**1. Introduction**

One of today's challenges in mechanical engineering is the environmentally friendly and resource-saving production of components [1–3]. Moreover, in the fields of energy technology, medical technology, automotive engineering, and the aerospace industry, the requirements for high-performance solid components are increasing steadily [4–6]. In the automotive industry, for example, the aim is to reduce CO2 emissions by reducing vehicle weight [7,8]. In addition, the development of components with a longer lifetime is one of the most important goals for the industry [9].

The choice of material is, therefore, always based on the requirements of the intended application of the component. Consequently, requirements such as a reduction in weight with a simultaneous increase in mechanical strength cannot be realized with the use of one material. Mono-material

components are thus increasingly reaching their material and production-specific limits. One strategy for reducing the component weight of highly stressed components is to combine di fferent materials in one component. This allows the design of components with di fferent materials that are locally adapted to the respective requirements. In this way, components can be realized that combine partly contradictory properties, such as high mechanical strength and simultaneous weight reduction. This is the focus of the Collaborative Research Centre (CRC) 1153, in which the new process chain, known as "tailored forming", is investigated [10]. In existing production technologies for hybrid solid components, the joining of the components takes place during or after the forming process. In the tailored forming process chain, the components are joined from semi-finished products, at the beginning of the production route, and their properties are specifically influenced by the subsequent forming processes. The simple geometry of the components also facilitates the joining process. In addition, considerably more complex geometries can be produced by this strategy. Within the framework of the CRC, the entire process chain, from the joining of the components until the final machining of the hybrid components, is considered, and the operating behavior of these components is investigated.

In the process chain of solid-part production, machining plays a quality-determining role as the final step. Research results of the last decades show a significant influence of machining on the operating behavior of components due to surface and subsurface modification [11].

In particular, for rolling contacts with high contact stresses >2 GPa, a strong influence on the wear and fatigue life behavior as a function of the surface and subsurface properties is known [12]. In components like roller bearings, an alternating stress field during over-rolling can propagate fatigue-crack growth in the contact zone after high cycle numbers exceeding 10<sup>7</sup> revolutions [13,14]. Component failure occurs when the crack network expands toward the surface, causing spalling of the raceway [15]. Although the topography makes a significant contribution to the operating behavior of components, not all e ffects can be explained by it. Wear resistance and fatigue life of components are also significantly influenced by residual stresses [11,16]. Residual stresses also have an e ffect on magnetizability and chemical resistance [17]. In general, compressive residual stresses in the subsurface lead to a proportional increase in fatigue strength [18]. Due to plastic deformations in the subsurface area, the residual stress distributions are of considerable importance, as they have a strong influence on the fatigue limit and the crack initiation tendency of the material. For roller bearings, for example, an increase in fatigue life from 40% to 250% could be achieved by introducing residual compressive stresses [19–23].

However, it must be considered that a residual-stress-induced increase in fatigue strength depends largely on the strength of the material [18]. Denkena et al. were able to show that the probability of failure of roller bearings can be reduced by the targeted introduction of compressive residual stresses [24]. The influences of hard turning and deep rolling was investigated by Pape et al. in terms of bearing fatigue tests. Additionally, an FEM-based model, in combination with a calculation routine, was set up to compute the influence of residual stresses on bearing fatigue [25,26]. However, current research results show that too-high residual compressive stresses can also lead to a shorter service life [27,28]. A cause for this is not ye<sup>t</sup> known and currently represents a research gap [29].

Surface and subsurface properties are significantly influenced by the choice of process parameters and tool microgeometry. The surface roughness is significantly influenced by the feed rate during turning. In addition, cutting speed and tool microgeometry also determine the final roughness of a component [30,31]. The mechanical and thermal e ffects during chip formation are the main factors influencing the subsurface condition. This can be specifically modified by subsequent processes, such as deep rolling [32]. In interaction with cutting speed and feed rate, the rounding of the cutting edge plays a dominant role in influencing the subsurface. Increasing rounding leads to a higher proportion of material being pressed under the cutting edge. This generally leads to compressive residual stresses below the surface. On the other hand, large cutting-edge radii lead to greater temperature development, which promotes the formation of tensile residual stresses [30,31]. The influence of process parameters

and tool microgeometry on the surface and subsurface properties of hybrid components and their influence on the operating behavior is currently inadequately investigated.

The hybrid design offers the potential to combine contradictory requirements in one component. However, there is no knowledge about the application behavior of hybrid components in comparison to mono-material components. Hence, the aim of this study is to investigate the production-related surface and subsurface properties of tailored forming components in comparison to mono-material components. Research questions that need to be addressed are as follows:


For this reason, the joining zone as a weak point of hybrid components is investigated in this work, in comparison to a reference sample set made from monolithic material. A possible influence of the application behavior of hybrid components by modifying the subsurface properties is further examined. A focus is set on the mechanical properties of finished components and their use case as hybrid machine elements under complex loads.

#### **2. Materials and Methods**
