The compression test has been used widely in obtailning flow stress data at hot working temperatures, in order to obtain reliable data, Uniform deformation must be ensured by having adequate lubrication tbrougbout testing, in tbis paper, the lubrication bebavior of graphite was investigated by ring compression tests of Ti-6A1-4V alloy at temperatures from 750 to 1000℃ and constant strain rates from 0.05 to 15s 1. it was found that the friction factor m increases with the increase of temperature and with the decrease of strain rate. At temperatures below 800℃, the friction factor m is less than 0.3, so that graphite can be used effectively as a luhiicant in this range of temperatm-e for compression tests.

Hot extrusion of Ti-6Al-4Valloy has been studied using 4nite element simulation and the results are compared with those obtained experimentally. First, the constitutive behavior of the material and friction at the extrusion temperatures are established based on the results obtained through cylindrical and ring compression tests, respectively. While the 8ow stress below βtransus temperature is expressed as a strain-dependent function, it is taken as strain-independent one at higher temperatures. The distribution of strain, temperature and effective stress has been simulated under different design and processing conditions. Simulation results show that heat generation due to deformation is signi4cant (as much as 160℃) in the hot extrusion of Ti alloys, and it mainly occurs at the beginning of the extrusion process. This leads to reduction in flow stress which, in turn, leads to enlarged deformation zone. A fair agreement has been found between the experimental results and those obtained through simulations.

Received 19 July 2002; accepted 11 December 2002 Abuminium extrusion involves the generation of free surface, thermal effects, large deformations and complex geometries. The establisbed finite alement method (FEM)-based 3D simulation tools using the updated Lagrangian approach, or the Eulerian approach or the arbitrary Lagrangian Eulerian approach all have limitations in describing the process that develops from the transient state to the steady state before reaching the end when the steady state is disturbedi As a result, the simulation of aluminium extrusion performed so fro has been restricted to simple geometries, small length to diameter (L/D) ratios, the beginning stage or steady state conditions. Tiris paper reports on an unprecedented attempt to simulate an entire cycle of alumimium extrusion from a billet with an L/D ratio of 4 to a solid cross-shaped profile, using the DEFORM 3D software based on the updated Lagrangian approach. Simulation successfolly predicts a complete extrusion pressure/ram displacement diagran that begins with a pressure breakthrough and ends with another pressure rise due to the inhihition of metal flow by the rigid dinmmy block. The developments of velocity, effective strain and temperature inside the deforming billet indicate that the processisnon steady, evenin the steady state, as a result of continuous heat generation and sticking condition at the billet-container interface. The non steady characteristics are reflected in the expanding defomation zone and shiinldng dead metal zone. Simulation also reveals the patterns of the maximum temperature variations in the workpiece and in the tooling, due to heat generation and exchange. Even at a relatively low ran speed of 2mm/s, the maximumn temperature of the workpiece, after an initial steep rise, increases gradually till the end of the process, which may well lead to the occurence of hot sbortness. On the basis of these results, a change of the conventional mode of aluminium extrusion is recommended, which at present operates almost all at a constant ram speed and often begins with a uniform billet temperature across the aluminium extrusion industry in the world.