Effects of initial microstructure on the microstructural evolution and stretch formability of warm rolled Mg–3Al–1Zn alloy sheets

Abstract

Microstructural evolution and stretch formability of Mg–3Al–1Zn (AZ31) magnesium alloy sheets warm rolled at 225 °C by a single differential speed rolling pass with two different initial microstructures were investigated. A cast ingot and that rolled by one pass at a high temperature of 550 °C with average grain sizes of ~470 μm and ~19 μm, respectively, were used as starting materials. Both warm rolled sheets exhibit deformation microstructure in as-rolled condition and the occurrence of texture randomization during subsequent annealing as a result of static recrystallization. New recrystallized grains with a large orientation spread tend to persist in the original orientations of parent deformed grains and twin hosts. Regarding the nucleation sites at pre-existing grain boundaries and double twins, static recrystallization preferentially occurs at the sides of deformed grains and within double twins with their c-axes inclining toward the rolling direction (RD), resulting in a stable end annealing texture with a basal pole tilting toward the RD. For the sheet warm rolled from a cast ingot, incomplete recrystallization originating from less stored energy in the interiors of large deformed grains results in an insufficient texture weakening and an inhomogenous microstructure. Both sheets possess an improved stretch formability compared with conventional AZ31 alloy sheets due to the weakened textures. Benefiting from a weaker basal texture and a more homogenous microstructure, the sheet rolled at 550 °C followed by warm rolling at 225 °C exhibits a higher Erichsen value of 8.2 compared with that (6.1) of the sheet warm rolled directly from a cast ingot.

Introduction

Even though magnesium (Mg) alloys are attractive as light-weight structural materials with some excellent specific properties, poor formability at ambient temperature originating from strong basal texture extremely hinders their widespread application. Nowadays, a great deal of work has been devoted to texture control due to its remarkable effect on enhancing sheet formability for wrought Mg alloys [1], [2], [3], [4], [5], [6], [7]. It is important to deeply understand microstructural and textural evolution during deformation and subsequent annealing for optimizing plastic forming process. Some work has been carried out on investigating microstructural and textural evolution during rolling for Mg alloys [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21]. It has been known that high activity of basal slip, tensile twinning in non-basal oriented grains as well as survived basal oriented grains from dynamic recrystallization (DRX) give rise to the development of basal texture with the progress of rolling. Moreover, unfortunately, subsequent annealing cannot change the basal texture remarkably but tends to strengthen it further during grain coarsening in most cases [22], [23].

Warm rolling is often carried out as a finish rolling process for Mg alloys because of its effect on grain refinement. However, warm rolled Mg alloy sheets generally exhibit very poor cold formability due to the formation of strong basal texture. Recently, we found that a combination of high temperature rolling and subsequent warm rolling can result in superior sheet formability comparable to aluminum alloys due to the significantly weakened annealing texture [4], which is originated from the formation of static recrystallized (SRX) grains with a large orientation spread during post-deformation annealing [24]. The microstructure formed by high temperature rolling (i.e. prior to the final warm rolling) is characterized by a relatively large grain size and a relatively weak texture compared with those conventional sheets rolled at lower temperatures [4]. As-cast microstructure possesses the two similar characteristics (i.e. coarse grains and random texture), suggesting as-cast microstructure might be suitable as an initial microstructure for the final warm rolling to achieve Mg alloy sheets with superior formability. Up to date, research on the changes in microstructure and texture caused by a single warm rolling pass for cast Mg alloys is still quite limited. This may be related to the fact that as-cast alloys are generally subjected by hot rolling as rough rolling process. On the other hand, recrystallization including DRX and SRX is a possible approach to modify the textures of deformed metals. However, there is still a lack of insight into the orientation relationship between recrystallized grains and parent deformed grains for wrought Mg alloys. The initial as-cast microstructure with coarse grains allows for an easy differentiation between small recrystallized grains and parent grains, which is beneficial for the detailed investigation of orientation change during recrystallization. In addition, differential speed rolling (DSR) can introduce unidirectional shear deformation and enhance sheet formability due to the inclination of basal pole [2], [13], [14]. This is convenient for revealing the relationship between slip activity and recrystallization behavior for individual grains compared with crossed shear banding introduced by conventional symmetric rolling.

In this study, the warm DSR processing was carried out using a cast ingot and a sheet previously processed by high temperature rolling as starting materials, in order to investigate the influence of initial microstructure on texture formation and stretch formability of warm rolled AZ31 Mg alloy sheets. The microstructural and textural evolution as well as recrystallization behavior during rolling and subsequent annealing were investigated.