Forming Limit Prediction of Micro Sheet Metal Forming due to Grain Size Effect

• Main Authors: Chi-HanChen, 陳麒翰
• Other Authors: Rong-Shean Lee
• Format: Others
• Language: en_US
• Published: 2010
• Online Access: http://ndltd.ncl.edu.tw/handle/86350503554601068524

博士 === 國立成功大學 === 機械工程學系碩博士班 === 98 === Due to the miniaturization trend of the devices in electronic, medical and 3C industries, the improvements for manufacturing efficiency and product quality are important means to keep competitive. Micro metal forming is the most suitable and cost effective manufacturing process for mass production of micro metal parts because of its high production rate, low material scrap rate, net shape production and superior mechanical properties. However, when the material deforms in the micrometer range, the forming feature may only have few grains. The microstructure becomes more important when miniaturization and exhibits grain size effect which influences material flow and formability in micro metal forming process. The plasticity theory and the know-how of conventional metal forming technology developed under macro scale cannot directly apply to micro metal forming. In this paper, the influence of grain size effect on forming process and formability of micro sheet metal forming was investigated. For the experimental study, a series of micro scale tensile tests, dome height tests and deep drawing experiments were conducted for investigating the grain size effect on mechanical properties and formability of stainless steel 304 and C2600 brass alloy foils. The FLDs of micro sheet metal forming were established by local strain measurements of the micro grids and spots through image measurement and management processes. In forming limit prediction, finite element code LS-DYNA was used to calculate the ductile fracture constants in the ductile fracture criteria and develop predicted forming limit curves. Based on the experimental results, simulation results and Oh’s fracture criterion, two new models were proposed in this paper for predicting the forming limit of stainless steel 304 and C2600 brass alloy foils in micro sheet metal forming. The first proposed model includes the effect of strain path while the second proposed model considers the coupling effect of strain path and thickness to grain size ratio. The first model is superior to the Oh’s criterion on predicting forming limit strain of the foils, but it is not suitable for the foils that are thinner than 100μm. However, the second proposed model can be used for forming limit prediction of the stainless steel 304 and C2600 brass alloy foils with the foil thickness less than 100μm where the grain size effect must be considered.