*Article* **Contact Geometry Modification of Friction-Welded Semi-Finished Products to Improve the Bonding of Hybrid Components**

**Bernd-Arno Behrens 1, Johanna Uhe 1, Tom Petersen 1, Florian Nürnberger 2, Christoph Kahra 2, Ingo Ross 1 and René Laeger 1,\***

> 1 Institute of Forming Technology and Machines, Leibniz Universität Hannover, 30823 Garbsen, Germany; behrens@ifum.uni-hannover.de (B.-A.B.); uhe@ifum.uni-hannover.de (J.U.);


**Abstract:** To improve the bond strength of hybrid components when joined by friction welding, specimens with various front end surface geometries were evaluated. Rods made of aluminum AA6082 (AlSi1MgMn/EN AW-6082) and the case-hardening steel 20MnCr5 (AISI 5120) with adapted joining surface geometries were investigated to create both a form-locked and material-bonded joint. Eight different geometries were selected and tested. Subsequently, the joined components were metallographically examined to analyze the bonding and the resulting microstructures. The mechanical properties were tested by means of tensile tests and hardness measurements. Three geometrical variants with different locking types were identified as the most promising for further processing in a forming process chain due to the observed material bond and tensile strengths above 220 MPa. The hardness tests revealed an increase in the steel's hardness and a softening of the aluminum near the transition area. Apparent intermetallic phases in the joining zone were analyzed by scanning electron microscopy (SEM) and an accumulation of silicon in the joining zone was detected by energy-dispersive X-ray spectroscopy (EDS).

**Keywords:** friction welding; hybrid components; tailored forming; surface geometry modification
