Improvement of microstructure and microhardness of AZ31 Mg alloy sheet by cross-forging-bending repeated deformation with sharply increasing temperature

Abstract

In this study, AZ31 Mg alloy sheets were processed by a severe plastic deformation (SPD) technique called forging-bending repeated deformation (FBRD). The effect on the microstructure and microhardness of AZ31 Mg alloy through FBRD was investigated with increasing temperature treatment and a 90° cross route. The results reveal that the effective strain increases with the number of passes. The flow uniformity is effectively enhanced due to alterations in shear deformation direction. After four passes of deformation, the average grain size is refined by 79.3% compared to the initial specimen. The grain refinement mechanism predominantly originates from the synergistic effects of discontinuous dynamic recrystallization (DDRX), continuous dynamic recrystallization (CDRX), and twinning-induced recrystallization (TDRX). The formation of {1012} extension twins (ET) significantly contributes to coarse grain subdivision and plastic deformation coordinated. Furthermore, pyramidal slip activation effectively enhances the plasticity of Mg alloys. By post four-pass processing, the alloy exhibits a microhardness of 81.9HV, primarily governed by fine grain strengthening and dislocation strengthening mechanisms.

1. Introduction

Magnesium (Mg) alloys have garnered significant research interest due to their exceptional specific strength/stiffness characteristics and recyclability. Mg alloys can meet the requirements of lightweight design, and they have great potential to improve energy efficiency in industries such as automotive and aircraft [1, 2]. Nevertheless, the hexagonal close-packed (HCP) structure of Mg alloy has low symmetry, and the activated slip system at room temperature is limited, resulting in weak plastic forming ability and low absolute strength. These adverse inherent characteristics restrict its large-scale commercial application [3−5]. Therefore, simultaneously enhancing the strength and plasticity of Mg alloys has received more and more attention.

Recent investigations have confirmed that severe plastic deformation (SPD) techniques including equal channel angular pressing (ECAP) [6, 7], multi-directional forging (MDF) [8, 9], and repeated upsetting-extrusion (RUE) [10, 11] effectively improve mechanical properties. In most SPD technologies, mult-pass deformation is one of the important methods to achieve grain refinement. ZHENG et al [12] applied ECAP technology to Mg-Gd-Nd-Zn-Zr alloy. After eight processes, the grain size was refined to 2.1 μm, and the strength and ductility were significantly improved. CUI et al [13] investigated a multi-directional forging experiment on ZK60 Mg alloy, in which continuous dynamic recrystallization (CDRX) dominated the grain refinement mechanism. Different processing routes are another way to achieve more refined and homogenized grains. KRAJNAK et al [14] found that processing routes had a significant impact on the average grain size and texture strength of AX41 Mg alloys. LI et al [15] conducted ECAP experiments on pure magnesium under the different routes, and the microstructure was the most uniform at the 90° routes, and {101ˉ2} extension twins played a leading role in the microstructure evolution. In addition, deformation temperature also has a significant impact on how the microstructure and texture are formed. MENG et al [16] reported that the Mg-Gd-Y-Zr alloy treated with four passes of variable temperature RUE reduced the basal texture strength from 5.303 to 2.582. When the deformation temperature i