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Corresponding author: Thabet Makhlouf E-mail: [email protected]. © University of ... bulk metallic alloys through the use of equal channel angu-.
International Journal of Minerals, Metallurgy and Materials Volume 19, Number 11, Nov 2012, Page 1016 DOI: 10.1007/s12613-012-0663-6

Microstructural evolution of a recycled aluminum alloy deformed by equal channel angular pressing process Thabet Makhlouf 1), Atef Rebhi1), Jean-Philippe Couzinié2), Yannick Champion2), and Nabil Njah1) 1) Applied Metallurgy Laboratory, Faculty of Sciences, University of Sfax, BP. 1171 (3000) Sfax, Tunisia 2) Institut de Chimie et des Matériaux Paris-Est (ICMPE), CNRS 2-8 rue Henri Dunant 94320 Thiais, France (Received: 15 October 2011; revised: 24 November 2011; accepted: 28 November 2011)

Abstract: The microstructural evolution of a recycled aluminum alloy after equal channel angular pressing (ECAP) up to four passes was investigated using X-ray diffraction (XRD) analysis and transmission electron microscopy (TEM). Microhardness tests were performed to determine the associated changes in mechanical properties. An ultrafine-grained material has been obtained with a microstructure showing a mixture of highly strained crystallites. A high density of dislocations was achieved as a result of severe plastic deformation (SPD) through the die. Changes in mechanical behavior are also revealed after ECAP due to strain hardening. Thermal analysis and TEM micrographs obtained after annealing indicate the succession of the recovery, recrystallization, and grain growth phenomena. Moreover, the energy stored during ECAP may be related to the dislocation density introduced by SPD. We finally emphasize the role played by the precipitates in this alloy. Keywords: aluminum alloys; equal channel angular pressing (ECAP); microstructural evolution; precipitates

1. Introduction Ultrafine-grained microstructure can be produced into bulk metallic alloys through the use of equal channel angular pressing (ECAP) [1-3]. During the process, a metal billet is pressed through a die consisting of two channels, equal in cross-section and intersecting at an angle Ф. By multiple passing, very large effective deformation can be developed in bulk products [4-7]. Extensive researches have been carried out on the microstructure development and mechanical property evolution during ECAP of several materials. However, the effects of ECAP on the properties and microstructure of recycled aluminum have not been widely investigated. It is well known that ECAP of commercial aluminum alloys leads to ultrafine-grained structures with improved mechanical properties [8-11]. The grain sizes obtained by ECAP are generally in the submicrometer/nanometer range [12]. Therefore, ECAP processed materials may be attractive for different applications, mainly those needing superplastic behavior and/or enhanced mechanical properties, i.e., fine-grained microstructure [13-14]. Corresponding author: Thabet Makhlouf

One of the most common severe plastic deformation (SPD) methods is ECAP, a technique that results in a homogeneous ultrafine-grained microstructure of the work piece [15-16]. It has been shown that an imposed equivalent strain of ~1 is introduced on one passage of the sample through the ECAP die of Φ=90° [15]. For understanding the mechanical behavior of materials produced by ECAP, it is necessary to characterize their microstructure. The crystallite size and the lattice strain in nanocrystalline materials can be determined by X-ray line profile analysis [17-18]. In SPD processed materials where the lattice distortions are primarily caused by dislocations, the characteristic parameters of the dislocation structure can be obtained by the evaluation of the strain broadening of X-ray line profiles [19-20]. The nanostructured materials produced by ECAP have very high strength due to their low grain size and high dislocation density [16]. For the pressed alloy, most of the energy is believed to come from the strain energy stored by SPD. For example, a small energy less than 0.5 J⋅g−1 was reported in the extruded

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© University of Science and Technology Beijing and Springer-Verlag Berlin Heidelberg 2012

T. Makhlouf et al., Microstructural evolution of a recycled aluminum alloy deformed by equal channel angular …

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ECAP via route B. In parallel, we demonstrate that the energy stored during ECAP can be related to the dislocation density introduced by SPD.

Cu at an equivalent strain of 1, namely that the shear modulus of Cu is larger than that of Al by about 1.7 times [21]. The stored energy was no more than 1 J⋅g−1 even after 4 passes. On the other hand, this measured stored enthalpy can also correspond to the full precipitation, which is expected to have already taken place during the slow cooling in a furnace before being subjected to ECAP [22].

2. Experimental The experiments were carried out using a recycled aluminum alloy received in the form of cast ingots. It was analyzed by inductively coupled plasma optical emission spectrometry. The chemical composition of the investigated material is given in Table 1. The material was annealed for 24 h at 500°C and quenched in iced water.

In the present study, we have used transmission electron microscopy (TEM), X-ray diffraction (XRD), differential scanning calorimetry (DSC), and Vickers hardness tests (Hv) techniques to control the evolution of the microstructure and of the mechanical properties of a recycled aluminum during

Table 1. Chemical composition of the investigated material

ppm

Fe

Si

Zn

Mg

Mn

Cu

Ti

Cr

Ni

Ca

Co

Pb

Al

2500

2190

1660

1071

600

558

60

56

36

14

2.6