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Cold metal transfer spot plug welding of AA6061-T6-to-galvanized steel for automotive applications
Journal of Alloys and Compounds 585 (2014) 622–632
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Cold metal transfer spot plug welding of AA6061-T6-to-galvanized steel for automotive applications R. Cao ⇑, Q. Huang, J.H. Chen ⇑, Pei-Chung Wang State Key Laboratory of Gansu Advanced Non-ferrous Metal Materials, Lanzhou University of Technology, Lanzhou 730050, China GM R&D Center, Warren, MI 48090, USA
a r t i c l e
i n f o
Article history: Received 29 April 2013 Received in revised form 23 September 2013 Accepted 29 September 2013 Available online 10 October 2013 Keywords: Spot welding Cold metal transfer Dissimilar alloys Orthogonal test Mechanical property
a b s t r a c t In this study, cold metal transfer (CMT) spot plug joining of 1 mm thick Al AA6061-T6 to 1 mm thick galvanized steel (i.e., Q235) was studied. Welding variables were optimized for a plug weld in the center of a 25 mm overlap region with aluminum 4043 wire and 100% argon shielding gas. Microstructures and elemental distributions were characterized by scanning electron microscopy with energy dispersive X-ray spectrometer. Mechanical testing of CMT spot plug welded joints was conducted. It was found that it is feasible to join Al AA6061T6-to-galvanized steel by CMT spot plug welding method. The process variables for two joints with Al AA6061T6-to-galvanized mild steel and galvanized mild steel-to-Al AA6061T6 are optimized. The strength of CMT spot welded Al AA6061T6-to-galvanized mild steel is determined primarily by the strength and area of the brazed interface. While, the strength of the galvanized mild steel-to-Al AA6061T6 joint is mainly dependent upon the area of the weld metal. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Aluminum alloys have been widely used in the automotive industry due to its many attractive properties such as low density, high specific strength along with good damping capacity [1,2]. Currently, steel, on the other hand, is still the automakers’ material of choice. This is because steel offers good formability, weldability and low overall component production costs. Moreover, the use of mild steel in automotive body applications over the past century has created a comprehensive technical knowledge base. The presence of zinc coating on these advanced steel surfaces sharply improves its corrosion resistance which contributes to widespread application of Zn coated steel in the automotive industry. Hybrid structures of aluminum alloy and steel have been suggested to improve the fuel efficiency and reduce air emissions by reducing the weight [3,4]. Therefore, it has become an attractive research field in recent years to join aluminum alloy and steel together. However, joining of aluminum to steel is a great challenge because of large differences in thermo-physical properties between the two materials and especially the formation of brittle Al–Fe intermetallic compounds (IMC) at elevated temperatures [5–7].
⇑ Corresponding authors Address: State Key Laboratory of Gansu Advanced Nonferrous Metal Materials, Lanzhou University of Technology, Lanzhou 730050, China. Tel.: +86 931 2973529; fax: +86 931 2976578. E-mail addresses: [email protected] (R. Cao), [email protected] (J.H. Chen). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.09.197
To join aluminum alloy and steel, various mechanical, solid state and fusion joining processes have been investigated. Resistance spot welding (RSW) is currently the dominant method used for joining steel vehicle structures, because it is fast, versatile, and easily automated [8]. Qiu et al. [9] indicated that it is feasible to weld aluminum alloy to steel sheets via a RSW method using a cover plate, and the lap-shear strength of the A5052/SPCC joint is influenced by the reaction layer formed at its interface. Friction stir spot welding (FSSW) is a relatively new alternative technology that has potential for joining multi-material structures [10–13], because it requires far less energy than RSW and, more importantly, reduces the IMC formation by avoiding liquid phase reactions. Bozzia et al. [13] investigated the interface Al 6016/IFsteel in FSSW joints. Results presented that the thickness of IMC layer increases with the rotational speed and the penetration depth, an optimal IMC layer thickness of 8lm has been measured for a rotational speed of 3000 rpm and a tool penetration depth of 2.9 mm. Chen et al. [14] showed that aluminum alloy 6111-T4 and 1 mm thick steel DC04 sheets can be successfully welded with a cycle time less than 1 s by ‘‘abrasion circle friction spot welding’’, a novel approach to joining dissimilar materials. On the other hand, friction stir lap welding (FSLW) of Al sheet to steel sheet has also been attempted in recent years, because a joint is produced in solid state and no cover gas or flux is used [15,16]. Many studies have shown that the formation of Fe–Al intermetallics in fusion welding, resistance spot welding, friction stir welding aluminum-to-steel is inevitable. To have a sound weld, it is
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Wt(Al)%
Magnetic
(a) Joint I (top aluminum-to-bottom steel)
At(Al)% Fig. 1. Phase diagram of Al–Fe [19].
critical to control the thickness of the intermetallic compounds less than 10 lm [17]. Although many methods have been proposed to minimize the thickness of the intermetallics, a high efficiency and low cost joining method is still lacking. Recently ‘‘Cold Metal Transfer’’ (CMT) joining technique [18] was introduced as a means to join aluminum to galvanized mild steel. The key feature of this process is that the wire motion has been integrated into the joining process and into the overall control of the process. The wire retraction motion assists the droplet detachment during the short circuit, thus the metal can transfer into the welding pool without the aid of the electromagnetic force. As a result, the heat input can be controlled, and as a consequence the IMP formation and thickness can be minimized as well thereby enabling optimization of the joint strength. From the Al–Fe phase diagram shown in Fig. 1, the melting points of aluminum and iron are 649 °C and 1539 °C, respectively. This huge difference in melting points makes it difficult to create a metallurgical bond between the aluminum alloy and steel only via fusion joining processes without forming intermetallic compounds. Previous study [20] showed that it is feasible to join the Al-galvanized steel by CMT method. If the thickness of the Fe–Al intermetallics on the brazing interface was controlled to 2–5 lm, welding–brazing joints were formed, and the strength of the welded Al–steel is comparable to that of the welded Al–Al joints. Besides the fillet weld, there is a need to spot joining the workpieces for automotive applications. In this study, the feasibility of CMT spot welding AA6061-T6 to galvanized mild steel (i.e. Q235) is investigated. Then, the process optimization, microstructure of the welded joints and bonding mechanism of CMT spot plug welded Al AA6061T6-to-galvanized mild steel are firstly indicated. Finally, the correlations between the joint strength, fracture modes for CMT spot plug welded Al AA6061T6-to-galvanized mild steel are also discussed.
(b) Joint II (top steel-to-bottom aluminum) Fig. 2. Schematics of joint configuration: (a) aluminum-to-steel, and (b) steel-toaluminum (dimensions in mm).
Table 2 The orthogonal experiment levels and factors for tests of AA6061-T6-to-galvanized steel shown in Fig. 2(a). Control factors
Levels
Wire feeder speed, A (m/min) Hole diameter on the Al, B (mm) Spot-welding time, C (s)
I
II
III
6 5.2 0.8
7 6.0 1.0
8 7.0 1.2
Table 3 The orthogonal experiment levels and factors for tests of galvanized steel-to-AA6061T6 shown in Fig. 2(b). Control factors
Levels
Wire feeder speed, A (m/min) Hole diameter on the steel, B (mm) Spot-welding time, C (s)
I
II
III
7 5.2 0.5
7.5 6.0 0.8
8 7.0 1.0
Table 1 Chemical compositions of aluminum alloy and wire (wt.%). Materials
Si
Fe
Cu
Mn
Mg
Zn
Cr
Ti
Al
C
P
S
Al6061 4043 Al wire Mild steel
0.4–0.8 4.5–6.0 0.01
0.70 0.80 Bal.
0.15–0.4 0.30
0.15 0.05 0.39
0.8–1.2 0.05
0.25 0.10
0.04–0.35 –
0.15 0.20
Bal. Bal.
– – 0.01
– – 0.03
– – 0.025
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Table 4 Input parameters of orthogonal array and output characteristics for tests of Joint I. Exp.
Wire feeder speed, A (m/min)
Hole diameter of Al, B (mm)
Spot-welding time, C (s)
Load1 (kN)
Load2 (kN)
Load-ave (kN)
1 2 3 4 5 6 7 8 9
6 6 6 7 7 7 8 8 8
5.2 6 7 5.2 6 7 5.2 6 7
0.8 1.0 1.2 1.0 1.2 0.8 1.2 0.8 1.0
1.80 1.46 2.96 1.20 1.20 3.14 4.96 2.66 3.76
1.60 1.18 2.66 2.54 1.16 2.30 4.48 2.98 2.98
1.70 1.32 2.81 1.87 1.18 2.72 4.72 2.82 3.37
Table 5 Input parameters of orthogonal array and output characteristics for tests of Joint II. Exp.
