Effects of Er on Hot Cracking Susceptibility of Mg-5Zn-xEr Magnesium Alloys
Yaohong LIU,Zhaohui WANG(),Ke LIU,Shubo LI,Wenbo DU
College of Materials Science and Engineering, Beijing University of Technology, Beijing 100124, China
Cite this article:
Yaohong LIU,Zhaohui WANG,Ke LIU,Shubo LI,Wenbo DU. Effects of Er on Hot Cracking Susceptibility of Mg-5Zn-xEr Magnesium Alloys. Acta Metall Sin, 2019, 55(3): 389-398.
Mg-Zn-Er casting magnesium alloys have good properties, such as high specific strength, high specific stiffness and remarkable temperature creep properties. Current researches mainly focused on the phases and mechanical properties at room and high temperatures. However, the effect of Er on hot cracking susceptibility of Mg-5Zn-xEr magnesium alloys was barely studied. In this work, a modified RDG (Rappaz-Drezet-Gremaud) model for predicting the hot cracking susceptibility of Mg-5Zn-xEr (x=0.83, 1.25, 2.5, 5, mass fraction, %) ternary alloys was proposed, which considered the effects of phase and solidification temperature range on the hot cracking susceptibility of the multiphase alloys. And, the hot cracking susceptibility was evaluated by the experiment of constrained rod casting (CRC). The results indicated that the modified RDG model could accurately predict the hot cracking susceptibility of Mg-5Zn-xEr magnesium alloys. The hot cracking susceptibility increased with the addition of Er up to 2.5%, and Mg-5Zn-2.5Er alloy showed the maximal hot cracking susceptibility; when the addition of Er increased to 5.0%, Mg-5Zn-5Er alloy exhibited the minimal hot cracking susceptibility. The calculated results were consistent with the experimental ones. Further analysis on the casting solidification curves, phases and microstructures showed that I-phase precipitated by peritectic reaction during solidification of Mg-5Zn-2.5Er alloy depleted liquid phases and extended the solidification temperature range of the alloy, leading to the hot cracking susceptibility increasing. The Mg-5Zn-5Er alloy underwent eutectic reaction of L→α-Mg+W during solidification, which reduced the solidification temperature range. Meanwhile, this process was beneficial to feeding the interdendritic hot cracking in the terminal period of solidification, which significantly decreased the hot cracking susceptibility of Mg-5Zn-5Er alloy.
Table 1 Compositions of Mg-5Zn-xEr alloys (mass fraction / %)
Fig.1 Schematics of hot cracking susceptibility factors including rod length factor (a), crack location factor (b) and crack width factor (c)
Fig.2 Schematic of hot cracking susceptibility experimental device (unit: mm)
Fig.3 Solid phase volume fraction (fs)- solidification temperature (T) curves of Mg-5Zn-xEr alloys (x=0.83, 1.25, 2.5, 5)
Alloy
Fitting function expression
R2
Mg-5Zn-0.83Er
Mg-5Zn-1.25Er
Mg-5Zn-2.5Er
Mg-5Zn-5Er
fs(T)=0.98886-3.854×10-12exp (0.042T)
fs(T)=0.971-3.09×10-9exp (0.031T)
fs(T)=0.9779-5.687×10-16exp (0.056T)
fs(T)=1.00089-9.178×10-10exp (0.033T)
0.987
0.969
0.991
0.978
Table 2 fs-T fitting function expression of Mg-5Zn-xEr alloys and correlation coefficients (R2) (x=0.83, 1.25, 2.5, 5)
Fig.4 Hot cracking susceptibilities predicted by optimized RDG model in this work () (a) and optimized RDG model of Easton (Sht) (b) of Mg-5Zn-xEr alloys (x=0.83, 1.25, 2.5, 5. fs,n―solid phase volume fraction corresponding to the coherency (fs,0) and coalescence (fs,co) points)
Fig.5 Hot cracking image of Mg-5Zn-0.83Er alloy (Arrows in Fig.5a show the macro-cracks) (a) and hot cracking susceptibilities of Mg-5Zn-xEr alloys (x=0.83, 1.25, 2.5, 5) (b) (Experiment via critical size method)
Fig.6 XRD spectra of Mg-5Zn-xEr alloys (x=0.83, 1.25, 2.5, 5)
Fig.7 SEM images (a, b, d, e) and EDS spectra of points A (c) and B (f) of Mg-5Zn-xEr alloys with x=0.83 (a, c), x=1.25 (b), x=2.5 (d) and x=5 (e, f)
Fig.8 Solidification curves of Mg-5Zn-xEr alloys (Experiment via critical size method)(a) x=0.83 (b) x=1.25 (c) x=2.5 (d) x=5
Alloy
α-Mg
W-phase
I-phase
ΔT
Mg-5Zn-0.83Er
616.4
-
438.7
177.7
Mg-5Zn-1.25Er
613.7
566.6
425.1
188.6
Mg-5Zn-2.5Er
615.3
544.9
419.8
195.5
Mg-5Zn-5Er
593.4
532.1
-
61.3
Table 3 Precipitation temperatures of various phases and solidification temperature range (ΔT) for Mg-5Zn-xEr alloys (x=0.83, 1.25, 2.5, 5. Experiment via critical size method) (℃)
Fig.9 Hot cracking fracture morphologies of Mg-5Zn-xEr alloys (longitudinal section)(a) x=0.83 (b) x=1.25 (c) x=2.5 (d) x=5
Fig.10 Hot cracking fracture morphologies of Mg-5Zn-xEr alloys (top view)(a) x=0.83 (b) x=1.25 (c) x=2.5 (d) x=5
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