An investigation on the formability of sheet metals in the micro/meso scale hydroforming process

Abstract

ABSTRACT The geometry and grain size effects have been revealed to affect the formability of metal materials during the micro/meso scale forming processes. To understand and characterize how these size effects influence practical forming process and further to utilize those knowledge in process design and optimization, an experimental and numerical investigation on a micro/meso scale hydroforming process of pure copper sheet metals was conducted as a case study. A hydroforming process experimental setup was first developed to form long multi-channel features with different dimensions. The experimental results reveal evident size effect: The pressure and maximum height onset of failure decrease as the grain size approaches the thickness. The size effect on the pressure was identified to be attribute to the reduction of flow stress of material as the grain size increases. The surface layer model has been employed to explain the mechanism. On the other hand, the decrease of ultimate height was revealed to be affected by the reduction of forming limit of sheet metals as the size effect becomes more significant. The interaction of geometry and grain sizes and the evolution of micro voids were discussed and a modified GTN–Thomason model with the consideration of grain size effect was employed in FE simulations to estimate the forming results. A reasonable agreement between the numerical and experimental results was observed. After that the method was further utilized in the process optimization for the fabrication of a fuel cell bipolar plate with typical micro/meso scale channel features. The dangerous area with a high risk of failure was predicted based on simulations. After optimizing the process parameters, the satisfying simulation result was obtained. Hence the hydroforming apparatus was developed accordingly and the bipolar plate was successfully fabricated with high quality, which verifies the applicability of the method in the present work.