A review on processing of aluminium and its alloys through Equal Channel Angular Pressing die

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Abstract

Severe Plastic Deformation (SPD) is a unique approach to processing the bulk material into ultrafine grain (UFG) structured material which possesses high ductility and strength. Equal Channel Angular Pressing (ECAP) is one of the preferred techniques of SPD, which induces high strains on the material without changing the dimensions of the material. Aluminium and its alloys are the most preferred materials for various applications due to its amendable properties. The objective of this review paper is to provide the effects of aluminium and its alloys processed through different process parameters of Equal Channel Angular Pressing die.

Introduction

Several factors determine the mechanical and physical properties of crystalline materials. Among these, the size of the grains is found to play a significant role. Nanocrystalline and ultrafine grain (UFG) structured materials have a large number of their atoms at the grain boundaries attributing to their improved properties. These materials exhibit increased strength/hardness and improved ductility/toughness amongst others when compared to coarse-grained materials [1], [2]. In the case of Aluminum, the coarse-grained Al has a yield strength of 56 MPa and UTS of 73 MPa while ultrafine grained Al has an improved yield strength of 173 MPa and UTS of 188 MPa [3]. Rapid development in research to produce UFGs in bulk materials has increased the application of severe plastic deformation (SPD) (See Fig. 1, Fig. 2, Fig. 3, Fig. 4).

SPD processing is unique as it imposes high strains without significant alterations in the dimensions of the workpiece. To process bulk samples, few requirements need to be met in SPD methods. These requirements include having, in the prepared sample, high-angle grain boundaries to produce qualitative changes in the properties, uniform nanostructures within its volume for having stable properties and no mechanical damage [4]. Conventional methods of SPD such as extrusion and rolling cannot meet all these requirements. For SPD processes, the Hall-Petch equation governs the relationship between the strength, regarding yield stress, σy, and the grain size of the particle. It is given as:σy=σ0+kyd-1/2where σ0 is termed the friction stress, d is grain size, and ky is a constant of yielding [5]. From this equation, we identify that strength improves with a decrease in grain size leading to increased interests to research this area.

Numerous methods for SPD processing such as accumulative roll-bonding (ARB), high-pressure torsion (HPT) and equal-channel angular pressing (ECAP) are now available. ECAP, currently, is the most developed SPD technique [6] as it has several advantages such as not introducing porosity to the material and obtaining grain refinement in bulk samples [7]. In this, the billet is extruded through a die with two channels, namely, the inlet channel and the exit channel, at an abrupt angle Ø (angle between the channels), also characterized by an additional angle Ψ, named corner angle, representing the outer curvature at the intersection of the two channels.

The relationship between the equivalent strain (ε) produced and the angles mentioned above is given as:=N2cot2+ψ2+ψcscϕ2+ψ23where N represents the number of passes [9].

The effect of grain refinement is mainly dependent on factors such as the angle between the channels (Ø), the angle representing the outer arc of curvature (Ψ), the number of passes and the processing route, namely, A, BA, BC or C [10].

Since SPD is ideally done to improve the strength in most materials, non-ferrous materials are the most researched upon as the need for strength improvement is commonly found in such materials rather than ferrous materials but not limited to these. For this reason, extensive research is carried out on the effects of ECAP on materials such as Aluminium [12], [13], Magnesium [14], [15] and Copper [16], [17] along with their respective alloys amongst others. Aluminum is relatively studied more due to its diverse range of applications in the industry and the constant demand for improving its strength that has resulted in several useful types of research.

This article aims at providing an overview of ECAP on aluminum and its alloys and its effect on the mechanical properties through different die geometry, but not limited to this. It also provides inferences suggesting avenues to extend research on ECAP.

Section snippets

Pure Aluminium and Al-1000 series

Y. Iwahashi et al. studied the microstructure and shearing characteristics of pure Aluminium that has been subjected to ECAP, as early as 1998 [18]. The experimental observations show that the shearing attributes associated with ECAP under different conditions (different routes and number of passes) are consistent for the primary direction of grain elongation after each pressing and the nature of shearing. Y. Iwahashi et al. provided more insight into the grain refinement process in another

Inferences

Thus, from the literature studied, the Summary of effects of ECAP on the mechanical properties of aluminum and its alloys as shown in following in Table 5.

From Table 5, the predominantly used route for processing aluminum and its alloys is the route BC possibly due to the route’s ability to produce homogeneous and equiaxed grains. Also, in almost all experiments, this route is used with a die channel angle (Ø) of 90° and 120° while the arc of curvature angle (Ψ) is usually 20°, 20.6° and 37° in

Conclusion

Equal channel angular pressing (ECAP) is capable of improving the mechanical properties (UTS, yield strength, and hardness) of the majority of aluminum alloys with a decrease in ductility and treated as a convention. However, several experiments are limited to testing tensile properties, and hence, research shall be extended into testing the compressive and torsional properties. Since the majority of the trials have a similar pattern of using a die with channel angle of 90°, through route BC

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