Al Dissimilar Metals Made by Cold Metal Transfer Welding with ER4043 Filler Metal

Al Dissimilar Metals Made by Cold Metal Transfer Welding with ER4043 Filler Metal

Rare Metal Materials and Engineering Volume 42, Issue 7, July 2013 Online English edition of the Chinese language journal Cite this article as: Rare M...

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Rare Metal Materials and Engineering Volume 42, Issue 7, July 2013 Online English edition of the Chinese language journal Cite this article as: Rare Metal Materials and Engineering, 2013, 42(7): 1337-1341.

ARTICLE

Microstructure Characteristics and Properties of Mg/Al Dissimilar Metals Made by Cold Metal Transfer Welding with ER4043 Filler Metal Shang Jing,

Wang Kehong,

Zhou Qi,

Zhang Deku,

Huang Jun,

Li Guangle

Nanjing University of Science and Technology, Nanjing 210094, China

Abstract: AZ31B magnesium alloy and 6061 aluminum alloy were joined in stable welding process by cold metal transfer welding with ER4043 as filler metal. The microstructure, morphology and phase composition of the welded joints were studied by OM, SEM, EDX and XRD. The results show that weld metal and aluminum substrate are combined with a good interface, while weld metal and Mg substrate are combined with a epitaxial solidification area where the intermetallic compounds of Mg2Al3, Mg17Al12 and Mg2Si are generated. The micro-hardness distribution shows a decreasing trend from Mg side to Al side in the weld. The joint is brittle fractured in the fusion zone of Mg side, where plenty of Mg2Si, Mg2Al3 and Mg17Al12 are distributed continuously. Key words: microstructure characteristics; cold metal transfer welding; intermetallic compounds; Mg/Al dissimilar metals

Mg alloys are the best and lightest metal materials which are used in high technology fields such as automotive, electron and aerospace industries[1]. However, the welding technology between Mg alloys and other metals especially Al alloys has been an important research field[2]. For example the Mg-Al laminated armor is used in tanks and armored vehicles, Mg-Al complex structure is used in aerospace engines and components, and Mg-Al connected pipe is used in bicycle manufacture. So the reliable connection of Mg/Al dissimilar metals can exert different performances of them, which will be widely used in military, aerospace, automotive and other fields[3,4]. At present, the joint of Mg/Al dissimilar metals with good performance is difficult to get. Intermetallic compounds which seriously impact the performance of the joint are inevitably generated in the joint during the welding process such as TIG welding, MIG welding, and diffusion bonding[5-7]. The generation of intermetallic compounds is effectively reduced by cold metal transfer (CMT) welding with the characteristics of low heat input[8] and Al/Steel dissimilar metals joint with good performance has been obtained by CMT process[9]. In this paper, CMT

welding was used to weld Mg alloy and Al alloy with ER4043 filler metal, and the microstructures and mechanical properties of the joints were studied. The study provided a theoretical reference and practical experience for Mg/Al dissimilar metals connection.

1

Experiment

Samples (100 mm×50 mm×3 mm) of extruded AZ31B Mg alloy and 6061 Al alloy were welded in this experiment. The ER4043 (Φ1.2 mm) filler metal was chosen. The chemical compositions of these two metals and filler metal are shown in Table 1. Some of the physical properties of base metals are shown in Table 2. Before welding, the oxide films on the substrates surface were removed by stainless steel wire brush, and acetone were wiped to remove the oil. The dimension of the welding plate and the shape of the weld bead are shown in Fig.1. The welding equipment was CMT welding machine of Fronius-5000 type. Wire feed speed and metal transfer process was digitally coordinated by CMT welding technology. When a short-circuit signal monitored by the DSP processor of welding machine fed back to the wire

Received date: July 21, 2012 Foundation item: National Natural Science Foundation of China (51075214) Corresponding author: Wang Kehong, Ph. D., Professor, School of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, P. R. China, Tel: 0086-25-84315776, E-mail: [email protected] Copyright © 2013 Northwest Institute for Nonferrous Metal Research. Published by Elsevier BV. All rights reserved.

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Table 1

Chemical composition of base metals and filler metal (ω/%)

Materials

Mg

Al

Zn

Si

Fe

Cu

Mn

Ca

Ti

Cr

Other

AZ31B 6061 ER4043

Bal. 0.8~1.2 ≤0.05

2.5~3.5 Bal. Bal.

