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Research Papers

Analysis and Observations of Current Density Sensitivity and Thermally Activated Mechanical Behavior in Electrically-Assisted Deformation

[+] Author and Article Information
James Magargee

Department of Mechanical Engineering,
Northwestern University,
2145 Sheridan Road,
Evanston, IL 60208

Rong Fan

Department of Mechanical Engineering,
Northwestern University,
2145 Sheridan Road,
Evanston, IL 60208
School of Automotive Engineering,
Dalian University of Technology,
No. 2 Linggong Road,
Dalian City 116024, China

Jian Cao

Department of Mechanical Engineering,
Northwestern University,
2145 Sheridan Road,
Evanston, IL 60208
e-mail: jcao@northwestern.edu

1Corresponding author.

Manuscript received April 2, 2013; final manuscript received October 28, 2013; published online November 27, 2013. Assoc. Editor: Yung Shin.

J. Manuf. Sci. Eng 135(6), 061022 (Nov 27, 2013) (8 pages) Paper No: MANU-13-1136; doi: 10.1115/1.4025882 History: Received April 02, 2013; Revised October 28, 2013

The flow of electric current through a metal during deformation has been observed to reduce its flow stress and increase its ductility. This observation has motivated the development of advanced “electrically-assisted” metal forming processes that utilize electric current to assist in the forming of high-strength and difficult-to-form materials, such as titanium and magnesium alloys. This method of heating provides attractive benefits such as rapid heating times, increased energy efficiency due to its localized nature, as well as the ability to heat the workpiece in the forming machine thus eliminating the transfer process between oven heating and forming. In this paper, a generalized method is proposed to relate applied electric current density to thermally activated mechanical behavior to better understand and improve the processing of metals during electrically-assisted deformation. A comparison is made of engineering metals studied experimentally as well as in the literature, and it is shown that the method provides insight into what some researchers have observed as the occurrence or absence of a “current density threshold” in certain materials. A new material parameter, “current density sensitivity,” is introduced in order to provide a metric for the relative influence of current density on a material's thermally activated plastic flow stress. As a result, the electric current necessary to induce thermal softening in a material can be estimated in order to effectively parameterize a wide range of advanced electrically-assisted forming processes. Thermally induced changes in material microstructure are observed and discussed with respect to the underlying deformation mechanisms present during electrically-assisted deformation. Finally, a strong correlation between thermally activated mechanical behavior and elastic springback elimination during sheet bending is demonstrated.

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Figures

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Fig. 1

Magnitude of steady-state temperature increase generated by Joule Heating as a function of effective current density. Plots end at material melting point.

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Fig. 2

Thermal softening parameter of low melting temperature metals as a function of homologous temperature compared with data from [31,32]

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Fig. 3

Thermal softening parameter of high melting temperature metals as a function of homologous temperature compared with data from [33-36]

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Fig. 4

Thermal softening parameter of compared metals as a function of homologous temperature

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Fig. 5

Thermal softening parameter as a function of effective current density

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Fig. 6

Current density sensitivity as a function of effective current density

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Fig. 7

Current density sensitivity as a function of homologous current density

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Fig. 8

Deformation mechanism map of pure titanium [37] compared with isothermal, high temperature uniaxial test data [34], and electrically-assisted tension data [16]

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Fig. 9

Deformation mechanism map of pure copper [37] compared with isothermal, high temperature uniaxial test data of 260 series brass [32], and electrically-assisted tension data of 260 series brass (NU)

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Fig. 10

Microstructure of 260 series brass after electrically-assisted tension test at 100 A/mm2. Increasing grain growth is observed along the tension specimen, approaching the fracture region.

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Fig. 11

Microstructure of 260 series brass after electrically-assisted tension test at 112 A/mm2 using pulsed current. Dendrite formation is observed near the fracture region.

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Fig. 12

Influence of temperature on springback elimination of Al 6111 sheet metal during bending tests compared with predicted thermal softening of Al 6061 (Eq. (7)). Experimental springback data from Ref. [10].

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