*2.2. Metallographic Characterization*

Cross-sections were extracted from the front and the back ends of the compound profiles to determine deviations of the positions of the reinforcing elements relative to their ideal center positions, as well as to characterize the microstructures of the extruded matrix material. The front end of the actual compound profile was defined as the location where all longitudinal weld seams appeared to be macroscopically closed. The position of the back end was dependent on the particular LACE experiment. If the extrusion was stopped before the entire reinforcing element was jacketed with aluminum, the area closest to the tool was examined in cross-section. For the macro- and microstructural examination of the compound profiles, the samples were prepared metallographically and treated with an etching solution consisting of HF and H2SO4 to contrast the secondary precipitates of the aluminum alloy.

#### *2.3. Push-Out Test and Shear Compression Test*

The mechanical properties of the bonding area of the compound profiles were measured with push-out tests and shear compression tests. For this purpose, samples were taken over the entire length of the composite profile. Due to the di fferent coe fficients of thermal expansion of aluminum and steel, the coaxially reinforced semi-finished products are assumed to have a force closure connection resulting from shrinking of the matrix material onto the reinforcing element [14]. Shear compression tests of sample segments served to determine whether the e ffective bond mechanism is mainly material closure or force and/or form closure. For the tests, samples were taken in an alternating order from one compound profile per extrusion ratio (Figure 3). The compound profile was divided into slices, each of which had a plane-parallel height of 10 mm after machining on both sides. The first sample was taken 25 mm behind the position, where all four longitudinal weld seams were macroscopically closed. The longitudinal weld seam that was used as the starting point of the sampling was the one closed last during extrusion and is referred to in the schematic illustration in Figure 3 as the relevant longitudinal weld seam.

**Figure 3.** Schematic illustration of a compound profile with alternating sampling for push-out tests and shear compression tests with planned geometry of the compound profile.

Starting at this location, samples were taken from the profile with a thickness of 15 mm each, taking into account the saw cut and the allowance for facing. These samples were used as full samples for the push-out tests or divided into several segments and then used for the shear compression tests. The measured shear strength of the sample segments was then compared with the de-bonding shear strengths from the push-out tests of the adjacent samples in order to be able to determine the contribution of the material bond over the compound profile length.

The push-out tests were carried out using a universal testing machine with a maximum force of 250 kN (type Z250, Zwick, Ulm, Germany) and the test setup is shown schematically in Figure 4. Centering of the sample was realized by a step in the punch. The compound specimens were positioned on a steel ring so that the contact surface with the aluminum alloy was as large as possible. By lowering the punch of the testing machine vertically, the reinforcing element was pressed out and the force-displacement curve was recorded.

**Figure 4.** Schematic illustration of the experimental setup of the push-out tests using a sample taken from a hollow compound profile.

The specimen discs for the shear compression test, which had a plane-parallel height of 10 mm, were separated by wire cutting into the segments with an angle of about 65◦ as shown in Figure 5a. This procedure resulted in two sample segments that did not contain a longitudinal weld seam and a sample that contained two longitudinal weld seams. The latter was taken so that the longitudinal weld seams no. 1 and no. 2 (cf. Figure 5a) were at the edges of the sample segment, and thus did not affect the test results significantly. In order to determine the actual test area, the bonding lengths of all sample segments were measured by using a laser microscope (type VK 9700, Keyence, Neu-Isenburg, Germany). From these data, the actual bonding areas were calculated.

**Figure 5.** Schematic illustration of (**a**) sample segmen<sup>t</sup> extraction from the cross-section of a compound profile with one segmen<sup>t</sup> having two longitudinal weld seams (LWS, highlighted by dotted lines) at the edges and two segments each without weld seams, (**b**) test setup used for the shear compression test.

For the characterization of the mechanical properties, the specimens were clamped in the test setup shown in Figure 5b and the steel portion of the specimens was pressed out with a universal testing machine (type Z250, Zwick, Ulm). As with the push-out tests on the entire specimen cross-sections, a test speed of 2 mm min−<sup>1</sup> was used. A drop in force of 80% was used as the break-off criterion for the push-out tests of the sample segments.
