Dynamic recrystallization in AZ31 magnesium alloy

doi:10.1016/j.msea.2006.11.095

Materials Science and Engineering: A

Volume 456, Issues 1–2, 15 May 2007, Pages 52-57

https://doi.org/10.1016/j.msea.2006.11.095 Get rights and content

Abstract

The effects of temperature and strain rate, as the most important thermomechanical processing (TMP) parameters, on the microstructural evolution of AZ31 magnesium alloy were studied. This was performed applying hot compression tests at a temperature range of 250–450 °C with various strain rates. The results indicated that the amount and the size of dynamically recrystallized grains are increased as Zener–Hollomon parameter decreased. In addition, the evolution of dynamically recrystallized grains was examined with increasing strain. In general, the amount of dynamically recrystallized grain is observed to increase with strain in a sigmoidal form. However, the related grain size was remained constant as strain increased.

Introduction

The main fabrication route of magnesium alloy parts and components has been realized to be die-casting methods. In addition, wrought magnesium alloys are increasingly used to produce components with superior mechanical properties. It is worth mentioning that the share of die-casting method in AZ91 parts production, as the most important magnesium die-casting alloy, was 81% of all production techniques in 1997 but was reduced to 67% by 2002 [1]. Nevertheless, to explain the aluminum and copper replacement by wrought magnesium alloys from economic point of view, precise study is necessitated to optimize fabrication processing.

Magnesium alloys, however, reveal poor mechanical and formability properties down to their HCP structure. This leads toward those fabrication processes, which rely on the hot deformation methods to increase the formability of the alloys. The previous investigations have shown that magnesium alloy might undergo the dynamic recrystallization (DRX) phenomena during hot working processes. Tan and Tan [2] studied the dominance of continuous recrystallization phenomena on Mg–3Al–1Zn in the temperature range of 250–400 °C. They reported that the grain refinement during DRX is insignificant at high temperatures due to the rapid grain growth. In addition, the grain refinement showed a maximum value at medium temperatures. Kaibyshev and Sitdikov [3], [4], [5], however, suggested a temperature and strain dependencies regime for grain size variation. This was related to the change of DRX mechanism by TMP parameters. However, Barnett [6] has shown that the dynamic recrystallization grain size is less sensitive to deformation conditions than that of other metals.

The present work aims to contribute clarifying the aforementioned debate through applying hot compression tests at different temperatures and strain rates on an as-rolled Mg–3Al–1Zn alloy.

Section snippets

Experimental procedure

The experimental material was AZ31 magnesium alloy (Mg–2.9%Al–0.85%Zn–0.3%Mn), which was received as-hot rolled plates with 22 mm thickness. The initial equiaxed microstructure of the as received alloy is shown in Fig. 1. The initial grain size was measured to be 35 μm. The cylindrical hot compression testing specimens were machined in the sizes of Ø8 mm × H 12 mm. In all specimens, the deformation axis was selected to be parallel to the rolling direction. It was well documented that rolled plates

Results and discussion

The typical true stress–true strain curves, which were obtained through hot compression testing, are presented in Fig. 3. This shows the effects of strain rate and deformation temperature on the flow behaviour of the experimental alloy. The flow stress initially increases with strain up to a peak (corresponding to ɛ_P) and then decreases by a rate that decays with increasing strain. The observed strain softening (after peak stress) is attributed to dynamic recrystallization (DRX) phenomena [7].

Conclusions

This work has focused on the influence of deformation conditions on the evolution of hot deformation microstructures in AZ31 magnesium alloy. The main points resulted from the present work are as follows:

(1)
The dynamically recrystallized grain size is reduced by increasing the Z values. An inverse power law may describe the relationship between the deformation condition (Z) and DRX grain size.
(2)
The DRX-G volume fraction was reduced as the Z value increased. This was described as a geometric

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Dynamic recrystallization in AZ31 magnesium alloy

Abstract

Introduction

Section snippets

Experimental procedure

Results and discussion

Conclusions

Mater. Sci. Eng. A

Mater. Sci. Eng. A

Acta Mater.

Phys. Met. Metall.

Mater. Trans.