Wire feeder speed, A (m/min)
Hole diameter of steel, B (mm)
Spot-welding time, C (s)
Load1 (kN)
Load2 (kN)
Load-ave (kN)
1 2 3 4 5 6 7 8 9
7 7 7 7.5 7.5 7.5 8 8 8
5.2 6 7 5.2 6 7 5.2 6 7
0.5 0.8 1.0 0.8 1.0 0.5 1.0 0.5 0.8
1.76 2.30 2.32 1.86 2.98 2.20 0.98 1.72 2.20
1.94 2.78 2.70 2.10 2.66 2.42 0.52 2.58 2.32
1.85 2.54 2.51 1.98 2.82 2.31 0.75 2.15 2.76
Table 6 Ranking of influential process parameters by Taguchi method for Joint I. Factors
A (wire feeder speed)
B (hole diameter of Al sheet)
C (spot-welding time)
Error
K1j K2j K3j Delta Rank
3.89 3.85 7.27 3.42 1
5.53 3.55 5.93 2.38 2
4.83 4.37 5.81 1.43 3
4.17 5.84 5.00 1.67
Table 7 Ranking of influential process parameters by Taguchi method for Joint II. Factors
A (wire feeder speed)
B (hole diameter of steel sheet)
C (spot-welding time)
Error
K1j K2j K3j Delta Rank
4.60 4.74 3.77 0.97 2
3.05 5.01 5.05 2.00 1
4.21 4.85 4.05 0.80 3
3.45 3.73 4.43 0.98
Table 8 Analysis of variance for joint strength using SS (sum of square) for Joint I. Source
DOF
SS (sum of squares)
S0 (pure sum of squares)
Variance
F
Contribution (%)
A B C Error Total
2 2 2 11 17
11.61 4.89 1.61 3.93 22.03
10.89 4.18 0.90 6.07 –
5.80 2.45 0.81 0.36 –
16.25 6.85 2.25 – –
49.4 19.0 4.1 27.5 100
Table 9 Analysis of variance for joint strength using SS (sum of square) for Joint II. Source
DOF
SS (sum of squares)
S0 (pure sum of squares)
Variance
F
Contribution (%)
A B C Error Total
2 2 2 11 17
0.82 3.91 0.54 2.53 7.80
0.36 3.45 0.08 3.91 –
0.41 1.95 0.27 0.23 –
1.78 8.48 1.17 – –
5 44 1 50 100
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4.0
A3 3.5
B: hole diameter on the Al sheet load (KN)
3.0
B3
B1
2.0
C3
C1
2.5
A1
2.3. Design-of-experiment
C2 A2 B2
1.5 A:wire feeder speed
sheets were degreased by acetone first and then polished by abrasive cloth and cleaned with 5–10% NaOH solution at a temperature range of 40–70 °C for 3– 7 min and rinsed with tap water afterward. This is followed by surface cleaning with 30% HNO3 solution at a temperature of 60 °C for 1–3 min and then rinsing with tap water. Two joints shown in Fig. 2 were fabricated. One is aluminum AA6061-T6 sheet was placed on the top of the steel shown in Fig. 2(a) and the other is galvanized steel is placed on the top of the Al AA6061-T6 sheet shown in Fig. 2(b). To obtain the process window for each joint configuration, design-of-experiment was performed and is described next.
C:spot-welding time
1.0 Fig. 3. Effect of welding variable on the joint strength for 1 mm thick AA6061 T6to-1 mm thick galvanized mild steel.
The Taguchi DOE technique [21] incorporating orthogonal array was utilized for the systematic evaluation of the welding variables. Tables 2 and 3 show the orthogonal array corresponding to three factors and three levels L9, where the subscript 9 denotes the number of experiments to be performed. Based on the output response (i.e., the joint strength in all nine investigated cases), Taguchi experimental design was analyzed. The design of orthogonal experiments can greatly reduce the time and increase the accuracy of assigning proper lines [21]. 2.4. Analytical analysis
4.0
3.5
Load (KN)
3.0
2.5
A:wire feeder speed
C:spot-welding time
B2
A2
B3
To examine the quality of CMT welding–brazing joint, the cross-sections of the specimens were prepared and examined. The polished aluminum and steel workpieces were etched by Dix-Keller’s and Nital (i.e., 4 vol.% HNO3 + 96 vol.% ethanol), respectively. The microstructures of the weld were examined by scanning electron microscope (i.e., SEM 6700F) equipped with energy dispersive X-ray spectrometer (EDS). The analyses of elemental distributions of the welds were carried out by Electron Probe Micro-Analysis (EPMA). 2.5. Mechanical testing
C2
A1 A3
2.0
1.5
C1
C3
B1 B: hole diameter on the steel sheet
•
1.0
Fig. 4. Effect of welding variable on the joint strength for CMT welded 1 mm thick galvanized mild steel-to-1 mm thick AA6061-T6.
Quasi-static tests were performed by loading each specimen shown in Fig. 2 to failure in a tensile tester. To minimize the bending stresses inherent in the testing of lap-shear specimens, filler plates were attached to both ends of the sample using masking tape to accommodate the sample offset. Load vs. displacement curves were recorded as the specimens were loaded at a stroke rate of 1 mm/min. Two replicates were performed, and all welded coupons were strained to fracture, and the maximum load was used to indicate the strength of the welds, and the average peak loads were reported. 2.6. Fracture analysis The fracture surface was analyzed by scanning electron microscope (SEM 6700F).
2. Experimental procedure 2.1. Materials
3. Results and discussion
The materials used in the study include 1 mm thick aluminum AA6061-T6 alloy and 1 mm thick hot dipped galvanized mild carbon steel. The compositions of galvanized mild steel, aluminum alloy and aluminum wire 4043 with a diameter of 1.2 mm, per the manufacturer’s data sheet, are shown in Table 1.
3.1. Optimization of welding variables
2.2. Sample fabrication The spot joint configuration, shown in Fig. 2, was selected for this study. The joints were fabricated from 125 50 1 mm sheets. A plug weld was located in the center of a 25 mm overlap region. A fixture was used to ensure consistent weld placement. Prior to welding, the steel sheets were degreased by acetone. Aluminum
Two aluminum–steel joint configurations shown in Fig. 2 were studied. To optimize the welding process for each joint configuration, quadratic regression analyses of various variables were performed. Tables 4 and 5 present the process parameters of orthogonal array and the estimated characteristics for AA6061T6-to-galvanized steel (i.e., Joint I) and galvanized steel-toAA6061-T6 (i.e., Joint II), respectively. The peak load to fracture
(a) Weld appearance Transient Brazed Interface
Weld Metal
Al 6061 Fusion zone
Al 6061 Galvanized Mild Steel
Middle Brazed Interface
1mm
(b) Cross section Fig. 5. (a) Appearance, and (b) cross section of optimized CMT welded 1 mm thick AA6061-T6-to-1 mm thick galvanized mild steel.
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Steel
Front side
Al sheet
Al sheet
Back side 10mm
10mm
(a) Weld appearance
Weld Metal
1mm
(b) Cross section Fig. 6. (a) Appearance and (b) cross section of optimized CMT welded 1 mm thick galvanized steel-to 1 mm thick AA6061-T6.
2
7 10
6
3 1 4 5
α-Al
8 9
Steel
Weld metal
Al-Si Brazing interface
10μm
Fe
100
40
40
30 60
Si
20
40
Si(%)
Fe(Al)(%)
50
Al
80
(a) Microstructure of weld metal
50
30
20
Mg(%)
(a)
10 20
Mg 0
10
0 0
10
20
Brazing interface
30
40
50
60
-10
0
Displacement ( μ m)
(b) Fig. 8. Line analysis results of the middle brazed interface for optimized CMT welded 1 mm thick AA6061 T6-to-1 mm thick galvanized mild steel: (a) crosssection, and (b) elemental distribution along the blue line in (a). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
(b) Microstructure of fusion zone Fig. 7. Microstructure of CMT welded Al-to-mild steel at the: (a) weld metal and (b) fusion zone.
of the joints (i.e., joint strength) is the metric used as the basis for the process optimization. Tables 6 and 7 show the analysis results on the rank of the process parameters using the Taguchi method for Joints I and II, respectively. As shown in Tables 6 and 7, K1j, K2j and K3j represent corresponding average values of the sum of loads with levels ‘‘1’’, ‘‘2’’ and ‘‘3’’ among the row j, respectively. The ‘delta’ value was estimated for each parameter from the difference of the maximum and minimum mean peak load values at
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R. Cao et al. / Journal of Alloys and Compounds 585 (2014) 622–632 Table 10 Energy spectrum analysis results of middle brazed interface in Fig. 8(a). Element (at.%)
1
2
3
4
5
6
7
8
9
10
AlK FeK SiK MgK Phases
46.48 50.64 2.88 – FeAl
70.50 26.45 3.05 – Fe2Al5
72.39 24.23 3.38 – Fe2Al5
74.13 23.76 2.11 – FeAl3
74.72 21.83 3.44 – FeAl3
90.47 9.53 – – FeAl3 and a-Al solid solution
95.35 4.65 – – FeAl3 and a-Al solid solution
78.09 – 21.91 – aAl+Si
94.28 2.59 2.12 1.00 a-Al solid solution
100 – – – a-Al solid solution
1000 Steel
Weld metal
Fe
1
Intensity (Cps)
800
3 6 2 4
5
7
Brazing interface
40
50
200
10
20
30
40
50
60
70
80
90
40
30
Al
Fig. 10. X-ray analysis at the brazed interface between the mild steel/weld metal for CMT welded 1 mm thick AA6061-T6-to-1 mm thick galvanized mild steel.
