Microstructure and mechanical properties of twin-roll cast Mg–4.5Al–1.0Zn sheets processed by differential speed rolling

Abstract

The microstructure and mechanical properties of the twin-roll cast (TRC) Mg–4.5Al–1.0Zn alloy sheets produced by differential speed rolling (DSR) were investigated by optical microscopy, transmission electron microscope and electron backscattered diffraction. The results are compared with those of the sheet processed by equal speed rolling (ESR). It is shown that twining played an important role at the initial stage of rolling. With the increase of the rolling reduction, the microstructures in the processed sheets become more homogeneous and they are more refined by DSR than by ESR. After annealing, the sheet processed by DSR shows a higher elongation and slightly lower yield strength than those of the ESR-processed sheet, which could be attributed to grain refinement and texture weakening. These results suggest that, in comparison with conventional casting and rolling, the combined technology of TRC and DSR is a more effective way to process magnesium sheets with enhanced formability after final annealing.

Introduction

Wrought magnesium (Mg) alloys have a great potential as lightweight structural materials, particularly in automobile industry, because of their high specific strength and excellent damping capacity [1], [2]. The high manufacturing cost and limited formability at room temperature, however, are two major barriers to the wide industrial applications of Mg alloys.

To reduce the production cost, twin-roll casting (TRC) technique has been applied to manufacture Mg alloy strips with mechanical properties equivalent to or better than those after warm rolling of conventional casting (CC) ingots [3], [4], [5], [6]. However, conventional warm rolling of TRC Mg strips preserves a strong basal texture as often found in the rolling of CC ingots. The basal texture is known to be detrimental to the formability of Mg alloys near the room temperature [7]. Thus, the room temperature ductility of Mg alloys strongly depends on the orientation of basal plane, and thus it is important to modify or weaken the basal texture in subsequent processing [8]. Many studies were reported to improve the formability of Mg alloys. Chang et al. [8] succeeded in enhancing the room temperature ductility of AZ31 alloy by asymmetric hot extrusion. Agnew et al. [9] performed equal channel angular pressing to improve tensile elongation of AZ31B alloy tremendously. However, such techniques are not suitable for processing thin sheets. Meanwhile, previous studies on CC ingots and hot-extruded plates implied that, compared to conventional or equal speed rolling (ESR), differential speed rolling (DSR) can impart more intense shear deformation and improve the formability of Mg alloys by grain refinement and basal texture weakening [10], [11]. It is thus of interest to combine TRC and DSR techniques in processing Mg alloy sheets with improved formability at a low relatively low cost. This concept has been tested for a ZK60 alloy in our previous work and it was shown that the DSR technique is more effective than ESR in improving the formability of the twin-roll casted ZK60 alloy [6].

In addition to ZK (zinc–zirconium), the other most common wrought alloy family is AZ (aluminum–zinc). The objective of this study is to explore the possibility of further enhancement of formability of TRC Mg alloys through the DSR technique in the AZ alloys. The materials selected here is Mg–4.5Al–1.0Zn–0.5Mn–0.5Ca (designated as AZ41 M in short, “M” denotes the modification of base material by adding Mn and Ca elements) alloy, which has improved elevated temperature properties and corrosion resistance [5], and is representative of the AZ alloy family with an intermediate Al content.