Formation mechanism and optimization strategy of surface back-end defects in miniature complex hollow extruded profile

Back-end defects present a crucial problem in the extrusion of aluminum alloy profiles, particularly in miniature complex hollow profiles. In this study, the formation mechanism and optimization strategy of surface back-end defects in miniature complex hollow extruded profile were revealed for the first time. The flow behavior of the billet skin, back-end defects, and effect of the porthole outer radius were studied via numerical simulation. The microstructure, chemical composition, and mechanical properties of the discarded billet and back-end defects were experimentally investigated. The results showed that the corrosion resistance and mechanical properties of the back-end defects were poor. Unlike regular-sized profiles, a rare forward flow of the billet skin dominates the extrusion of miniature hollow profiles, resulting in the premature formation of severe surface back-end defects. The primary cause of the forward flow is the significant weakening of the radial flow of billet metal under an ultra-large extrusion ratio (e.g., 137). Nevertheless, a small porthole outer radius effectively delays the entry of billet skin into the profile along the forward flow path owing to a larger storage choke zone that increases the blocking effect and storage capacity of billet skin. In summary, a new dimensionless parameter η was formulated to evaluate the ability of the extrusion parameters (e.g., porthole outer radius) in delaying the formation of surface back-end defects in miniature hollow profiles. The larger the value of η, the later the back-end defects form.