*3.2. Metallography*

In the following, cross sections of the specimens of Geometries B, C, D, E and F are depicted to show exemplary bonding defects. The Geometries A, G and H feature the desired bonding quality and visible defects such as gas pores, inclusions or cracks are not present in the joining zone.

Figure 7 gives an overview of the Geometries B (a) and D (b). The plotted angles mark the direction of the material flow when the aluminum alloy is detached from the steel. The bond of sample Geometry B is almost complete. At an angle of approximately 15◦, the bond starts detaching and closes again in the shoulder area. This results in air inclusions and is a weak point over the complete circumference of the joint. The reason for this is the material flow which is indicated schematically by the violet arrows in Figure 7a.

**Figure 7.** (**a**) Cross section of Geometry B and (**b**) cross section of Geometry D; the angles mark the detachment of the aluminum.

At Geometry D (Figure 7b), the aluminum alloy peels off at an angle of 35◦ and does not ge<sup>t</sup> into further contact. The material flow and the applied forces possibly cause the detachment in both geometries.

Geometry C is depicted in Figure 8. A complete filling of the holes was not achieved and gaps on the circumference occur with increasing depth; additionally, fragments of the aluminum alloy are visible.

**Figure 8.** Geometry C, (**left**) schematic draft, (**middle**) overview of a filled borehole, and (**right**) exemplary gap at the borehole flank.

A section of Geometry E is depicted in Figure 9. The bond is complete except for higher radii, where air inclusions at diameters of 37 to 40 mm can be seen. For Geometry F, a small air inclusion appears near the undercut. This area is displayed in Figure 10.

The hardness of the samples was measured at different distances across the joining zone to characterize the influence of the generated heat and the forming during the friction welding process. It can be assumed that the size of the grains and the concentration of elements are influenced by the heat resulting in varying hardnesses compared to the basic materials. The space between two recording points in the aluminum alloy was chosen according to DIN EN ISO 6507-1 [17]. For simplification, the same distance of 0.5 mm was used in the steel. Figure 11 gives an example (Geometry F) of the measurements. The transition area could not be narrowed down due to the limiting conditions.

**Figure 9.** Geometry E, (**left**) schematic draft, (**middle**) overview of a half-cut sample, and (**right**) example of air inclusions in regions of the increased diameter.

**Figure 10.** Geometry F, (**left**) schematic draft, (**middle**) overview of a nearly half-cut sample, and (**right**) exemplary air inclusion at the undercut.

**Figure 11.** Diagram of the hardness of a sample with Geometry F including the variance, range between recording points is approximately 0.5 mm.

On the steel side, almost all samples show a small increase in hardness for the measuring point closest to the transition area—for example, recording point 4 in Figure 11. The soft annealed base material has an average hardness of 170 HV0.1 and is marked in Figure 11 as a horizontal dotted line. It can be concluded that some samples have experienced a slight softening and others an increase in steel hardness further from the interface in the axial direction.

The aluminum alloy has an average hardness of 113 HV0.1 in the T6 condition. Close to the joining zone the aluminum becomes softened and has an average hardness below 75 HV0.1, as can be seen for Geometry F in Figure 11. Geometry E is the only exception where a hardness of 103 HV0.1 was determined, possibly caused by a lower heat generation.

To investigate the joining zone, micrographs were examined. Two different types of interlayers between steel and aluminum alloys were found in the metallographic analyses. The first layer is located on the aluminum side near the friction welding surface and has a darker color. Figure 12 depicts an analyzed example of such a layer. Its thickness varies up to 1.5 μm. It is mainly found on flat areas of the friction surfaces—for example, in the undercut in Geometry A around the central axis. Since it was not possible to characterize the layer in detail by light microscopy, scanning electron microscopy was applied (confer Section 3.3).

**Figure 12.** (**Left**) overview of Geometry C, and (**right**) dark layer on the aluminum side close to the joining zone.

The second layer found close to the joining zone, is a layer of fine-grained steel microstructures with increasing degree of fineness from the basic steel to the interface. Its thickness increases with the diameter from about 0.5 up to 3 μm (Figure 13).

**Figure 13.** (**Left**) fine-grained layer on the steel side close to the joining zone and (**right**) overview of Geometry A.

#### *3.3. Scanning Electron Microscopy*

To identify the darkened layers described in Section 3.2, EDS analyses were carried out via a scanning electron microscope using a sample of Geometry C. This sample was chosen due to the clear formation of the darkened layer (Figure 12). Figure 14 depicts the cross section prepared by a FIB with the highlighted recording line of the EDS measurement. The results of the EDS analysis are given in Figure 15 and illustrate the chemical composition of the elements along the marked line.

**Figure 14.** Scanning electron microscopy (SEM) image of a cross section prepared by a focused ion beam (FIB) in-lens detector (sample of Geometry C).

**Figure 15.** Energy-dispersive X-ray spectroscopy (EDS) analysis of the transition area in a specimen of Geometry C, SEM image of the recording line (yellow), Electron High Tension (EHT) = 12 kV, probe current = 1.7 nA, and working distance = 4.9 mm.

The left side of the graph in Figure 15 depicts the base material composition of the aluminum alloy. A content of almost 5 at.% of diffused iron is noticeable. On the right side of the graph, the composition of the steel base material is displayed, which contains a certain amount of alloying elements. Akin to the diffused iron on the aluminum side, aluminum diffused into the steel side with a content of about 5 at.%. Furthermore, an increased occurrence of manganese, magnesium and silicon can be observed in the transition zone. A mapping of the silicon content reveals its enrichment within a zone of about 0.5 μm as can be seen in Figure 16.

**Figure 16.** EDS analysis of a sample with Geometry C, (**a**) distribution of silicon, and (**b**) silicon content in wt.% near the joining zone.

High silicon contents on sample surfaces can result from conventional sample preparation with silicon carbide grinding discs when a slope is formed in the joining zone during preparation due to the large differences in strength between aluminum and steel [19]. Silicon carbide particles can thus accumulate at the slope. Since in this case the EDS measurement was carried out on a cross section prepared using a FIB, such an influence of the preparation can be excluded.

Here, the increased silicon content measured in the joining zone by an EDS analysis is in accordance with observations of Liu et al. and Wang et al., who also reported increased silicon concentrations in the intermetallic compound (IMC) layer in the joining zone in investigations on friction welding of aluminum and steel [20,21]. The silicon is incorporated into the IMC layer and slows down the growth of the IMC layer [22]. With increasing silicon content in the aluminum alloy, the thickness of the IMC layer is reduced, and the phase constitution of the aluminide layers is altered [23].