60
20
40
20
Zn(%)
30
Si(%)
Fe(Al)(%)
400
2θ
80
10
Si 20
10
Zn 0
0 0
20
30
40
50
Displacement ( μm)
(b) Fig. 9. Line analysis results of the transient brazing interface for optimized welded aluminum-to-galvanized mild steel: (a) cross-section, and (b) element distribution along the blue line in (a). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Table 11 Energy spectrum analysis results of transient brazed interface in Fig. 9(a). Element 1 (at.%)
2
3
4
5
6
7
AlK FeK SiK Phases
14.02 85.98 – a-Fe solid Solution
69.43 24.91 5.66 Fe2Al5
75.39 24.61 – FeAl3
91.29 8.71 – FeAl3 and aAl solid solution
98.35 1.65 – a-Al solid solution
98.41 1.59 – a-Al solid solution
2.89 97.11 – c-Fe solid solution
AlFe
0
Fe
0 10 Brazing interface
Al5Fe2Zn0.4
600
10μm
(a) 100
Al5Fe2
different levels of the same factor. The rank of each parameter generated by the Taguchi method was determined from the delta values of all the parameters. The rank of the parameters signifies their relative importance in terms of their influence on the output response, i.e., peak loads in this investigation. As shown in Table 6, the variables in the order of importance for Joint I are as follows: wire feeder speed (A) > hole diameter of Al sheet (B) > spot-welding time (C). When a single factor is considered, optimized welding variables can be obtained. i.e., a wire fee-
der speed of 8 m/min, a hole diameter of 7 mm on the Al sheet, and a welding time of 1.2 s. As shown in Table 7, the variables in the order of importance for Joint II are as follows: hole diameter of steel sheet (B) > wire feeder speed (A) > spot-welding time (C). When a single factor is considered, optimized welding variables can be obtained, i.e., a wire feeder speed of 7.5 m/min, a hole diameter of 7 mm on the steel sheet, a welding time of 0.8 s. A statistical analysis of variance (ANOVA) was performed to determine the statistical significance of the process parameters [21] and the effect of an individual input parameter on the output parameters. Tables 8 and 9 show the results of ANOVA analysis for Joints I and II, respectively. Based on these results, the wire feeder speed was found to be the most influential process parameter with a 49.4% contribution followed by the hole diameter on the Al sheet 19.0% for Joint I. The effect of single factor on the peak load of the joints is shown in Fig. 3. The optimized welding window (i.e., a wire feeder speed of 8 m/min, a hole diameter of 7 mm on the Al sheet, a spot-welding time of 1.2 s) was developed by combining the results shown in Fig. 3 and Table 6. As shown in Table 9 for steel-Al Joint II in Fig. 2(b), the hole diameter of steel sheet is found to be the most influential process parameter with a 44% contribution. The effect of single factor on the peak load of the joints is shown in Fig. 4. The optimized welding window (i.e. a wire feeder speed of 7.5 m/min, a hole diameter of 7 mm on the steel sheet, a spot-welding time of 0.8 s) was developed by combining the results shown in Fig. 4 and Table 7. 3.2. Weld appearance of optimized CMT welded Al 6061-mild steel joint The weld appearance and corresponding cross-sections of optimized CMT welded AA6061-T6-to-galvanized steel welds are shown in Fig. 5(a) and (b), respectively. As shown, the welded joint is composed of the weld metal, aluminum fusion zone on the
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6 5
CMT welded Al6061-gavanized mild steel, Joint I
5
4
Load (KN)
Load (kN)
4
CMT welded Al6061-Al6061
3
2
1 0 0.0
3
2
1
CMT welded gavanized mild steel-Al6061, Joint II
0 40
50
60
70
80
90
100
2
Brazing area(mm ) 0.4
0.8
1.2
1.6
2.0 Fig. 13. Effect of the brazed area on the strength of CMT welded 1 mm thick AA6061-T6-to-1 mm thick galvanized mild steel.
Displement (mm) Fig. 11. Effect of material stacking sequence on the strength of CMT weld-brazed 1 mm thick AA6061-T6–1 mm thick galvanized mild steel.
AA6061-T6, and the brazed interface (including transient brazed and middle brazed interfaces) between the weld metal and un-molten galvanized mild steel. The weld appearance and corresponding cross-sections of optimized galvanized steel-to-AA6061T6 joints are shown in Fig. 6(a) and (b), respectively. The fusion zone was formed between the aluminum weld metal and aluminum substrate, and little brazed interface zone was formed between the aluminum weld metal and steel substrate. As shown in Fig. 6(b), a narrow gap existed between the aluminum weld metal and the edge of the hole of steel substrate. The steel-to-aluminum joining joint in Fig.6 is similar to rivet joint in Ref. [22]. Detailed microstructures of the Joints I and II will be analyzed next.
3.3. Microstructure of optimized AA6061-galvanized mild steel joint To understand the bonding mechanisms of CMT spot welding AA6061-T6l-to-galvanized mild steel with filler wire Al 4043, the microstructures of the welds were analyzed. Fig. 7 presents the microstructures of the weld metal and fusion zone for Joint I. As can be observed in Fig. 7(a), the weld metal zone labeled by a solid rectangle in Fig. 5(b) is composed of equi-axed pro-eutectic a-Al solid solutions (i.e., grey base metal) and Al–Si second phases (i.e., bright white). The microstructure of aluminum fusion zone
labeled by a circle in Fig. 5(b) is shown in Fig. 7(b). As shown, coarse grain is produced at this region. Similar microstructures appeared for the welded galvanized steel-to-AA6061-T6 joint shown in Fig. 7(b). The microstructures of middle brazed interface and transient brazed interface shown in Fig. 5(b) will be analyzed next by combining line analysis and point analysis results to further explain the bonding mechanism of CMT spot welded Al6061-T6-togalvanized steel. 3.4. Bond mechanism of Al-steel joints Since the IMC formed at the interface between the AA6061-T6 and galvanized steel during CMT joining process, it is important to understand the microstructures of this interphase. As the molten aluminum (melting point is about 649 °C) deposited onto the galvanized steel workpiece, the liquid aluminum molten metal wetted the galvanized steel substrate. However, the rate of forming the Fe–Al intermetallic is quicker than the spread of the molten aluminum onto the galvanized steel surface which would have dissolved the Fe–Al intermetallics [23]. Therefore, the interface reaction–diffusion dominates the Fe–Al diffusion process [23]. Based on the Fe– Al phase diagram, refer to Fig. 1, the Fe–Al interaction reaction forms c-Fe and a-Al solid solutions, and intermetallic compounds Fe3Al, FeAl, FeAl2, Fe2Al5, FeAl3 and FeAl6. Previous studies showed that the Fe3Al and FeAl based alloys have the best oxidation resistance, sulfidation resistance, high specific strength and mid-tem-
Al sheet
steel
5mm
5mm
(a) Al sheet
steel
10mm
10mm
(b) Fig. 12. Fracture surface of CMT welded (a) AA6061-T6-galvanized mild steel, and (b) galvanized mild steel-AA6061-T6.
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Al sheet
Galvanized steel sheet
Brazed interface
(a) Schematic of fracture location
Al sheet
C
B
A
D 5mm
(b) Fracture surface of CMT welded AA6061 T6-galvanized mild steel
Point 1 Point 2
(c) Region A
(d) Region B Point 5 Point 6 Point 4 Point 3
(e) Region C
(f) Region D
Fig. 14. Fracture surface of CMT welded 1 mm thick AA6061 T6-to-1 mm thick galvanized mild steel: (a) schematic of fracture location, (b) fracture aluminum–steel brazing interface, and (c) magnified view of region A, (d) region B, (e) region C, and (f) region D.
