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Research Papers

Microstructure Stability of a Fine-Grained AZ31 Magnesium Alloy Processed by Constrained Groove Pressing During Isothermal Annealing

[+] Author and Article Information
Kai Soon Fong

Forming Technology Group,
Singapore Institute of
Manufacturing Technology,
71 Nanyang Drive,
Singapore 638075, Singapore
e-mail: ksfong@SIMTech.a-star.edu.sg

Ming Jen Tan

School of Mechanical and
Aerospace Engineering,
Nanyang Technological University,
50 Nanyang Avenue,
Singapore 639798, Singapore
e-mail: mmjtan@ntu.edu.sg

Fern Lan Ng

Measurements and Characterization Unit,
Singapore Institute of
Manufacturing Technology,
71 Nanyang Drive,
Singapore 638075, Singapore
e-mail: flng@SIMTech.a-star.edu.sg

Atsushi Danno

Forming Technology Group,
Singapore Institute of
Manufacturing Technology,
71 Nanyang Drive,
Singapore 638075, Singapore
e-mail: danno@SIMTech.a-star.edu.sg

Beng Wah Chua

Forming Technology Group,
Singapore Institute of
Manufacturing Technology,
71 Nanyang Drive,
Singapore 638075, Singapore
e-mail: bwchua@simtech.a-star.edu.sg

Manuscript received August 17, 2016; final manuscript received April 6, 2017; published online May 8, 2017. Assoc. Editor: Yannis Korkolis.

J. Manuf. Sci. Eng 139(8), 081007 (May 08, 2017) (9 pages) Paper No: MANU-16-1434; doi: 10.1115/1.4036529 History: Received August 17, 2016; Revised April 06, 2017

In this study, an AZ31 magnesium alloy plate was processed by constrained groove pressing (CGP) under three deformation cycles at temperatures from 503 to 448 K. The process resulted in a homogeneous fine grain microstructure with an average grain size of 1.8 μm. The as-processed microstructure contained a high fraction of low-angle grain boundaries (LAGB) of subgrains and dislocation boundaries that remained in the structure due to incomplete dynamic recovery and recrystallization. The material's yield strength was found to have increased from 175 to 242 MPa and with a significant weakening of its initial basal texture. The microstructure stability of the CGP-processed material was further investigated by isothermal annealing at temperature from 473 to 623 K and for different time. Abnormal grain growth was observed at 623 K, and this was associated with an increased in nonbasal grains at the expense of basal grains. The effect of annealing temperature and time on the grain growth kinetics was interpreted by using the grain growth equation,  Dn+D0n=kt, and Arrhenius equation, k=k0exp((Q/RT)). The activation energy (Q) was estimated to be 27.8 kJ/mol which was significantly lower than the activation energy for lattice self-diffusion (QL = 135 kJ/mol) and grain boundary diffusion (Qgb = 92 kJ/mol) in pure magnesium. The result shows that grain growth is rapid but average grain size still remained smaller than the as-received material, especially at the shorter annealing time.

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Figures

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Fig. 1

(a) Schematic of the CGP cycle (black arrow indicates the rolling direction of the plate) and (b) dimensions of the grooved dies

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Fig. 2

Microstructure of the (a) as-received and CGP-processed AZ31 alloy after (b) first, (c) second, and (d) third cycles

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Fig. 3

EBSD measurement of the microstructure after CGP: (a) orientation map, (b) misorientation distribution profile, (c) grain boundary map (LAGB in the range of 2–15 deg were delineated by thin lines, and HAGB above 15 deg were delineated by thick lines), and (d) grain orientation spread map

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Fig. 4

Pole figures of the (a) as-received and (b) CGP-processed alloy

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Fig. 5

The true stress–strain curves of the as-received and CGP-processed AZ31 alloy after first to third cycle along the (a) RD and (b) TD directions

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Fig. 6

Microstructures at different annealing temperatures and time

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Fig. 7

Superposition of normalized grain size distributions at (a) 473 K, (b) 523 K, (c) 573 K, and (d) 623 K for different annealing time

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Fig. 8

Relationship between texture and annealing time at different temperatures: (a) 473 K, (b) 523 K, (c) 573 K, and (d) 623 K

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Fig. 9

Plot of (a) hardness against time and (b) average grain size against time at different temperatures

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Fig. 10

Plot of ln(dD/dt) against ln(D) (a), D5.4 − D05.4 against annealing time (b), and lnK against 1000/T (c)

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