Solidification microstructure and mechanical properties of hot rolled

and annealed Mg Sheet produced through twin roll casting route

M. Aljarrah1,a, E. Essadiqi1,b, D. H. Kang2 and In-Ho Jung2

1Materials Technology Laboratory-CANMET, Hamilton, Ontario L8P 0A1, Canada

2Mining and Materials Engineering, McGill University, 3610 University Street,

Montreal, Quebec, H3A 2B2, Canada

amaljarra@nrcan.gc.ca;

bessadiqi@nrcan.gc.ca,

Keywords: Twin Roll casting (TRC), Segregation, Dendrite arm spacing, solidification, Annealing and Hot Rolling

Abstract The use of wrought magnesium for automobile structural components is an important

component of the mass reduction strategy for automobiles to improve their fuel efficiency.

Compared to Direct chill casting, Twin Roll Casting (TRC) allows major reduction of hot rolling

steps in the production of Mg sheet due to the thin thickness of the as-cast strip. This TRC route can

substantially reduce the time and cost to produce Mg alloy sheet product. In this work, AZ31

magnesium alloy was casted to 5 and 6 mm thick strips under different process conditions.

Microstructure of these strips was analyzed using optical microscopy, SEM and EPMA. TRC strip

was annealed under two different conditions: 2 hours at 330 and 1 hour at 400°C. It has been found

that heat treatment at 400°C for 1 hour reduces centerline segregation significantly. TRC strips were

rolled down to 2 mm and annealed at 450°C for 2 minutes. The average grain size was 4-6 µm and

mechanical properties were comparable with commercial AZ31 sheet.

Introduction

The production of Mg sheet from TRC (Twin Roll Casting) is much cost competitive than

conventional production route using Mg slab produced by chill casting. POSCO/RIST already

commercialized the TRC process to produce Mg sheet coil [1]. Large national research projects on

TRC process technology of Mg alloys suitable for TRC process are being carried out in many

countries including Canada, Germany and Korea [2-5].

CANMET installed TRC for 250mm width in 2008. The casting line also includes 400kg Mg

melting furnace and automated melt delivery system. Fully integrated HMI system collected all

process information during the casting process to facilitate the development of TRC process

technology and development of new Mg wrought alloys. In the present study, some of our early

experimental results for AZ31 alloy produced by TRC process will be presented. In particular, the

microstructural features of as cast and as annealed TRC AZ31 alloys will be presented. In addition,

the preliminary results of the hot rolled and annealed TRC strip will be presented in comparison

with commercial AZ31 alloy

Experiment

Commercial AZ31 ingots were melted in an electric resistance furnace under the protective gas

mixture of SF6 and N2. The molten metal was transferred to the headbox by the melt delivery system

and was maintained at about 710 oC. Then, the melt was poured into the twin roll gap through the

feeding tip, which gives the combined effect of solidification and hot rolling as schematically shown

in Figure 1. It should be noted that the temperature control of feeding tip is important to obtain

sound alloy strip. In the present research, electrically heated steel nozzle tip was used. AZ31 strip,

4-6 mm thick and 250 mm wide, was twin roll casted. The casting speed was 2-2.5 m/min and the

strip exit temperature was 390-410°C. Melt temperature and feeding speed was carefully controlled.

The AZ31 strip was rolled down to 2mm at 450°C.

Materials Science Forum Vol. 690 (2011) pp 331-334Online available since 2011/Jun/14 at www.scientific.net© (2011) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/MSF.690.331

All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,www.ttp.net. (ID: 87.236.232.197-14/01/15,11:27:30)

Figure 1. Schematics of twin roll casting and CANMET 250mm wide twin roll caster.

Results and Discussions

Typical as cast microstructure of TRC AZ31 alloy sheet is shown in Figure 2. As can be seen in

this figure, the microstructure can be divided into three different areas through the strip thickness.

Fine dendritic microstructure (chill zone) is observed near surface of the strip due to the very fast

cooling rate by the direct contact with roll surface. Columnar dendrites can be observed beneath

chill zone to the center. The dendrites are oriented 15° to the transverse axis of the strip and rotated

by plastic deformation to 45° to this axis. In the central area, equiaxed dendrites and coarse eutectic

structure are frequently observed. The trace of severe deformation can often be found at the

columnar and central areas. The measured secondary dendrite arm spacing (SDAS) through strip

thickness is in the range of 4-12 µm.

The EPMA backscattered image for sound surface of the TRC strip is presented in the Figure 3a.