Table 12 Energy spectrum analysis results of fracture surface in Fig. 14. Element (at. %)
1
2
3
4
5
6
Al Fe Si Zn Mg Phases
77.33 20.46 2.22 – – FeAl3
67.59 29.98 2.43 – – Fe2Al5
97.95 – – 02.05 – a-Al solid solution
88.65 – – 09.24 02.10 Al–Zn eutectic phase
71.65 1.65 21.36 05.98 01.01 Al–Zn– Si eutectic phase
96.74 – – 03.26 – a-Al solid solution + Al– Zn eutectic phase
perature strength [24]. Fig. 8(a) and (b) present the micrographs of the middle brazed interface shown in Fig. 5(b). The results showed that the interfaces with a thickness range of 5–8 lm which is less than the critical thickness of 10 lm presented by Schubert et al. [17] were formed between the weld metal and galvanized steel substrate. As shown, while the interface regions near the steel workpiece had a relative straight edge and uniform thickness, the interface zones near the weld metal had various thicknesses and grew into the weld metal, and some dispersed needle-shaped compounds were produced at the weld metal. To analyze the composition of the middle brazed interfacial zones, energy dispersive X-ray spectrometer (EDS) analysis was performed and the results are presented in Table 10 and
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Galvanized steel sheet
Al sheet
Weld metal
Fracture location
(a) schematic of fracture location
Al sheet B
A
C 5mm
Point 1
Point 2
(c) Region A
(d) Region A
Point 4
Point 7
Point 3 Point 6
Point 5
(e) Region B
(f) Region C
Fig. 15. Fracture surface of CMT welded 1 mm thick galvanized mild steel-to-1 mm thick AA6061-T6: (a) schematic of fracture location, (b) pull-out aluminum, (c) magnified view of region A, (d) magnified view of region A, (e) magnified view of region B, and (f) magnified view of region C.
Table 13 Energy spectrum analysis results of fracture surface in Fig. 15(a). Element (at.%)
1
2
3
4
5
6
7
Al Fe Si Zn Mg Phases
100 – – – – a-Al solid solution
96.87 – 03.13 – – a-Al solid solution and small Al–Si eutectic phase
63.26 – 00.32 34.31 02.11 Al–Zn eutectic phase
64.70 – 01.05 27.76 06.49 Al–Zn eutectic phase
20.78 – – 59.82 19.39 Al–Zn eutectic phase
12.57 – – 87.43 – a-Al solid solution+ Al–Zn eutectic phase
45.59 –
Fig. 8(b). As shown, the Fe element content decreased whereas the Al element content increased along the interface zones. The change is attributed to the diffusion of Fe and Al elements. By combining the phase diagram in Fe–Al and the features shown in Fig. 8(a) and the compositions in Table 10, it is most likely that the regions
54.41 – Al–Zn eutectic phase
1–10 in Fig. 8(a) contain FeAl, Fe2Al5, FeAl3, FeAl3 intermetallics and a-Al solid solution, a-Al+Si, and a-Al solid solution, respectively. Fig. 9(a) and (b) is the micrographs of the transient brazed interface shown in Fig. 5(b). Similar results have been observed in Table 11, the brazed interface is also composed of Fe2Al5 and
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FeAl3 intermetallics denoted by regions 3 and 4 shown in Fig. 9(a) and Table 11, and only the thinner interfaces with a thickness of 4 lm was formed. As the results shown above, the welding–brazing interface is composed of more layers with various compositions and phases for AA6061-T6-galvanized steel joints. In order to confirm the bonding mechanism of the weld-brazed joints, X-ray analyses of the interface of the specimens fractured along the interface between the weld metal and galvanized steel were conducted and the results are presented in Fig. 10. It was found that the interface is mainly composed of intermetallics Al5Fe2Zn0.4 and Fe2Al5 and FeAl.
3.5. Strength and fractures of CMT spot welded aluminum-galvanized steel Static testing of optimized CMT weld-brazed Al6061T6-galvanized steel joints was performed and the results are presented in Fig. 11. For the purpose of the comparison, test results of CMT spot plug welded joint of AA6061-T6-to-AA6061-T6 with a hole with the same width and thickness were also included in Fig. 11. For the sake of clarity, only one representative result was presented. As shown, the strength of CMT spot plug welded AA6061-T6-galvanized mild steel is significant stronger than that of CMT spot plug welded AA6061-T6 joints. Thorough analyses of the fractured surfaces of the tested specimens, shown in Fig. 12(a), the results indicated that the failure is located at the weld-brazed interface. Moreover, we carefully examined the thickness of the IMC at the weld-brazed interface of the optimized AA6061-T6-to-galvanized steel joints and found that the thicknesses of the intermetallics were about 8 lm which is slightly less than the critical value 10 lm [17]. With this thin intermetallic layer, we examined the effect of the brazed area at the interface on the joint strength and the results are presented in Fig. 13. It can be seen that in general the joint strength increased with increasing the areas of the brazed interface. These results suggest that the joint strength is related to the area of the AA6061-T6-galvanized steel brazed interface when the intermetallics of the brazed interface were kept below 10 lm. As shown in Fig. 6(b), CMT welded galvanized steel-toAA6061-T6 is similar to that of AA6061-AA6061 joints, and a weld nugget pull-out failure was observed in Fig. 12(b). Furthermore, the strength is comparable to that of CMT spot plug welded AA6061-T6. Fig. 14 presents the fracture features of the optimized AA6061 T6-to-galvanized mild steel. Point analyses of various regions were performed and the results are shown in Table 12. Fig. 14(a) presents that the optimized AA6061 T6-to-galvanized mild steel joint is fractured at the brazed interface. The fractured brazed interface at Al side is shown in Fig. 14(b). The magnified view of region A in Fig. 14(b) is shown in Fig. 14(c). As shown, coarser fracture feature was observed at the middle of aluminum–steel interface. Region B in Fig. 14(b) represents the fracture feature of typical aluminum– steel brazed interface. From Fig. 14(b)–(d), though the fracture features are different for regions A and B, the same intermetallic FeAl3 shown in Table 12 induced the final fracture. For region C in Fig. 14 (b), a-Al solid solution was formed and is shown in Fig. 14(e). For region D in Fig. 14 (b), it can be seen from Table 12 that Al–Zn eutectic phase, Al–Zn–Si eutectic phase were produced at the fracture surface and are shown in Fig. 14(f). Regions C and D could not be properly connected due to the fact that zinc vapor produced during CMT joining and produced significant amount of porosities shown in Fig. 14(b) and (f). These observations suggest that the strength of the CMT Al-steel spot welded joints primarily depend on the connected area of regions A and B.
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Fig. 15 presents the fracture features CMT welded 1 mm thick galvanized steel-to-1 mm thick AA6061-T6. Point analyses of various regions were conducted and the results are shown in Table 13. As shown in Fig. 15(a), bold line presents fracture location of steel-Al joint, i.e. weld pull-out fracture was observed shown in Fig. 15(b). From analyses of points 1 and 2 of the fracture surface as shown in Fig. 15(b)–(d), the joints fractured at the edge (denoted by region A) of aluminum weld metal which is composed of a-Al solid solution and small amount of Al–Si eutectic phase. Unsatisfied bonding was formed at the regions B and C. This observation is supported by the results shown in Fig. 15(e) and (f) and Table 13. More zinc element was produced at these regions. As much as 87.43% zinc element was formed at Point 6. Though Al–Zn phases were formed at these regions B and C, these phases may contribute little in joint strength because a large gap existed between the aluminum weld metal and the edge of the hole on the steel substrate. Furthermore, points 3–6 contain little Fe element and these results confirmed that poor bonding developed between the regions B and C. By analyzing the joint strength, fracture features and bonding mechanism, the results showed that while the strength of spot plug welded 1 mm thick AA6061 joints is lower than that of 1 mm thick Al AA6061-to-1 mm thick galvanized mild steel joint, it is comparable to that of 1 mm thick galvanized steel-to-1 mm thick Al AA6061 welded joints. In short, it is recommended to choose the stacking sequence that the AA6061-T6 sheet is placed on the top of the steel. 4. Conclusions According to the study on the CMT welded Al AA6061-T6 and galvanized mild steel dissimilar alloys, the followed results can be summarized: (1) CMT spot plug welding of 1 mm thick Al AA6061-T6-1 mm thick galvanized mild steel with filler wire Al 4043 was conducted. Test results showed that it is feasible to obtain a sound joint if the wire feeder speed is properly controlled. (2) The optimized welding variables for AA6061-T6-to-galvanized mild steel are: a wire feeder speed of 8 m/min, a hole diameter of 7 mm on the Al sheet and a welding time of 1.2 s. Similarly, the optimized welding variables for galvanized mild steel-to-AA6061-T6 are: a wire feeder speed of 7.5 m/min, a hole diameter of 7 mm on the steel sheet, and a welding time of 0.8 s. (3) CMT spot plug welded Al AA6061-to-galvanized steel joints were composed of the fusion zone of Al AA6061 sheet and Al 4043 filler wire, Al weld metal, and the brazing interface between the Al weld metal and galvanized mild steel. The brazing interface consists of about 5–8 lm thick FeAl3 intermetallic. For CMT spot plug welded galvanized mild steel-toAl AA6061 joints, the joints were only composed of Al weld metal. (4) Material stacking sequence affects the strength of CMT spot plug welded joints. While the strength of spot plug welded 1 mm thick AA6061 joints is lower than that of 1 mm thick Al AA6061-to-1 mm thick galvanized mild steel joint, it is comparable to that of 1 mm thick galvanized steel-to 1 mm thick Al AA6061 welded joints.
