This investigation focuses on simulation of the extrusion process and in particular the effect of the initial billet temperature on friction and its consequences on material ow. The simulation is compared with data obtained from an experimental extrusion press. All the simulations are performed with the implicit nite element codes FORGE2 and FORGE3. 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 comparison with experiments is made to assess the relative importance of some extrusion parameters in the extrusion process and to ensure that the numerical discretisation provided a true simulation of the process. A speci c functional relationship to directly measure 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 from 0 . 65 at 300°C to a 0 . 91 at 450°C after reaching the peak pressure. The dead metal zone is observed to vary in form and has a greater volume at high temperatures. The increase in friction results in an increase of initial extrusion load. The nite element program appears to predict all the major characteristics of the ow observed macroscopically. MST/5520The authors are in Bournemouth University,

This communication reviews previous work on the extrusion of hollow shapes and uses a three-dimensional (FEM) solution to predict load-required, temperature of the extrudate and material¯ow during the process. A comparison with experiments is made to assess the relative importance of some extrusion parameters in the extrusion process and to ensure that the numerical discretisation yields a realistic simulation of the process. The usefulness and limitations of FEM when modelling complex shapes is also discussed. Methods to assess the dif® culty of extrusion of hollow extrusions in general are presented. The paper also illustrates the essentials of numerical analysis to assist the reader in the comprehension of the thermomechanical events occurring during extrusion through bridge dies. Results are presented for velocity distribution in the extrusion chamber, iso-temperature contours and pressure/ displacement traces. These are compared with experiments conducted using a 5 MN press. It is shown that the ® nite element program predicts the pressure requirement: the pressure/displacement trace showing a double peak which is discussed in some detail. The ® nite element program appears to predict all the major characteristics of the¯ow observed macroscopically. MST/5236Mr Flitta (i¯itta@bournemouth.ac.uk) and Professor Sheppard (tsheppar@bournemouth.ac.uk) are at Bournemouth University,

The extrusion process is complex, involving interaction between the process variables and the material's high temperature properties and is typically conducted at relatively high temperatures because the lower flow stress of the material permits larger section reductions to be achieved. This lowers the power requirements and processing times. Temperature is, perhaps, the most important parameter in extrusion. The flow stress is reduced if the temperature is increased and deformation is, therefore, easier, but at the same time, the maximum extrusion speed is reduced because localised temperatures must be well below any incipient melting temperature. The present investigation focuses on the evolution of the temperature in the billet from upsetting and until the end of the extrusion cycle is reached. The extrusion pressure and the temperature rise are predicted and the pressure-displacement trace and the events which take place in the deformed material during the extrusion process are also simulated. The simulation is compared with data obtained from an experimental extrusion press. All simulations are performed with the implicit finite element code FORGE2. A comparison with experiments is made to validate the predicted temperatures readings from FORGE2 to ensure that the numerical discretisation provides a true simulation of the process. It was found that the extrusion parameters (friction, heat transfer, etc.) are significantly influenced by the temperature gradients produced in the billet during transfer to the container, and after upsetting in the container. These parameters are thus clearly extremely sensitive input data when attempting to simulate the extrusion process.

Materials which form the surface and subcutaneous layers of an extrudate experience large deformations when they traverse the die land, which, added to the inhomogeneous caused by the dead metal zone, leads to considerable modifications to the deformation parameters when compared with the remainder of the extrusion. The distribution of structure is therefore greatly inhomogeneous. Reference to both empirical and physical models of the recrystallisation process indicates that nucleation and growth will differ at these locations in those alloys that are usually solution treated and aged subsequent to the deformation process. Since static recrystallisation (SRX) has a significant influence on many of the properties of the extrudate, it is therefore essential to provide the methodology to predict these variations. In the present work, a physical model based on dislocation density, subgrain size and misorientation is modified and integrated into the commercial FEM codes, FORGE2 and FORGE3 to study the changes of the microstructure. Axisymmetrical and shape extrusion are presented as examples. The evolution of the substructure influencing SRX is studied. The metallurgical behaviours of axisymmetric extrusion and that of shape extrusion are compared. The predicted results show an agreement with the experimental measurement. The distribution of equivalent strain, temperature compensated strain rate and temperatures is also presented to aid in interpretation. Importantly the properties of hard alloys improve with the increase in the temperature of the extrusion. This phenomenon is discussed and theoretically justified.

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