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TRANSCRIPT
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
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
[1] I.-H. Jung, W. Bang, I.J. Kim, H.J. Sung, W.J. Park, D. Choo, S. Ahn, Magnesium Technology
2007. (Eds. R.S. Beals, A.A. Luo, N.R. Neelameggham, M.O. Pekguleryuz) p. 85-88.
[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