Acknowledgement This work was financially supported by National Nature Science Foundation of China (No. 51265028), and GM-Research and Development Center, MI, USA.
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Cold metal transfer spot plug welding of AA6061-T6-to-galvanized steel for automotive applications R. Cao ⇑, Q. Huang, J.H. Chen ⇑, Pei-Chung Wang State Key Laboratory of Gansu Advanced Non-ferrous Metal Materials, Lanzhou University of Technology, Lanzhou 730050, China GM R&D Center, Warren, MI 48090, USA
a r t i c l e
i n f o
Article history: Received 29 April 2013 Received in revised form 23 September 2013 Accepted 29 September 2013 Available online 10 October 2013 Keywords: Spot welding Cold metal transfer Dissimilar alloys Orthogonal test Mechanical property
a b s t r a c t In this study, cold metal transfer (CMT) spot plug joining of 1 mm thick Al AA6061-T6 to 1 mm thick galvanized steel (i.e., Q235) was studied. Welding variables were optimized for a plug weld in the center of a 25 mm overlap region with aluminum 4043 wire and 100% argon shielding gas. Microstructures and elemental distributions were characterized by scanning electron microscopy with energy dispersive X-ray spectrometer. Mechanical testing of CMT spot plug welded joints was conducted. It was found that it is feasible to join Al AA6061T6-to-galvanized steel by CMT spot plug welding method. The process variables for two joints with Al AA6061T6-to-galvanized mild steel and galvanized mild steel-to-Al AA6061T6 are optimized. The strength of CMT spot welded Al AA6061T6-to-galvanized mild steel is determined primarily by the strength and area of the brazed interface. While, the strength of the galvanized mild steel-to-Al AA6061T6 joint is mainly dependent upon the area of the weld metal. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Aluminum alloys have been widely used in the automotive industry due to its many attractive properties such as low density, high specific strength along with good damping capacity [1,2]. Currently, steel, on the other hand, is still the automakers’ material of choice. This is because steel offers good formability, weldability and low overall component production costs. Moreover, the use of mild steel in automotive body applications over the past century has created a comprehensive technical knowledge base. The presence of zinc coating on these advanced steel surfaces sharply improves its corrosion resistance which contributes to widespread application of Zn coated steel in the automotive industry. Hybrid structures of aluminum alloy and steel have been suggested to improve the fuel efficiency and reduce air emissions by reducing the weight [3,4]. Therefore, it has become an attractive research field in recent years to join aluminum alloy and steel together. However, joining of aluminum to steel is a great challenge because of large differences in thermo-physical properties between the two materials and especially the formation of brittle Al–Fe intermetallic compounds (IMC) at elevated temperatures [5–7].
⇑ Corresponding authors Address: State Key Laboratory of Gansu Advanced Nonferrous Metal Materials, Lanzhou University of Technology, Lanzhou 730050, China. Tel.: +86 931 2973529; fax: +86 931 2976578. E-mail addresses: [email protected] (R. Cao), [email protected] (J.H. Chen). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.09.197
To join aluminum alloy and steel, various mechanical, solid state and fusion joining processes have been investigated. Resistance spot welding (RSW) is currently the dominant method used for joining steel vehicle structures, because it is fast, versatile, and easily automated [8]. Qiu et al. [9] indicated that it is feasible to weld aluminum alloy to steel sheets via a RSW method using a cover plate, and the lap-shear strength of the A5052/SPCC joint is influenced by the reaction layer formed at its interface. Friction stir spot welding (FSSW) is a relatively new alternative technology that has potential for joining multi-material structures [10–13], because it requires far less energy than RSW and, more importantly, reduces the IMC formation by avoiding liquid phase reactions. Bozzia et al. [13] investigated the interface Al 6016/IFsteel in FSSW joints. Results presented that the thickness of IMC layer increases with the rotational speed and the penetration depth, an optimal IMC layer thickness of 8lm has been measured for a rotational speed of 3000 rpm and a tool penetration depth of 2.9 mm. Chen et al. [14] showed that aluminum alloy 6111-T4 and 1 mm thick steel DC04 sheets can be successfully welded with a cycle time less than 1 s by ‘‘abrasion circle friction spot welding’’, a novel approach to joining dissimilar materials. On the other hand, friction stir lap welding (FSLW) of Al sheet to steel sheet has also been attempted in recent years, because a joint is produced in solid state and no cover gas or flux is used [15,16]. Many studies have shown that the formation of Fe–Al intermetallics in fusion welding, resistance spot welding, friction stir welding aluminum-to-steel is inevitable. To have a sound weld, it is
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Wt(Al)%
Magnetic
(a) Joint I (top aluminum-to-bottom steel)
At(Al)% Fig. 1. Phase diagram of Al–Fe [19].
critical to control the thickness of the intermetallic compounds less than 10 lm [17]. Although many methods have been proposed to minimize the thickness of the intermetallics, a high efficiency and low cost joining method is still lacking. Recently ‘‘Cold Metal Transfer’’ (CMT) joining technique [18] was introduced as a means to join aluminum to galvanized mild steel. The key feature of this process is that the wire motion has been integrated into the joining process and into the overall control of the process. The wire retraction motion assists the droplet detachment during the short circuit, thus the metal can transfer into the welding pool without the aid of the electromagnetic force. As a result, the heat input can be controlled, and as a consequence the IMP formation and thickness can be minimized as well thereby enabling optimization of the joint strength. From the Al–Fe phase diagram shown in Fig. 1, the melting points of aluminum and iron are 649 °C and 1539 °C, respectively. This huge difference in melting points makes it difficult to create a metallurgical bond between the aluminum alloy and steel only via fusion joining processes without forming intermetallic compounds. Previous study [20] showed that it is feasible to join the Al-galvanized steel by CMT method. If the thickness of the Fe–Al intermetallics on the brazing interface was controlled to 2–5 lm, welding–brazing joints were formed, and the strength of the welded Al–steel is comparable to that of the welded Al–Al joints. Besides the fillet weld, there is a need to spot joining the workpieces for automotive applications. In this study, the feasibility of CMT spot welding AA6061-T6 to galvanized mild steel (i.e. Q235) is investigated. Then, the process optimization, microstructure of the welded joints and bonding mechanism of CMT spot plug welded Al AA6061T6-to-galvanized mild steel are firstly indicated. Finally, the correlations between the joint strength, fracture modes for CMT spot plug welded Al AA6061T6-to-galvanized mild steel are also discussed.
(b) Joint II (top steel-to-bottom aluminum) Fig. 2. Schematics of joint configuration: (a) aluminum-to-steel, and (b) steel-toaluminum (dimensions in mm).
Table 2 The orthogonal experiment levels and factors for tests of AA6061-T6-to-galvanized steel shown in Fig. 2(a). Control factors
Levels
Wire feeder speed, A (m/min) Hole diameter on the Al, B (mm) Spot-welding time, C (s)
I
II
III
6 5.2 0.8
7 6.0 1.0
8 7.0 1.2
Table 3 The orthogonal experiment levels and factors for tests of galvanized steel-to-AA6061T6 shown in Fig. 2(b). Control factors
Levels
Wire feeder speed, A (m/min) Hole diameter on the steel, B (mm) Spot-welding time, C (s)
I
II
III
7 5.2 0.5
7.5 6.0 0.8
8 7.0 1.0
Table 1 Chemical compositions of aluminum alloy and wire (wt.%). Materials
Si
Fe
Cu
Mn
Mg
Zn
Cr
Ti
Al
C
P
S
Al6061 4043 Al wire Mild steel
0.4–0.8 4.5–6.0 0.01
0.70 0.80 Bal.
0.15–0.4 0.30
0.15 0.05 0.39
0.8–1.2 0.05
0.25 0.10
0.04–0.35 –
0.15 0.20
Bal. Bal.
– – 0.01
– – 0.03
– – 0.025
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Table 4 Input parameters of orthogonal array and output characteristics for tests of Joint I. Exp.
Wire feeder speed, A (m/min)
Hole diameter of Al, B (mm)
Spot-welding time, C (s)
Load1 (kN)
Load2 (kN)
Load-ave (kN)
1 2 3 4 5 6 7 8 9
6 6 6 7 7 7 8 8 8
5.2 6 7 5.2 6 7 5.2 6 7
0.8 1.0 1.2 1.0 1.2 0.8 1.2 0.8 1.0
1.80 1.46 2.96 1.20 1.20 3.14 4.96 2.66 3.76
1.60 1.18 2.66 2.54 1.16 2.30 4.48 2.98 2.98
1.70 1.32 2.81 1.87 1.18 2.72 4.72 2.82 3.37
Table 5 Input parameters of orthogonal array and output characteristics for tests of Joint II. Exp.
