**4. Discussion**

In order to comply with the requirements of the intended subsequent die forging process, specifications for the compound profiles with regard to the internal geometry of the reinforcing element as well as the position of the joining partners in relation to each other were defined prior to toolset and

process development. Thus, the co-extruded semi-finished products were required to have an internal diameter Øi of 32 mm as well as an aluminum cladding with a uniform wall thickness so that the hybrid semi-finished products can be heated by means of an internal inductor prior to die-forging [16]. Furthermore, a maximum deviation in the coaxial arrangemen<sup>t</sup> of the joining partners of 0.2 mm (0.3%) was aimed for with a height of the semi-finished product of 85 mm. This was not fully achieved, as in some cases there was an o ffset of up to 0.6 mm (0.9%). By turning the semi-finished product and correspondingly increasing the length, the mold filling could still be realized in the subsequent die forging process.

In the present study, coaxially reinforced hollow profiles were produced, with a reinforcement content of 14 vol.% and 34 vol.%. With the extrusion ratio of 9:1, it was possible to envelope a steel tube having a wall thickness of 3 mm in an outer aluminum cladding without the reinforcing element collapsing due to the pressure acting on the tube during the LACE process. At the front end of the compound profile, a slight deviation of the steel tube from the desired circular geometry was detected. This deformation was no longer present at the end of the profile. A similar problem did not occur with the higher reinforcement content due to the higher wall thickness of the steel tube. Compared with previous works [12], in which a steel rod was fed into the process, a significantly improved coaxial positioning of the reinforcing element was achieved using the new guiding mandrel part and the resulting, more uniform material flow inside the novel tool. It should be noted that in the original LACE process with a titanium sheet used as external reinforcement there is no o ffset, since the compound profile was manufactured asymmetrically for process-related reasons [11]. However, in the case of co-extrusion processes that use wire reinforcements, deviations from the expected position of the reinforcing elements could also be detected [17]. The slight deviations observed in the present study can be explained in terms of the position on the feeding of the reinforcement via a mandrel part. For the application in the process chain of the CRC 1153, the o ffset could be compensated by further adjustment of the tool or using turned semi-finished products.

The metallographic characterization has revealed that the gaps between the matrix material and the reinforcing element were present on the recipient side in the initial parts of the compound profile. However, these gaps between aluminum and steel were no longer detected in any tests at the end of the compound profile. Thus, these gaps will not be relevant in a Tailored Forming process chain for the production of hybrid bearing bushings once steady-state conditions are attained.

At 42 MPa ± 7 MPa, the average shear strength of the profile produced with the lower extrusion ratio was slightly lower than that of the profile with the higher extrusion ratio, i.e., the one with the higher volume fraction of reinforcement. For the latter, an average shear strength of 48 MPa ± 9 MPa was measured. These values are below the composite strengths that were determined for a LACE process conducted on a laboratory scale [18]. However, since the compound profiles manufactured in these earlier experiments showed a clear warping, their higher bond strengths are attributed to the more pronounced force and form closure.

Compared with the shear strengths achieved in push-out tests on compound forged bearing bushings investigated by Behrens et al. [16], the shear strength achieved with semi-finished products manufactured by LACE was about 20 MPa lower. The bond formation in the compound forging was thus somewhat better, which can be substantiated by the joint forming, and thus by the more extensive formation of new surfaces in the process [14]. In the LACE process, on the other hand, only the aluminum is formed and the relative movement of the materials to each other [11] forms the joint between the partners. The strength of the joining zones is nevertheless promising to withstand the subsequent die forging process.

Due to the coaxial arrangemen<sup>t</sup> of the joining partners inside the LACE tool, it can be assumed that the higher thermal expansion coe fficient of aluminum causes shrinking of the outer EN AW-6082 cladding onto the steel tube, which results in a form closure as well as a force closure [14]. However, a firm connection that will withstand subsequent bulk forming processes such as forging, requires material closure, as this connects the joining partners by means of physical or chemical bonds, so that they function as one body [19]. Whether the desired material closure is present in the LACE samples, was therefore tested using shear compression tests on segments taken from the sample cross-sections. The existing form and force closures were released by cutting out 65◦ segments from the sample cross-section.

