**6. Results and Discussion**

Copper-copper and copper-aluminum bimetallic tubes were produced using the methodology described. The initially stacked copper-copper and copper-aluminum tubes were measured to have an average outer diameter of 28.58 mm and an inner diameter of 22.1 mm before processing. After the AEB process, the outer diameters were reduced to an average of 25.324 mm for the 52% extrusion and 24.257 mm for 68% extrusion. For all extrusions, the inner diameters were found to be 22.073 mm on average. This demonstrates that 49.9% and 66.3% radial deformation was achieved which is less than the targeted 52%, and 68% deformation, respectively. This is due to initial air gaps between the stacked layers, the mandrel, and the die, as well as expansion of the die during extrusion.

Table 7 summarizes all bimetallic tubes produced and whether bonding was achieved or not. The copper-copper tube serves as a baseline and represents the easiest possible chance to achieve full-bonding since metallographic substructures are consistent and the material exhibits identical material behavior. Copper-aluminum, however, represents a combination that is more difficult to bond due to differences in material behavior and different naturally occurring oxide layers. Four-layer tubes of copper-copper, and higher, were not attempted since the 2-layer copper-copper tube demonstrates that bonding is possible using AEB. Further processing of a copper-copper tube would provide no more desired insight as future iterations are expected to bond as the first 2-layer.


**Table 7.** Summary of bimetallic tubes produced.

\* = no bonding.

Despite adhering to the methodology described above, all samples extruded at 52% did not bond. The only successful bonding occurred at 68% deformation in the coppercopper bimetallic tube. Based on the extrusions performed, deformation greater than 52% is required for bonding since the processing remained constant between all tests. The processing of the 8-layer copper-aluminum tube demonstrates that the extrusion and expansion process can create bimetallic tubes, but deformation percentage needs to be of sufficient strain to enact bonding as shown in the 68% deformed copper-copper tube. Most literature in ARB reports 50% as the low-end of deformation required to enact bonding. Evidently, bonding in the AEB process did not occur at 50% revealing the role of more complex geometry of tube relative to sheet. Mechanical fields in the tube during AEB are different than those in the sheet during ARB making the strain levels required for bonding greater in AEB process than in ARB process.

The copper-aluminum bimetal tube cross-section, shown in extrusion direction, is displayed in Figure 12. As shown in this paper, the layer thickness decreased exponentially, while the tube wall thickness remained constant. Tabulated in Table 8 are the minimum and maximum layer thicknesses measured at the cross-section taken. The 2-layer bimetal tube has very consistent layer thicknesses as well as the 4-layer bimetal. At 8-layers, the layer thickness varied greatly where some layers completely thinned to obsolesce. Beyond this, layer thickness was very inconsistent in the 8-layer bimetal.

**Figure 12.** Bimetallic tube of (**a**) 2-layer, (**b**) 4-layer, and (**c**) 8-layer copper-aluminum, shown in the extrusion direction, produced at 52% deformation.


**Table 8.** Minimum and maximum layer thickness (µm) at 52% deformation.

In each sample produced at 52% deformation, bonding did not occur and is observed as the dark voids at each interface, as shown in Figure 12. Each subsequent extrusion pass did not further promote bonding as observed in the 8-layer bimetal. For this reason, it is necessary to achieve bonding on the first extrusion iteration. Since bonding did not occur, the material layers acted independently for each future extrusion, and the thin layers did not handle the imposed plastic strain, which ultimately caused significant wrinkling and tearing on the innermost and outermost layers, as well as layer thinning inside the bimetal.

Copper-copper bimetallic tubes were attempted at both 52% and 68% deformation. As mentioned previously, no bonding was achieved at 52% deformation when attempting a copper-copper bimetal. However, using the same method described, bonding was achieved using 68% deformation. As shown in Figure 13, the 2-layer copper-copper bonded interface is observed normal to the extrusion direction. Unlike Figure 12, the copper-copper cross section, shown in Figure 13, required acid-etching to view the interface using microscopy. The layer thickness was measured and found to be 510 µm for the outer layer of copper and 568 µm for the inner layer of copper where the expected thickness was 527 µm. This is attributed to the outer layer being pulled in front of the extrusion ledge during the initial extrusion start which is exhibited predominantly in Figure 10a,b.

