Physical and Numerical Analysis of Extrusion Process for Production of Bimetallic Tubes Page: 26 of 108
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Fig. 3.2. Eccentricity definitions for tubes (a-d) and bimetallic tubes (e-1). (a) tube with no
eccentricity, (b) off-center hole in tube, (c) oval center hole in tube, (d) oval off-center hole in tube,
(e) bimetallic tube with no eccentricity, (f) core material off center in bimetallic tube, (g) off-center hole in
bimetallic tube, (h) off-center hole and core in bimetallic tube, (i) oval center hole in bimetallic tube, (j) oval
center hole and off-center core in bimetallic tube, (k) oval off-center hole in bimetallic tube, and (I) oval off-
center hole as well as core material off-center in bimetallic tube.
container prior to extrusion. In order to maintain structural integrity and minimize tooling wear, the
extrusion chamber is usually at a lower temperature than the billet to be extruded. The container
temperature is based on the material to be extruded and its thermal characteristics, including thermal
conductivity and heat capacity. As the billet sits in the chamber, the bottom portion of the billet is in
contact with the chamber allowing for heat flow from the higher temperature material into the lower
temperature chamber. As the material locally cools, the flow stress in that region increases; thus, the
material flow during extrusion becomes heterogeneous giving rise to eccentricity. Similarly, heat flow
from the inner diameter (ID) of the billet to the mandrel may cause changes in material flow and
result in eccentricity.
Gravity plays a role in the development of eccentricity as well; since the center hole in the billet must
be larger than the mandrel for it to be inserted, uneven flow may occur in the center of the billet
during initial upsetting. This is caused by the billet settling in the chamber causing the mandrel to be
off center in the billet center hole.
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Misiolek, W. Z. & Sikka, V. K. Physical and Numerical Analysis of Extrusion Process for Production of Bimetallic Tubes, report, August 10, 2006; United States. (digital.library.unt.edu/ark:/67531/metadc884646/m1/26/: accessed February 21, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.