Physical and Numerical Analysis of Extrusion Process for Production of Bimetallic Tubes Page: 36 of 108
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consideration must be given to the potential microstructural transformation products that will be a
result of the process. Since no melting occurs in co-extrusion, there are no solidification concerns like
there are in fusion welding. Diffusional transformations are of concern, and it is important to consider
the effects on mechanical, electrical, or thermal properties of the final product.
3.1.2 Diffusion and Co-extrusion Bonding: Microstructural Development
Solid-state welding processes are used to join two (or more) work pieces together without heating the
materials to their melting temperature or by using an intermediate liquid phase. The most common
metals welded by the co-extrusion process include low-carbon and stainless steel, aluminum,
aluminum alloys, copper, and copper alloys. Additional applicable materials include nickel,
zirconium, titanium, tantalum, niobium, and their alloys.26 The benefits of using co-extrusion are
several-fold and include the ability to have a semi-continuous process to produce large products, the
ability to join dissimilar and difficult to join materials, the ability to minimize the formation of
undesirable phases (such as brittle intermetallics), and no phase melting during deformation
processing. The formation of undesirable phases is often a limitation that is associated with fusion
welding because of liquid formation. Additionally, in fusion welding processes, an undesirable cast or
solidification microstructure may evolve that requires additional post processing heat treatments to
ensure acceptable physical and mechanical properties. Co-extrusion differs from diffusion bonding
because pressure and metal flow is used in addition to chemical transport at the interface to join the
The interface between the materials is of the highest importance because it is the most likely region
for failure to occur and will exhibit the most complex microstructure. In the case of two materials
joined via co-extrusion, failure may occur at the exact interface, or in either of the two materials in
the region adjacent to the new bond. However, the failure will be a result of the interaction between
the materials and their chemistry, and microstructure that developed as a result of processing. An
understanding of the development of the interfaces between materials systems is critical for meeting
production needs; however, even for simple systems that form solid solution alloys there is a lack of
knowledge of the interface reactions.26 Much like a HAZ forms in a fusion weld from elevated
temperatures, a thermomechanically affected zone (TMAZ) forms during co-extrusion. It
encompasses the region of microstructure adjacent to the bond affected by the pressure-temperature
history during extrusion.
Solid-state welding processes include friction welding, friction stir welding, impact or explosion
welding, forge welding, roll welding, clad welding, friction bonding, and diffusion bonding.
Diffusion bonding is the process most similar to the bonding that occurs during co-extrusion.
Figure 3.10 illustrates the various stages of diffusion bonding that occur without the application of an
external pressure. Initial contact between the two materials is limited to a few asperities, which occurs
at room temperature, Figure 3.10(a). During the first stage of bonding, Figure 3.10(b), surface
asperities begin to deform from either plastic flow, creep, or a combination of both. In co-extrusion
welding, an external force is applied to the bond due to the nature of the geometry of the process as
the billet is forced through the extrusion die. As the asperities deform, the interface becomes a
discontinuous network of pores. The second stage of diffusion bonding, Figure 3.10(c), begins with
grain boundary diffusion of atoms to the voids and grain boundary migration. Pores may shrink as
well as become trapped in the interiors of grains as the grain boundaries begin their movement. The
third and final stage of bonding involves volume diffusion of atoms to the voids, Figure 3.10(d).
The interfacial bonding process during co-extrusion can be separated into three different stages:
primary bonding, bond-surface extension, and elimination of original joining surface.26 Figure 3.11 is
a schematic representation of the development of the interface and the different stages: (a) faying
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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/m1/36/: accessed February 23, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.