3D FEM simulation of the whole cycle of aluminium extrusion throughout the transient state and the steady state using the updated Lagrangian approach

Abstract

Aluminium extrusion involves the generation of free surface, thermal effects, large deformations and complex geometries. The established finite element 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 disturbed. As a result, the simulation of aluminium extrusion performed so far has been restricted to simple geometries, small length-to-diameter ( L/ D) ratios, the beginning stage or steady-state conditions. This paper reports on an unprecedented attempt to simulate an entire cycle of aluminium 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 successfully predicts a complete extrusion pressure/ram displacement diagram that begins with a pressure breakthrough and ends with another pressure rise due to the inhibition of metal flow by the rigid dummy block. The developments of velocity, effective strain and temperature inside the deforming billet indicate that the process is non-steady, even in 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 deformation zone and shrinking 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 ram speed of 2 mm/s, the maximum temperature of the workpiece, after an initial steep rise, increases gradually till the end of the process, which may well lead to the occurrence of hot shortness. 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.