2.2.1. Surface Preparation

The stacking operation is the simple action of inserting one metal tube into the other. Before this operation is performed, it is critically important to prepare the surfaces, which will become the metal-to-metal interface. In ARB, the interfacial surfaces are typically degreased using acetone and then scratch-brushed with stainless steel bristles [13,53–55]. This surface preparation is reportedly one of the most important steps to achieve full bonding, since it removes the naturally occurring oxide layer and hardens the surface simultaneously. The scratch brushing creates a slightly hardened and brittle outer surface in comparison to the bulk material, due to local strain hardening occurring at the surface. The brittle surface, which will be prone to cracking during extrusion, will allow virgin sub-surface metal to pass through the cracks to contact the virgin material of the other metal to enact bonding. Scratch brushing is applied transverse to the extrusion direction to help promote crack opening.

Before stacking and scratch brushing, acid cleaning is performed to remove any surface impurities. The copper was cleaned by pickling using a solution of 10% sulfuric acid and 90% distilled water per volume. This was done at room temperature for 10 min. After, the acid was neutralized with cool distilled water. The copper was then degreased in an ultrasonic acetone bath for 30 min, where the acetone was drained and replenished halfway through the cleaning process. The aluminum only received degreasing using the ultrasonic acetone bath. After degreasing in acetone, the metal tubes were scratch-brushed and stacked together.

To achieve the most optimal hardened surface, tubes were scratched with stainless steel bristles. The outer diametrical interface was scratched with a handheld stainless-steel brush, while the inner diametrical interface was brushed with a rotary stainless-steel brush. The stainless-steel bristles were 25.4 mm long on the handheld brush with a diameter of 0.305 mm. The stainless-steel bristles on the rotary brush were 13.97 mm long with a diameter of 0.152 mm and rotated at a constant RPM during application. The two different methods of applications were employed due to the curved geometry of each surface.

The surface roughness was measured before and after brushing and is tabulated in Table 1. Surface roughness increased 29.6% and 46.8% respectively on the inner, and outer surfaces, respectively. To find the average surface finish, measurements were taken in 10 random locations on each surface using a Pocket Surf III profilometer (Mahr Federal Inc., Providence, RI, USA). All surface finish values reported before and after brushing are representative of cold extruded aluminum using lubricant for context [57].

**Table 1.** Average surface roughness before and after scratch brushing.


The intent of the brushing is not to induce visible asperities and significantly increase the surface roughness, but to clean the surface of oxidation while hardening it at the same time. Oxidation layer minimization is necessary to aid in bonding, but significantly over-brushing did not improve the amount of bonding. Not quantified by the authors, but reported in literature [52], minimizing the build-up of oxidation is critical to aid in bonding and to reduce additional foreign material inclusion. Beyond scratch brushing, minimizing contact with the atmosphere was employed to discourage further oxidation growth.

After scratch brushing the metal tubes are not cleaned of any debris caused by brushing. Instead, the tubes were lightly tapped to remove any loose particles. Immediately following the acetone cleaning and brushing, the tubes were stacked and extruded. On average, this was performed within 2 min to prevent the naturally occurring oxide layer to fully reform. After 15 min of exposure to air, as reported in another study [58] regarding aluminum bonding, the natural oxide layers begin to markedly interfere with the bonding process.

The scratch brushing method does not produce any noticeable debris from the stainless steel bristles which helped promote cleanliness. Other methods beyond scratch brushing were attempted but were ultimately not used. Metal files, Scotch-BriteTM pads (3M Company, Saint Paul, MA, USA), and various sanding papers were also used with no success. The major issue with these methods is cleanliness control and the lack of versatility to be applied to curved surfaces of the inner and outer diameters. These methods created a lot of non-metallic debris during application and caused too much inconvenience during processing.
