**1. Introduction**

Bimetallic materials have been used for components delivering different material properties by their geometry (e.g., inside versus outside of a tube) [1–4]. Such components achieve benefits, such as lower cost for the consumer and producer, reduced weight, simplification of design, and/or reduced number of parts in a structure or assembly [3]. Bimetallic materials could be manufactured in the form of tubular geometries to serve desired applications. When employing bimetallic tubes, for example, one material can provide strength and stability, while the other can offer better corrosion resistance. A twolayer copper-steel tube, for example, can handle high loads via the steel and the corrosion resistance via the copper [4].

Bonding between the bimetals is not always necessary, and ultimately depends on the application. The two-layer designs typically rely on each material to perform one aspect of the intended design function independent of the other. In this case, a very tight compression fit between the two constituent metals may be appropriate. Evolving from the two-layer concept, multilayered material is envisioned for even more demanding or unique applications. To this end, multilayered materials provide a blended, and most often optimized, set of material properties but require each layer to be bonded to the next to do so.

Multilayered bimetallic manufacturing is a relatively new frontier in manufacturing and delivers superior material characteristics when compared to the constituent materials.

**Citation:** Standley, M.R.; Knezevic, M. Towards Manufacturing of Ultrafine-Laminated Structures in Metallic Tubes by Accumulative Extrusion Bonding. *Metals* **2021**, *11*, 389. https://doi.org/10.3390/ met11030389

Academic Editors: Andrew Kennedy, Gabriel Centeno Báez and Maria Beatriz Silva

Received: 15 January 2021 Accepted: 23 February 2021 Published: 27 February 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Many material combinations have been reported as bonded using accumulative roll bonding (ARB) such as Cu/Ti [5], Al/Cu [6], Al/Zn [7], Mg/Al [8], Cu/Zn/Al [9], Cu/Zn [10], Zr/Nb [11,12], Mg/Nb [13,14] and Zn/Sn [15] in plate form. When the layering is pushed to the ultrafine micron, or ultimately nanometer scale, the multilayered bimetallic material exhibits significantly improved strength [16–24], thermal stability [25,26], resistance to shock damage [27], and resistance to radiation damage [28,29]. Beyond this, the authors of [30] summarize the history of laminated metal composites and other benefits of bimetallic materials, such as improved fracture resistance, delayed fatigue crack growth, and ballistic energy absorption.

This work explores a processing methodology for manufacturing multilayered bimetallic tubing to achieve similar improvements in material properties to ARB sheets. To the knowledge of the authors at the time of publication, no research has reported producing multilayered bimetallic tubing using any severe plastic deformation processes. Following previous research in extrusion to achieve bimetallic tubing [31], this work takes inspiration from their design to develop a more complete manufacturing process. The process is termed accumulative extrusion bonding (AEB) and is used, in an iterative sense, to create single metal and bimetallic tubing with several layers. AEB, like ARB, is a severe plastic deformation process, which is defined as a metal forming process that creates very high strain without significant change to the overall dimensions to produce substantial grain refinement using severe straining and high levels of hydrostatic pressure [11,32–39]. By doing so, a decrease in grain size can improve the material properties following the Hall-Petch relationship [40–42] and increase the material strength by a factor of three to eight [43]. Additionally, as reported in [44], materials with ultrafine grains can have good damping properties, exhibit lower temperature super-plasticity, and high magnetic properties.

The AEB process involves iterative extrusion, cutting, expanding, restacking, and annealing. Due to the increased complexity of maintaining the geometrical shape of tubing relative to sheets, AEB is a much more challenging process than ARB. Other severe plastic deformations processes exist to produce extruded tube, and are well documented in the following review article [45]. Such processes include Equal Channel Angular Pressing (ECAP), Tube Channel Pressing (TCP), Tubular Channel Angular Pressing (TCAP), among others [45–47]. Since restacking and reprocessing is essential in producing ultrafine grains in multilayered tubing, AEB is described herein.

When this AEB process is compared to other AEB processes, the main differences include geometry of the extruded material, the custom dies and setup used, and the expansion process. Additionally, AEB performed in [48] did not use intermittent annealing, and because of this, tracked the true strain increase as samples were continuously processed. The AEB process used in the present research, utilizes annealing after every severe plastic deformation step such that accounting for continuous strain is not needed as strain effects are removed. Additionally, no material specimen or die preheat is used as indicated in the research performed in [49]. Most unique, no research has reported using AEB to manufacture multilayered bimetals in the form of tube as current research utilizes plate or sheet as done in [48–51]. For this reason, as part of the AEB process used for this research, expansion is necessary to facilitate restacking.

Two sets of specialty tooling are designed for a hydraulic press: One imparting 52% radial strain and another imparting 68% radial strain to produce the tubing. The design process is aided by finite element (FE) simulations to better understand the mechanics of the severe plastic deformation operations. After making the tooling, several similar and dissimilar metallic tubes are created to evaluate the extent of bonding, microstructure, and material properties. Hardness of the materials, yield strength, and ultimate tensile strength are compared before and after processing. Moreover, grain structure and the amount of bonding at the metal-to-metal interfaces are measured using optical microscopy. Surface preparation and the amount of deformation imposed per extrusion pass are discussed as critical to successful metal-to-metal bonding at the interface. Future work will attempt to create ultrafine multilayered bimetallic tubes with micron to nano radial layers of alternating material with many metal-metal interfaces governing a unique set of material properties.
