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Research Papers

Effects of Process Parameters on Friction Stir Spot Welding of Aluminum Alloy to Advanced High-Strength Steel

[+] Author and Article Information
Kai Chen

Mechanical Engineering Department,
University of Michigan,
Ann Arbor, MI 48105
e-mail: chenkai@umich.edu

Xun Liu

Mechanical Engineering Department,
University of Michigan,
Ann Arbor, MI 48105
e-mail: xunxliu@umich.edu

Jun Ni

Mechanical Engineering Department,
University of Michigan,
Ann Arbor, MI 48105
e-mail: junni@umich.edu

Manuscript received October 20, 2016; final manuscript received February 24, 2017; published online May 25, 2017. Assoc. Editor: Wayne Cai.

J. Manuf. Sci. Eng 139(8), 081016 (May 25, 2017) (9 pages) Paper No: MANU-16-1557; doi: 10.1115/1.4036225 History: Received October 20, 2016; Revised February 24, 2017

This paper studies a friction stir spot welding (FSSW) process that has been successfully applied to join aluminum alloy 6061-T6 to transformation-induced plasticity steel (TRIP) 780/800 steel. Cross sections of weld specimens show the formation of a hook with a swirling structure. A higher magnified scanning electron microscope (SEM) view of the swirling structure with energy dispersive X-ray spectroscopy (EDS) analysis reveals that it is composed of alternating thin layers of steel and Al–Fe intermetallic compounds (IMCs). To check the effect of different process parameters on the weld strength, the effects of tool plunge speed and dwell time were studied through the design of experiments (DOE) and analysis of variance (ANOVA) method. It shows that dwell time is a more dominant parameter in affecting the weld strength than plunge speed. Furthermore, investigation of failure using a lap shear tests reveals that cross nugget failure is the only failure mode. It also shows that cracks are initiated in the swirling structure at the tensile side of the weld nugget. After failure, a cleavage feature can be observed on the fractured surface.

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References

Figures

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Fig. 1

Dimensions of the FSSW tool

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Fig. 2

Dimensions of lap shear test (unit: mm)

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Fig. 3

Plunge force as a function of the plunge distance (Plunge speed 5 mm/min, dwell time 10 s)

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Fig. 4

Maximum plunge force at different plunge speeds

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Fig. 5

(a) Cross section view of the weld nugget, (b) geometry of the hook, and (c) detailed view of the right end side of welding zone (plunge speed 10 mm/min, dwell time 5 s)

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Fig. 6

(a) Typical hook geometry on the left side, (b) EDS line test on the swirling structure, and (c) element weight percentage from EDS line test (plunge speed 5 mm/min, dwell time 10 s)

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Fig. 7

Al–Fe phase diagram [17]

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Fig. 8

(a) Groove line shows up after etching and (b) EDS line test from point A to point B (plunge speed 10 mm/min, dwell time 3 s)

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Fig. 9

ROI count for zinc from EDS line test

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Fig. 10

(a) EDS line test on the coating of steel and (b) element distributions from the test result

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Fig. 11

Vickers hardness distribution (plunge speed 10 mm/min, dwell time 5 s)

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Fig. 12

Joint shear force from different plunge speeds

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Fig. 13

Cross-sectional view of FSSW joint with plunge speed of 15 mm/min

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Fig. 14

Main effects of different plunge speeds and dwell time on joint shear force

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Fig. 15

Interaction plot of different plunge speeds and dwell time on joint shear force

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Fig. 16

(a) Cross nugget failure mode and (b) schematic illustration of tensile shear test

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Fig. 17

Cross section view of the fracture: (a) At the beginning of tensile test: Cracks formed in the IMCs and (b) at the end of tensile test: Cracks propagate and fracture occurs

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Fig. 18

Illustration of the failure process in the right side of the nugget, where the aluminum is subjected to tensile stress

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Fig. 19

Illustration of the failure process in the left side of the nugget, where the aluminum is subjected to compressive stress

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Fig. 20

Failure on the left side of the nugget

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Fig. 21

(a) Failure happened at the tensile side while the compressive side is still connected and (b) enlarged view of the fracture region

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Fig. 22

SEM images of the fracture surface on the aluminum side

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