Physical and Numerical Analysis of Extrusion Process for Production of Bimetallic Tubes Page: 24 of 108
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R = extrusion ratio,
Ao = cross-sectional area of the upset billet, and
A1 = cross-sectional area of the extrudate.
The amount of deformation in the process is proportional to the extrusion ratio. The initial billet
cross-sectional area, A;, refers the billet prior to any deformation. During upsetting (prior to
extrusion), the billet expands to fill the extrusion chamber, shortening in length and increasing in
cross-sectional area. Higher values of R can be obtained successfully with materials that exhibit lower
flow stress (or by reducing flow stress caused by increased temperature), up to the point where
extrusion defects occur. High-temperature defects include "hot shortness" where localized surface
melting or cracking occurs, which results in surface tearing, galling, or die pickup that occurs when
the lubricant breaks down. Low temperature/high strain rate/flow stress failures include chevron
cracking, in which a series of v-shaped cracks appear in the center of the extrudate. Typical extrusion
ratios for aluminum are above 40, and for steel typical values are lower than 40. Both material
families are generally extruded at elevated temperatures.
Typical load versus stroke behavior during extrusion (both indirect and direct) can be observed in
Fig. 3.1(a). In Fig. 3.1(b) the individual components of the overall work to extrude a material in direct
extrusion are noted. Section A describes the work (force x ram displacement) needed to upset the
material to fill the extrusion chamber. The amount of work needed is influenced by both the diameter
of the billet and the chamber as well as the flow stress of the material at the upsetting temperature.
Area B describes the work needed to initiate extrusion, also referred to as "break through," with the
maximum load defined as the break-through force (or pressure). It is the least understood component
due to the complex flow of material during the initial, non-steady state flow conditions of extrusion.'
Region C is the work associated with material deformation and is directly influenced by the flow
stress of the material at the extrusion temperature. The final area, D, represents the work associated
with friction in the process. Generally, this region decreases in magnitude as extrusion progresses due
to the decrease in surface contact of the billet with the press chamber. This region is influenced by the
type and amount of lubricant on the billet during extrusion as well as the lubricant's stability.
Friction plays an extremely important role in extrusion. As shown in Fig. 3.1(b), region D, it can play
a significant role in extrusion force. Lubricants are used in order to reduce the friction during
extrusion. This is important not only to lower the extrusion force that is necessary, but it also to
reduce the wear on the tooling (extrusion chamber, mandrel, and extrusion die) as well as to minimize
any surface temperature increase associated with frictional heating during deformation that may result
in surface-defect generation. Additional roles of lubricants during deformation include thermal
shielding of the heated billet from temperature loss to tooling (especially the extrusion chamber) as
well as acting to prevent or minimize oxidation of the billet due to the high temperatures associated
with the preheat. The lubricant that is used should not react chemically with the work piece.
One defect associated with the extrusion of hollow cross sections or multi-material extrudates is
eccentricity. Eccentricity is defined differently for each of the two cases. Figure 3.2 shows examples
of some of the different types of eccentricity that may develop during the extrusion of tubes or
bimetallic tubes. As expected, there are more definitions of eccentricity for the bimetallic tubes due to
the additional variables (core location and thickness). In general, there are two types of eccentricity:
the location of the center hole and the shape of the center hole. For the bimetallic tubes, the additional
types of eccentricity are the location and shape of the core layer. The different types of eccentricity
may be present alone or in combination with other types. Eccentricity is affected by many different
factors. One major cause is the localized temperature loss of the billet while sitting in the extrusion
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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/m1/24/: accessed September 19, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.