*3.2. Metallographic Characterization*

In Figure 7, representative cross-sections extracted from the front and the back ends of a compound profile are shown. Here, a steel tube with an outer diameter of Øa = 38 mm was used, which resulted in an extrusion ratio of 9:1, and thus a volume fraction of the reinforcement of 14 vol.%. The outer aluminum matrix of the compound profile had a slightly elliptical cross-section at the start of the compound profile. There was a slight material overlap on the side facing the recipient (recipient side), which is interspersed with oxide lines (Figure 7a, left-hand area). The overlap on aluminum extended over a circumference of 55 mm for a total circumference of the cross-section of 202 mm. This overlap resulted from incidental clamping of the steel tube, which in turn resulted in temporarily faulty material flow. This illustrates that is of paramount importance to accurately control the local material flow in the die. For the determination of the lengths of the main and secondary axes of the aluminum jacket, this section was not taken into account. Thus, the main axis had a length *l*y of 63.3 mm at the start of the profile and the secondary axis a length *l*z of 63.6 mm. The outer contour was thus 1.5% larger in the y-direction and 2.0% larger in the z-direction than the theoretical diameter of the aluminum jacket. No bond was formed between the matrix material and the reinforcing element on the recipient side; instead, there was a 0.5 mm wide gap. The longitudinal weld seam on the side facing away from the recipient (rear side) also showed a gap in the bonding area between the aluminum alloy and the steel tube. By contrast, the cross-section taken from the end of the compound profile showed an almost ideal circular contour without any material overlap. The main axis had a length of 63.2 mm (deviation +1.4%) and the length of the secondary axis was 61.9 mm (deviation 0.7%).

**Figure 7.** Cross-section of a sample taken from (**a**) the start of the compound profile, i.e., after a macroscopically closed longitudinal weld seam had formed and (**b**) the end of the compound profile, which was produced with an extrusion ratio of 9:1; etching: HF/H2SO4 mixture; the position of the longitudinal weld seams is highlighted by dotted lines; the interpolated outer contour of the steel tube is highlighted with black dashed lines; the (in case of (**a**) interpolated) outer contour of the aluminum is highlighted with white dashed lines.

In the compound profile shown here, the reinforcing element was not truly embedded coaxially in the matrix. The aluminum metal stream inside the die flowed to the side facing away from the recipient preferentially. This is evident in the greater wall thickness of the matrix material on the right-hand side of the cross-sections shown in Figure 7. In the initial area of the compound profile, this led to a slight deformation of the reinforcing element, which can also be observed in Figure 7a. The reinforcing elements used in these LACE experiments were deep-hole drilled tubes with an uneven wall thickness over the tube circumference. In the compound profile shown here, the wall thickness deviated from the intended 3 mm by up to 0.5 mm, i.e., the wall thickness was between 2.7 mm and 3.5 mm. For the shown cross-section from the start of the compound profile with elliptical reinforcing element, the outer contour of the steel tube was interpolated using the theoretical outer diameter Øa of 38 mm. The o ffset of the steel tube at the start of the profile was thus 0.4 mm or 0.6% in the y-direction and 0.8 mm or 1.2% in the negative z-direction. The cross-section from the end piece of the compound profile showed no geometrical deviation of the reinforcing element caused by the LACE process, despite variations in the wall thickness. The o ffset in the y-direction was 0.4 mm or 0.6% at the start of the profile. In the negative z-direction, the steel tube was shifted by 1.7 mm or 2.7%.

Metallographic etching was used to contrast the secondary precipitates and make the longitudinal weld seams visible. The two longitudinal weld seams that are running horizontally in the metallographic image were caused by the material flowing into the portholes of the tool entry and subsequent welding after flowing around the mandrel part or reinforcing element. Two additional longitudinal weld seams are expected on the side facing the recipient, each of which should be located at an angle of 120◦ to each other and to the longitudinal weld seam on the rear side. As seen in Figure 7, the weld seams appear close to the expected positions.

The material combination EN AW-6082 and 20MnCr5 was also extruded to a compound profile with an outer diameter of 44.5 mm for the steel tube, and thus an extrusion ratio of 11:1. The cross-sections taken from the start and end of the compound profile are shown in Figure 8a,b. Both cross-sections had almost the desired circular cross-section and did not show any deviations in the wall thickness of the steel tube, which could be attributed to deep hole drilling. The outer contour of the aluminum jacket had a length of 63.4 mm of the main axis at both the start and end of the profile (which had a length of 215 mm), which is 1.6% greater than the expected outer diameter. For the secondary axis, a length of 62.1 mm (−0.4%) could be determined at the start and 62.0 mm (0.5%) at the end of the compound profile. Residual oxides could still be detected inside the matrix material at the front end, but no longer at the back end of the compound profile. In addition, there was no complete bond between the aluminum alloy and the steel tube in the initial area, which became apparent in form of a gap with a width of 15 μm (detail in Figure 8c). At the end of the compound profile, this gap was no longer so pronounced (detail in Figure 8d).

