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.

extrusion1

The extrusion process involves aluminium metal being forced through a die with a shaped opening. This is made possible by preheating the billet to 450-500C 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.

Porthole

The extrusion leaves the die at a temperature of around 500C 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.

extrusion3

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.

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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.

Filling


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).

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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.

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