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Research Papers

Characterizing the Hemming Performance of Automotive Aluminum Alloys With High-Resolution Topographic Imaging

[+] Author and Article Information
M. R. Stoudt

Materials Performance Group,
National Institute of Standards and Technology,
100 Bureau Drive,
Gaithersburg, MD 20899-8553
e-mail: stoudt@nist.gov

J. B. Hubbard

Materials Performance Group,
National Institute of Standards and Technology,
100 Bureau Drive,
Gaithersburg, MD 20899-8553
e-mail: Joseph.hubbard@nist.gov

J. E. Carsley

General Motors Technical Center,
30500 Mound Road,
Warren, MI 48090-9005
e-mail: john.carsley@gm.com

S. E. Hartfield-Wünsch

General Motors Technical Center,
30001 Van Dyke Road,
Warren, MI 48090
e-mail: susan.hartfield-wunsch@gm.com

2Certain commercial equipment, instruments, software, or materials are identified in this paper to foster understanding. Such identification does not imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose.

3Based on the range observed in the topographic data, the specimen curvature produced an offset where the edges of the 400 μm wide image were approximately 20 μm lower in the z-plane than the apex region.

1Corresponding author.

Contributed by the Materials Division of ASME for publication in the JOURNAL OF ENGINEERING MATERIALS AND TECHNOLOGY. Manuscript received June 19, 2013; final manuscript received February 26, 2014; published online April 1, 2014. Assoc. Editor: Joost Vlassak.

This material is declared a work of the U.S. Government and is not subject to copyright protection in the United States. Approved for public release; distribution is unlimited.

J. Eng. Mater. Technol 136(3), 031001 (Apr 01, 2014) (11 pages) Paper No: MATS-13-1100; doi: 10.1115/1.4027093 History: Received June 19, 2013; Revised February 26, 2014

We used high-resolution quantitative surface analysis to evaluate the surfaces of two aluminum automotive closure panel alloys, which were bent to a 180 deg angle in a simulated hemming test. Maps of the displacements normal to the sheet were superimposed on the topographies to correlate the location of the maximum displacements and the surface morphology. While the alloys had similar mechanical properties, quantitative analyses yielded considerable differences in the deformed surface morphologies. One alloy had a greater density and broader size distribution of constituent particles, which increased the likelihood for particle decohesion. This resulted in large surface displacements that were uncorrelated with the underlying microstructure. While no splitting was observed in either alloy, large uncorrelated surface displacements could indicate the presence of short surface cracks.

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Figures

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Fig. 1

An actual aluminum hem joint presented in a cross-sectional view

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Fig. 2

The three-point bend fixture used to evaluate hemming behavior. (a) Initial positions of the rollers, the punch, and the specimen. (b) The punch has bent the sample to a 180 deg angle. (c) The punch is presented schematically.

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Fig. 3

The as-polished microstructures presented for the two alloys in the RD plane

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Fig. 4

The (0.2 to 8.0) μm2 segment of the particle distributions are presented for the IBR and IH alloys. The histograms are the results of image analyses performed on the as-polished microstructures in Fig. 3. The analyses were based on 723 total particles for the IBR alloy and 425 total particles for the IH alloy.

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Fig. 5

The microstructure of the IBR alloy (modified Keller's etch) is presented as an isometric perspective view of the three sheet orientations (i.e., rolling plane, long transverse, and short transverse)

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Fig. 6

The microstructure of the IH alloy (modified Keller's etch) is presented as an isometric perspective view of the three sheet orientations (i.e., rolling plane, long transverse, and short transverse)

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Fig. 7

The mechanical behavior in uniaxial tension is presented for the IBR and IH alloys. RD denotes the tensile direction is parallel to the rolling direction and TD denotes the tensile direction is perpendicular to the rolling direction.

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Fig. 8

Representative optical micrographs acquired for each of the four alloy/strain conditions after bending to a 180 deg angle in the simulated hemming test. The surface morphologies with 0% prestrain are presented for the IBR and IH alloys in (a) and (b), and with 10% prestrain in (c) and (d), respectively. The orientations of the RD and the bend direction are indicated in (a) and are the same for the other images in this figure. The dashed line indicates the approximate location of the bend apex.

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Fig. 9

An illustration of the technique used to construct a strain localization map using the image in Fig. 8(c). (a) is the original deformed surface, (b) is the original Rt map constructed from the surface data in (a), (c) are the remaining data after the application of a 95% threshold, and (d) is the composite image after overlaying (c) onto (a). The color bar in the figure is a standard 8-bit color scale (where blue is low and red is high) used to map the Rt data so that the color of an individual cell directly correlates with the magnitude of Rt.

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Fig. 10

The Rt-map/deformed surface composite images constructed for the IBR and IH alloys with the overlay procedure outlined in Fig. 8. The 0% prestrain condition is presented in (a) and (b), and for the 10% prestrain condition in (c) and (d). The dashed line again indicates the approximate location of the bend apex.

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Fig. 11

The influence of prestrain on the overall character of the surface roughness is presented for the IBR alloy. Each data point in the figure is the maximum Rt value plotted as a function of position with respect to the apex of the bend.

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Fig. 12

The influence of prestrain on the overall character of the surface roughness is presented for the IH alloy. Each data point in the figure is the maximum Rt value plotted as a function of position with respect to the apex of the bend.

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