The surface is flat and fine columnar dendrites are well distributed in the microstructure near the

surface. In some areas, however, segregated defects and microvoids along the grain boundaries were

detected (Figure 3b). WDS analysis confirmed the formation of large amount of Mg17Al12 phase and

ternary Mg-Al-Zn phases, which can be formed due to non-uniform melt flow near solidification

front. Figure 3c shows the microstructure in the central zone. Large amount of coarse segregations

(~1,000µm) are found in the central zone parallel to the casting direction. WDS analysis in higher

magnification image (Figure 3d) shows that these coarse eutectic particles are a mixture of α-Mg

and ternary Mg-Al-Zn compounds, which may be formed in the last stage of the solidification in the

remaining solute-enriched liquid. About 8 wt. % Al and 2.5 wt. % Zn was detected in this heavily

segregated central region. Therefore, careful heat treatment is required to prevent stress localization

or remelting in this solute enriched area.

500µm

Top Bottom

Casting

Columnar zone

Chill zone Central equiaxed

zone

Columnar zone

Chill zone

Figure 2. TRC microstructure of AZ31 alloy observed through strip at the transverse direction.

332 Light Metals Technology V

a b

c d

Figure 3. EPMA micrographs of as TRC AZ31 alloy strip. (a) sound surface area, (b) cracks in

surface area, (c) and (d) center line segregations. The casting direction is indicated by the arrow.

Annealing of the TRC strip was performed under the two following conditions: 330°C for 2

hours and 400° C for 1 hour. After the annealing at 330°C for 2 hours (Figure 4a), coarse eutectic

structures (white area) are still observed (dark regions are artifacts). On the other hands, when the

Mg alloy strip was annealed at 400°C for 1 hour, coarse particles were mostly dissolved into Mg

matrix and small amount of their traces were left. It is obvious that 400°C, 1 hour is better heat

treatment condition than 330°C, 2 hours to reduce this heavy centerline segregation. It is worth

noting that the heat treatment at 400°C may induce partial melting because the eutectic

microstructures (centerline segregation) has low melting temperature (about 360°C calculated from

equilibrium calculation using FactSage thermo-chemical software). Conventional annealing

condition of AZ31 alloy is also known as 345°C for 2 hours for full annealing [6]. However,

considering the enrichment of Al and Zn solute in central area, this temperature is not high enough

within a reasonable annealing time.

a b

Figure 4. EPMA backscattered image of central region of the as annealed strip. (a) 330°C for 2

hours, and (b) 400oC for 1 hour.

TRC strip was hot rolled to 2 mm thick sheet at 450°C and subsequently annealed for two

minutes at the same temperature. Optical micrographs of hot rolled and annealed AZ31 sheet are

shown in Figure 5. It can be noticed that a finer structure is observed at the surface of the sample

while a larger grain size microstructure dispersed with smaller grains are seen at the center. Average

grain size varies from 4-6 µm from the surface to the sheet centre. Table 1 summarizes the

Materials Science Forum Vol. 690 333

mechanical properties in longitudinal direction of AZ31 sheet produced by TRC and hot rolled and

heat treated for 2 minutes at 450°C. Yield and tensile strength of AZ31-TRC was comparable to

commercial AZ31 sheet.

Table 1: Mechanical properties along longitudinal direction of AZ31sheet produced by TRC route

and heat treated at 450°C for 2 min.

Alloy Yield stress

(MPa)

Ultimate tensile

stress (MPa)

Elongation

(%)

AZ31-TRC

hot rolled and annealed

(this study)

197 272 21

AZ31B-H24 [6] 220 290 15

Acknowledgments

The authors would like to thank the casting and rolling technologists for their help in TRC process

and financial supports of NSERC and GM of Canada.

References

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[2] S.S. Park, G.T. Bae, D.H. Kang, I.-H. Jung, K.S. Shin, N.J. Kim, Scripta Mater. 57(9) 2007.

793-796.

[3] D. Liang, C.B. Cowley, J. Min. Met. Mater. Soc. 56 (5) (2004) p. 26-28.

[4] Press release, ThyssenKrupp Steel AG (26 September 2002).

[5] O. Duygulu, S. Ucuncuoglu, G. Oktay, D. S. Temur, O. Yucel, A. A. Kaya, Magnesium

Technology 2009, (Eds. E. A. Nyberg, S. R. Agnew, N. R. Neelameggham, M. O. Pekguleryuz) p.

385-390.

[6] Magnesium and Magnesium Alloys, ASM Specialty Handbook, ASM International. Sep 1999.

Figure 5: Optical micrograph of AZ31 TRC strip after rolling and annealed for two

minute at 450 o

C (a) surface and (b) centre.

a b

334 Light Metals Technology V

Light Metals Technology V 10.4028/www.scientific.net/MSF.690 Solidification Microstructure and Mechanical Properties of Hot Rolled and Annealed Mg Sheet

Produced through Twin Roll Casting Route 10.4028/www.scientific.net/MSF.690.331

DOI References

[2] S.S. Park, G.T. Bae, D.H. Kang, I. -H. Jung, K.S. Shin, N.J. Kim, Scripta Mater. 57(9) 2007. 793-796.

doi:10.1016/j.scriptamat.2007.07.013