Physical and Numerical Analysis of Extrusion Process for Production of Bimetallic Tubes Page: 60 of 108
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4.1.6 Discussion of Results for Finite Element Modeling Analysis of New Billet
The underlying motivation for this activity was to determine the feasibility of utilizing a commercial
FEM package such as DEFORM-2D to design multi-component billets for co-extruded products.
Typically co-extrusion is a process used to produce a composite product that involves one or more
expensive alloys coupled with a support alloy that is less expensive. Uneven metal flow leading to
front end defects produces waste that minimizes the impact of using a less expensive "backing" alloy.
The use of a FEM package to perform initial modeling experiments allows for the design of new
billets and reduces the amount of actual physical experiments that need to be performed. This
eliminates costly experiments (including billet assembly), press-down time, and unnecessary
As shown in Fig. 4.17, the results from a series of FEM experiments for a particular billet set are in
good agreement with physical experiments, showing differences only in the early stage extrusion
behavior and consistent results during steady state extrusion. The most important and evident
difference between the three simulations occurs at the core thickness distribution near the front of the
extrudate. Although all three show front-end core thicknesses that are higher than the steady state
values, the 80% core length sample shows little material that would be out of tolerance and therefore
scrapped. This trend is repeated in all of the simulations, including both extrusion ratios.
When the core thickness data from the actual extrudates is compared to the FEM results, the
agreement is good. Samples extruded using the lower extrusion ratio show the same general core
thickness distribution as the simulations, but it is shifted back slightly in the extrudate.
The FEM simulations show that a billet with a core length 80% (out of the three conditions simulated)
of the overall length produces the optimal extrusion results. For both the low and high extrusion
ratios, this proves to be true in the actual extrusion experiments as well. Although low extrusion ratios
promote concurrent flow of material,4 an extrusion ratio of 3.3 appears to be high enough to benefit
from a billet with a recessed core. This is illustrated in Figs. 4.9 through 4.11. When the core is the
full length of the billet (samples Lowl and Highl), the core extrudes at a thickness equal to or greater
than its initial value. This is caused by a radial flow of the sleeve material into the core, forced by the
full extrusion ratio imposed by the die. This radial flow causes the core material to be upset outward
towards the die orifice, even though the initial core thickness may be much less than the die orifice.
At lower extrusion ratios (larger die orifice), this phenomenon can be exaggerated more. Figures 4.12
and 4.16 illustrate the core "ballooning." When the core is recessed to a value 80% of the initial billet
length, the sleeve upsets to fill the gap left by the missing core material and extrudes ahead of the
core material, causing scrap material. By reducing the core by only 10%, concurrent flow of both
materials is promoted in both the low and high extrusion ratio scenarios. A reduction of 20% results
in better or comparable material flow (depending on the extrusion ratio) and a savings in material in
the initial billet that would become scrap.
Processing variables, such as ram speed, extrusion temperature, and material variables, such as flow
stress, are either limited by the extrusion press or by the desired properties of the final product.
According to Sliwa27 in the case of a soft-sleeve/hard-core material, as is the case for this research,
the material flow is simplified. For the case where there is a soft core material, the core flow will be
promoted more because the sleeve material flow is more difficult. Core/sleeve thickness ratio in the
initial billet is limited by the desired ratio in the final extrudate. The extrusion ratio is used to control
the final extrudate size (and shape) and is limited by both the press container size and final product
geometry. In addition to the length of the core material in the initial billet, front end material flow
may be controlled by the die entrance angle. As shown by Apperley et al.,4 low die angles force the
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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/m1/60/: accessed October 16, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.