Wire feeder speed, A (m/min)
Hole diameter of steel, B (mm)
Spot-welding time, C (s)
Load1 (kN)
Load2 (kN)
Load-ave (kN)
1 2 3 4 5 6 7 8 9
7 7 7 7.5 7.5 7.5 8 8 8
5.2 6 7 5.2 6 7 5.2 6 7
0.5 0.8 1.0 0.8 1.0 0.5 1.0 0.5 0.8
1.76 2.30 2.32 1.86 2.98 2.20 0.98 1.72 2.20
1.94 2.78 2.70 2.10 2.66 2.42 0.52 2.58 2.32
1.85 2.54 2.51 1.98 2.82 2.31 0.75 2.15 2.76
Table 6 Ranking of influential process parameters by Taguchi method for Joint I. Factors
A (wire feeder speed)
B (hole diameter of Al sheet)
C (spot-welding time)
Error
K1j K2j K3j Delta Rank
3.89 3.85 7.27 3.42 1
5.53 3.55 5.93 2.38 2
4.83 4.37 5.81 1.43 3
4.17 5.84 5.00 1.67
Table 7 Ranking of influential process parameters by Taguchi method for Joint II. Factors
A (wire feeder speed)
B (hole diameter of steel sheet)
C (spot-welding time)
Error
K1j K2j K3j Delta Rank
4.60 4.74 3.77 0.97 2
3.05 5.01 5.05 2.00 1
4.21 4.85 4.05 0.80 3
3.45 3.73 4.43 0.98
Table 8 Analysis of variance for joint strength using SS (sum of square) for Joint I. Source
DOF
SS (sum of squares)
S0 (pure sum of squares)
Variance
F
Contribution (%)
A B C Error Total
2 2 2 11 17
11.61 4.89 1.61 3.93 22.03
10.89 4.18 0.90 6.07 –
5.80 2.45 0.81 0.36 –
16.25 6.85 2.25 – –
49.4 19.0 4.1 27.5 100
Table 9 Analysis of variance for joint strength using SS (sum of square) for Joint II. Source
DOF
SS (sum of squares)
S0 (pure sum of squares)
Variance
F
Contribution (%)
A B C Error Total
2 2 2 11 17
0.82 3.91 0.54 2.53 7.80
0.36 3.45 0.08 3.91 –
0.41 1.95 0.27 0.23 –
1.78 8.48 1.17 – –
5 44 1 50 100
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4.0
A3 3.5
B: hole diameter on the Al sheet load (KN)
3.0
B3
B1
2.0
C3
C1
2.5
A1
2.3. Design-of-experiment
C2 A2 B2
1.5 A:wire feeder speed
sheets were degreased by acetone first and then polished by abrasive cloth and cleaned with 5–10% NaOH solution at a temperature range of 40–70 °C for 3– 7 min and rinsed with tap water afterward. This is followed by surface cleaning with 30% HNO3 solution at a temperature of 60 °C for 1–3 min and then rinsing with tap water. Two joints shown in Fig. 2 were fabricated. One is aluminum AA6061-T6 sheet was placed on the top of the steel shown in Fig. 2(a) and the other is galvanized steel is placed on the top of the Al AA6061-T6 sheet shown in Fig. 2(b). To obtain the process window for each joint configuration, design-of-experiment was performed and is described next.
C:spot-welding time
1.0 Fig. 3. Effect of welding variable on the joint strength for 1 mm thick AA6061 T6to-1 mm thick galvanized mild steel.
The Taguchi DOE technique [21] incorporating orthogonal array was utilized for the systematic evaluation of the welding variables. Tables 2 and 3 show the orthogonal array corresponding to three factors and three levels L9, where the subscript 9 denotes the number of experiments to be performed. Based on the output response (i.e., the joint strength in all nine investigated cases), Taguchi experimental design was analyzed. The design of orthogonal experiments can greatly reduce the time and increase the accuracy of assigning proper lines [21]. 2.4. Analytical analysis
4.0
3.5
Load (KN)
3.0
2.5
A:wire feeder speed
C:spot-welding time
B2
A2
B3
To examine the quality of CMT welding–brazing joint, the cross-sections of the specimens were prepared and examined. The polished aluminum and steel workpieces were etched by Dix-Keller’s and Nital (i.e., 4 vol.% HNO3 + 96 vol.% ethanol), respectively. The microstructures of the weld were examined by scanning electron microscope (i.e., SEM 6700F) equipped with energy dispersive X-ray spectrometer (EDS). The analyses of elemental distributions of the welds were carried out by Electron Probe Micro-Analysis (EPMA). 2.5. Mechanical testing
C2
A1 A3
2.0
1.5
C1
C3
B1 B: hole diameter on the steel sheet
•
1.0
Fig. 4. Effect of welding variable on the joint strength for CMT welded 1 mm thick galvanized mild steel-to-1 mm thick AA6061-T6.
Quasi-static tests were performed by loading each specimen shown in Fig. 2 to failure in a tensile tester. To minimize the bending stresses inherent in the testing of lap-shear specimens, filler plates were attached to both ends of the sample using masking tape to accommodate the sample offset. Load vs. displacement curves were recorded as the specimens were loaded at a stroke rate of 1 mm/min. Two replicates were performed, and all welded coupons were strained to fracture, and the maximum load was used to indicate the strength of the welds, and the average peak loads were reported. 2.6. Fracture analysis The fracture surface was analyzed by scanning electron microscope (SEM 6700F).
2. Experimental procedure 2.1. Materials
3. Results and discussion
The materials used in the study include 1 mm thick aluminum AA6061-T6 alloy and 1 mm thick hot dipped galvanized mild carbon steel. The compositions of galvanized mild steel, aluminum alloy and aluminum wire 4043 with a diameter of 1.2 mm, per the manufacturer’s data sheet, are shown in Table 1.
3.1. Optimization of welding variables
2.2. Sample fabrication The spot joint configuration, shown in Fig. 2, was selected for this study. The joints were fabricated from 125 50 1 mm sheets. A plug weld was located in the center of a 25 mm overlap region. A fixture was used to ensure consistent weld placement. Prior to welding, the steel sheets were degreased by acetone. Aluminum
Two aluminum–steel joint configurations shown in Fig. 2 were studied. To optimize the welding process for each joint configuration, quadratic regression analyses of various variables were performed. Tables 4 and 5 present the process parameters of orthogonal array and the estimated characteristics for AA6061T6-to-galvanized steel (i.e., Joint I) and galvanized steel-toAA6061-T6 (i.e., Joint II), respectively. The peak load to fracture
(a) Weld appearance Transient Brazed Interface
Weld Metal
Al 6061 Fusion zone
Al 6061 Galvanized Mild Steel
Middle Brazed Interface
1mm
(b) Cross section Fig. 5. (a) Appearance, and (b) cross section of optimized CMT welded 1 mm thick AA6061-T6-to-1 mm thick galvanized mild steel.
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Steel
Front side
Al sheet
Al sheet
Back side 10mm
10mm
(a) Weld appearance
Weld Metal
1mm
(b) Cross section Fig. 6. (a) Appearance and (b) cross section of optimized CMT welded 1 mm thick galvanized steel-to 1 mm thick AA6061-T6.
2
7 10
6
3 1 4 5
α-Al
8 9
Steel
Weld metal
Al-Si Brazing interface
10μm
Fe
100
40
40
30 60
Si
20
40
Si(%)
Fe(Al)(%)
50
Al
80
(a) Microstructure of weld metal
50
30
20
Mg(%)
(a)
10 20
Mg 0
10
0 0
10
20
Brazing interface
30
40
50
60
-10
0
Displacement ( μ m)
(b) Fig. 8. Line analysis results of the middle brazed interface for optimized CMT welded 1 mm thick AA6061 T6-to-1 mm thick galvanized mild steel: (a) crosssection, and (b) elemental distribution along the blue line in (a). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
(b) Microstructure of fusion zone Fig. 7. Microstructure of CMT welded Al-to-mild steel at the: (a) weld metal and (b) fusion zone.
of the joints (i.e., joint strength) is the metric used as the basis for the process optimization. Tables 6 and 7 show the analysis results on the rank of the process parameters using the Taguchi method for Joints I and II, respectively. As shown in Tables 6 and 7, K1j, K2j and K3j represent corresponding average values of the sum of loads with levels ‘‘1’’, ‘‘2’’ and ‘‘3’’ among the row j, respectively. The ‘delta’ value was estimated for each parameter from the difference of the maximum and minimum mean peak load values at
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R. Cao et al. / Journal of Alloys and Compounds 585 (2014) 622–632 Table 10 Energy spectrum analysis results of middle brazed interface in Fig. 8(a). Element (at.%)
1
2
3
4
5
6
7
8
9
10
AlK FeK SiK MgK Phases
46.48 50.64 2.88 – FeAl
70.50 26.45 3.05 – Fe2Al5
72.39 24.23 3.38 – Fe2Al5
74.13 23.76 2.11 – FeAl3
74.72 21.83 3.44 – FeAl3
90.47 9.53 – – FeAl3 and a-Al solid solution
95.35 4.65 – – FeAl3 and a-Al solid solution
78.09 – 21.91 – aAl+Si
94.28 2.59 2.12 1.00 a-Al solid solution
100 – – – a-Al solid solution
1000 Steel
Weld metal
Fe
1
Intensity (Cps)
800
3 6 2 4
5
7
Brazing interface
40
50
200
10
20
30
40
50
60
70
80
90
40
30
Al
Fig. 10. X-ray analysis at the brazed interface between the mild steel/weld metal for CMT welded 1 mm thick AA6061-T6-to-1 mm thick galvanized mild steel.
