Plasma-MIG Hybrid Welding Hot Cracking Susceptibility of 7075 Aluminum Alloy Based on Optimum of Weld Penetration
Yingkai SHAO, Yuxi WANG, Zhibin YANG(), Chunyuan SHI
School of Materials Science and Engineering, Dalian Jiaotong University, Dalian 116028, China
Cite this article:
Yingkai SHAO, Yuxi WANG, Zhibin YANG, Chunyuan SHI. Plasma-MIG Hybrid Welding Hot Cracking Susceptibility of 7075 Aluminum Alloy Based on Optimum of Weld Penetration. Acta Metall Sin, 2018, 54(4): 547-556.
The 7075 aluminum alloys have major applications in commercial, transportation industry and military air carriers, owing to their associated light weight, high strength, good machinability, high fracture toughness and low fatigue crack growth. Several welding techniques, such as metal inert gas (MIG) welding, tungsten inert gas (TIG) welding, laser welding and friction stir welding (FSW), have been applied to weld the 7075 aluminum alloys. However, their applications are limited because of the lower weld strength, slower welding speed and other significant limitations of them. Among the different welding techniques, plasma-MIG hybrid welding is a new fabrication technique with many advantages such as stable welding process, no weld spatter, the decreased pores, small grain size and high joint quality. Up to now, the study mainly focuses on coaxial plasma-MIG hybrid welding, and it is rare in dealing with the hot cracking susceptibility of 7000 series aluminum alloys welded by paraxial plasma-MIG hybrid welding. In this work, the paraxial plasma-MIG hybrid welding system was used to weld 7075-T6 aluminum alloy plates. The quantitative relationship between plasma-MIG hybrid welding parameters of 7075 aluminum alloy and weld penetration was established by linear regression orthogonal test. Hot ductility tests were studied by using the thermal simulated test to determine the brittleness temperature range of the alloy. Welding hot cracking susceptibility tests were conducted by using the fish bone method, and the type and cause of the hot cracking were analyzed by SEM, EDS and OM. The results indicated that the brittleness temperature range of 7075 aluminum alloy was 470~620 ℃. When the heat inputs of plasma-MIG hybrid welding were 2.52, 2.95 and 3.42 kJ/cm respectively, the welding hot cracking susceptibility decreased and then increased with the heat input increasing. The type of cracking in partially melted zone of base metal was liquation cracking, and that of weld zone was solidification cracking. When the heat input was 2.95 kJ/cm, the welding hot cracking sensitivity was the least, and the welding cracking was solidification cracking. Compared to MIG welding joints, the hot cracking susceptibility of plasma-MIG hybrid welding joints decreased by 47% under the same conditions.
Table 6 Results of welding hot cracking susceptibility tests
Fig.10 Microstructures of WZ (a, c, e) and PMZ (b, d, f) of hot cracking at the heat inputs of 2.52 kJ/cm (a, b), 2.95 kJ/cm (c, d) and 3.42 kJ/cm (e, f) (WZ—weld zone, FZ—fusion zone, PMZ—partially melted zone)
Fig.11 Microstructures of fracture surface of samples at the heat input of 2.52 kJ/cm (a), 2.95 kJ/cm (b) and 3.42 kJ/cm (c)
Fig.12 SEM image (a) and EDS element maps of Al (b), Mg (c), Zn (d) of the area around the crack in PMZ of base metal at 2.52 kJ/cm
Position
Al
Cu
Mg
Zn
1
93.22
1.73
2.51
2.54
2
93.05
2.30
2.25
2.40
3
92.25
2.11
2.89
2.75
Mean
92.84
2.05
2.55
2.56
Table 7 EDS analyses of grain boundary of PMZ in Fig.12a (atomic fraction / %)
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