was 527 μm. This is attributed to the outer layer

**Figure 13.** Bimetallic tube of 2-layer copper-copper, shown in the transverse direction, produced at 68% deformation.

μ

layer of copper and 568 μm for the inner layer of copper where the expected thickness

510 μm

Full bonding, however, did not occur as there are voids at the interface. A 15,240 μm 148 voids were identified with an average length of 15.4 μm. No identifiable pattern was μm. Typical vo Full bonding, however, did not occur as there are voids at the interface. A 15,240 µm long section was surveyed and 85.0% of the length was found to be bonded. In this section, 148 voids were identified with an average length of 15.4 µm. No identifiable pattern was observed regarding the location of the voids, and the largest void was found to be 49.7 µm. Typical voids are shown in Figure 14, which are represented by black at the interface. These voids are expected to collapse during further iterations.

**Figure 14.** Typical voids found at bonding interface of 2-layer copper-copper tube shown in transverse direction. Bimetal was extruded at 68% deformation.

The grain structure before and after extrusion is displayed in Figure 15. As shown in the transverse direction, the annealed grain structure became highly elongated due to the extrusion process. The copper-copper tube after extrusion is expected to exhibit

an anisotropic material behavior where grains no longer have uniformity in all spatial directions, which is expected for non-heat-treated metal after drawing or extrusion.

**(a)** 

**(b)**

**Figure 15.** Microstructure of copper, (**a**) before extrusion, and (**b**) after extrusion at 68% deformation.

The copper-aluminum and copper-copper bimetal underwent significant strain-hardening during extrusion at 52% and 68% deformation. As tabulated in Tables 9 and 10, the hardness of each metal constituent increased significantly. The 8-layer copper-aluminum was not tested for hardness since the individual layers were too small for micro-hardness testing. Interestingly, the copper-copper layers experienced approximately the same increase (165.1% vs. 168.5%) in hardness even though the deformation percentage was 52% and 68% respectively. This suggests there is a hardening limit as no significant increase in hardness was observed with strain.



**Table 10.** Hardness (HK) before and after extrusion of copper-aluminum bimetal at 52% deformation.


The copper-copper tube, extruded at 68%, exhibits significantly improved material strength as indicated by the material behavior displayed in Figure 16. A second tensile test was performed to confirm results; a difference of ~2% was identified between the two ultimate tensile strengths found. Tensile tests were performed per ASTM E8 using custom sidewall specimens. The 0.2% offset yield strength improved from 83 MPa to 481 MPa; a 480% increase compared to the annealed material. Due to the work hardening experienced during extrusion, ductility is sacrificed for the improved material strength.

The ultimate tensile strength of the copper-copper tube, extruded at 68% using AEB, is compared to pure copper experiencing ARB, as reported by [70], and tube cyclic extrusioncompression (TCEC), as reported by [71]. TCEC is a severe plastic deformation technique where tubes are fully constrained and deformed between an external chamber and an internal mandrel [71]. The pure copper undergoing ARB and TCEC achieved ultrafine grain size after four passes of severe plastic deformation processes. Pure copper experiencing 68% AEB, on the other hand, has a grain size approximately one order of magnitude greater but exhibits the greatest improved ultimate tensile strength in one iteration. The significant improvement of strength is attributed to the hyper-elongated grains shown in Figure 15b, which are oriented in the extrusion direction, where dislocation structures, and underlying residual stress fields expected to form during the process similar to ARB [72]. We observe that grain size obtained is an order of magnitude larger than other severe plastic deformation processes (Table 11), but the largest improvement in ultimate tensile strength was obtained. Therefore, grain size refinement alone does not govern improvements in material strength, but also other features such as dislocation density and low angular boundaries in sub-grain structure also impact strength. Anisotropic material properties are expected since the average grain size is 2.3 µm in the extrusion direction and an average length of 40 µm is in the transverse direction.

**(a)** 

**(b)**

**Figure 16.** Engineering stress-strain, (**a**) and true stress-strain, (**b**) of 68% deformation 2-layer copper-copper compared to annealed curves.


**Table 11.** Ultimate tensile strength of pure copper deformed using different severe plastic deformation processes.
