A novel strategy for improving the formability of surface arrayed micro-grooves by constructing dual-gradient microstructures

Uniform grain refinement has been widely recognized as a strategy for enhancing the formability of surface arrayed micro-grooves by increasing the number of grains involved in the filling process. However, excessive grain refinement impeded the efficient formation of micro-grooves due to increased flow stress. In order to address this dilemma, a new strategy for constructing dual-gradient (DG) microstructures was proposed, to improve the formability of micro-grooves. This unique dual-gradient microstructure, featuring both grain size and dislocation density gradients, exhibited a soft surface (∼200 HV) and a hard core (∼350 HV). In addition, the effect of dual-gradient microstructure on the formability of micro-grooves in 316 L stainless steel was investigated by micro-rolling, in comparison with homogeneous coarse-grained (CG) and fine-grained (FG) sheets. The experimental results showed that the micro-grooves in the DG sheet achieved comparable surface quality to that of the FG sheet, while exhibiting significantly greater forming heights. Smaller differences in the surface quality and greater increases in forming heights were observed with increasing reduction or micro-groove width. Microstructural analysis revealed that the surface quality was governed by grain size, while the forming height was influenced by both the surface grain structure and through-thickness dual-gradient structure. The synergistic effect of initially favorable grain orientation of the surface grains, as well as intensified localized stress concentrations near the micro-grooves induced by the through-thickness dual-gradient structure, facilitated dislocation slip within the transverse direction-normal direction plane, thereby contributing to greater forming heights. This study offers promising a practical approach for micro-forming applications in stainless steels and other metallic materials.