Recent Progress in Extrusion and Related SPD Technologies for Metallic Materials: A Review

Extrusion is a core branch of advanced metal forming. Over the past century, it has evolved from conventional hot and cold extrusion into a diversified family of processes that now encompasses severe plastic deformation (SPD) techniques. The industrial limits of these processes are governed by a small set of dominant physical mechanisms: dynamic recrystallization (DRX), which dictates grain refinement; adiabatic heating and the associated hot shortness (surface cracking), which bound the maximum usable extrusion speed; and, in porthole-die and hybrid extrusion, the pressure-temperature-time criteria that govern solid-state seam-weld integrity. This review examines metal extrusion and the closely related SPD technologies from this mechanism-oriented perspective. First, it elucidates the fundamentals governing metal flow and deformation behavior (MFDB) during extrusion, including the primary shear zone, the dead metal zone, and container-wall friction, together with the influence of material-intrinsic factors (alloy composition and initial grain morphology) and external conditions (die design and process parameters). Second, it discusses the key role of die design and manufacturing in regulating metal flow and ensuring product precision, including the application of finite element analysis (FEA) and intelligent algorithms to die optimization. The core of this review is a comparative analysis of state-of-the-art processes, organized into two kinematically distinct categories: continuous profile-forming extrusion, in which the billet is forced through a die orifice (direct, indirect, lateral, field-assisted and shear-assisted variants), and discrete SPD-based conditioning, in which large strains are accumulated at constant cross-section, covering cyclic extrusion compression (CEC), high-pressure torsion (HPT), equal-channel angular extrusion (ECAE), field-assisted extrusion (FAE), and shear-assisted processing and extrusion (ShAPE). For each technology, the working principle, die structural features, and distinctive effects on microstructural evolution (grain refinement, phase transformation, texture regulation) and mechanical properties (strength, ductility, wear resistance) are detailed. Finally, current industrial applications are summarized, and future development trends are outlined, including intelligent monitoring systems, modular equipment, and hybridization with additive manufacturing, with the aim of achieving green and efficient forming of metallic materials.