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Acta Metall Sin  2018, Vol. 54 Issue (4): 547-556    DOI: 10.11900/0412.1961.2017.00357
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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
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Abstract  

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.

Key words:  7075 aluminum alloy      plasma-MIG hybrid welding      weld penetration      welding hot cracking susceptibility     
Received:  29 August 2017     
ZTFLH:  TG406  
Fund: Supported by General Project of Education Department of Liaoning Province (No.JDL2016009)

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.

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https://www.ams.org.cn/EN/10.11900/0412.1961.2017.00357     OR     https://www.ams.org.cn/EN/Y2018/V54/I4/547

Material Si Fe Cu Mg Zn Ti Pb Cr Mn Al
7075-T6 0.09 0.29 1.45 2.23 5.34 0.05 0.03 - - Bal.
ER5183 0.3 0.1 0.1 4.5 - 0.1 - 0.1 0.8 Bal.
Table 1  Chemical compositions of 7075-T6 aluminum alloy and ER5183 (mass fraction / %)
Material Yield strength / MPa Tensile strength / MPa Elongation / %
7075-T6 ≥435 ≥505 ≥8
ER5183 ≥125 ≥275 ≥17
Table 2  Mechanical properties of 7075-T6 aluminum alloy and ER5183
Fig.1  Schematic of samples for hot ductility tests (unit: mm)
Fig.2  Schematic of samples for fish-bone welding hot cracking tests (unit: mm)
Level zj IP (x1) IMIG (x2) v (x3) Q (x4)
Lower -1 80 120 90 5
Zero 0 100 150 100 6
Upper 1 120 180 110 7
Range Δj 20 30 10 1
Table 3  Levels of nature factors and its codings
Fig.3  Partial weld formation of orthogonal test schemes

(a) No.1 (b) No.2 (c) No.3

No. z1 z2 z3 z4 IP (x1) IMIG (x2) v (x3) Q (x4) y Forming condition
1 1 1 1 1 120 180 110 7 4.00 Over penetration
2 1 1 -1 -1 120 180 90 5 4.00 Well-formed
3 1 -1 1 -1 120 120 110 5 2.46 Lack of penetration
4 l -1 -1 1 120 120 90 7 4.00 Over penetration
5 -1 1 1 -1 80 180 110 5 3.04 Lack of penetration
6 -1 1 -1 1 80 180 90 7 4.00 Well-formed
7 -1 -1 l 1 80 120 110 7 1.68 Lack of penetration
8 -1 -1 -1 -1 80 120 90 5 2.14 Lack of penetration
9 0 0 0 0 100 150 100 6 3.30 Lack of penetration
10 0 0 0 0 100 150 100 6 3.51 Lack of penetration
11 0 0 0 0 100 150 100 6 3.06 Lack of penetration
Table 4  Orthogonal test scheme and its results
Source of variance SS Df MS F Significance
z1 1.6200 1 1.6200 15.55 * *
z2 2.8322 1 2.8322 27.18 * *
z3 1.0952 1 1.0952 10.51 *
z4 0.5202 1 0.5202 4.99 *
Regression 6.0676 4 1.5169 14.56 * *
Residual 0.6253 6 0.1042
Sum 6.6929 n-1=10
Table 5  Analyses of significance of the linear multivariate regress equation
Fig.4  Weld formation at optimized parameters
Fig.5  Rm-T curve during heating process (Rm—tensile strength, T—temperature)
Fig.6  SEM image of fracture surface of sample at zero strength temperature
Fig.7  Z-T curve during cooling process (Z—reduction of area)
Fig.8  SEM image of fracture surface of sample at zero ductility temperature
Fig.9  Macrostructures of samples after hot cracking susceptibility tests

(a) 2.52 kJ/cm (b) 2.95 kJ/cm (c) 3.42 kJ/cm (d) MIG-2.95 kJ/cm

Welding process Heat input / (kJcm-1) Cracking sensitivity / % Mean cracking sensitivity / %
Plasma
-MIG







2.52 72.92 86.39
92.50
93.75
2.95 39.58 45.14
50.00
45.83
3.42 84.58 85.14
86.25
84.58
MIG 2.95 88.56 88.86
90.15
87.86
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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