Research Papers: JOINING

Formation and Structure of Work Material in the Friction Stir Forming Process

[+] Author and Article Information
Sladjan Lazarevic

Department of Mechanical Engineering,
University of Hawaii at Manoa,
2540 Dole Street,
Honolulu, HI 96822
e-mail: laz@hawaii.edu

Kenneth A. Ogata

Department of Mechanical Engineering,
University of Hawaii at Manoa,
2540 Dole Street,
Honolulu, HI 96822
e-mail: kogata009@gmail.com

Scott F. Miller

Department of Mechanical Engineering,
University of Hawaii at Manoa,
2540 Dole Street,
Honolulu, HI 96822
e-mail: scott20@hawaii.edu

Grant H. Kruger

Department of Mechanical Engineering,
University of Michigan,
2350 Hayward Street,
Ann Arbor, MI 48105
e-mail: ghkruger@umich.edu

Blair E. Carlson

Manufacturing Systems Research,
General Motors Technical Center,
Warren, MI 48092
e-mail: blair.carlson@gm.com

Contributed by the Manufacturing Engineering Division of ASME for publication in the JOURNAL OF MANUFACTURING SCIENCE AND ENGINEERING. Manuscript received December 21, 2014; final manuscript received May 12, 2015; published online September 4, 2015. Assoc. Editor: Edmund Chu.

J. Manuf. Sci. Eng 137(5), 051018 (Sep 04, 2015) (9 pages) Paper No: MANU-14-1694; doi: 10.1115/1.4030641 History: Received December 21, 2014

Friction stir forming (FSF) is a new environmentally friendly manufacturing process for lap joining of dissimilar materials. Fundamentally, this process is based on frictionally heating and mechanically stirring work material of the top piece in a plasticized state to form a mechanical interlocking joint within the bottom material. In this research, the significant process parameters were identified and optimized for Al 6014 alloy and mild steel using a design of experiments (DOE) methodology. The overall joint structure and grain microstructure were mapped as the FSF process progressed and the aluminum work material deformed through different stages. It was found that the work material within the joint exhibited two layers, thermomechanical affected zone, which formed due to the contact pressure and angular momentum of the tool, and heat affected formation zone, which was composed of work material formed through the hole in the steel sheet and into the anvil cavity. Two different geometries of anvil design were employed to investigate geometrical effects during FSF of the aluminum. It was found that the direction and amount of work material deformation under the tool varies from the center to the shoulder.

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Fig. 1

Illustration of steps and joint of the FSF process [17]: (a) the three stages of the joining procedure and (b) A and B are the sheet thicknesses, C is the diameter of the indention and tool, D is the pin diameter, E is the cavity diameter, F is the upset material, and G and H are the heat affected zone

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Fig. 2

FSF tool and a finished sample: (a) tool, (b) front view of a sample, and (c) back view of a sample. On the left, the black arrows show the working diameter of the tool (20 mm).

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Fig. 3

Setup of FSF experiments: on the left is the CNC machine used for the experimentation and on the right is a closer look of the setup

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Fig. 4

Anvil designs with different cavities: (a) slot cavity and (b) hole cavity

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Fig. 5

Factorial interaction plot of two way ANOVA analysis showing effect of the parameters on lap-shear strength: (a) tool diameter, (b) tool depth, (c) tool position, and (d) anvil cavity depth

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Fig. 6

Force and torque measured during the FSF process for joint between aluminum and steel with lines A–E marking where the process was stopped to produce metallurgical samples

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Fig. 7

Cross-sectional view of samples for different tool depth in joint: (a) 0.4 mm tool depth, (b) the neck is formed, tool depth: 0.6 mm, (c) the initial stage of the head formation, tool depth: 0.8 mm, (d) the material started to move laterally, tool depth: 1.0 mm, and (e) head formation was finished, tool depth: 1.6 mm

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Fig. 8

Area of different boxes shown in Fig. 7: (a) box in Fig. 7(a) with tool depth of 0.4 mm, (b) box in Fig. 7(b), tool depth: 0.6 mm, (c) box in Fig. 7(c), tool depth: 0.8 mm, (d) box in Fig. 7(d), tool depth: 1.0 mm, (e) left box in Fig. 7(e), tool depth: 1.6 mm, and (f) right box in Fig. 7(e), this is a closer look on the flesh on the top of the aluminum piece

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Fig. 9

The underside of the Al 6014 samples showing deformation into the slot cavity (bottom view): (a) 0.3 mm, (b) 0.6 mm, and (c) 0.9 mm

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Fig. 10

The underside of the Al 6014 samples showing deformation into the hole cavity (bottom view): (a) 0.4 mm, (b) 0.6 mm, and (c) 0.9 mm

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Fig. 11

FSF process axial force and torque measurement for slot anvil design with lines marked F, G, and H where separate tests were stopped for metallurgical samples. Distance refers to depth of the tool as it plunges into the work material from first contact.

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Fig. 12

FSF process axial force and torque measurement for hole anvil design with lines marked I, J, and K where separate tests were stopped for metallurgical samples

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Fig. 13

Grain structure of FSF material formed in slot cavity: (a) 0.3 mm slot sample, (b) 0.6 mm slot sample, and (c) 0.9 mm slot sample

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Fig. 14

Grain structure of FSF material formed in four hole cavity: (a) 0.4 mm holes sample, (b) 0.6 mm holes sample, and (c) 0.9 mm holes



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