**1. Introduction**

If a component has to withstand diverse local loads or a lightweight design is demanded [1], the combination of different materials offers the use of a load-adapted component. Components consisting of at least two materials are called hybrid components. Due to different material-specific properties such as melting points or flow stresses, these components require adapted joining methods. Depending on the specific material combination, this can be, for example, a fusion welding process or a friction welding process.

The most important technical advantages of friction welding compared to fusion welding are the high reproducibility and the wide variety of possible material combinations, such as aluminum and steel, since the joining process is based on plastic deformation instead of melting. Compared to friction welding, fusion-welded products have much larger heat-affected zones which can result in undesired microstructures and reduces the resilience of parts [2]. The molten phase may cause defects such as gas porosity, which leads to brittle fracture.

Common multimaterial components are produced by joining several individual parts that are already in a near-net shape. Therefore, the joining process takes place at the end of the process chain—for example, splicing or riveting of sheet metal components in the production of automobile chassis [3]. Another approach is joining by forming, such as the

**Citation:** Behrens, B.-A.; Uhe, J.; Petersen, T.; Nürnberger, F.; Kahra, C.; Ross, I.; Laeger, R. Contact Geometry Modification of Friction-Welded Semi-Finished Products to Improve the Bonding of Hybrid Components. *Metals* **2021**, *11*, 115. https:// 1doi.org/0.3390/met11010115

Received: 17 November 2020 Accepted: 4 January 2021 Published: 8 January 2021

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consolidation of powder with simultaneous bonding with steel during forming to produce hybrid gears [4] or the application of ultrasound enhanced friction stir welding to join different materials [5].

As part of the collaborative research center 1153 (CRC 1153) "Tailored Forming", a novel process chain was developed, in which various materials are joined at an initial stage before being subjected to further processing [6]. The aim of this concept is to further improve the joining zone by the subsequent processing steps resulting in a load-adapted component. The CRC 1153 maps several process chains in their entirety, to improve components such as shafts, bevel gears or bearing disks [6]. The process chain for manufacturing hybrid shafts by applying friction welding is depicted in Figure 1. Within this process chain, joining is followed by impact extrusion, which requires a homogenous formability in both material sections. Hence, an inhomogeneous temperature distribution in the joined parts prior to the forming has to be ensured to align the flow curves of the investigated 20MnCr5 steel and of the AA6082 aluminum alloy (EN AW-6082). Therefore, a customized inductive heating strategy was developed to achieve the material-specific forming temperatures of 900 ◦C in steel and 20 ◦C in aluminum simultaneously [7].

**Figure 1.** Schematic tailored forming-process chain of the collaborative research center 1153 (CRC 1153) [8], material combination EN AW-6082 (AA6082) and 20MnCr5.

Friction welding was selected based on various reports which concluded that the successful joining of aluminum alloys and steels and the free designability of joining zone geometry—e.g., Ashfaq et al. detected an increased bond strength when using a conical geometry instead of flat surface. They found that this modification benefits material flow and results in an improved bond quality [9]. Fukumoto et al. investigated the influence of different parameters on the completeness of the bond. The most significant result was that the highest bond strength is achieved by certain friction times of 1 s with a pressure of 50 MPa and 6 s with a pressure of 150 MPa. Higher or lower friction times resulted in lower bond strength [10]. Lee et al. focused on the resulting microstructures and their correlations with the friction parameters. Besides the base metals, they identified different regions—that is, a region of dynamic recrystallization—a heat-affected zone (HAZ) and a deformation zone, and how these are formed due to different forming pressures (70 to 150 MPa) and friction times (0.1 to 3.0 s) [11]. Fukumoto et al. studied the properties of the bonds created by a friction welding process of the aluminum alloy EN AW 1050 and the stainless steel 1.4301 (AISI 304). They were able to show that the extension of the frictional time from 0.1 to 0.2 s increased the bond strength from 85 to 96 MPa [12]. Sahin characterized the bond by different test methods such as tensile tests and hardness measurements and found a significant influence of contaminants at the interface on the joint quality. He recommended a statistical analysis as an economical and reliable method for selecting optimized welding parameters [13]. Behrens et al. investigated the influence of surface geometry by using a conical shape. They found out that at room temperature a sharper shape with an increased friction path results in a higher bond strength. Compared to specimens with flat surfaces, bond strength could be improved from 252 to 294 MPa using a conical surface of 30◦ [14]. So far, only a few studies such as [9], [14] or [15] took an adaption of the surface geometry into account. In [15], the effects of frictional contact

surfaces on the formation of an intermetallic phase were studied. Since most investigations are focused on flat surfaces, which often show compound defects in the zone around the central axis after joining [10], on other material combinations or without focusing the bonding strength [15], further research is required regarding alternatives such as a combination of material bond and form locking by varying the friction contact geometries. Comparison these results with additional references is only possible to a limited extent, since parameters of the friction welding process differ as well as the material combinations.

Friction welding processes are divided into three sequences: contact phase, friction phase and deformation phase [16]. In the contact phase, the geometries are aligned and brought into contact with a specific pressure. The heat is generated in the friction phase, in which one component begins to rotate—in this case, the steel side. This phase can be adjusted by controlling the friction time or the relative friction path of the welding components covering in the axial direction. In the deformation phase, the rotation stops and the welding components are joined by generating high axial pressure.

To improve the bonding strength of the steel–aluminum specimen and thus manufacture semifinished parts suited to subsequent impact forging, this work is mainly concerned with varying the contact geometries. In addition to increasing the contact areas between both materials or increasing the contact times and contact pressures in the sample center, possibilities for generating a form closure are also investigated in addition to the pure material bond. Different combinations of friction surface geometries are tested experimentally in the following and their impact on the bond strength is determined. For example, the applicability of undercuts is examined to implement the additional bonding mechanisms such as form locking.

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

The following subsections describe the applied materials and the performed methods of the investigation. For this purpose, the basic conditions are explained and clarified with the help of illustrations.