0.6~1.4 0.25 ≤0.1

0.1 0.4~0.8 4.5~6.0

0.005 0.7 ≤0.8

0.05 0.15~0.4 ≤0.3

0.2 0.15 ≤0.05

0.04 — —

— 0.15 ≤0.2

— 0.04~0.35 —

0.3 0.15 ≤0.05

Table 2 Materials AZ31B 6061-O

Tensile strength/MPa 263.3 124.2

Physical and mechanical properties of the base metals

Density/g·cm-3 1.77 2.70

Solidus/ ℃ 605 582

Liquidus/ ℃ 630 652

Thermal conductivity (25 ℃)/W·(m·K)-1 96.3 180

Front

100

100

6061

6061

AZ31 Welding direction Weld seam

50º

Back

6061

AZ31B 3 Fig.1

AZ31

Diagram of the form of welded joints (mm)

feeder to withdraw wire, wire and droplet would be separated to make the droplet transfer in the absence of current state. So the metal transfer process was a high frequency of "hot-cold-hot" alternating process, and the heat input was significantly reduced. The main welding parameters were as follows: current of 77 A, voltage of 12.3 V, wire feed speed of 4.3 mm/s and welding speed of 0.5 m/min. Metallographic sample was prepared in bakelite for microstructure examination. Mg alloy side of the joint was etched in a solution of 1 mL oxalic acid, 1 mL nitric acid, 1 mL acetic acid and 150 mL distilled water. Al alloy side of the joint was etched in a solution of 2 mL hydrofluoric acid, 5 mL nitric acid and 95 mL distilled water. Microstructure, fracture morphology and phase composition of joints were observed and analyzed respectively by OLYMPUS (GX41) optical microscope, S-3400N II scanning electron microscope (SEM), EX-250 energy dispersive spectroscopy (EDX) and D8 ADVANCE X-ray diffraction (XRD). The micro-hardness distribution was measured by the HVS-1000 Digital Display Micro-hardness Tester with a load of 1 N for 10 s. SANS CMT5105 universal testing machine was used for tensile test with loading rate of 1 mm/min.

2 2.1

Results and Discussion Microstructure of the joint

Fig.2 shows the macro-photograph of the weld in front and back side. It can be seen that the weld seam is in water-wave shape with a good formation. Base metal of Mg side is melted more than Al side because of its lower

Heat capacity (20 ℃)/J·(kg·K)-1 1130 896

Fig.2

10 mm

Macrophotograph of welding seam a

Fusion line 6061

Black particles

b Columnar crystals Weld zone

Weld zone AZ31B Fusion line

100 µm Fig.3

20 µm

Microstructure of welding seam: (a) Al side and (b) Mg side

thermal conductivity. Optical micrograph in Al side of the joint is shown in Fig.3a. Al substrate and the weld is combined well and the fusion line can clearly be seen. Small black particles are chaotically distributed in the weld. Fig.3b shows the optical micrograph in Mg side of the joint. Crystalline area of epitaxial solidification between the weld metal and the Mg substrate is generated near the fusion zone of Mg side, where the grains are grown perpendicular to the weld and the uniform columnar crystals are formed. Owing to the super low heat input of the CMT welding process, rapid heating and rapid cooling can be realized. There is no sufficient time for Mg and Al to mix; thereby, the thickness of intermetallic compounds layer is thin, and the structure of heat affected zone is not observed in the both sides of the weld. SEM micrograph in the fusion zone of Mg side is shown in Fig.4a. It can be seen that the columnar grain structure is grown along the weld center. EDX analysis results which

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AZ31

B

2 µm

1.2

2.0

b

Mg

Intensity/×103 cps

Columnar crystals A

1.5 Intensity/×103 cps

a

0.9 0.6 0.3 0.0

Al Si

2

6

10

14

1.6 1.2 0.8

Al

0.4 0.0

18

1

Energy/keV

5

7

9

11

SEM micrograph in the fusion zone of Mg side (a) and (b) EDX analysis results for area A (b) and area B (c) in the Fig.4a

are obtained from different locations shown in Fig.4a are illustrated in Fig.4b and Fig.4c. Combined with the graph theory, it can be known that area A consists of 34.12 at% Al, 65.01 at% Mg and 0.87 at% Si, which indicates that this location should be composed of α-Mg solid solution, γ(Mg17Al12) and dispersed Mg2Si. Area B consists of 35.59 at% Al and 64.41 at% Mg, which suggests that this location mainly consists of α-Mg solid solution and γ(Mg17Al12) eutectic structure. Line scan result of the joint is shown in Fig.5. Some air holes can be seen in the joint. The reason is that the melt heat loss is serious; therefore, it is subjected to rapid solidification, and in some cases air holes are formed[10]. The content of Al decreases from Al side to Mg side, while Mg increases gradually. The reason is that Mg element is melted and diffuses into the weld during the welding process. In addition, the diffusion coefficient of Si element in the filler metal is large compared to the Mg and Al element, and its affinity with Mg is stronger than Al, so the content of Si increases to generate Mg2Si with Mg element in the fusion zone of Mg side[11].

