A multi-stage micro deep drawing process for double-stage stepped micro-cups based on pre-optimization of forming limits

The development of microforming processes for stepped micro-cup components with intricate geometries remains a formidable challenge. This study proposes a multi-stage micro deep drawing (MMDD) process incorporating forming limit pre-optimization and successfully fabricate double-stage stepped micro-cup components with satisfactory dimensional accuracy. However, at the micro/meso scale, the formability and fracture mechanisms of TA1 pure titanium foil exhibits pronounced grain size sensitivity. Specifically, as grain size increases, both the forming limit and the limiting drawing depth of the double-stage stepped micro-cup are reduced. This deterioration in formability is primarily attributed to the weakening of macroscopic mechanical properties and capacity for microstructural strain coordination. Concurrently, a transition from ductile to brittle fracture modes is observed with increasing grain size. This transition is attributed to twinning-induced dynamic recrystallization (TDRX) behavior: larger grains are associated with reduced dislocation density and less pronounced twinning fragmentation, which suppresses the generation of fine dynamic recrystallized (DRXed) grains and hindering the evolution of defects into spherical pores. Based on forming limit pre-optimization, process feasibility verification, and elucidation of mechanism at the micro/meso scale, the following material selection guidelines are proposed: to balance economic efficiency with forming requirements, the as-received material is preferred when the target component’s aspect ratio, defined by the height of the upper large-diameter section (H) relative to the lower small-diameter section (h), satisfies H/h ≥ 0.810. For more demanding drawing depths, corresponding to 0.810 > H/h ≥ 0.439, the material annealed at 500 °C is recommended, as it provides optimal formability.