Physical and Numerical Analysis of Extrusion Process for Production of Bimetallic Tubes Page: 39 of 108
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Precipitates form preferentially at grain boundaries due to increased diffusivity (pipe diffusion) as
well as the boundary acting as a site for heterogeneous nucleation. Heterogeneous nucleation is
energetically favorable to homogeneous nucleation due to the reduction of surface energy that occurs
by grain boundary nucleation. Each precipitate that nucleates and grows reduces the amount of high
energy surface at the intersection between adjacent grains.
3.2 Modeling and Simulation
3.2.1 Finite Element Modeling
The FEM is a method used to solve engineering problems through numerical analysis. The process
relies on the use of fundamental equations related to material behavior and established mathematical
approaches to iteratively solving the equations to determine state variables such as stress, strain, strain
rate and temperature related to a given deformation problem.
Decreasing costs associated with computer hardware and the increased availability of software and
increasing costs associated with "in-house" experimentation are driving forces for use of FEM in
industrial practice. Proper use of FEM can result in lower turn around times from design to product.
Commercial and in-house/self-programmed versions of FEM code are used to solve a large number of
engineering related problems. Each version has its own advantages and disadvantages which must be
understood before applying the solutions. For the work performed in this project, DEFORM-2D
commercial FEM package was used.
With the increasingly important role of FEM in the manufacturing process for both design and
process and product optimization, it is important to understand how to approach the engineering or
scientific problem using this technique and how to interpret the results. Additionally, the drawbacks
must be identified and methods to rectify them should be determined. Ultimately, the results of the
research will be used to couple data gathered from experimentation and physical simulation enhanced
by numerical simulations in order to improve analysis that can be performed using commercially
available simulation packages.
Currently, there are several pieces of information that are not predicted by commercial FEM packages
and therefore additional experimental and characterization resources are still needed. The ability of
FEM code to simulate complex microstructural reactions and chemical changes is still limited.
Microstructure development and the resultant mechanical properties cannot easily if at all be modeled
today and normally only average or general mechanical properties are given for a deformed work
piece. By utilizing FEM software to determine processing variables stress, temperature, strain, and
strain rate, physical models can be performed without using actual extrusions.
3.2.2 Thermomechanical Simulation
In order to simulate complex microstructural development on a laboratory scale, thermomechanical
simulation is used. State of the art FEM packages are limited in the amount of microstructural
development simulation that can be successfully achieved and thus the use of thermomechanical
simulation is still widespread.16'17 Typically, a material is subjected to a processing path that involves
a series of time at temperature, strain, strain rate, and pressure steps. This can be achieved by using a
unit such as a Gleeble. The Gleeble uses resistive heating so that temperatures can be monitored and
controlled with a high degree of accuracy. Samples can be subjected to tension, compression, or
torsional loading. Strain rates can be varied over several orders of magnitude. Operations are
controlled by a computer, and multiple operations may be performed on a single sample. Data is
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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/m1/39/: accessed December 17, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.