*2.2. Manufacturing*

– To achieve multilayered bimetallic tubing, it is essential to introduce an iterative process to obtain layers expediently. As the process is iterated, the layer thicknesses within the tubular wall exponentially decreases, while the number of layers exponentially increases using the process flow detailed in Figure 2. Unlike the research in [56], which manufactured bimetallic tube from solid billets of copper and aluminum, the starting base material is tube since tubes are ubiquitous and can be purchased such that they initially stack together.

The first process step is surface preparation of initially annealed tubes followed by stacking. Surface preparation is important to remove foreign material and naturally occurring oxides which can hinder bonding. Additionally, during this step, the surfaces to be bonded must be hardened as described by the thin film theory. Once prepared, one metal tube is inserted into the other metal tube and then the stacked tube is extruded by decreasing the outer diameter and maintaining the inner.

If the desired laminated layer thickness is not achieved, then the metals are prepared for another iteration as shown in Figure 2. The first step to prepare the extruded tube for reprocessing is annealing to restore ductility and bisection at the midpoint to create two tubes of approximately half the extruded length. One of the two metal tubes is then expanded such that it can fit over its extruded diameter. After another annealing of the expanded tube, the initial process is repeated. This continues until the desired layer thickness is achieved. The exponential decrease in layer thickness, occurring at 2<sup>i</sup> , is critical in achieving very thin layer thicknesses in a reasonable way.

The two annealing steps, and the initial annealing, are critical to processing and are tactically used to restore ductility before all severe plastic deformation process steps. Based on testing performed, herein, the process flow shown in Figure 2 is the minimum process flow required. Bimetallic tubes produced when omitting any of the annealing steps, for example, caused blistering and tearing during extrusion or expansion.
