Aluminium extrusion: Simalex
Aluminium extrusions are made from solid aluminium cylinders called billets, which are continuously cast from molten aluminium. Billets are available in a wide variety of alloys, pre-treatments and dimensions, depending upon the requirements of the manufacturer. Aluminium extrusions are used throuout the construction industry as door and window-frames. Aluminium extrusions can also be found in heat exchangers, road en rail vehicles and so on.
The extrusion process involves aluminium
metal being forced through a die with a shaped opening.
This is made possible by preheating the billet to 450-500°C
and then applying a pressure of between 100 and 1000 MPa
(equivalent to the pressure found at the bottom of a 10km
-100km high water tank!). The heated and softened metal is
forced against the container walls and the die by a
hydraulic ram, the only exit is the geometric cross-section
of the die opening, and the metal is squeezed out.
The extrusion leaves the die at a
temperature of around 500°C and the exit temperature is
carefully controlled in order to achieve specified
mechanical properties, a high quality surface finish and
good productivity.
At the present time, design of extrusion
dies and operation in extrusion companies is primarily
based on trial and error. The experience of the die
designer, the press operator and the die corrector to a
large extent determine the performance of the process. In
order to improve the performance, it is necessary to have
more knowledge about the extrusion process. Numerical
simulations can be a valuable tool in obtaining such
knowledge.
In this research some new developments in
the simulation of aluminium extrusion with the finite
element method are investigated.
During extrusion very large deformations occur. In a finite
element simulation these deformations can result in severe
mesh distortions if an updated Lagrangian formulation is
used. Therefore expensive remeshing operations would be
necessary. When using an Eulerian formulation the mesh
remains undeformed and the material flows through the mesh
avoiding the time consuming remeshing operations. The
disadvantage of an Eulerian formulation is the difficulty
of following the interface between aluminium and air, or
more common the interface in a two phase flow.
A
large part of the extrusion process is a semi-stationary
process. Shortly after the start-up of the process almost
until the end of the billet the flow aluminium through the
die can be regarded as steady state. However the start-up,
filling of the die, will influence the deformation of the
die. To simulate this part of the extrusion process the
free surface of aluminium should be tracked.
In 1986 Thomson [1] [2] introduced the pseudo-concentration
method for tracking the interface in two phase flows. At
the start of the simulation in the complete domain the
pseudo-concentration function (C) is equal to 0. All the
material flowing into the domain will get a
pseudo-concentration function value C=1 assigned. At any
time the phase material can be determined by the value of
the pseudo-concentration function. The interface can be
found on the C=0.5 isoline (2D) or isosurface (3D).
In the Simalex project the improvement
and implementation of this concentration technique in DiekA
is investigated. The final goal is to compare steady state
simulations with transient simulations and to create design
rules for the dies based on those simulations.
[1] Thompson, E. (1986). "Use of Pseudo-concentrations to
follow creeping viscous flows during transient analysis."
International Journal for Numerical Methods in Fluids
6(10): 749-761.
[2] Thompson, E. (1988). "Transient analysis of forging
operations by the pseudo-concentration method."
International Journal for Numerical Methods in Engineering
25(1): 177-189.