2.2.2. Extrusion

The extrusion process bonds the two metals together through severe plastic deformation as described by the thin film theory. Previous research in die geometry has determined an outer ring die paired with a straight mandrel produces the least peak stresses within the die and can successfully achieve enough plastic strain to promote bonding [31]. Therefore, the die angle and geometry are adopted for this research. Figures of the die are presented in the next section. The extrusion process is performed in a 4-pronged die set which was customized to support a self-aligning die and punch. The die, punch, and mandrel are used to maintain the tubular shape and are described in more detail in the die design section. The extrusion was performed at room temperature (approximately 20.5 ◦C) with an extrusion speed of 2.73 mm/s. No heat was added during extrusion to promote bonding.

The die, mandrel, and punch are coated with a thin layer of extrusion oil to reduce friction and discourage material adhesion. Non-diluted Drawsol® WM 4740 (Houghton International, Manchester, England) was used for its ability to maintain high film strength when under extreme pressure. This oil is also recommended for various metals including steel, stainless-steel, titanium, and aluminized alloys. From a processing perspective, this synthetic lubricant is water soluble, which is easily removed with running tap water.

Bimetals are extruded at 52% and 68% deformation which represent how much the outer diameter is reduced during extrusion. These values represent the minimum (50%) [5–9] and mid-range of reported deformation employed in previous research, which achieved bonding using ARB. These deformation values also promote stacking as well; at 52% deformation a 2-layer tube can be re-stacked, and the original die can be reused, while at 68% deformation, 3-layer tube stacking can be utilized. This was done intentionally to reduce the number of required dies. In both cases, the deformation percentage values are slightly more than 1/2 and 2/3 as to provide clearances from the nominal stacking fraction to assist in processing.

## 2.2.3. Cutting

The second step in the iterative loop, if the bimetallic layer thickness is not achieved, is cutting. Simply, the bimetal is cut to remove the non-bonded section at the end of the bimetallic extrusion, the non-bonded initial section at the start, and then equally in half perpendicular to the extrusion direction. By performing cutting the total material volume is not conserved. Therefore, it is necessary to quantify the expected losses per iteration to ensure a viable end-product is produced. Cutting is performed using a material specimen preparation sawmill using a diamond infused metallurgical cutting disc. After, all edges were deburred with 320 grit sandpaper.

## 2.2.4. Expansion

One of the two extruded tubes require diametrical expansion to facilitate stacking. The chosen tube is expanded such that the inner diameter is increased to a size that is slightly larger than the outer diameter of the extruded tube. This is performed by pushing the tube over a diametrical expansion mandrel. The expansion mandrel utilizes a cylindrical section that tapers at 10◦ to an enlarged diameter. The punch, which pushed the bimetal into the extrusion die, is the same punch used to push the tube over the expansion ledge. The bimetal is passed over the expansion post multiple times to fully remove spring-back and achieve a cylindrical tube for stacking. The expansion mandrel is described in more detail in the die design section. Other options to expand the tube were considered such as metal spinning as described in [59,60]. Ultimately, it is more desirable to expand the tube using a mandrel because the process is simple, easy to control, and contained to the same experimental setup (i.e., the hydraulic press and die set).

The expansion step does not promote or impact bonding at the interface since bonding occurred during extrusion where significantly large strains (compared to strains experienced during expansion) are imposed. Additionally, from a conservation of volume perspective, the wall thickness of the expanded tube will decrease depending on the amount of deformation imposed (~11% and ~8% for the 52%, and 68% deformation cases, respectively), which will impact layer thickness consistency. Even though wall thicknesses become slightly inconsistent as iterations continue, the intent is to create many interfacing layers, i.e., the ultrafine structures independent on local layer thicknesses. The local deformation conditions also cause non-uniformity in layer thickness.