60
20
40
20
Zn(%)
30
Si(%)
Fe(Al)(%)
400
2θ
80
10
Si 20
10
Zn 0
0 0
20
30
40
50
Displacement ( μm)
(b) Fig. 9. Line analysis results of the transient brazing interface for optimized welded aluminum-to-galvanized mild steel: (a) cross-section, and (b) element distribution along the blue line in (a). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Table 11 Energy spectrum analysis results of transient brazed interface in Fig. 9(a). Element 1 (at.%)
2
3
4
5
6
7
AlK FeK SiK Phases
14.02 85.98 – a-Fe solid Solution
69.43 24.91 5.66 Fe2Al5
75.39 24.61 – FeAl3
91.29 8.71 – FeAl3 and aAl solid solution
98.35 1.65 – a-Al solid solution
98.41 1.59 – a-Al solid solution
2.89 97.11 – c-Fe solid solution
AlFe
0
Fe
0 10 Brazing interface
Al5Fe2Zn0.4
600
10μm
(a) 100
Al5Fe2
different levels of the same factor. The rank of each parameter generated by the Taguchi method was determined from the delta values of all the parameters. The rank of the parameters signifies their relative importance in terms of their influence on the output response, i.e., peak loads in this investigation. As shown in Table 6, the variables in the order of importance for Joint I are as follows: wire feeder speed (A) > hole diameter of Al sheet (B) > spot-welding time (C). When a single factor is considered, optimized welding variables can be obtained. i.e., a wire fee-
der speed of 8 m/min, a hole diameter of 7 mm on the Al sheet, and a welding time of 1.2 s. As shown in Table 7, the variables in the order of importance for Joint II are as follows: hole diameter of steel sheet (B) > wire feeder speed (A) > spot-welding time (C). When a single factor is considered, optimized welding variables can be obtained, i.e., a wire feeder speed of 7.5 m/min, a hole diameter of 7 mm on the steel sheet, a welding time of 0.8 s. A statistical analysis of variance (ANOVA) was performed to determine the statistical significance of the process parameters [21] and the effect of an individual input parameter on the output parameters. Tables 8 and 9 show the results of ANOVA analysis for Joints I and II, respectively. Based on these results, the wire feeder speed was found to be the most influential process parameter with a 49.4% contribution followed by the hole diameter on the Al sheet 19.0% for Joint I. The effect of single factor on the peak load of the joints is shown in Fig. 3. The optimized welding window (i.e., a wire feeder speed of 8 m/min, a hole diameter of 7 mm on the Al sheet, a spot-welding time of 1.2 s) was developed by combining the results shown in Fig. 3 and Table 6. As shown in Table 9 for steel-Al Joint II in Fig. 2(b), the hole diameter of steel sheet is found to be the most influential process parameter with a 44% contribution. The effect of single factor on the peak load of the joints is shown in Fig. 4. The optimized welding window (i.e. a wire feeder speed of 7.5 m/min, a hole diameter of 7 mm on the steel sheet, a spot-welding time of 0.8 s) was developed by combining the results shown in Fig. 4 and Table 7. 3.2. Weld appearance of optimized CMT welded Al 6061-mild steel joint The weld appearance and corresponding cross-sections of optimized CMT welded AA6061-T6-to-galvanized steel welds are shown in Fig. 5(a) and (b), respectively. As shown, the welded joint is composed of the weld metal, aluminum fusion zone on the
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6 5
CMT welded Al6061-gavanized mild steel, Joint I
5
4
Load (KN)
Load (kN)
4
CMT welded Al6061-Al6061
3
2
1 0 0.0
3
2
1
CMT welded gavanized mild steel-Al6061, Joint II
0 40
50
60
70
80
90
100
2
Brazing area(mm ) 0.4
0.8
1.2
1.6
2.0 Fig. 13. Effect of the brazed area on the strength of CMT welded 1 mm thick AA6061-T6-to-1 mm thick galvanized mild steel.
Displement (mm) Fig. 11. Effect of material stacking sequence on the strength of CMT weld-brazed 1 mm thick AA6061-T6–1 mm thick galvanized mild steel.
AA6061-T6, and the brazed interface (including transient brazed and middle brazed interfaces) between the weld metal and un-molten galvanized mild steel. The weld appearance and corresponding cross-sections of optimized galvanized steel-to-AA6061T6 joints are shown in Fig. 6(a) and (b), respectively. The fusion zone was formed between the aluminum weld metal and aluminum substrate, and little brazed interface zone was formed between the aluminum weld metal and steel substrate. As shown in Fig. 6(b), a narrow gap existed between the aluminum weld metal and the edge of the hole of steel substrate. The steel-to-aluminum joining joint in Fig.6 is similar to rivet joint in Ref. [22]. Detailed microstructures of the Joints I and II will be analyzed next.
3.3. Microstructure of optimized AA6061-galvanized mild steel joint To understand the bonding mechanisms of CMT spot welding AA6061-T6l-to-galvanized mild steel with filler wire Al 4043, the microstructures of the welds were analyzed. Fig. 7 presents the microstructures of the weld metal and fusion zone for Joint I. As can be observed in Fig. 7(a), the weld metal zone labeled by a solid rectangle in Fig. 5(b) is composed of equi-axed pro-eutectic a-Al solid solutions (i.e., grey base metal) and Al–Si second phases (i.e., bright white). The microstructure of aluminum fusion zone
labeled by a circle in Fig. 5(b) is shown in Fig. 7(b). As shown, coarse grain is produced at this region. Similar microstructures appeared for the welded galvanized steel-to-AA6061-T6 joint shown in Fig. 7(b). The microstructures of middle brazed interface and transient brazed interface shown in Fig. 5(b) will be analyzed next by combining line analysis and point analysis results to further explain the bonding mechanism of CMT spot welded Al6061-T6-togalvanized steel. 3.4. Bond mechanism of Al-steel joints Since the IMC formed at the interface between the AA6061-T6 and galvanized steel during CMT joining process, it is important to understand the microstructures of this interphase. As the molten aluminum (melting point is about 649 °C) deposited onto the galvanized steel workpiece, the liquid aluminum molten metal wetted the galvanized steel substrate. However, the rate of forming the Fe–Al intermetallic is quicker than the spread of the molten aluminum onto the galvanized steel surface which would have dissolved the Fe–Al intermetallics [23]. Therefore, the interface reaction–diffusion dominates the Fe–Al diffusion process [23]. Based on the Fe– Al phase diagram, refer to Fig. 1, the Fe–Al interaction reaction forms c-Fe and a-Al solid solutions, and intermetallic compounds Fe3Al, FeAl, FeAl2, Fe2Al5, FeAl3 and FeAl6. Previous studies showed that the Fe3Al and FeAl based alloys have the best oxidation resistance, sulfidation resistance, high specific strength and mid-tem-
Al sheet
steel
5mm
5mm
(a) Al sheet
steel
10mm
10mm
(b) Fig. 12. Fracture surface of CMT welded (a) AA6061-T6-galvanized mild steel, and (b) galvanized mild steel-AA6061-T6.
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Al sheet
Galvanized steel sheet
Brazed interface
(a) Schematic of fracture location
Al sheet
C
B
A
D 5mm
(b) Fracture surface of CMT welded AA6061 T6-galvanized mild steel
Point 1 Point 2
(c) Region A
(d) Region B Point 5 Point 6 Point 4 Point 3
(e) Region C
(f) Region D
Fig. 14. Fracture surface of CMT welded 1 mm thick AA6061 T6-to-1 mm thick galvanized mild steel: (a) schematic of fracture location, (b) fracture aluminum–steel brazing interface, and (c) magnified view of region A, (d) region B, (e) region C, and (f) region D.
