Mg-Al-Ca base alloys have great potential for application because of its low cost and good high temperature creep properties, but its higher hot cracking susceptibility greatly limits the application of the alloy. The effect of Ca addition on the hot cracking susceptibility of Mg-5Al-xCa (x=0.5, 1.0, 2.0, 3.0, 4.0, 5.0, mass fraction, %) alloys at the pouring temperature 700 ℃ and mold temperature 200 ℃ was studied by using hot cracking curve test, solidification curve test, OM, XRD and SEM. The results showed that the hot cracking susceptibility of alloys decreased with increasing Ca addition until to 4.0%, and the Mg-5Al-4.0Ca alloy had minimal hot cracking susceptibility, which cracking susceptibility coefficient was only 0.824. But when Ca addition increased to 5.0%, the hot cracking susceptibility of the alloy increased, and the cracking susceptibility coefficient increased to 0.96. The appropriate Ca addition can reduce the precipitation temperature of α-Mg in Mg-5Al-xCa alloys, inhibit precipitation of Mg17Al12 phase, narrow solidification temperature range of the alloy and increase eutectic content, which are helpful for the alloy having a stronger ability of compensation at the solidification end to decrease hot cracking susceptibility. But excessive Ca addition will increase the numbers of brittle phases containing Ca and coarsen the microstructure, resulting in the increase of hot cracking susceptibility.
Fund: Supported by National Natural Science Foundation of China (Nos.51504153 and 51571145) and Natural Science Foundation of Liaoning Province (No.201602548)
Table 1 Chemical compositions of Mg-5Al-xCa alloys (mass fraction / %)
Fig.1 Schematic of hot cracking system (a) and hot cracking specimen (b)
Fig.2 Photographs of hot cracking specimens for Mg-5Al-xCa alloys (a) x=0.5 (b) x=1.0 (c) x=2.0 (d) x=3.0 (e) x=4.0 (f) x=5.0
Fig.3 Hot cracking curves of Mg-5Al-xCa alloys (T—temperature) (a) x=0.5 (b) x=1.0 (c) x=2.0 (d) x=3.0 (e) x=4.0 (f) x=5.0
Alloy
Hot crack initiation
Hot crack propagation
θi / ℃
fs-i / %
Fr / N
tp / s
vP / (Ns-1)
Mg-5Al-0.5Ca
467
98.2
0.614
0.72
0.853
Mg-5Al-1.0Ca
451
97.4
0.536
1.26
0.425
Mg-5Al-2.0Ca
430
97.1
0.371
0.91
0.408
Mg-5Al-3.0Ca
434
97.6
0.215
1.17
0.184
Mg-5Al-4.0Ca
-
-
-
-
-
Mg-5Al-5.0Ca
441
98.5
2.079
2.67
0.779
Table 2 Results of hot cracking test for Mg-5Al-xCa alloys
Fig.4 Solidification curves of Mg-5Al-xCa alloys (ΔT—solidification temperature range) (a) x=0.5 (b) x=1.0 (c) x=2.0 (d) x=3.0 (e) x =4.0 (f) x=5.0
Alloy
Peak A
Peak B
Peak C
Peak D
Peak E
α-Mg
L→α+C14
L→α+C36
L+C36 → α+A12
L→α+C14+C36
Mg-5Al-0.5Ca
621
525
513
424
-
Mg-5Al-1.0Ca
619
525
519
-
-
Mg-5Al-2.0Ca
614
528
521
-
-
Mg-5Al-3.0Ca
610
-
528
-
-
Mg-5Al-4.0Ca
598
-
528
-
511
Mg-5Al-5.0Ca
598
-
528
-
512
Table 3 Key point reactions and reaction temperatures corresponding to Mg-5Al-xCa alloys solidification curves in Fig.4 (℃)
Fig.5 Solidification temperature range and hot cracking susceptibility coefficient for Mg-5Al-xCa alloys
Fig.6 Microstructures of initial hot crack (a~c) and crack extension end (d~f) of Mg-5Al-xCa alloys (a, d) x=3.0 (b, e) x=4.0 (c, f) x=5.0
Fig.7 SEM images of Mg-5Al-xCa alloys (a) x=0.5 (b) x=1.0 (c) x=2.0 (d) x=3.0 (e) x=4.0 (f) x=5.0
Fig.8 XRD spectra of Mg-5Al-xCa alloys
Fig.9 SEM images of the crack extension ends (a) and local magnification (b) for Mg-5Al-4.0Ca alloy (C36: hexagonal (Mg, Al)2Ca Laves phase, C14: hexagonal Mg2Ca Laves phase)
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