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Research Papers

The Springback Characteristics of a Porous Tantalum Sheet-Metal

[+] Author and Article Information
Paul S. Nebosky

 Department of Aerospace and Mechanical Engineering, University of Notre Dame, Notre Dame, IN 46556pnebosky@sitesmedical.com

Steven R. Schmid1

 Department of Aerospace and Mechanical Engineering, University of Notre Dame, Notre Dame, IN 46556schmid.2@nd.edu

M.-A. Sellés

 Polytechnic University of Valencia, Department of Mechanical and Materials Engineering, Alcoy, Spainmaselles@dimm.upv.es

1

Corresponding author.

J. Manuf. Sci. Eng 133(6), 061022 (Dec 21, 2011) (9 pages) doi:10.1115/1.4005356 History: Received October 28, 2010; Revised October 11, 2011; Published December 21, 2011; Online December 21, 2011

This study examines the elastic recovery (springback) of a porous tantalum foam after sheet forming operations. The foam and sheet-like form is applicable to bone ingrowth surfaces on orthopedic implants and is desirable due to its combination of high strength, low relative density, and excellent osteoconductive properties. Forming of the foam improves nestability during manufacture and is essential to have the material achieve the desired shape. Experimentally, bending about a single axis using a wiping die is studied by observing cracking and measuring springback. Die radius and clearance strongly affect the springback properties, while punch speed, embossing, die radius, and clearance all influence cracking. To study the effect of the foam microstructure, bending also is examined numerically. A horizontal hexagonal mesh comprised of beam elements is employed, which allows for the densification that occurs during forming. The flow strength of individual tantalum struts is directly measured in an atomic force microscope. The numerical results show that as the hexagonal cells are elongated along the sheet length, elastic springback decreases. By changing the material properties of the struts, the models can be modified for use with other open-cell metallic foams.

Copyright © 2011 by American Society of Mechanical Engineers
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Figures

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Figure 1

Trabecular Metal (a) SEM image of Trabecular Metal; (b) SEM image of trabecular bone at the same scale, showing the microstructural similarities between bone and Trabecular Metal; and (c) the Implex Proxylock total hip replacement incorporating Trabecular Metal

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Figure 2

Illustrating springback. The shaded shape shows the workpiece prior to elastic recovery, and the dashed lines represent the final geometry after springback has occurred [4].

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Figure 3

The three categorical levels for embossing. (a) An unembossed sample. (b) A single embossed sample. (c) A double embossed sample. All double embossings were symmetric and identical.

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Figure 4

The wiping die setup, shown prior to the forming operation

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Figure 5

Indentations generated during testing. From the left, the indentation forces were 308.8, 231.6, 115.8, 77.21, and 38.61 μN.

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Figure 6

The measured flow strength of tantalum as a function of indentation depth

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Figure 7

A cross-section of the finite element mesh

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Figure 8

Geometry of a horizontal hexagonal cell, showing side length, height, width, and hexagon angle. All sides have the same length. The aspect ratio of the hexagon can be altered for different anisotropy ratios.

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Figure 9

A three-dimensional portion of the horizontal hexagonal mesh

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Figure 10

A representation of the two support beams as a half hexagon. The lower support beam is removed and the upper beam is replaced with a half hexagon. To ensure identical stiffnesses in the two different types of hexagonal columns, the modulus of the support beams are calculated to make them half as stiff as the half hexagon.

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Figure 11

The half hexagon, shown with one of the original support beams. All three legs of the half hexagon have the same length.

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Figure 12

A free-body diagram of beam 1

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Figure 13

The boundary conditions of beam 1 in the half hexagon. The right end is fixed, and the left end is guided. The forces have also been included.

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Figure 14

Springback factor as a function of R/T. The only two significant factors were die radius and clearance. Variation of R/T for a given R value is due to slight differences in sheet thickness between samples.

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Figure 15

Springback factor as a function of aspect ratio. The confidence regions display the least significant difference as determined by the statistical software.

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Figure 16

Interaction between embossing and die radius. (a) With the smaller die radius, the indentation tended to close upon itself, reducing the bend radius. (b) This was not the case for the larger die radius.

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Figure 17

The observed workpiece behavior during wiping die bending. (a) The experimentally observed workpiece behavior for the larger clearance at full punch extension. The deformation is localized in the region of the bend radius. (b) The deformation behavior as predicted by the numerical models. In addition to deformation occurring near the bend radius, extensive elastic deformation could be seen away from this region.

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