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Research Papers

Material Flow Visualization of Dissimilar Friction STIR Welding Process Using Nano-Computed Tomography

[+] Author and Article Information
Xun Liu

Welding Engineering Program,
Department of Material Science and Engineering,
The Ohio State University,
Columbus, OH 43221

Sheng Zhao

Shanghai Jiao Tong University,
Shanghai 200240, China

Kai Chen, Jun Ni

Department of Mechanical Engineering,
University of Michigan,
Ann Arbor, MI 48105

Manuscript received November 18, 2017; final manuscript received June 17, 2018; published online August 31, 2018. Assoc. Editor: Wayne Cai.

J. Manuf. Sci. Eng 140(11), 111010 (Aug 31, 2018) (8 pages) Paper No: MANU-17-1717; doi: 10.1115/1.4040915 History: Received November 18, 2017; Revised June 17, 2018

In this study, the friction stir welding (FSW) of aluminum alloy 6061-T6511 to TRIP 780 steel is analyzed under various process conditions. Two FSW tools with different sizes are used. To understand the underlying joining mechanisms and material flow behavior, nano-computed tomography (nano-CT) is applied for a 3D visualization of material distribution in the weld. With insufficient heat input, steel fragments are generally scattered in the weld zone in large pieces. This is observed in a combined condition of big tool, small tool offset, and low rotating speed or a small tool with low rotating speed. Higher heat input improves the material flowability and generates a continuous strip of steel. The remaining steel fragments are much finer. When the volume fraction of steel involved in the stirring nugget is small, this steel strip can be in a flat shape near the bottom, which generally corresponds to a better joint quality and the joint would fracture in the base aluminum side. Otherwise, a hook structure is formed and reduces the joint strength. The joint would fail with a combined brittle behavior on the steel hook and a ductile behavior in the surrounding aluminum matrix.

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References

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Figures

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Fig. 5

Fracture surface of the specimen failed in the Al side: (a) overview, (b) higher magnified view of the center of the fracture surface showing the dimple structure, and (c) a further higher magnified view of the dimple structure (welding condition: big tool, tool offset 1.65 mm, 1800 rpm, 60 mm/min; joint strength: 249.00 MPa)

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Fig. 4

Four different failure modes of the obtained joints: (a) type I: failed in the Al side (big tool, tool offset 1.65 mm, 1800 rpm, 60 mm/min), (b) type II: failed at the outside boundary of the steel hook (big tool, tool offset 1.65 mm, 1200 rpm, 120 mm/min), (c) type III: failed at the large steel fragments (big tool, tool offset 1.00 mm, 1800 rpm, 60 mm/min), and (d) type IV: failed at the Al–Fe interface in the bulk steel side (small tool, tool offset 1.00 mm, 1200 rpm, 120 mm/min)

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Fig. 3

(a) Typical dissimilar material weld (big tool, tool offset 1.65 mm, 1800 rpm, 60 mm/min) and (b) weld with large surface groove defect (big tool, tool offset 1.00 mm, 1800 rpm, 120 mm/min)

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Fig. 6

Fracture surface of the specimen failed at the steel fragments: (a) region near the steel fragments, (b) overview, and (c) main region of the aluminum matrix (welding condition: big tool, tool offset 1.00 mm, 1800 rpm, 60 mm/min; joint strength: 141.40 MPa)

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Fig. 7

Effects of rotating and welding speed on temperature distribution (big tool and small tool offset 1.00 mm)

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Fig. 8

Effects of rotating and welding speed on temperature distribution (small tool and small tool offset 1.00 mm)

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Fig. 9

Effects of the tool size on temperature distribution (rotating speed 1800 rpm, welding speed 60 mm/min and tool offset 1.00 mm)

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Fig. 10

Effects of tool offsets on temperature distribution (big tool, rotating speed 1800 rpm, and welding speed 60 mm/min)

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Fig. 11

(a) Two-dimensional cross-sectional and (b) 3D view of the steel distribution in the weld with the type I failure mode, where fracture occurs in the aluminum side (welding condition: big tool, tool offset 1.65 mm, 1800 rpm, 60 mm/min; Joint strength: 249.00 MPa)

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Fig. 12

Three-dimensional view of the steel distribution in the weld with large surface groove defects (welding condition: big tool, tool offset 1.00 mm, 1200 rpm, 120 mm/min)

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Fig. 13

(a) Two-dimensional cross-sectional and (b) 3D view of the steel distribution in the weld with type II or type III failure modes associated with hook or large steel fragment (welding condition: big tool, tool offset 1.00 mm, 1800 rpm, 60 mm/min; joint strength: 141.40 MPa)

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Fig. 14

Two-dimensional cross section and 3D view of the steel distribution in the weld for the type IV failure mode where fracture occurs along the Al–Fe interface in the bulk steel side (welding condition: small tool, tool offset 1.00 mm, 1200 rpm, 60 mm/min, joint strength: 160.51 MPa)

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Fig. 2

Schematic illustration of the dog bone tensile specimens and nano-CT samples with regard to the FSW weld

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Fig. 1

Schematic illustration of the experimental setup: Ds is the shoulder diameter, and Dp is the pin diameter at the tip; four thermocouples are attached to the bottom surface of the workpiece

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