2.2 Mechanical properties of the joints 2.2.1 Micro-hardness analysis The micro-hardness test points are taken every 0.2 mm on the weld and every 0.4 mm on both sides of the substrates. The distribution of micro-hardness is shown in Fig.6. Two dotted lines represent the fusion lines of Al side and Mg side. The micro-hardnesses in both sides of the substrates are distributed uniformly, about 540 MPa in Mg side and 350 MPa in Al side. The micro-hardness in the fusion zone of Mg side is increased sharply and the maximal value is 2380 MPa. The micro-hardness in the weld from Mg side to Al side shows a decreasing trend. The reason is that Mg and Al have infinitely mutual solubility in the melting state, while in the solidification process eutectic reaction occurs to precipitate β(Mg2Al3) and γ(Mg17Al12) whose property is brittle and hard[12].

2.2.2

3

Energy/keV

Air holes Weld zone Mg Al Scan line 6061

AZ31B

Si

500 µm

Fig.5

Micro-hardness, HV/MPa

Fig.4

c

Mg

Line scan results of the joint

2500 2000

Weid zone

AZ31

6061

1500 1000 500 0 –2

–1

0

1

2

Distance from the Center of Weld Joint/mm Fig.6

Distribution of microhardness of the joint

50 µm

Tensile strength and fracture analysis

Tensile strength test shows that bonding strength is less than 20 MPa and fracture occurs in brittle intermetallic

Fig.7

Fracture morphology of welded joint

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60.70 at% Mg, 14.40 at% Al, and 20.90 at% Si. A certain amount of Si element appears because of the diffusion and recrystallization between Si and Mg element in the welding process, which suggests that this location mainly consists of Mg 2Si and γ(Mg17 Al 12). Fig.8d shows the EDX result of point 3 in Fig.8a consisting of 61.11 at% Mg, 12.12 at% Al, and 26.76 at% Si, whose content is similar to that of point 2. This result verifies that cavities with edges and corners are formed by the pull-out particle phases. The phase composition of the fracture analyzed by XRD is shown in Fig.9. It can be seen that Mg, Mg17Al12, Mg2Al3 and Mg2Si exist on the fracture surface. So the EDX analysis results of Fig.4, Fig.5 and Fig.8 correspond with XRD result. It can be confirmed that a large amount of intermetallic compounds distributed continuously in the fusion zone are responsible for the fracture.

compound layer of the fusion zone adjacent to Mg substrate. There is no phenomenon of necking and plastic deformation in the fracture area. Fracture morphology is shown in Fig.7. A large number of cleavage steps and a small number of dimples are distributed on the fracture. Some particle phases with edges and corners are embedded in the fracture surface. Meanwhile, some cavities with edges and corners left by the pull-out intermetallic particles can be observed on the fracture surface. It can be confirmed that the fracture is in brittle fracture mode. EDX analysis results from the fracture are shown in Fig.8. Particle phase (point 3) and cavity with edges and corners (point 2) are clearly seen in Fig.8a. Fig.8b shows the EDX result for rectangle zone 1 in Fig.8a, which contains 55.04 at% Mg and 44.96 at% Al. There is no Si element, which indicates that there should be β(Mg2Al3) and γ(Mg17Al12). Fig.8c shows the EDX result of point 2 in Fig.8a consisting 1.5 Intensity/×103 cps

a

2 1 3

5 µm

Mg

b

1.2 0.9 Al

0.6

1.5 Mg

1.6

0.9

1.2

0.3 2

6

10

14

18

Intensity/cps

Si

0.0 0

0.4 Al 4

8

12

16 0.0

2

Energy/keV

6

10

14

18

Energy/keV

SEM micrograph of the fracture (a) and EDX analysis for rectangle zone 1 (b), point 2 (c) and point 3 (d) in Fig.8a

400

Mg Mg2 Al3 Mg17 Al12 Mg2Si

300 200 100 0 10

30

50

70

90

2θ/(º) Fig.9

X-ray diffraction analysis of the fracture

are distributed uniformly, about 540 MPa in Mg side and 350 MPa in Al side. The highest value of micro-hardness is 2380 MPa in the fusion zone of Mg side. The micro-hardness in the weld from Mg side to Al side shows a decreasing trend with reduction of intermetallic compounds. 3) The joint with low bonding strength is brittle fractured in the intermetallic compound layer of the fusion zone of Mg side. Intermetallic compounds of Mg2Si, Mg2Al3 and Mg17Al12 distributed continuously in the fusion zone are responsible for the fracture.

References 1

3

d

Mg

0.8 Si Al

Energy/keV Fig.8

2.0

1.2

0.6

0.3 0.0

c

Conclusions

1) AZ31B magnesium alloy and 6061 aluminum alloy are successfully welded by cold metal transfer welding with ER4043 as filler metal. Filler metal and aluminum substrate are combined with a good interface, while weld metal and Mg substrate are combined with a epitaxial solidification area where the intermetallic compounds of β(Mg2Al3), γ(Mg17Al12) and Mg2Si are generated. 2) The micro-hardnesses in both sides of the substrates

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