Shen et al. / J Zhejiang Univ SCIENCE A 2006 7(8):1343-1350
1343
Simulation study on fluctuant flow stress scale effect
SHEN Yu
†
, YU Hu-ping, RUAN Xue-yu
(
1
National Die and Mold CAD Engineering Research Center, Shanghai Jiao Tong University, Shanghai 200030, China)
†
E-mail: shenyu@sjtu.edu.cn
Received Dec. 16, 2005; revision accepted Feb. 24, 2006
Abstract: Crystal plasticity theory was used to simulate upsetting tests of different dimensions and grain size micro copper
cylinders in this study on the fluctuant flow stress scale effect. Results showed that with the decrease of billet grain quantity, flow
stress fluctuation is not always increased, but there is a maximum. Through this study, the fluctuant flow stress scale effect can be
understood deeper, and relevant necessary information was obtained for further prediction and control of this scale effect and to
design the microforming process and die.
Key words: Microforming, Scale effect, Numerical simulation, Fluctuation
doi:10.1631/jzus.2006.A1343 Document code: A CLC number: TG301
INTRODUCTION
With the continuous development of science and
technology, especially the rapid progress of electronic
industry and micromachine, microparts are needed
more and more, and their technical requirements are
higher and higher (Geiger et al., 1994; 1996; Geiger
and Engel, 2002). However, traditional processing
technologies, like cutting, etc., do not meet these
requirements. Traditional plastic forming technology
has merits like high production rate, excellent me-
chanical properties of products, so it is suitable to
produce mass micro parts. Under such circumstance,
in the 1990’s, microforming came into being, tradi-
tional plastic forming process is applied in micro
scale to batch produce micro metal parts. As a com-
pletely new plastic processing technology, micro-
forming is a technology that can be used to produce
submillimetre parts in at least two dimensions by
plastic deformation of materials (Geiger et al., 2001).
Microforming inherits many merits which traditional
plastic forming technology owns, and has good pro-
ducing potentials, so its application range is broad.
Typical processes of microforming can be classified
into micro extrusion, micro sheet metal, micro die
forging, etc. Up to now, this technology has been
partially used in electronic industry for the production
of microparts like micro screws, contact springs, IC
socket, etc. With the continuous development of
technologies, applications of microforming are rap-
idly extended to machinery, chemical industry,
aerospace, biology, etc., and its status is more and
more important in these fields.
However, existing studies show that in micro
scale, with the downscaling of billet dimensions,
grain size is increased relatively, so material is no
more traditional isotropic continuum. Accordingly,
material mechanical properties are explicitly different
from that in traditional macro scale. With the down-
scaling of billet size, process parameters, flow stress,
etc. are very sensitive to billet size, and a series of
explicit scale effects emerged, which result in the
failure of traditional (macro) forming process and
theories. Of these scale effects, the fluctuant flow
stress scale effect, defined as an increasing fluctuation
of flow stress with the downscaling of billet size,
further results in the non-uniform distribution of loads
on billet and die, as well as a big fluctuation of
forming force and formed part quality. Thus, the de-
sign of process and die in microforming is influenced,
even its development and application are hindered.
Due to the problems mentioned above, fluctuant
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