Mechanism of Crack Initiation and Propagation in Single-side Piercing Process for Hollow Forged Parts

Hollow parts are demanded to reduce the weight of automotive body, and it is required to develop manufacturing technology of hollow parts with high efficiency. Thus the process, which forged part is pierced, has been applied to produce hollow parts since the forging process has advantages compared to conventional processes e.g. casting, machining. However the process, which is combined forging and piercing process, is single-side piercing process since the die is not placed. In this research, the influences of the bottom thickness of pre-form and clearance on the crack propagation are investigated. 6XXX series aluminum alloy is used as a material, and specimen is extruded as a preforming process. Extruded specimens are punched under several clearance conditions. Additionally, finite element analysis is performed to consider the stress and strain state, which affects the initiation and propagation of macroscopic crack. The experimental result shows that sheared surface length increases with increase of clearance between punch and material inner wall. Analytical result indicates that compressive stress state, which occurs at sheared area, interferes the propagation of initiated crack. As a result, secondary sheared surface is generated in case of large bottom thickness of extruded pre-form.