Simulation of aluminium extrusion
Introduction
Modern computer programs for simulation of a luminium extrusions can effectively improve the quality of extruded aluminium profiles already at the design stage of extrusion dies. For the average user, these models often remain “black box”, whose internal structure is unknown or unclear. Below we will try to explain some of the properties of these models, and also to show their potential usefulness for production of aluminium extruded products.
Computer simulation
Finite element method
Almost all computer models are based on the finite element method (FEM). The Finite Element Method is a numerical method for solving problems in various branches of physics and engineering, such as, mechanics of materials, heat transfer, mechanics of liquids and so on. The analytical solution of these problems require complex solutions of partial differential equations, which is possible only in the simplest cases. The formulation of the same tasks in the finite element method reduces to the solution of large systems of algebraic equations, which modern computers can easily cope with [1].
To solve the problem of modeling the object is divided into small elements of simple shape – finite elements. For two-dimensional problems, such simplest elements are triangles (Figure 1) and quadrangles, for three-dimensional – tetrahedra and parallelepipeds. Finite elements interconnected, but only at the connection points. Using mathematical relationships, which define the relationship between stresses and strains (in the simplest case – Hooke’s law), simulate the behavior of each of the finite elements during loading or moving its components.
Then all of these finite elements are combined into larger systems of algebraic equations, which already model the entire object as a whole (figure 2). These systems of equations and solves computer.
Figure 1 – Triangular finite elements
with components of nodal displacements [1]
Figure 2 – Three-dimensional finite element mesh
for simulation of aluminium extrusion
in the computer program QForm [2]
Material model
The flow of material through the container and die most strongly depends on:
- aluminium temperature and
- aluminium strain rate.
Therefore, most often the behavior of aluminium is represented by models of a rigid-viscous-plastic material [2-4]. Typically, these models do not take into account the elastic deformation of the material, that in general it is justified, as in the process flow of aluminium through the container and the elastic deformation of the matrix is negligibly small fraction of the total deformation. However, in the channel operating belts elastic behavior of the material can play an important role and have a significant impact on the simulation results. Therefore, to more accurately simulate aluminium velocity at the exit of the die is often used visco-elastic-plastic material model.
Friction model
An important condition for adequate modeling is also a model of friction between the aluminium and pressing tool, which reflects the true interaction with the aluminium walls of the container and the elements of matrix, including, with its running web.
Optimization of aluminium profile
With the help of modern computer simulation of aluminium extrusion is already at the stage of designing the matrix measures are taken, design and technological, to improve the quality of the profile according to the following indicators [2, 3]:
- dimensional accuracy and profile shape;
- location, strength and appearance of welds;
- optimization of the grain microstructure of the profile material;
- efficiency thermal hardening profile material while minimizing its buckling under cooling at the outlet from the press.
The size and shape of the profile
Simulation shows a possible distortion of the shape of the profile and possible deviations thickness of its walls and shelves at the output of matrix. These phenomena can largely be exacerbated by the elastic deformation of the elements of the die. Examples of such troughs are troughs so-called “tongue” in the planar array, as a result of which there is a significant change in the angle of inclination of the planes of the bearings relative to the extrusion axis (Figure 3).
Figure 3 – Deflection of the tongue in a flat matrix [4]
Welds
There are two types of welds on extruded aluminum profiles (picture 4):
- transverse welds
- longitudinal welds.
Transverse welds
Transverse welds occur, when the leading end of a new billets comes into contact with metal remaining in the die from the previous billet. These metals are metallurgically welded under high pressure and high temperature. Depending on the size and shape of the cross-sectional profile and the die design transverse the seam can have a considerable length, which may be about one meter or more (Figure 4).
Figure 4 – The extrusion welding process showing the seam weld and charge weld in a longitudinal direction [5]
Longitudinal welds
The longitudinal welds formed, when a hot aluminum alloy is separated in matrix ports into separate streams porthol, which are then metallurgically welded just before leaving the die (Figure 5).
