Physical and Numerical Analysis of Extrusion Process for Production of Bimetallic Tubes Page: 35 of 108
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not constant must be discarded as scrap due to the lack of geometrical tolerances. These regions can
be very large depending on processing. Understandably, there is a need to minimize the amount of
scrap material that is generated during extrusion due to the inherent costs of billet preparation and
assembly, as well as the potential financial losses associated with the core material. In addition to
cost, the scrap produced from defects discussed above also cause significant energy waste. The
energy waste results from a combination of factors, including (1) loss of energy required during billet
heating and extrusion process and (2) energy required to replace the wasted material. As stated
earlier, it is common to produce composite structures in an effort to minimize the amount of an
expensive material in a given product. In the case where there is an expensive alloy as the core
material, losses due to premature extrusion are greater and minimize the benefits of composite
During steady state material flow, the bond location is constant. Because steady state material flow
has been managed by the variables listed above, a new approach has been proposed in the current
project to promote an earlier onset of steady state material flow as well as to reduce material that is
extruded "out of tolerance" during the initial breakthrough.14 Figure 3.9 illustrates a conventional and
a novel billet design for a bimetallic tube. In the novel design, the core material is recessed (Fig. 3.9)
at the nose of the billet. This is to minimize the amount of core material that tends to extrude
prematurely (before the sleeve), which results in the ballooning of the core at the front of the
extrudate (Fig. 3.8). The findings are presented in the next chapter, Results and Discussion.
Eccentricity in co-extruded materials is similar to that in normal extrusion but complicated by the
variance in flow of different materials. Sensitivity to temperature/flow stress gradients is increased
due to the potentially greater difference in flow stresses at a constant temperature. A greater number
of eccentricity cases exist as a result of the additional variable (location of the bond between the two
materials) as shown in Fig. 3.2.
An additional and important consideration in preparing a co-extruded product is the chemical
compatibility of the component alloys. Much like fusion welding of dissimilar metals, great
Fig. 3.9. Schematic cross section of unextruded billet: (a) traditional design and (b) novel
design with shortened core.
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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/m1/35/: accessed September 18, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.