*2.1. Materials*

For the friction welding, the aluminum alloy AA6082 (EN AW-6082) and the case hardening steel 20MnCr5 were chosen. 20MnCr5 is a chromium–manganese alloyed steel. During friction welding, the steel was employed in its delivery condition (soft annealed) with a tensile strength of 554 MPa. The aluminum alloy used featured the T6 condition with a tensile strength above 360 MPa. The mechanical properties, tested in prior investigations, are listed in Table 1 and the chemical compositions are given in the content lists in Table 2, measured by optical emission spectrometry.


**Table 1.** Mechanical properties of the 20MnCr5 steel and AA6082 aluminum alloy.

**Table 2.** Chemical composition of the 20MnCr5 steel and AA6082 aluminum alloy in wt.%., measured by optical emission spectrometry.


The microstructures of both base materials prior to friction welding are shown in Figure 2. On the left side (a) the ferritic–pearlitic microstructure of the steel 20MnCr5 is

depicted. To visualize the grain boundaries and the different microstructures, the sample was etched with Nital, a solution of nitric acid (3%) and alcohol. A micrograph of the aluminum alloy in its T6 condition is shown on the right side (b).

**Figure 2.** (**a**) Ferritic–pearlitic microstructure of the 20MnCr5 steel in the soft annealed condition, etched with Nital (3%), and (**b**) unetched micrograph of the AA6082 aluminum alloy (T6 condition).

#### *2.2. Friction Surface Geometries*

The different surface geometries of the semifinished products investigated were chosen to improve the joining zone properties by surface enlargements, undercuts and shrinkage. An increase in the friction surface leads to a higher temperature generation, from which a lower demand for the yield forces results. As described in the literature [10], air pockets can occur in the center of the specimen for flat surfaces. With higher temperatures, these can be avoided. An undercut results in a form fit or force fit, depending on whether the aluminum fills a hole by flowing or encloses a shape by shrinking. The geometries, manufactured by machining, are depicted in Figure 3.

Geometry A (Figure 3) was selected for a form-locking connection to enhance the bonding strength. During friction welding, the undercut of the cavity located in the steel part with an angle of 75◦ was filled with aluminum. On the basis of preliminary tests, an angle of 75◦ was determined to be optimum, since at this angle complete mold-filling can be ensured, despite a relatively concise form fitting. Geometry B offers an enlarged friction surface due to the hemispherical geometry, which results in a higher heat generation due to friction. The shoulder at the transition from the hemisphere was designed with a beveled edge to improve material flow. Geometry C features four drilled holes intended to increase the torsional stiffness by means of flowing aluminum entering the holes, thus achieving a form lock. Geometry D forms a hemispherical surface, resulting in an enlarged friction surface analogous to Geometry B. The difference to Geometry B is the absence of a shoulder to examine its necessity for the material flow.

Conical geometries were welded with varying angles of 30◦ (Geometry E) and 45◦ (Geometry G) using an increased friction contact surface and reduced manufacturing effort compared to the hemispherical Geometry B. The conical Geometry G is additionally truncated to simplify production and to combine an axial force with directed material flow during the forming process. Compared to Geometry A, Geometry F has no cavity in the steel component. The undercut was formed by a protruding elevation with an angle of 80◦, while the aluminum is of a flat geometry. The aluminum was intended to flow around the shoulder and shrink to the steel due to the greater thermal expansion coefficient. In addition to the form lock and material bond, this geometry provides a force-locked connection to enhance the bonding strength. Preliminary tests have shown that too large a pin or an angle smaller than 80◦ will result in air pockets. Geometry H has a pin on the aluminum side to investigate the influence of the expected deformation ratio and the high friction path on the bonding strength (Table 3).

**Figure 3.** Geometries of friction surfaces (**A**–**H**), outer diameters of 40 mm.


**Table 3.** Main parameters of the friction welding process.
