PREDICTION OF THE SOLIDIFICATION PATH OF Al-6.32Cu-25.13Mg ALLOY BY A UNIFIED MICROSEGREGATION MODEL COUPLED WITH THERMO-CALC
Erhu YAN1,2(),Lixian SUN1,Fen XU1,Daming XU2
1 School of Materials Science and Engineering, Guilin University of Electronic Technology, Guilin 541004, China 2 School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China
Cite this article:
Erhu YAN,Lixian SUN,Fen XU,Daming XU. PREDICTION OF THE SOLIDIFICATION PATH OF Al-6.32Cu-25.13Mg ALLOY BY A UNIFIED MICROSEGREGATION MODEL COUPLED WITH THERMO-CALC. Acta Metall Sin, 2016, 52(5): 632-640.
The solidification path of alloy reveals the detailed relationship between the solute concentration in liquid and the temperature during the solidification process. The best and most accurate method to predict the solidification path of multicomponent/multiphase alloys is to establish proper microsegregation modeling coupled with phase diagram calculations according to the CALPHAD method. Recently, several alloy systems such as Al-Cu, Al-Mg and Cu-Mg have been developed, which have aroused the interest of many researchers. Up to now, the research about Al-Cu-Mg ternary alloy, especially containing higher Mg content, is relatively rare. The purpose of the present work is to investigate the solidification path of Al-6.32Cu-25.13Mg (mass fraction, %) ternary eutectic alloy at different cooling rates and solid back diffusion coefficients by an extended unified microsegregation model coupled with Thermo-Calc. Solidification experiments and subsequent microstructural characterization are combined with numerical calculation of solidification paths. It was shown that the cooling rates Rf had no obvious effect on the solidification path which was (L+α)→(L+α+T)→(L+α+β+T); but the solid back diffusion coefficient Φ had a great effect on the solidification path, which evolved gradually from (L+α)→(L+α+T)→(L+α+β+T) into (L+α)→(L+α+T) when Φ increased from 0 to 1. The volume fractions of primary α phase Vα, binary eutectic V2E and ternary eutectic V3E at each solidification path were calculated. It was shown that V2E decreased with the increase of Rf whereas V3E increased and Vα was almost invariant. The dependence of V2E, V3E and Rf were determined by linear regression analysis given as: V2E=-2.5lgRf+64.9, V3E=2.5lgRf+22.12. The increase in Φ led to increases in Vα and V2E and decrease in V3E. The predicted solidification paths and volume fractions of Al-6.32Cu-25.13Mg ternary eutectic alloy at different cooling rates were in good agreement with experimental results.
Fund: Supported by National Natural Science Foundation of China (No.51361005), Natural Science Foundation of Guangxi Province (Nos.2015GXNSFBA139208, 2014GXNSFDA118005 and UF14023Y) and Guangxi Key Laboratory of Information Materials (No.1210908-217-Z)
Fig.1 Phase diagram of the Al-Cu-Mg ternary alloy system[25] and the position of Al-6.32Cu-25.13Mg ternary alloy on the phase diagram which has been marked as A (The box areas represent the previous studies)
Fig.2 Schematic growth of α-dendrites and binary eutectic (α+β) in a ternary solidification system (ternary eutectic (α+β+T) may form from a part of the interdendritic liquid (L-phase) in a later solidification stage, λ represents the space between α and β phases and d2 represents the space between primary α phases)
Fig.3 Calculated results for Al-6.32Cu-25.13Mg alloy with different solidification rates Rf
Parameter
Value
Literature
Solidification shrinkage
0.043
[19]
Distance of secondary dendrite / mm
0.1
Calculated
DCuα/ (mm2s-1)
29exp(-15600/T)
[20]
DMgα/ (mm2s-1)
37exp(-14900/T)
[20]
Rf
0.08
Calculated
Step length of α ΔfS
0.0025
Initial value
Step length of binary eutectic ΔT / ℃
0.25
Initial value
Table 1 Physical and thermodynamic data used in the present solidification paths calculation (DCuαandDMgα represent the solute diffusion coefficients of Cu and Mg in α phase, respectively)
Fig.4 Fitted curves of volume fraction of binary eutectic V2E (a) and ternary eutectic V3E (b) vs Rf
Fig.5 Calculated results for Al-6.32Cu-25.13Mg alloy with different solid back diffusion coefficients Φ
Fig.6 Comparison of CL-fS curves for Al-6.32Cu-25.13Mg alloy with different Φ
Fig.7 Cooling curves of the Al-6.32Cu-25.13Mg alloy in different molds
Fig.8 SEM images of Al-6.32Cu-25.13Mg ternary alloy under cooling rates of 0.1 ℃/s (a), 0.06 ℃/s (b), 0.005 ℃/s (c) and 0.0007 ℃/s (d)
Mold
Cooling rate ℃s-1
Primary phase α-Al
Binary eutectic
Ternary eutectic
Calculated
Measured
Calculated
Measured
Calculated
Measured
Graphite mold
0.1
13.2
15.1
66.8
68.2
20.0
16.7
Sand mold
0.06
13.3
14.7
69.6
70.1
17.1
15.2
Insulated mold
0.005
13.2
14.5
71.6
72.8
15.2
12.7
Constant temperature mold
0.0007
13.2
14.1
75.1
76.7
11.7
9.2
Table 2 Comparisons of the volume fractions of α-primary, binary eutectic and ternary eutectic between the calculated and measured results on the Al-6.32Cu-25.13Mg samples solidified under different cooling rates (%)
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