Mechanism of Grain Refinement of Pulse Current Assisted Plasma Arc Welded Al-Mg Alloy
YUAN Tao, ZHAO Xiaohu, JIANG Xiaoqing(), REN Xuelei, LI Boyang
Engineering Research Center of Advanced Manufacturing Technology for Automotive Components, Ministry of Education, Beijing University of Technology, Beijing 100124, China
Cite this article:
YUAN Tao, ZHAO Xiaohu, JIANG Xiaoqing, REN Xuelei, LI Boyang. Mechanism of Grain Refinement of Pulse Current Assisted Plasma Arc Welded Al-Mg Alloy. Acta Metall Sin, 2024, 60(3): 323-332.
During welding, the vibration effect of applying a pulse current on the molten pool can effectively improve weld formation and refine grains. The effect of pulse current on grain refinement and its mechanism were studied for Al-Mg alloy welds fabricated by conventional plasma welding (PAW), PAW with conventional pulse current, and PAW with composite pulse current. The grain size produced by conventional PAW was 78.2 μm, whereas the average grain size was reduced from 78.2 μm to 53.3 μm with increasing conventional pulse current frequency from 0 Hz to 100 Hz; in addition, the degree of grain refinement increased by about 30%. However, the minimum grain size was 48.2 μm, and the grain refinement effect can reach nearly 40% by combining low-frequency pulse current with conventional pulse current. The proportion of small grains and high-angle grain boundaries increased significantly after applying the composite pulse current. The additional oscillation effect of the composite pulse current can effectively eliminate coarse grains during the solidification of the weld pool. The main mechanism of grain refinement is dendrite fragmentation, which is discussed through thermodynamics and composition.
Fund: National Natural Science Foundation of China(51704013);Beijing Municipal Education Commission Fund(KM201810005016);Technology Fund of Beijing University of Technology(ykj-2018-00325)
Fig.1 Welding equipment (a), current waveforms of conventional pulse (b) and composite pulse (c), current acquisition waveforms of conventional pulse (d) and composite pulse (e), and sampling positions (f) (PAW—plasm arc welding)
Sample
No.
Base
current / A
Peak
current / A
First pulse frequency / Hz
First peak time percent / %
Plasma gas
flux / (L·min-1)
Shielding gas
flux / (L·min-1)
C0
180
- -
10
10
S1
90
210
20
75
10
10
S2
90
210
40
75
10
10
S3
90
210
60
75
10
10
S4
90
210
80
75
10
10
S5
90
210
100
75
10
10
Table 1 Experimental design of the conventional plasma pulse current
Sample
No.
Base
current / A
Peak
current / A
First pulse frequency / Hz
Second pulse frequency / Hz
First peak time percent / %
Second peak time percent / %
C0
180
-
-
-
-
D1
90
210
50
2
75
50
D2
90
210
50
4
75
50
D3
90
210
50
6
75
50
D4
90
210
50
8
75
50
D5
90
210
50
10
75
50
Table 2 Experimental design of the plasma composite pulse current
Fig.2 Polarization micrographs of upper surface of conventional pulse current PAW (a) C0 (b) S1 (c) S2 (d) S3 (e) S4 (f) S5
Fig.3 Change of grain size under conventional pulse current PAW
Fig.4 Polarization micrographs of cross-section of conventional pulse current PAW (a) C0 (b) S1 (c) S2 (d) S3 (e) S4 (f) S5
Fig.5 Polarization micrographs of upper surface under composite pulse current PAW (a) C0 (b) D1 (c) D2 (d) D3 (e) D4 (f) D5
Fig.6 Change of grain size under composite pulse current PAW
Fig.7 Grain orientations (a1-c1), grain size distributions (a2-c2), and grain boundary angle distributions (a3-c3) of C0 (a1-a3), S4 (b1-b3), and D4 (c1-c3) samples
Fig.8 Influence of different types of pulse current on grain size and the proportion of large-angle grain boundaries (LAGBs)
Fig.9 Pole figures of C0 (a), S4 (b), and D4 (c) samples (TD—transverse direction, RD—rolling direction)
Fig.10 Influence of temperature gradient and growth rate on the morphology of solidification structure
Fig.11 EDS analysis results of C0 (a), S4 (b), and D4 (c) samples
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