Flow behavior of the billet surface layer in porthole die extrusion of aluminum

In this study, the effects of some direct extrusion parameters on the flow behavior of the billet surface layer in porthole die direct extrusion of aluminum were investigated. Extrusion parameters to be changed were friction conditions on the container wall, temperature, and stroke. To detect the flow pattern of the surface layer, two types of billets were used. The flow of the surface layer could be divided into an inward flow along the back face of the billet and a forward flow along the boundary of the dead zone. The container friction strongly influenced the balance between two flows, while the extrusion temperature had little influence on the flow pattern. The paths that the billet surface layer followed in the welding chamber through the porthole were revealed clearly. Once the surface layer flows into the porthole, it appears on the outer face of the extrudate or just below it. Especially, the surface layer, which follows the inward-flow path, penetrates into even the welding plane of the tube, resulting in the welding defect.