after the heat treatment are displayed in Figure 1b).

The difference in the hardness of both materials after the heat treatment is evident. SAE5140 is significantly harder than SAE1020 due to the higher carbon content. Therefore, in the finishing operation, different machining behavior of the respective materials is expected. Knowledge about the influence of different material properties on the machining behavior is consequently important in order to comply with shape, dimension, and position tolerances of the hybrid components. In the second and thus final machining step, the hybrid shafts are then turned to size with a required surface quality of Ra = 0.14 μm and with adapted process parameters. With higher roughness values, the load-carrying capacity of the bearings would not be fully utilized, since contacting roughness peaks in the mixed friction area can lead to surface-induced early failures.

**Figure 1.** (**a**) Process chain for the production of hybrid shafts, and (**b**) material properties and composition.
