Metal Flow and Fracture in an Extrusion-Forging Process

A study was conducted of the metal flow and fracture in a forging process involving both extrusion flow into a hub and lateral flow into a flange. In the initial stage of deformation, depending on the die angle and friction, either the extrusion mode of flow or the upsetting mode occurs, but never both. If the upsetting mode acts initially, tensile strains occur on the hub surface, which lead to fracture. The extrusion mode results in compressive strains and fracture does not occur. Slip-line field analysis is utilized to predict the combinations of die angle and friction coefficient that delineate between the initial extrusion and upsetting modes of deformation. In addition, an experimental study was performed using sintered powder billets of 601 AB aluminum alloy as the workpiece material. Measurements of the surface strains and workpiece deformation profiles confirm the slip-line field analytical predictions of initial deformation mode, even though the material properties do not match those required for slip-line analysis. After the initial stage of deformation, both extrusion and upsetting flow occur. Surface strain states on the hub surface that were compressive in the initial stage of deformation now become tensile and lead to fracture in the later stages of deformation. The strains at fracture obey a fracture criterion established previously by the authors. Results of this study can be utilized to aid preform and die design in extrusion-forging processes. In addition, the experimental techniques may aid the analysis of other deformation processes.