# Accumulative Roll Bonding--A Review
## 1 Introduction  
Nowadays, bulk materials with high strength to weight ratio are in high demand particularly in weight-critical industrial applications. Average grain size plays a significant role in determining the mechanical properties of almost all crystalline materials and hence ultra-fine grained (UFG) materials, i.e., materials with the sub-micron mean grain size, are widely used owing to their superior mechanical properties. In order to produce such UFG structures using severe plastic deformation (SPD) procedures, exceptionally high strain value should be considered so that a high density of dislocations can be inducted in the bulk of material resulting in re-arranged grain boundaries and new array of UFGs. The most common SPD procedures are equal channel angular pressing (ECAP) [1], high-pressure torsion (HPT) [2] and accumulative roll bonding (ARB). ECAP can be applied to various materials where it employs simple shear without reducing cross-sectional area by pressing a metal billet through an angled channel [3; 4; 5; 6]. HPT, however, imposes torsional/compressional stresses to a relatively small and thin metal disk [7; 8; 9], usually with no dimensional changes. Both ECAP and HPT are limited to producing small parts, relatively expensive, poor in productivity especially in continuous mass production, and usually require considerably large load capacities and dies. However, having an ability to bond similar and/or dissimilar materials, ARB is emerging as a cost-effective SPD technique to obtain large UFG sheets or multi-layered composites structures by accumulating large strains usually by repeating the procedure. It can also be easily scaled up industrially as a continuous operation. ARB is applicable for processing a large number of different materials including those that cannot be bonded by traditional bonding methods. Invented by [PERSON] et al. [10] in 1999, ARB is a solid-state multi-stage process including surface preparation, cutting, stacking, rolling and sometimes followed by a post-rolling heat treatment to improve bonding quality of the stacked sheets. Figure 1a schematically shows the process where two or more sheets are stacked together and passed through the rolls to apply plastic deformation; the deformation must be enough to produce a solid-state bonding. ARB has a great industrial potential due to a high production rate and continuous procedure. For instance, a two-layer copper/aluminum (Cu/Al) clad sheet composites manufactured by ARB can almost reduce 40% of weight, maintaining equivalent electrical and heat conductivities like a Cu alloy. The ARBed clad composites are frequently used in armored cables, yoke coils in TV sets, air-cooling fin and bus-bar conductor joints [11]. [PERSON] et al. [12] employed ARB to produce a closed-cell Al foam wherein a reinforcing agent is blown between the sheets. [PERSON] et al. [13] used ARB to fabricate composite filler metals of tungsten inert gas welding where the results revealed that the yield strength of welds was significantly improved when employing Al\({}_{2}\)O\({}_{3}\)/TiC/TiO\({}_{2}\) nanoparticle/Al composite filler metals. A similar effort was made by [PERSON] et al. [14] to produce ZrO\({}_{2}\)/AZ31 nanocomposite fillers of gas tungsten arc welding. Using ARB, [PERSON] et al. [15] fabricated inexpensive composites commonly used as acid containers with a low-thickness stainless steel (for inner side) stacked to a thick low-carbon metal (for outer side). To improve bonding quality, contacting surfaces are to be degreased and wire-brushed to remove oxides or any other surface layers. As a result of rolling pressure, thickness reduction and frictions, a great amount of heat is generated wherein a synchronous presence of pressure and heat results in the bonding of cleaned surfaces. The interfacial bond is the consequence of mechanical and atomic affinity of the sheets [16; 17]. To improve bonding quality, ARB can be performed at elevated temperatures or below the recrystallization temperature.