**4. Discussion**

The presented results reveal that the surface geometry of friction-welded semifinished products and the parameters of the friction welding process have a decisive impact on the resulting bond and its strength. For example, Figure 14 shows microinterlocking, which may result from the different contact areas of the various geometries generating different temperatures during the friction process. This interlocking can result in an improvement of the bonding strength. Additionally, the different generated temperatures can lead to different microstructures, such as the grain size or the thickness of a possible intermetallic phase, and further in different bonding strengths.

Micrographs of the cross sections reveal that the material flow has an impact on the completeness of the bond. For example, for Geometry B (Figure 7a), air inclusions occur due to the resulting material flow. The aluminum alloy flowed outwards over the dome and detached from the 20MnCr5 steel. In addition, it can be assumed that some of the aluminum alloy was pushed back inwards onto the shoulder due to the colder outer zone, which has a higher deformation resistance. Another example of the importance of the material flow are the boreholes of Geometry C (Figure 8), which were not fully filled by flowing aluminum. The pressure of the enclosed air inside the boreholes inhibited a complete filling. Higher temperatures generated by rotational speed or pressure would increase the degree of deformation. This could lead to a better material flow.

The hardness tests show a decisive influence of the processing as a small increase in the hardness on the steel side near transition area (confer Figure 11). This hardness increase is probably caused by a combination of strain hardening due to the deformation process and grain refinement in the joining zone or by the occurrence of harder phases such as an intermetallic phases. An indication for the latter might be the multiple changes of the slope visible in the EDS line scan at the aluminum side in Figure 15 and the similarity of the darker layer in Figure 12 compared to the literature, such as [15].

Contrary to the hardness increase in the steel, the aluminum became softened close to the transition area which can probably be attributed to recrystallization or overageing of the T6 state due to heat generation during the friction welding process. In the transition area, the hardness decreased gradually between steel and aluminum, caused by mutual diffusion of aluminum and iron. An example of the concentration profile across the transition area is depicted in Section 3.3, Figure 15.

Hardness itself does not account for the quality of a bond, but it correlates with the tensile strength. Summarized, the hardness measurements reveal a heat-affected zone in both materials and in between them but could not be narrowed down due to the limiting conditions.

For the intended following impact extrusion process, Geometries A, F and H show the most promising results in mechanical tests and metallographic analyses. They feature a nearly complete bonding and high tensile strengths above 220 MPa. Only Geometry F contains a small volume of air inclusions at the undercut, which can possibly be avoided using modified parameters of the friction welding process. All specimens exhibited brittle fractures except for Geometry A. Brittle fractures underline the possible presence of (brittle) intermetallic phases [15]. Geometry H has the highest tensile strength of almost 280 MPa which is 100 MPa lower than the tensile strength of the base material (over 360 MPa), thus reaching about 77% of the base strength. Regarding the future processing by subsequent heating and impact extrusion, a recrystallization of the microstructure in the transition area can be expected and might further increase the bonding strength. With the evaluated bonding strength, Geometries A, F and H are hence suited for the subsequent inductive heating and forming process.

#### **5. Summary and Conclusions**

Based on the presented results, the following conclusions can be drawn:


Table 4 presents the geometries investigated, their bond strengths and the main comments with regard to the further process chain of the CRC 1153.


**Table 4.** Results of the different geometries with regard to the further process.


**Table 4.** *Cont.*

The influence of various factors, such as friction welding parameters and material choice, leads to a large spectrum of possible improvements for enhancing the bonding strength. For example, the integrity of the joining zone might be improved by increasing heat generation during processing and thus diffusion of the alloying elements, though grain growth is to be expected. Hence, further investigations will focus on specimens with fixed surface geometries but varied friction welding parameters—e.g., a modification of Geometry F with an undercut angle of 80◦ on the steel side is to be expected promising regarding a further increase in the bonding strength. With this geometry, not only material bonds but force and form locks as well can be accomplished without a significant penetration of the aluminum alloy on the steel side, which otherwise could lead to a premature melting of the aluminum during induction heating in the further processes within the process chain of the CRC 1153 [8].

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

**Funding:** Funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation)— CRC 1153, subproject B3—252662854. The authors thank the German Research Foundation (DFG) for their financial support of this project.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available in the article.

**Acknowledgments:** The authors appreciate the support of the subproject A2 regarding tensile testing and of Torsten Heidenblut regarding the SEM investigations.

**Conflicts of Interest:** The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
