Hot Deformation Stability of Extruded AZ61 Magnesium Alloy Using Different Instability Criteria

doi:10.1007/s40195-015-0334-5

Abstract

Abstract:

The hot deformation stability of extruded AZ61 magnesium alloy was investigated by means of hot compression tests at the temperature range of 250-400 °C and strain rate range of 0.001-1 s^-1. The 3D instability maps considering the effect of strain were developed to delineate the regions of unstable flow on the basis of Jonas’s, Semiatin’s, Prasad’s, Murty’s, Gegel’s and Alexander’s criteria. Since non-uniform deformation occurs due to the initial microstructure inhomogeneity, the friction, etc., finite element simulations were performed to determine the position of the specimens which can mostly represent the preset deformation parameter. Detailed microstructural investigation on such position was carried out to examine the validity of the instability maps, and the results indicate that for extruded AZ61 magnesium alloy: (1) Jonas’s and Semiatin’s criteria conservatively predict the instability regions; (2) Gegel’s and Alexander’s criteria inadequately predict the instability regions; (3) Prasad’s and Murty’s criteria provide more effective predictions of the instability regions than Jonas’s, Semiatin’s, Gegel’s and Alexander’s criteria.

Key words: Magnesium alloy, Hot deformation stability, Finite element simulation, Microstructure

Ju-Qiang Li, Juan Liu, Zhen-Shan Cui. Hot Deformation Stability of Extruded AZ61 Magnesium Alloy Using Different Instability Criteria[J]. Acta Metallurgica Sinica (English Letters), 2015, 28(11): 1364-1372.

Figures/Tables 8

Fig. 1 Initial microstructure of extruded AZ61 magnesium alloy

Fig. 2 True stress versus true strain curves of extruded AZ61 magnesium alloy corrected for the friction at 0.001 s-1 a, 0.01 s-1 b, 0.1 s-1 c, 1 s-1 d

Fig. 3 3D instability maps developed according to Jonas’ criterion a, Semiatin’s criterion b, Prasad’s criterion c, Murty’s criterion d, Gegel’s criteria e, Alexander’s criteria f

Table 1 Simulation parameters

Simulation parameter	Value
Young’s modulus (MPa)	43,000
Poisson’s ratio	0.35
Thermal conductivity [W/(m K)]	79.55
Heat capacity [kJ/(kg K)]	1.21
Friction coefficient between specimen and die	0.2
Heat transfer coefficient between specimen and die [W/(m² K)]	1100
Heat transfer coefficient between specimen and air (W/m² K)	20
Emissivity	0.1

Fig. 4 Effective strain distributions under the temperature of 300 °C and strain rate of 0.1 s-1 at strokes of: a 3.20 mm, b 5.72 mm, c 7.70 mm, d 9.48 mm

Fig. 5 The variations of the strain with stroke at the selected points

Fig. 6 The variations of the strain at point B with the set value under various temperatures and strain rates

Fig. 7 Microstructures of the specimens deformed at various temperatures and strain rates

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