An experimental investigation of the material flow inside the bearing channel in aluminium extrusion

The metal flow inside various bearing channels in aluminium extrusion has been investigated, using different grid pattern techniques. After extrusion the die channel was partitioned and removed from the metal rest. The contact conditions in the aluminium/die land interface during the press cycle were then evaluated from the visual appearance of the surface of the die channel after removal of the metal rest. Experiments performed in long choked die channels showed that the metal would stick to the die wall in the inlet region. However, in the outlet region the extrusion metal would slide against the die wall. Between these two regions there was a sticking/sliding transition point (line). In the sticking region there was a layer of metal adjacent to the die channel wall, which flowed with a considerably lower velocity than the rest of the cross–section which showed plug flow. Experiments performed in a die with zero die bearing length, showed that an outer surface layer of 0.2 mm thickness, would have a slightly lower axial flow velocity than the rest of the cross–section which showed plug flow. Finally, experiments performed in a laboratory porthole extrusion die working with low pressures in the outlet region and the welding chamber showed that the contact conditions of the aluminium/die interface would change from sticking deep inside the die to sliding along a sticking/sliding transition line located near the die outlet. In all cases a more or less completely covering adhesion layer of extrusion material was observed to form on the die surfaces exposed to sliding metal.