Reduction in tension–compression asymmetry via grain refinement and texture design in Mg–3Al–1Zn sheets

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Abstract

Twin rolled cast Mg–3Al–1Zn Mg sheets were processed using equal channel angular plate extrusion (ECAPE) at 200 °C and 150 °C following different processing schemes. Grain size was effectively refined from 9 µm down to 1.9 µm, and the starting strong basal texture was weakened after multi-step processing. A crystal plasticity model was utilized to reveal the operating deformation modes in the ECAPE processed sheets under uniaxial loading at room temperature. The results show that tension compression asymmetry due to tensile twinning activity notably decreased due to both grain refinement and texture modifications after ECAPE.

Introduction

The interest in lightweight structural materials has significantly increased in recent years, especially in the automotive industry for fuel economy and low CO2 emission. Due to their low density and high specific strength, Mg alloys have recently attracted a significant attention. However, several challenges still exist: the high cost of Mg sheet production and the limited formability at ambient temperatures, which is a consequence of the few active slip systems available due to the hexagonal close-packed structure. In addition, considerable differences between the activation barriers for the possible deformation modes further limit the low temperature formability of Mg alloys.

Twin-roll casting (TRC) can notably reduce the cost of traditional Mg sheet production by combining casting and additional rolling/annealing processes. TRC processed AZ31 Mg sheets were shown to have equivalent or superior mechanical properties to conventionally processed ones [1]. However, similar to conventionally processed AZ31 Mg sheets, TRC AZ31 Mg alloys again exhibit a strong basal texture (i.e. c-axes are mostly perpendicular to the rolling plane), which negatively influences the formability and causes high mechanical anisotropy near room temperature.

Formability can be improved by weakening the basal texture in Mg alloy sheets through different processing techniques [2], [3], [4], [5], [6], [7], such as multiple step rolling at different temperatures and differential speed rolling. Differential speed rolling is a new processing technique that introduces intense shear deformation and modifies texture and microstructure of Mg sheets [5], [8]. However, in addition to thickness limitations, this technique has certain disadvantages such as texture gradient and non-uniform deformation through thickness [9], [10]. Alloying with rare-earth elements is another method for improving the formability, which increases the non-basal slip activity and weakens the basal texture [11], [12], [13], [14], [15], [16]; however, those metals are expensive and not readily available.

Ductility of Mg alloys can be enhanced by microstructural refinement. Koike et al. [17] have attributed the ductility improvement at room temperature after grain refinement through severe plastic deformation to the activation of non-basal slip systems. In fine-grain Mg alloys, more non-basal slip activity should take place to satisfy the plastic compatibility across large number of grain boundaries [17]. In addition, the activation of twinning, the source of tension–compression asymmetry at low temperatures, is more difficult in fine-grained microstructures [18], [19].

The equal channel angular processing (ECAP) can refine the microstructure and induces various deformation textures in contrast to conventional sheet rolling processes in Mg alloys [20], [21]. In addition, since the cross section of the work piece does not change during ECAP, multiple passes and various ECAP routes are possible, allowing grain refinement and modification of deformation texture. To date, ECAP of Mg alloys have been limited to bar/billet forms and scale up of the ECAP technology has not been widely attempted via Mg alloy plate and sheet processing.

In the present work, hot-rolled AZ31 plates and sheets were successfully processed via equal channel angular plate extrusion (ECAPE) which provides a new unconventional processing route and relatively large samples (plate and sheets) for further testing and forming that was not possible before with conventional billet samples. The texture of TRC AZ31 sheets was altered and the grain size was refined via different ECAPE routes at 200 °C and 150 °C. In addition, resulting texture and grain size effect on tension–compression asymmetry were evaluated at room temperature, which may assist in designing AZ31 alloy microstructures for better formability.

Section snippets

Experimental procedure

A 3 mm-thick AZ31 TRC sheet was cut into square specimens of 90×90 mm2. In order to process such a thin sheet using the available ECAPE tool, the sheet was embedded in a 152×152×13 mm3 commercial AZ31 plate such that rolling direction is aligned with the longitudinal direction, as illustrated in Fig. 1. Note that, there is no limitation on batch processing thin sheet of Mg alloys using ECAPE, but the present sheets were embedded into the plates, with comparable mechanical flow response (not shown

Results

Fig. 2a shows OM micrographs of as-received (As-R) TRC AZ31 Mg sheet. The average grain size is 9 µm. SEM micrographs of the ECAPE processed samples are given in Fig. 2b–d, for 4A@200 °C(4A), 4Bc-P@200 °C(4Bc-P), and 4A@200 °C+2F@150 °C(4A+2F) samples, respectively. Average grain size dropped to around 3.6 µm after the 4A route. A more uniform grain size distribution is evident after the 4B-P c route, as shown in Fig. 2c and e, where the average grain size is ~2.8 µm. Fig. 2d presents the

Discussion

In the present study, the plate ECAP (ECAPE) technology is successfully applied to Mg alloy sheets. The twin roll cast AZ31 Mg sheets have been ECAPE processed for the first time at 200 °C and 150 °C using different processing routes. The As-R texture has been modified, and the grain size was refined upon ECAPE processing. A slight texture change and more uniform grain size distribution were detected after the new 4Bc-P route available in ECAPE, however, a more significant change in texture was

Conclusions

In this study, texture evolution and mechanical flow response of twin-roll cast magnesium AZ31 alloy sheets have been investigated after equal channel angular plate extrusion (ECAPE) at multiple temperatures. Main findings and conclusions can be summarized as follows:

  • 1.

    The ECAPE is successfully applied to Mg alloy sheets and twin roll cast AZ31 Mg sheets have been ECAPE processed for the first time at 200 °C and 150 °C using different processing routes.

  • 2.

    The average grain size was effectively refined

Acknowledgment

This research was supported by NPRP Grant no. 4-1411-2-555 from the Qatar National Research Fund (a member of Qatar Foundation). The statements made herein are solely the responsibility of the authors. The authors acknowledge the assistance of Prof. T. Hartwig, D. Foley and R. Barber for ECAPE processing.

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