Study on size effect affected progressive microforming of conical flanged parts directly using sheet metals

Progressive forming of microparts directly using sheet metals is an efficient approach for realization of mass production. However, there are many unknown behaviors and phenomena affecting the decision-making in process and tooling design, because the specimen generally comprises only a few grains in the wall thickness, where the mechanical response and deformation of material behave differently compared with those in macro-scale due to size effects. In this research, a progressive forming system including punching, extrusion, and blanking operations with three-scaled punches and dies was developed to fabricate cylindrical parts and conical flanged parts by directly using copper sheets with different grain sizes. Through physical experiments, measurement and analysis of the deformations and related variables, grain and specimen size effects affected deformation behaviors and the characteristics of the conical flanged parts were comprehensively studied in term of forming pressure, microstructural evolution, dimensional accuracy, undesirable geometries and surface qualities. It is revealed from the experimental results that the forming pressure increases with the decrease of specimen and grain size. For the cylindrical parts, their scaled length increases with the decrease of grain size and increase of scaling factor. For the conical flanged parts, four deformation zones were divided based on microstructural evolution and a worse forming quality appeared on the conical surface, where the defects including microcracks, microbulges, and micropits were observed. Properly coarse grains can reduce the undesirable geometries, but at a slight cost of deterioration of the quality of the formed conical surface. When scaled down to micro-scale, the undesirable geometries and the surface qualities deteriorate considerably. The anisotropy of individual grain is not significant on the constrained features and surfaces. All these understandings facilitate the fabrication of conical flanged microparts directly using sheet metal in the aspects of deformation behaviors and controlling product qualities and enrich the knowledge of this unique microforming process.