1. Light Alloys Research Institute, Central South University, Changsha 410083, China 2. Aerospace Engineering Equipment Suzhou Co., Ltd., Suzhou 215100, China 3. School of Materials Science and Engineering, Central South University, Changsha 410083, China
Cite this article:
Hua JI,Yunlai DENG,Hongyong XU,Weiqiang GUO,Jianfeng DENG,Shitong FAN. The Influence of Welding Line Energy on the Microstructure and Property of CMT Overlap Joint of 5182-Oand HC260YD+Z. Acta Metall Sin, 2019, 55(3): 376-388.
In recent years, the welding of dissimilar metals such as steels and aluminum alloys has attracted much more attentions due to weight reduction, especially in automobile and railway vehicle manufacturing industry. However, many challenges and problems need to be addressed in order to obtain high quality welding joints between steels and aluminum alloys resulting from their differences of thermal-physical properties. The formation of intermetallic compounds (IMCs) in the course of welding will lower the mechanical properties of the joints. Up to now, a few techniques have been tried to weld aluminum alloys and steels, including solid welding and fusion welding. In this work, dissimilar metals of 5182-O and HC260YD+Z were welded by cold metal transfer (CMT) arc-brazing using AlSi5 as filler metal. The macro and micro morphologies of the overlap joint were investigated using OM, XRD, SEM and EDS analyses. The hardness and shear strength of the joints were tested. Results show that welding line energy can affect the thickness of IMCs existing on the brazing interface and thus depress the combination properties because of the different fracture modes. When the welding speed and wire feed speed are 9 mm/s and 5 m/min respectively, the IMCs thickness is about 6 μm, and the shear strength of the jonts can reach to 160 MPa. Two typical fracture modes of fusion interface fracture and brazing interface fracture were observed. The fracture mode of the position near arc striking is "fusion interface". With the increasing of welding energy, the thickness of IMCs is increased and the fracture mode near arc extinguishing is changed from "fusion interface" to "brazing interface". When the output power of CMT equipment is 150~210 J/mm at welding beam length, the IMCs thickness is less than 9 μm, which benefits the shear strength performance of the joints, and the fracture mode of "fusion interface" can be easily obtained.
Fund: National Key Research and Development Program of China(2016YFB0300901);National Natural Science Foundation of China(51375503);Special Funds for BaGui-Scholar of Guangxi Province(2013A017)
Table 1 Specimen groups of overlap joint of 5182-O and HC260YD+Z
Fig.1 Schematic of welding process
Fig.2 Typical zones of an Al alloy/steel CMT lap joint (S3) (CMT—cold metal transfer)
Fig.3 XRD spectra of different regions of an Al alloy/steel CMT lap joint (S3)
Fig.4 Microstructures and EDS of the brazing interface under different wire feed speeds and welding speeds(a) S1 (b) S3 (c) S5 (d) S6 (e) S9 (f) EDS along line D
Point
Atomic fraction / %
Possible phase
Al
Fe
Mg
Si
Others (C, O et al)
A
24.08
52.78
-
0.99
22.15
γ-Fe+FeAl
B
37.45
22.10
-
1.71
38.74
FeAl2
C
53.99
15.52
0.17
1.88
28.44
FeAl3
E
34.94
31.98
-
2.14
30.94
FeAl
Table 2 EDS of the points in Fig.4
Fig.5 Schematics of the brazing interface formation showing liquid molten pool (a), primary γ-Fe phase, γ-Fe+FeAl mixed phase precipitation (b), fine columnar crystal FeAl precipitation (c), planar FeAl2 precipitation (d) and atypical columnar crystal FeAl3 precipitation (e)
Fig.6 Microstructures of the fusion interface zones under different wire feed speeds and welding speeds(a) S1 (b) S3 (c) partial magnification of the area in Fig.6b (d) S5 (e) S6 (f) S9
Fig.7 Microstructures of the fusion zones under different wire feed speeds and welding speeds(a) S1 (b) S3 (c, d) S5 (e) S6 (f) S9
Fig.8 EDS mapping analyses of Fig.7c (a) and Fig.7d (b)
Point
Atomic fraction / %
Possible phase
Al
Fe
Mg
Si
Others (C, O et al)
A
96.15
0.35
0.20
1.24
2.06
α-Al
B
73.97
3.42
0.20
17.40
5.01
(α-Al+Si)
C
62.72
18.50
0.18
2.53
16.07
FeAl3
D
62.66
19.65
0.11
2.84
14.74
FeAl3
Table 3 EDS of different points in Fig.7
Fig.9 Schematics of welding area metallurgical process showing liquid molten pool (a), primary γ-Fe phase and γ-Fe+FeAl mixed phase precipitation (b), fine columnar crystal FeAl precipitation (c), planar FeAl2 precipitation (d) and atypical columnar crystal FeAl3 precipitation (e)
Fig.10 Microstructures and EDS mapping of the Zn-rich zones(a) weld toe of S5 (b) partial magnification of area in Fig.10a (c) EDS mapping of Fig.10b (d) weld root of S3
Point
Atomic fraction / %
Possible phase
Al
Fe
Zn
Si
Others (C, O et al)
A
71.16
0.13
12.41
8.39
7.91
α-Al
B
81.95
0.18
14.79
0.58
2.50
(α-Al+Zn)
Table 4 EDS analyses of different points in Fig.10
Fig.11 Microhardness of heat affected zones (HAZs) of Al alloy (a) and steel (b) of joint S3 (Insets shows the microstructures in the horizontal direction of aluminum alloy (a) and vertical direction of steel (b))
Fig.12 Effect of wire feed speed and welding speed on welding line energy factor (a) and shear strength (b, c) of the joints (N—output work per unit welding length which produced, R—wire feed speed, V—welding speed)
Fig.13 Typical fracture morphologies of samples S3 (a~c) and S5 (d~f)(a) "fusion interface" fracture (b) SEM image of middle area (c) partial magnification of area in Fig.13b (d) "brazing interface" fracture (e, f) partial magnifications of area in Fig.13d
Specimen
Near arc striking
Near arc extinguishing
S1
A
A
S2
A
A
S3
A
A
S4
A
B
S5
A
B
S6
A
B
S7
A
B
S8
A
A
S9
A
A
Table 5 Fracture types of different joints
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