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Research Papers

Local Mechanical Properties of a Magnesium Hood Inner Component Formed at Elevated Temperature

[+] Author and Article Information
Vesna Savic1

 General Motors LLC, 30200 Mound Road, Warren, MI 48090-9010

Louis G. Hector, Sooho Kim, Ravi Verma

 General Motors LLC, 30200 Mound Road, Warren, MI 48090-9010

1

Corresponding author.

J. Eng. Mater. Technol 132(2), 021006 (Feb 17, 2010) (10 pages) doi:10.1115/1.4000222 History: Received January 13, 2009; Revised July 24, 2009; Published February 17, 2010; Online February 17, 2010

There is considerable worldwide interest in magnesium (Mg) sheet as a replacement for heavier steel and aluminum alloys in vehicle closure components. As Mg gains acceptance in the automotive industry, there will be an increasing demand for accurate material properties for finite element simulations of Mg structures. In this paper, we investigate the extent to which average grain size and postformed tensile properties vary across a Mg AZ31B hood inner component formed at 485°C for 20 min under a constant gas pressure. Tensile specimens were extracted from six regions of the hood inner, which underwent varying degrees of thinning. A state-of-the-art digital image correlation (DIC) algorithm and custom image acquisition software provided true stress-true strain data for each specimen. Tensile data acquired during room temperature testing was compared with that from baseline (undeformed) Mg AZ31B in a fully recrystallized condition (O-temper). Due to its importance in finite element simulations, particular emphasis was placed on the variation of postformed yield strength with specimen thickness and average grain size. Finally, we compute local strain fields during fracture in a tensile specimen with DIC grids positioned in the failure region.

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Copyright © 2010 by American Society of Mechanical Engineers
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Figures

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Figure 1

Micrograph of the as-received Mg AZ31B in the H24 condition. Note the high density of twins.

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Figure 2

Mg AZ31B hood inner following the elevated temperature gas blow forming process at 485°C. The numbers label six regions of the component from which tensile specimens were extracted after forming. The dashed white arrows denote the raised (rib) regions, which underwent substantial thinning during forming under a constant gas pressure. The solid white arrow shows the rolling direction of the unformed sheet.

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Figure 3

Tensile specimen geometry. All dimensions are in mm.

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Figure 4

Miniature tensile testing apparatus with tensile specimen moments after fracture

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Figure 5

True stress-true strain curves for H24, O-temper, and selected groups from the hood inner: (a) for tensile specimens cut along the RD and (b) for tensile specimens cut along the TD. Group 1 curves are added strictly for qualitative comparison. All curves terminate prior to the onset of fracture.

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Figure 6

Yield strength variation with postformed thinning (%). RD means rolling direction and TD means transverse direction.

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Figure 7

(a) Micrograph of AZ31B after O-temper heat treatment (grain size: 8.6 μm, aspect ratio: 1.10); (b) micrograph of the section 6 grain structure (grain size: 13.1 μm, aspect ratio: 1.07); (c) micrograph of the section 4 grain structure: (grain size: 16.7 μm, aspect ratio: 1.10); and (d) micrograph of the rib section grain structure (grain size: 20.4 μm, aspect ratio: 1.20)

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Figure 8

Yield strength variation with grain size for O-temper material and groups 4 and 6

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Figure 9

Cumulative strain maps for a specimen taken from the rib area of the hood inner (prior to fracture): (a) axial strain contours, (b) transverse strain contours, and (c) shear strain contours

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Figure 10

Selected images from a tensile test of a rib specimen following the onset of fracture. Corresponding crosshead loads and displacements are also listed. Tensile axis is along the horizontal. (a) Image 64, a crack has developed as indicated in the figure (F=640 N and d=1.5 mm), (b) Image 70, a crack in the lower specimen surface is now evident (F=252 N and d=1.7 mm), and (c) Image 92 (F=112 N and d=2.2 mm).

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Figure 11

Cumulative strain maps corresponding to Fig. 1: (a) axial strain contours, (b) transverse strain contours, and (c) shear strain contours

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Figure 12

Cumulative strain maps corresponding to Fig. 1: (a) axial strain contours, (b) transverse strain contours, and (c) shear strain contours

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Figure 13

Cumulative strain maps corresponding to Fig. 1: (a) axial strain contours, (b) transverse strain contours, and (c) shear strain contours

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