Influence of dead metal zone on dislocation strengthening effect during micro-progressive forming

Micro-scale plastic deformation (microforming) is a promising manufacturing technology in modern industries. To put microformed parts in a more competitive position compared with micro-products made by other micro-manufacturing methods (such as micromachining and 3D printing), micro-progressive forming must be fully studied for the mass production of micro-parts to obtain good grain texture and decrease product costs. Simultaneously, the undiscovered dislocation strengthening effect of micro-scale progressive forming should be investigated. Using micro-universal joint progressive forming as a case study, the influence of the dead metal zone–induced strengthening effect on ductile fracture in micro-progressive forming is discussed. Based on the results of upsetting experiment and finite element simulation, it was found that the specimen produced via metal foil forward-extrusion-blanking process eliminated the ductile fracture when applying the same experiment condition to the specimen produced by micromachining. Based on the micro-indentation and X-ray diffraction results, a dead metal zone–induced dislocation strengthening model was established, and the mechanism for the strengthening effect during micro-progressive forming was revealed. This research thus provides an in-depth understanding of the micro-scale progressive forming process and takes an important step in the field of ductile fracture in multi-stage plastic deformation.