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 $\eta$, the later the back-end defects form.

后端缺陷是铝合金型材挤压中的一个关键问题，特别是在微型复杂空心型材中。本研究首次揭示了微型复杂空心挤压型材表面后端缺陷的形成机理和优化策略。通过数值模拟研究了钢坯蒙皮的流动行为、后端缺陷以及舷窗外半径的影响。对废弃钢坯和后端缺陷的微观结构、化学成分和力学性能进行了实验研究。结果表明，后端缺陷的耐腐蚀性能和力学性能较差。与常规尺寸的型材不同，一种罕见的坯皮向前流动主导着微型空心型材的挤压，导致过早形成严重的表面后端缺陷。前向流动的主要原因是在超大挤压比（如137）下金属坯料径向流动显着减弱。然而，较小的舷窗外半径有效地延迟了坯料沿前向流动路径进入型材的时间，这是由于较大的存储节流区增加了坯料蒙皮的阻塞效果和存储容量。总之，一个新的无量纲参数 较小的舷窗外半径有效地延迟了坯皮沿前向流动路径进入型材，这是因为较大的储存节流区增加了坯皮的阻塞效果和储存容量。总之，一个新的无量纲参数 较小的舷窗外半径有效地延迟了坯皮沿前向流动路径进入型材，这是因为较大的储存节流区增加了坯皮的阻塞效果和储存容量。总之，一个新的无量纲参数η用于评估挤压参数（例如，舷窗外半径）在延迟微型空心型材中表面后端缺陷形成的能力。的值越大$\eta$，后端缺陷形成的时间越晚。