Asymmetric cold rolling of AA7075 alloy: The evolution of microstructure, crystallographic texture, and mechanical properties

Abstract

In the present work, for the first time, the effect of asymmetric rolling at room temperature on the microstructural evolution, texture, hardness, tensile properties, and fracture surface of AA7075 alloy was investigated. It was found that the width of grains significantly decreased to 2.6 μm by 60 % asymmetric rolling. Also, shear bands in the RD-ND (rolling direction – normal direction) plane of AA7075 after 40 % and 60 % asymmetric rolling were evident. In addition, the size of iron-rich intermetallic particles was reduced with increasing thickness reduction. Microstructure and texture revealed that new grains through continuous dynamic recrystallization (CDRX) are created after 40 % and 60 % strain. On the one hand, after applying only 10 % thickness reduction, average hardness demonstrated 75 % enhancement. On the other hand, maximum yield (611 MPa) and ultimate tensile (644 MPa) strengths achieved in the 60 % deformed sample, demonstrating 287 % and 73 % improvement compared to the initial sample. These results were due to strain hardening, dynamic/static precipitation, and Fe-rich particle fragmentation. The Portevin-Le Chatelier (PLC) effect is reduced with increasing the strain. Interestingly, the 40 % deformed sample revealed the serrated flow. For this reason, the 40 % rolled sample has a lower total elongation (4.4 %) than the 60 % deformed sample (11.1 %). The tensile strength of the 40 % deformed sample (575 MPa) was lower than the 10 % and 20 % rolled samples (581 and 594 MPa, respectively). This result was owing to the occurrence of softening mechanisms, strengthening the Goss {110}⟨001⟩ component, and weakening the Copper {112}⟨111⟩ orientation after 40 % asymmetric rolling. By increasing the thickness reduction up to 40 %, the number of dimples decreased. However, the fracture surface of the 60 % deformed sample showed a noticeable amount of small dimples.

Introduction

Al–Zn–Mg–Cu or 7xxx aluminum alloys due to their excellent fracture toughness, high strength-to-weight, and good machinability are widely used in transportation and aircraft industries [[1], [2], [3], [4]]. In 7xxx alloys, Zn, Mn, and Cu form ternary and quaternary precipitations that their size, distance, and distribution affect the strength of the alloy [5]. The low room temperature formability, the intense decrement of ductility in the high-strength AA7075 alloy, and the severe anisotropy in mechanical properties are their most important limitations [[6], [7], [8]].

To manufacture high-strength AA7075 alloy, severe plastic deformation (SPD) techniques including accumulative roll bonding (ARB), high-pressure torsion (HPT), and equal channel angular pressing (ECAP) have been investigated extensively in the past decades [[9], [10], [11], [12], [13], [14]]. But their industrial applications due to their limitations are suppressed. For instance, both ECAP and HPT methods are not suitable for manufacturing large-scale samples and it is hard for the ARB method to reach a good interfacial bond between sheets [15].

Asymmetric rolling (ASR) is a powerful metal forming process for industrial applications that can be done using existing rolling mills, which are commonly used in symmetrical rolling. The first investigation was carried out in the middle of the past century [16]. Generally, there are four types of ASR including (1) different diameters of the rolls; (2) different velocities of rolls; (3) single roll drive; and (4) different lubricated roll surfaces [17]. ASR can have positive effects on texture, grain refinement as well as microstructure homogeneity [[17], [18], [19], [20], [21], [22], [23]]. Also, it can improve the formability of materials. ASR can save energy via the reduction in rolling force and torque. Compared to symmetrical rolling, asymmetric rolling could decrease the rolling force by 5%–30 % [17].

Asymmetric rolling of 5182 Al alloy showed finer microstructure as compared with symmetric hot rolling [24]. However, asymmetric rolling has no remarkable influence on the mechanical properties of AA5182 alloy. Asymmetric and symmetric rolling of AA6061 Al alloy in the room and cryogenic temperatures, as well as aging, was conducted by Magalhães et al. [25]. Artificial aging after symmetric rolling in the room temperature was more effective on strength and hardness whereas asymmetric rolling at cryogenic temperature with the same subsequent heat treatment led to increase uniform elongation and reduce texture intensity [25]. Goli and Jamaati [26] found that the most important texture component after single roll drive rolling of AA2024 Al alloy consists of a strong α-fiber. The Goss/Brass ratio after 20 % and 40 % thicknesses reduction was 1.44 and 1.32, respectively. It is noted that the evolution of texture in AA2024 alloy significantly depends on the mode of rolling [26].

To the best of our knowledge, the asymmetric rolling of 7075 Al alloy has not yet been investigated. Hence, the focus of this study is to study the influence of single roll drive rolling on the microstructural evolution, crystallographic texture, hardness, and room-temperature tensile properties of AA7075 alloy.