With respect to the subsequent forging operation, the interface properties are of paramount importance. Ideally, a firm metallurgical bond should be formed. Depending on the process conditions, brittle intermetallic phase can growth at the interface between the joining partners. Herbst et al. reported that intermetallic phase seams narrower than 1 μm have no negative effect on the strength of the composite [3]. In the present study, no intermetallic phase seams were detected metallographically. In addition, due to the coaxial arrangemen<sup>t</sup> of the joining partners inside the LACE tool, it can be assumed that the higher thermal expansion coefficient of aluminum causes shrinking of the outer EN AW-6082 cladding onto the steel tube. This will result in a form closure as well as a force closure [14]. However, subsequent bulk-forming processes, such as forging, require material closure, i.e., a firm connection by means of physical or chemical bonds, so that they function as one body [19]. Whether the desired material closure was realized in the LACE samples, was therefore tested using shear compression tests on segments taken from the sample cross-sections. The existing form and force closures were released by cutting out 65◦ segments from the sample cross-section.

The LACE profile with a steel tube made of 20MnCr5 had a shear strength of 54 MPa ± 5 MPa over the entire profile length, which was determined in the push-out test. The segments without longitudinal weld seams showed almost identical shear strengths with values of 56 MPa ± 6 MPa and the samples with two longitudinal weld seams showed no change over the averaged total profile length, resulting in a value of 58 MPa ± 15 MPa. The strength of the specimen with longitudinal weld seams was both above and below the strength of the segments without longitudinal weld seams. Only the increased deviation in the values shows a slight influence of the longitudinal weld seams on the shear strength. It can therefore be assumed that the proportion of form-fit or frictional connection in the samples manufactured using LACE is low. The shear compression test of the specimens with longitudinal weld seams has shown that the longitudinal weld seams can have a positive influence on the shear strength. The aluminum adhered clearly to the steel near a longitudinal weld seam, which was formed by splitting by a support arm of the mandrel part. The splitting of the aluminum flow and re-welding in the welding chamber thus produced juvenile metal surfaces, which had a positive influence on the formation of the bonding area. According to Weidenmann et al., material closure is also assumed if the reinforcing element is covered by residues of the matrix material after a shear compression test [20], which was the case here for most of the tested samples.

#### **5. Conclusions and Outlook**

It could be shown that quasi-continuous hybrid hollow profiles made of EN AW-6082 and 20MnCr5 can be produced on an industrial-relevant scale by employing a lateral angular co-extrusion process together with a new tool concept. The modular design allows the extrusion ratio to be increased, e.g., from 9:1 to 11:1, and thus the reinforcement content could be varied between 14 vol.% and 34 vol.%. The placement of the steel tube inside the aluminum cladding deviated slightly from the desired ideal coaxial position. The de-bonding shear strengths determined by push-out tests were between 42 MPa and 47 MPa. The shear compression tests on sample segments showed that not only form-fit and force-fit is present between the aluminum alloy and the steel tube. The aluminum residues adhering to the steel after the shear compression tests also indicate a material-locking connection, which was observed especially in the areas next to the longitudinal weld seams. The bonding area of these samples needs be investigated more closely in the future to be able to fully exploit the potential of the LACE process.

**Author Contributions:** Conceptualization, H.J.M., C.K. and B.-A.B.; methodology, S.E.T., J.P. and F.C.B.; validation, S.E.T., N.H. and J.U.; investigation, S.E.T.; writing—original draft preparation, S.E.T.; writing—review and editing, H.J.M., J.U., B.B. and C.K.; visualization, S.E.T.; supervision, H.J.M., C.K. and B.-A.B.; project administration, H.J.M., C.K. and B.-A.B.; funding acquisition, H.J.M., C.K. and B.-A.B. All authors have read and agreed to the published version of the manuscript.

**Funding:** This study was funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation)—CRC 1153, subproject A1—252662854. The authors thank the DFG for financial support.

**Acknowledgments:** The results presented were obtained within the subproject A1 "Influence of local microstructure on the formability of extruded composite profiles" of the Collaborative Research Centre 1153 "Process chain to produce hybrid high performance components by Tailored Forming".

**Conflicts of Interest:** The authors declare no conflict of interest.
