Formation of shearing bands in the hot-rolling process of AZ31 alloy

doi:10.1016/j.jallcom.2011.09.021

Journal of Alloys and Compounds

Volume 512, Issue 1, 25 January 2012, Pages 73-78

https://doi.org/10.1016/j.jallcom.2011.09.021 Get rights and content

Abstract

Three types of AZ31 alloy samples, numbered with A, B, C, with various texture and microstructure condition were hot-rolled in single pass to investigate the different mechanism of shearing bands formation. Shearing bands came into being via twinning related grain fragmentation and DRX in Sample A while via rotational recrystallization in Sample B. Twinning played the most important role in shearing bands formation in Sample C. DRX and twinning are the two major elements in the formation of shearing band in magnesium alloys. Contrastive study indicated that sharper texture would increase the influence of twining while small size would promote the recrystallization in shearing bands formation.

Highlights

► Shearing bands forming process of AZ31 alloy was examined in this study. ► We prepared three types of samples with different grain size and texture condition. ► Sharper texture strengthened the influence of twining on shearing bands formation. ► Small size promoted the recrystallization in shearing bands formation.

Introduction

In recent years, magnesium alloys have attracted an increasing interest in the field of automotive, astronomy due to their low densities and other excellent properties such as heat dissipation, electro-magnetic shielding, etc. As the most widely used magnesium alloy products, magnesium sheets were usually produced with hot/warm rolling methods. A characteristic and inevitable phenomenon in rolling process is the formation of shearing bands [1], [2], [3]. The study on the forming mechanism of shearing bands would be of great help in understanding the mechanism of plastic deformation in magnesium alloys and improving the quality of their sheets products. Different phenomenon have been reported for shearing band formation in magnesium alloys and a number of models have been introduced, such as double-twinning [3], [4], [5], [6], rotational recrystallization (RDRX) [7], etc. However, previous studies mainly concentrate on only one type of samples, little attention has been paid to the influence of initial state on bands forming mechanism. Since such material factors as grain size, texture, etc., always played a most important role in the deformation behavior of mg alloys, they were expected to exert a profound influence on the forming process of shearing bands.

In the present work, three types of samples with different texture and mean grain size were obtained for single pass hot-rolling experiment to investigate the influence of initial state on the shearing bands formation in rolling process.

Section snippets

Materials and methods

The material used in this study was AZ31B sheets fabricated by twin-roll casting (TRC) method. The nominal composition was 3 wt% Al, 1 wt% Zn and balance Mg. Three types of samples, numbered as A, B, C, were prepared following the procedures as described in Table 1. Single pass hot-rolling experiments were conducted on the three types of samples at 375 °C. This temperature was chosen in order to avoid prevalent precipitation of β-phase (Mg₁₇Al₁₂) [1], [8]. The rolling facility was furnished with

Microstructure and texture before rolling

Fig. 1 shows the optical microstructure of the three samples prior to rolling. All the samples had equal-axial grains with a mean grain size of 20 μm, 5 μm and 20 μm respectively. Their textures were presented in (0 0 0 2) pole figures, as shown in Fig. 2. Although all of these samples exhibited typical basal texture, their texture intensity showed significant discrepancy. It could be seen clearly that Sample A had a maximum basal pole density of only 5.5, while Samples B and C owned much shaper

Conclusion

Dynamic recrystallization and twinning are two major elements in the formation of shearing band in magnesium alloys. Which one dominates in the formation process depends on the grain size and basal texture of magnesium alloys. It could come to a conclusion that sharper texture will increase the influence of twining, while small size will promote the recrystallization in shearing bands formation.

Acknowledgements

The authors acknowledge financial support from Program 2004BA311A11-1 under the Ministry of Science and Technology of the PRC.

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