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Research Papers

Microstructure and Mechanical Performance of Cold Metal Transfer Spot Joints of AA6061-T6 to Galvanized DP590 Using Edge Plug Welding Mode

[+] Author and Article Information
Haiyang Lei, Zhongqin Lin

State Key Laboratory of Mechanical
System and Vibration,
Shanghai Key Laboratory of Digital
Manufacture for Thin-Walled Structures,
School of Mechanical Engineering,
Shanghai Jiao Tong University,
Shanghai 200240, China

Yongbing Li

Mem. ASME
State Key Laboratory of Mechanical
System and Vibration,
Shanghai Key Laboratory of Digital
Manufacture for Thin-Walled Structures,
School of Mechanical Engineering,
Shanghai Jiao Tong University,
Shanghai 200240, China
e-mail: yongbinglee@sjtu.edu.cn

Blair E. Carlson

Manufacturing Systems Research Lab,
General Motors Research
and Development Center,
30500 Mound Road,
Warren, MI 48090

1Corresponding author.

Manuscript received May 21, 2015; final manuscript received October 30, 2015; published online March 9, 2016. Assoc. Editor: Wayne Cai.

J. Manuf. Sci. Eng 138(7), 071009 (Mar 09, 2016) (13 pages) Paper No: MANU-15-1246; doi: 10.1115/1.4032082 History: Received May 21, 2015; Revised October 30, 2015

Dissimilar joining of aluminum to steel poses a challenge for arc welding. In this study, aluminum AA6061-T6 and hot dipped galvanized DP590 steel were joined using the Fronius cold metal transfer (CMT) welding process applying an edge plug welding mode (EPW). The correlation of the welding parameters, weld characteristics, and weld strength was systematically investigated. It was found that the EPW mode created a zinc-rich zone at the weld root along the Al–steel faying interface which transitioned to a continuous and compact intermetallic compounds (IMC) layer in the middle portion of the joint. The fracture propagation in lap-shear specimens was affected by this increase of IMC layer thickness. At a wire feed speed (wfs) of 5.6 m/min, the fracture initiated along the zinc-rich layer at the faying interface and then, upon meeting the compact IMC layer, propagated into the aluminum weld nugget. Propagation followed a path within the weld nugget along the boundary between columnar and equiaxed grains leading to weld nugget pullout upon fracture. For IMC layer peak thicknesses below 10 μm, the strength increased as a function of weld nugget diameter. However, larger heat inputs resulted in IMC layer thicknesses greater than 10 μm and interfacial fracture.

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Figures

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Fig. 1

CMT robotic welding system

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Fig. 2

Schematic of CMT spot weld coupons: 1 mm-thick AA6061-T6 aluminum and 1.2 mm-thick HDG DP590 steel (dimension in mm)

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Fig. 3

Illustration of the EPW mode of CMT spot welding

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Fig. 4

Appearance, cross section, and fractography of CMT welded 1 mm-thick AA6061-T6 to 1.2 mm-thick bare DP590 made with a wfs of 5.6 m/min: (a) top view of the weld, (b) polished cross section of the joint, (c) close examination of the IMC of region c shown in (b), (d) close examination of the IMC of region d shown in (b), (e) close examination of the IMC of region e shown in (b), and (f) fractography of a lap-shear specimen exhibiting interfacial fracture

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Fig. 5

Effect of wfs on the cross section of CMT welded 1 mm-thick AA6061-T6 aluminum to 1.2 mm-thick HDG DP590 steel joints: (a) 4.2 m/min, (b) 5.6 m/min, and (c) 6.4 m/min

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Fig. 6

Photos of joint fracture surfaces corresponding to different wfs settings during welding: (a) 4.2 m/min, (b) 5.6 m/min, and (c) 6.4 m/min. Note that the steel sheet faying surface is exposed with the fractured weld in the center and that the photomicrographs of polished weld cross sections exhibit combined crack initiation and propagation resulting from lap-shear testing of EPW mode welded 1 mm-thick AA6061-T6 to 1.2 mm-thick HDG DP590 joints.

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Fig. 7

Microhardness testing of different areas of the EPW mode weld metal at a wfs of 5.6 m/min: (a) illustration of the test areas and Vickers hardness results from (b) traverse #1, (c) traverse #2, and (d) traverse #3. Note that the 0 position is the starting point on the left-hand side of the photo.

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Fig. 8

Interface region between columnar and equiaxed grain structures along the crack propagation path of the weld nugget fabricated using the EPW mode and a wfs of 5.6 m/min: (a) lower half and (b) upper half of weld nugget

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Fig. 9

CMT welded 1 mm-thick AA6061-T6 and 1.2 mm-thick galvanized DP590 steel joint made with a wfs of 5.6 m/min: (a) cross section of the weld, (b) enlarged view of the region b presented in (a), (c) enlargement of region c as indicated in (b), and (d) close-up view of the region d shown in (b)

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Fig. 10

Photomicrograph of a polished cross section of a weld which had undergone interrupted lap-shear testing. Crack propagation (from left to right in the photo) is observed along the Zn-rich layer at the faying interface up to point A whereupon the crack deviates upward into the weld metal.

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Fig. 11

Microhardness results of different areas of the EPW mode weld metal at a wfs of 4.2 m/min: (a) identification of the test areas; Vickers hardness results from (b) traverses #1 and #2, (c) traverse #3, and (d) traverse #4. Note that the 0 position is the starting point on the left-hand side of (a).

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Fig. 12

Transition zones of grain structure along the crack propagation path in the weld metal using the EPW mode at wfs of 4.2 m/min: (a) left lower region, (b) right lower region, and (c) upper right region

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Fig. 13

Weld using EPW mode at a wfs of 6.4 m/min: (a) enlarged view of the brazed interface at the root of the weld and (b) enlarged view of the crack along the interface shown in Fig. 6(c)

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Fig. 14

Measurements of the IMC layer thickness distributions of CMT plug welded 1 mm AA6061-T6 to 1.2 mm galvanized DP590 steel joint: (a) welds at wfs of 4.0–4.8 m/min with partial thickness fracture mode, (b) welds at wfs of 5.2–5.6 m/min with button fracture mode, and (c) welds at wfs of 5.8–6.4 m/min with interfacial fracture mode

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Fig. 15

Schematic of a CMT Al-steel plug weld cross section exhibiting a Zn-rich eutectic region and IMC peak thickness (T) on the right side of the weld, and a weld of size (D)

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Fig. 16

Plots of CMT plug welded 1 mm AA6061-T6 to 1.2 mm galvanized DP590 steel joint strength, IMC layer peak thickness, and weld size versus wfs and heat input

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