A Phenomenological Mechanism for the Occurrence of the Extrusion Central Burst Defect.

Six billets of 7075 Al were partially extruded in a horizontal extrusion press using a three-inch diameter container. Extrusion conditions were selected such that the central burst defect would occur. A rigid ram stop mechanism was employed to halt the extrusion process abruptly after selected increments of deformation were achieved in each billet. Both metallographic and scanning electron microscopy analyses were used to identify the mechanism for occurrrence of the defect. The occurrence of the defect was found to be preceded by the development of tangential velocity discontinuity surface in the deformation zone. Enhanced metal flow in the radial direction on the die exit side of this discontinuity surface caused tensile stresses to develop along the axis of the billet initiating the defect by a tensile overload mechanism. The defect was found to propagate along the discontinuity surface but was halted when continued ram motion caused continued plastic flow along this discontinuity surface to become diffused. Author