(**a**)

(**b**)

**Figure 8.** *Cont*.

**Figure 8.** Cross-sections of (**a**) start and (**b**) end of a compound profile made of EN AW 6082 and 20MnCr5 with an extrusion ratio of 11:1; the position of the longitudinal weld seams is highlighted by dotted lines; the outer contour of the steel is highlighted with black dashed lines; the outer contour of the aluminum is highlighted with white dashed lines; with (**c**) detailed image of the bonding area with a gap between the joining partners from the front end of the compound profile; with (**d**) detailed image of the bonding area from the back end of the compound profile; etching: (**<sup>a</sup>**–**<sup>c</sup>**) HF/H2SO4 mixture, (**d**) HNO3/Ethanol mixture.

The position of the reinforcing element remained unchanged over the entire profile length and showed an offset of 0.4 mm or 0.6% in the negative y-direction and an offset of 0.6 mm or 0.9% in the negative z-direction. The position of the longitudinal weld seams did not ye<sup>t</sup> correspond perfectly to the expected position at the start of the compound profile. On the one hand, the longitudinal weld seam on the recipient side was not in a perfectly horizontal position but offset in the negative y-direction. On the other hand, the two longitudinal weld seams, which were formed by the support arms of the mandrel part on the side facing the recipient, had a smaller angle to each other than expected. At the end of the compound profile, the longitudinal weld seams, which are formed horizontally due to the splitting of the matrix material at the portholes and subsequently by passing by one of the support arms of the mandrel part, were on the expected horizontal plane. The angle between the two longitudinal weld seams no. 2 and 4 remained unchanged.

## *3.3. Mechanical Properties*

The strength of the bonding area was determined for the compound profiles with different reinforcement content using push-out tests. Figure 9 shows an exemplary force-path graph from a push-out test on a representative sample that was produced with an extrusion ratio of 11:1. At the beginning the measured force *F* increases almost linearly until the curve flattens out slightly towards the end and finally reaches its maximum *F*max. After the maximum, the force decreases rapidly and runs out in a plateau. Based on these data, the shear strength was calculated as [15]

$$
\tau\_{\text{max}} = \frac{F\_{\text{max}}}{\pi d \, h} \tag{1}
$$

where *d* is the diameter of the reinforcement and *h* the height of the sample.

Figure 10 shows the de-bonding shear strength calculated by using measured data from all the push-out tests executed over the profile length of both compound profiles. The de-bonding shear strength of the profile with the lower reinforcement content of 14 vol.% was determined over a profile length of 250 mm and varied between 29 MPa and 55 MPa with an average shear strength of 42 MPa ± 7 MPa. In the case of the profile having a reinforcement content of 34 vol.%, the shear strength determined in the push-out test varied between 45 MPa and 63 MPa over the entire profile length of 445 mm with an average shear strength of 54 MPa ± 5 MPa.

**Figure 9.** Force-displacement diagram of a push-out tests on a sample sectioned from the front end of a compound profile produced with an extrusion ratio of 11:1.

**Figure 10.** De-bonding shear strength for compound profiles produced via LACE with an extrusion ratio of 9:1 or 11:1, respectively. The respective mean values are indicated by dashed lines, which were determined over the entire length of the profile with macroscopically intact longitudinal weld seams.

For the shear compression test, two segments without longitudinal weld seams and one segmen<sup>t</sup> containing two longitudinal weld seams were available for each specimen cross-section. The segments without longitudinal weld seams showed a similar shear strength curve progression as the de-bonding shear strength curves determined by the push-out tests, as it can be seen in Figure 11. The values determined in the shear compression test fluctuated between 47 MPa and 69 MPa. The tested segments, which had two longitudinal weld seams, showed similar behavior over the profile length. However, there were two outliers at 100 mm and 220 mm, which, at 92 MPa and 83 MPa, respectively, had the highest strength values for the compound profiles made of the material combination EN AW-6082 and 20MnCr5. In general, the shear strength of the segmen<sup>t</sup> with longitudinal weld seams was below that of the segments without longitudinal weld seams from 250 mm onwards. However, the average shear strength determined in the shear compression test was 56 MPa ± 6 MPa for the segments without longitudinal weld seams and 58 MPa ± 15 MPa for the segments with longitudinal weld seams.

**Figure 11.** Determined shear strength of the compound profile with a reinforcement content of 34 vol.% starting 25 mm after macroscopically closed longitudinal weld seam with de-bonding shear strength averaged over the entire compound profile length (dashed line); dotted lines represent one standard deviation both for sample segments with and without longitudinal weld seams.

Figure 12 shows one of the two outliers in the shear compression test with longitudinal weld seams, extracted 100 mm behind the location where the longitudinal weld seam was considered to be macroscopically intact. The arrows and dotted lines mark the position of the longitudinal weld seams no. 1 and 2. At the position of the longitudinal weld seam no. 1, which is on the left-hand side in Figure 12, however, a gap between the joining partners can be seen. At the location of the longitudinal weld seam no. 2, which is on the right-hand side in Figure 12, it can be seen that the aluminum still adheres to the reinforcing element after the test. This demonstrates that the separation of the materials did not take place in the bonding zone.

**Figure 12.** Sample after the shear compression test with adhering aluminum at the level of a longitudinal weld seam (LWS; position marked with arrows and highlighted with dotted lines).