Table 12 Energy spectrum analysis results of fracture surface in Fig. 14. Element (at. %)
1
2
3
4
5
6
Al Fe Si Zn Mg Phases
77.33 20.46 2.22 – – FeAl3
67.59 29.98 2.43 – – Fe2Al5
97.95 – – 02.05 – a-Al solid solution
88.65 – – 09.24 02.10 Al–Zn eutectic phase
71.65 1.65 21.36 05.98 01.01 Al–Zn– Si eutectic phase
96.74 – – 03.26 – a-Al solid solution + Al– Zn eutectic phase
perature strength [24]. Fig. 8(a) and (b) present the micrographs of the middle brazed interface shown in Fig. 5(b). The results showed that the interfaces with a thickness range of 5–8 lm which is less than the critical thickness of 10 lm presented by Schubert et al. [17] were formed between the weld metal and galvanized steel substrate. As shown, while the interface regions near the steel workpiece had a relative straight edge and uniform thickness, the interface zones near the weld metal had various thicknesses and grew into the weld metal, and some dispersed needle-shaped compounds were produced at the weld metal. To analyze the composition of the middle brazed interfacial zones, energy dispersive X-ray spectrometer (EDS) analysis was performed and the results are presented in Table 10 and
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Galvanized steel sheet
Al sheet
Weld metal
Fracture location
(a) schematic of fracture location
Al sheet B
A
C 5mm
Point 1
Point 2
(c) Region A
(d) Region A
Point 4
Point 7
Point 3 Point 6
Point 5
(e) Region B
(f) Region C
Fig. 15. Fracture surface of CMT welded 1 mm thick galvanized mild steel-to-1 mm thick AA6061-T6: (a) schematic of fracture location, (b) pull-out aluminum, (c) magnified view of region A, (d) magnified view of region A, (e) magnified view of region B, and (f) magnified view of region C.
Table 13 Energy spectrum analysis results of fracture surface in Fig. 15(a). Element (at.%)
1
2
3
4
5
6
7
Al Fe Si Zn Mg Phases
100 – – – – a-Al solid solution
96.87 – 03.13 – – a-Al solid solution and small Al–Si eutectic phase
63.26 – 00.32 34.31 02.11 Al–Zn eutectic phase
64.70 – 01.05 27.76 06.49 Al–Zn eutectic phase
20.78 – – 59.82 19.39 Al–Zn eutectic phase
12.57 – – 87.43 – a-Al solid solution+ Al–Zn eutectic phase
45.59 –
Fig. 8(b). As shown, the Fe element content decreased whereas the Al element content increased along the interface zones. The change is attributed to the diffusion of Fe and Al elements. By combining the phase diagram in Fe–Al and the features shown in Fig. 8(a) and the compositions in Table 10, it is most likely that the regions
54.41 – Al–Zn eutectic phase
1–10 in Fig. 8(a) contain FeAl, Fe2Al5, FeAl3, FeAl3 intermetallics and a-Al solid solution, a-Al+Si, and a-Al solid solution, respectively. Fig. 9(a) and (b) is the micrographs of the transient brazed interface shown in Fig. 5(b). Similar results have been observed in Table 11, the brazed interface is also composed of Fe2Al5 and
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FeAl3 intermetallics denoted by regions 3 and 4 shown in Fig. 9(a) and Table 11, and only the thinner interfaces with a thickness of 4 lm was formed. As the results shown above, the welding–brazing interface is composed of more layers with various compositions and phases for AA6061-T6-galvanized steel joints. In order to confirm the bonding mechanism of the weld-brazed joints, X-ray analyses of the interface of the specimens fractured along the interface between the weld metal and galvanized steel were conducted and the results are presented in Fig. 10. It was found that the interface is mainly composed of intermetallics Al5Fe2Zn0.4 and Fe2Al5 and FeAl.
3.5. Strength and fractures of CMT spot welded aluminum-galvanized steel Static testing of optimized CMT weld-brazed Al6061T6-galvanized steel joints was performed and the results are presented in Fig. 11. For the purpose of the comparison, test results of CMT spot plug welded joint of AA6061-T6-to-AA6061-T6 with a hole with the same width and thickness were also included in Fig. 11. For the sake of clarity, only one representative result was presented. As shown, the strength of CMT spot plug welded AA6061-T6-galvanized mild steel is significant stronger than that of CMT spot plug welded AA6061-T6 joints. Thorough analyses of the fractured surfaces of the tested specimens, shown in Fig. 12(a), the results indicated that the failure is located at the weld-brazed interface. Moreover, we carefully examined the thickness of the IMC at the weld-brazed interface of the optimized AA6061-T6-to-galvanized steel joints and found that the thicknesses of the intermetallics were about 8 lm which is slightly less than the critical value 10 lm [17]. With this thin intermetallic layer, we examined the effect of the brazed area at the interface on the joint strength and the results are presented in Fig. 13. It can be seen that in general the joint strength increased with increasing the areas of the brazed interface. These results suggest that the joint strength is related to the area of the AA6061-T6-galvanized steel brazed interface when the intermetallics of the brazed interface were kept below 10 lm. As shown in Fig. 6(b), CMT welded galvanized steel-toAA6061-T6 is similar to that of AA6061-AA6061 joints, and a weld nugget pull-out failure was observed in Fig. 12(b). Furthermore, the strength is comparable to that of CMT spot plug welded AA6061-T6. Fig. 14 presents the fracture features of the optimized AA6061 T6-to-galvanized mild steel. Point analyses of various regions were performed and the results are shown in Table 12. Fig. 14(a) presents that the optimized AA6061 T6-to-galvanized mild steel joint is fractured at the brazed interface. The fractured brazed interface at Al side is shown in Fig. 14(b). The magnified view of region A in Fig. 14(b) is shown in Fig. 14(c). As shown, coarser fracture feature was observed at the middle of aluminum–steel interface. Region B in Fig. 14(b) represents the fracture feature of typical aluminum– steel brazed interface. From Fig. 14(b)–(d), though the fracture features are different for regions A and B, the same intermetallic FeAl3 shown in Table 12 induced the final fracture. For region C in Fig. 14 (b), a-Al solid solution was formed and is shown in Fig. 14(e). For region D in Fig. 14 (b), it can be seen from Table 12 that Al–Zn eutectic phase, Al–Zn–Si eutectic phase were produced at the fracture surface and are shown in Fig. 14(f). Regions C and D could not be properly connected due to the fact that zinc vapor produced during CMT joining and produced significant amount of porosities shown in Fig. 14(b) and (f). These observations suggest that the strength of the CMT Al-steel spot welded joints primarily depend on the connected area of regions A and B.
631
Fig. 15 presents the fracture features CMT welded 1 mm thick galvanized steel-to-1 mm thick AA6061-T6. Point analyses of various regions were conducted and the results are shown in Table 13. As shown in Fig. 15(a), bold line presents fracture location of steel-Al joint, i.e. weld pull-out fracture was observed shown in Fig. 15(b). From analyses of points 1 and 2 of the fracture surface as shown in Fig. 15(b)–(d), the joints fractured at the edge (denoted by region A) of aluminum weld metal which is composed of a-Al solid solution and small amount of Al–Si eutectic phase. Unsatisfied bonding was formed at the regions B and C. This observation is supported by the results shown in Fig. 15(e) and (f) and Table 13. More zinc element was produced at these regions. As much as 87.43% zinc element was formed at Point 6. Though Al–Zn phases were formed at these regions B and C, these phases may contribute little in joint strength because a large gap existed between the aluminum weld metal and the edge of the hole on the steel substrate. Furthermore, points 3–6 contain little Fe element and these results confirmed that poor bonding developed between the regions B and C. By analyzing the joint strength, fracture features and bonding mechanism, the results showed that while the strength of spot plug welded 1 mm thick AA6061 joints is lower than that of 1 mm thick Al AA6061-to-1 mm thick galvanized mild steel joint, it is comparable to that of 1 mm thick galvanized steel-to-1 mm thick Al AA6061 welded joints. In short, it is recommended to choose the stacking sequence that the AA6061-T6 sheet is placed on the top of the steel. 4. Conclusions According to the study on the CMT welded Al AA6061-T6 and galvanized mild steel dissimilar alloys, the followed results can be summarized: (1) CMT spot plug welding of 1 mm thick Al AA6061-T6-1 mm thick galvanized mild steel with filler wire Al 4043 was conducted. Test results showed that it is feasible to obtain a sound joint if the wire feeder speed is properly controlled. (2) The optimized welding variables for AA6061-T6-to-galvanized mild steel are: a wire feeder speed of 8 m/min, a hole diameter of 7 mm on the Al sheet and a welding time of 1.2 s. Similarly, the optimized welding variables for galvanized mild steel-to-AA6061-T6 are: a wire feeder speed of 7.5 m/min, a hole diameter of 7 mm on the steel sheet, and a welding time of 0.8 s. (3) CMT spot plug welded Al AA6061-to-galvanized steel joints were composed of the fusion zone of Al AA6061 sheet and Al 4043 filler wire, Al weld metal, and the brazing interface between the Al weld metal and galvanized mild steel. The brazing interface consists of about 5–8 lm thick FeAl3 intermetallic. For CMT spot plug welded galvanized mild steel-toAl AA6061 joints, the joints were only composed of Al weld metal. (4) Material stacking sequence affects the strength of CMT spot plug welded joints. While the strength of spot plug welded 1 mm thick AA6061 joints is lower than that of 1 mm thick Al AA6061-to-1 mm thick galvanized mild steel joint, it is comparable to that of 1 mm thick galvanized steel-to 1 mm thick Al AA6061 welded joints.
Acknowledgement This work was financially supported by National Nature Science Foundation of China (No. 51265028), and GM-Research and Development Center, MI, USA.
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