**1. Introduction**

The realization of lightweight constructions to increase resource e fficiency and reduce CO2 emissions is of paramount interest to the automotive and aviation industries [1]. In particular, the use of light metals such as aluminum is attractive due to the high specific strength of the respective alloys. Merklein et al. reported that one promising approach to integrate aluminum in automotive designs is the use of hybrid components [2]. In this context, the integration of Tailored Blanks in sheet metal forming has become state of the art in the automotive industry. These allow meeting conflicting design challenges by providing sheet metal components with locally adapted properties. As well as reducing weight by combining sheets of di fferent material grades, thicknesses, etc., Tailored Blanks can also offer improved crash performance [2].

However, the concept of hybrid semi-finished products is not ye<sup>t</sup> widely used in bulk forming of metals. Innovative processing technologies are required for the production of hybrid bulk metal components made of di fferent metals such as aluminum alloys and steel. As part of the novel concept of Tailored Forming, process chains are being developed in which the various bulk materials are first joined before a subsequent forming process such as die forging or impact extrusion is applied. As discussed by Herbst et al., this di ffers significantly from conventional process chains in which the individual parts of the components are joined together after the forming step or at the end of the process chain [3].

In contrast to sheet metal forming processes used for the production of components made of Tailored Blanks, the Collaborative Research Center 1153 (CRC 1153) "Tailored Forming" aims at developing suitable processes for the production of three-dimensional solid components with locally adapted properties. The use of aluminum instead of steel in the bulk metal component can result in a reduction of mass of the component. Specifically, only the functional surface, which must be wear-resistant in the solid component, consists of material like hardened steel. A key feature in the present approach is the subsequent joint forming step. The advantage of this process combination lies in the positive influence of the subsequent forming on the local microstructure in the joining zone, and thus further the mechanical properties. This is a key aspect for materials that are di fficult to form such as aluminum and steel. An exemplary process chain is shown in Figure 1 for the manufacture of a hybrid bearing bushing made of aluminum and steel that is investigated as a demonstrator part.

**Figure 1.** Tailored forming process chain for the production of hybrid bearing bushings.

Similar to Tailored Blanks, the production of hybrid solid semi-finished products can be realized by welding. Pressure welding is one of the processes that enables the formation of bonds in which the intermetallic phase seam is su fficiently small enough to have no negative impact on properties such as tensile strength [4]. In the context of continuous hybrid profiles, a promising approach for joining di fferent materials is co-extrusion, which was used in the present study to manufacture the semi-finished products for the bearing bushing. Co-extrusion enables the production of composite profiles consisting of at least two materials [5]. Co-extrusion can be assigned to joining by forming according to DIN (Deutsches Institut für Normung e. V.—German Institute for Standardization) 8593-5, in which the parts to be joined are formed locally or completely. In principle, co-extrusion can be divided into two di fferent process variants:


the support arms of the mandrel. Hence, the wire reinforcement was present in the longitudinal weld seams of the profiles only [10].

With the LACE process (Lateral Angular Co-Extrusion) developed by Grittner et al., reinforcing elements such as titanium sheets and flat profiles that are already relatively rigid can be fed laterally, and thus continuously into the extrusion process [11]. A laboratory-scale LACE process has already been developed within the CRC 1153, which provided a round rod made of 20MnCr5 steel with an aluminum cladding made of EN AW-6082. However, due to the design of the tool the resulting geometry of the compound profile showed significant deviations from the desired coaxial arrangemen<sup>t</sup> [12]. For subsequent die forging, the steel reinforcement must be embedded coaxially within the aluminum matrix. In the present study, this issue was addressed by a novel extrusion tool design, which also featured an industry relevant scale. The mechanical properties of the bonding zones of the compound profiles produced with the new tool were characterized in push-out tests. In addition, the influence of the bonding mechanisms, e.g., material closure or form-closure, on the composite strength were determined by shear compression tests on sample segments.

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

## *2.1. LACE Process*

A schematic section through the developed modular tool is shown in Figure 2a. Since the LACE process involves the feeding of a rigid reinforcing element instead of a wire, a mandrel part was designed, which is supported by three support arms in the tool housing. In this concept, the aluminum alloy is divided into two metal streams by the portholes in the middle of the symmetrically designed entry. Both metal strands are then directed into pockets milled into each half of the tool cavity. This is intended to change the material flow in such a way that the aluminum alloy evenly envelops the reinforcing element and displacement and/or distortion of the compound profile is avoided. The aluminum alloy then flows around the mandrel part inside the tool. The reinforcing element, in this case a steel tube, is inserted into the tool orthogonally to the movement direction of the extrusion punch and guided through the clamping cover and the mandrel part. This also ensures a coaxial position of the tube in the compound profile.

**Figure 2.** (**a**) Schematic illustration with sectional plane lengthwise through the tool with c: clamping cover, d: pocket, e: deflection, f: die, g: mandrel part with three support arms, h: inlet (the geometry of the inlet and deflection are highlighted); (**b**) schematic illustration of the concept for the 10 MN extrusion press in longitudinal section with i: reinforcing element, j: die housing, k: thermocouple bore, l: container, m: aluminum billet.

The process is shown in Figure 2b as a schematic sectional view including the reinforcing element in the tool and the billet inside the container. The LACE direction is orthogonal to the direction of the movement of the ram.

The LACE experiments were performed on a 10 MN extrusion press (SMS Meer GmbH, Düsseldorf, Germany). A non-heated tool holder specially modified for this project was used, which allowed the reinforcing element to be fed laterally. Aluminum EN AW-6082 billets as well as tubes consisting of the case-hardening steel 20MnCr5 were used as joining partners. To keep the process chain as short as possible and to avoid additional drilling of the hybrid semi-finished products, reinforcing elements with the desired inner diameter were used. Furthermore, the reinforcement content was varied by using steel tubes with di fferent wall thickness. With an inner diameter of the container of 146 mm, an opening diameter of 62.7 mm for the die and 38 mm or 44.5 mm for the outer diameter of the steel tubes, the extrusion ratio equaled 9:1 and 11:1, respectively. This corresponds to a reinforcement content of 14 vol.% for the extrusion ratio of 9:1 and 34 vol.% for the extrusion ratio of 11:1. Since the objective was to achieve a metallurgical bond between the joining partners, the reinforcing element was ground with 40 grit paper and cleaned with ethanol prior to the extrusion process. Previous numerical studies have shown that a bond by material closure can be achieved by employing relatively high temperatures together with long contact times of the joining partners [13]. This translates to high process temperatures and low extrusion speed. Thus, the billets were preheated to 530 ◦C for 4.5 h, whereas the steel tube had room temperature at the beginning of each experiment. The die was preheated to 490 ◦C and the container had a temperature of 440 ◦C. A ram speed of 1.5 mm/s was used at the beginning of the experiment during the upsetting of the billet in the container and the filling of the tool, and it was reduced stepwise until the ram speed reached 0.3 mm/s. In this way, the cooling of the aluminum billet during filling of the tool was kept to a minimum.
