Simulation of aluminium extrusion process.

Abstract

The aim of this thesis is to model the extrusion process conditions for some aluminium alloys using Finite Element Modelling (FEM) program. All the simulations were performed with the implicit finite element code FORGE20 (2-D) and FORGE3® (3-D). In this work only the alloys AA2024, AA2014, Al-1 %Cu and AA6063 where experimental work is available were considered. The FORGE2® program was used to investigate and select an appropriate flow stress constitutive equation to describe the material behaviour to model the process conditions. The extrusion pressure and the temperature rise were predicted and the pressure-displacement trace and the events which take place in the deformed material during the extrusion process were also simulated. The effect of the initial billet temperature on friction, and the extent of the surface zone affected by surface friction and the consequence changes in material flow were investigated. The changes in the subgrain size during quasi-static deformation were predicted. This allows a construction of velocity-displacement profiles which would ensure consistent properties over the length of the extrudate. The FORGE3® program was used to simulate the effect of changing the die geometry on material flow during extrusion for rod, shapes and tube extrusion and the effect of the initial temperature on the deformation zone. The load required, temperature evolution, surface formation of the extrudate and material flow during the process, were also predicted. These included solid sections and the production of tubes using bridge die. Two most commonly used constitutiveflow stress equations,the Zener-Hollomon and the Norton-Hoff were analysed and compared with experimental results. It was found that the Zener-Hollomon relationship provided a better representation of the experimental flow stress under high working conditions than the Norton-Hoff relationship. FEM has been successfully applied to model the deformation patterns in the load/displacement traces and temperature evolution during the extrusion cycle. The effect of the initial billet temperature on the deformation zone pattern and its consequent effect on friction using both numerical simulation and experimental work are presented. A specific function relationship to measure directly interfacial friction under conditions approaching those encountered in the quasi-static deformation process is described. The results revealed that the friction factor increases with increase in initial billet temperature and varies during the extrusion cycle. The dead metal zone (DMZ) is observed to vary in form and has a greater volume at high temperatures. FEM proved to be a very effective and efficient way to design the ram speed profile to control the extrudate properties. The control of the properties of the extrudate under a constant (Z) parameter resulted in a more uniform distribution of the subgrain size across and along the extrudate cross-section. Furthermore, the speed profile under constant Z conditions resulted in an improved extrusion speed and hence greater productivity coupled with better control of the subgrain size and the exit temperature. This new extrusion process is termed iso-Z Extrusion, and is considered an improvement on Iso-Thermal extrusion. The usefulness and the limitation of FEM when modelling complex shapes are discussed. Methods to assess the difficulty of hollow and section shapes are presented. The work also illustrates the essentials of numerical analysis in the comprehension of the thermo-mechanical events occurring during extrusion through bridge and shape dies. Results are presented for velocity distribution in the extrusion chamber, Iso-temperature contours and pressure/displacement traces. It is shown that for most of the shapes investigated, the material making up the extrudate cross-sections originated from virgin material within the billet. The outside surface of the extrudate originates from the material moving along the DMZ and the core of the extrudate from the central deformation zone. When simulating tube extrusion, it is shown that the FE program is able to predict the pressure requirements: the pressure/displacement trace showing a double peak for tube extrusion which is discussed in some detail. The FE program appears to predict all the major characteristics of the flow observed macroscopically.