Figure 5 – Formation of longitudinal welds in the porthole die [6]
Design of welds
These welds, as longitudinal, and transverse, can significantly spoil the appearance of facial surfaces anodized profiles, since they often differ significantly from the rest of the surface in terms of gloss or haze (picture 6). This is due to the different sensitivity of the grains of the material to alkaline pickling, which is made before anodizing operation.
Figure 6 – Stripes on matte anodized surface
along longitudinal welds [6]
Computer simulation provides the following possibilities for optimal design of welds:
- Changing the location of the welds, for example, transferring them with faces of the profile at the corner portions.
- Achievement of the specified strength of the weld by creating the required pressure and temperature in the welding chamber.
- Optimization of the parameters of plastic deformation in the welding chambers to reduce the degree of nonuniformity of microstructure.
The grain microstructure
Recovery and recrystallization
During the plastic deformation the original microstructure of the workpiece material is subjected to considerable rearrangement under the influence of such mechanisms, how the return and different types of recrystallization (Figure 7). The dynamic crystallization occurs during continuous energy supply as a result of plastic deformation. Static recrystallization occurs after plastic deformation and the energy controlled, which is already stocked in the material [7, 8].
Figure 7 – Recovery and recrystallization processes during extrusion
of materials with high and low stacking fault energy.
Partly re-drawn from Bauser et al. (2006) [7]
The computer simulaton makes it possible to perform optimization a die design and extrusion parameters to minimize the possibility of coarse recrystallized microstructure.
Quenching
Thermal hardening of aluminium profiles of 6xxx series alloys, includes two stages:
- quenching at the press
- artificial aging.
Quenching at the press is sufficiently rapid cooling of the profile immediately after the release from the die to a temperature of about 250 ºС (Fig 8).
The object of quenching is to keep in in solid solution state the maximum amount of alloying elements. For 6xxx series alloys such elements are magnesium and silicon. From this stage, the level of the achieved strength properties by ageing depends (Figure 9).
Figure 8 – The simulation of press quenching of aluminium profiles [9]
Figure 9 – Press quenching and artificial ageing of 6063 aluminium profiles [10]
At the same time, the choice of cooling parameters during quenching necessarily take into account the profile of resistance to warping and distortion of the shape. In practice the cooling parameters of quenching are a compromise between achieving high strength profile and minimizing its buckling from profile shape tolerances (Figure 10).
Figure 10 – Cooling limitations when quenching an aluminium profile at the press [9]
Conclusion
- Modern computer simulation of aluminum extrusion are effective tools for improving the quality of extruded sections.
- They give a deeper understanding of the processes, which occur within the die during pressing of a profile.
- This saves time and money in both the design and implementation of new matrices, and in solving problems in the operation of existing matrices.
Sources:
- Applied Finite Element Analysis / Larry J. Segerlind – Wiley, 1976
- Application of QForm Program for Improvement of the Die Design and Profile Extrusion Technology /N. Biba, S. Stebunov, and A. hair – QuantorForm Ltd. – Proc. of ET2008 – 2008.
- Quality Prediction and Improvement of Extruded Profiles by means of Simulations /N. Biba, R. Rezvykh, I. Kniazkin – Aluminium Extrusion, 2/2019.
- CAD Implementation of Design Rules for Aluminium Extrusion Dies / G. van Ouwerkerk – University of Twente, 2009.
- Microstructural Caracterization of Extrusion Welds in 6xxx Aluminium Alloys /X. Ren et al – IC3ME 2015.
- Influence of Al Microstructure on Hard Anodising Quality– Profile Material/ Tom Hauge, Hydro Aluminium, Norway IHAA Symposium, 25th of September 2014, New York.
- Characterization of Microstructure in Aluminum Alloys Based on Electron Backscatter Diffraction /T. Kayser – PhD Thesis – Technical University of Dortmund – 2011.
- Effects of Extrusion Parameters on Coarse Grain Surface Layer in 6xxx Series Extrusions /E.D. Sweet te al – Proc. of ET2004 – 2004.
- Distorsion Mechanisms due to the Cooling Process in Aluminum Extrusion /S. Bikass et al- Proc. of ET 2012 – 2012
- Thermal Treatments During Processing of Aluminum— AEC Webinar Presentation /R.E. Sanders (Alcoa) – 2010