Study On The Constitutive Behavior In Micro Forming And Micro Bending Of Ultra-thin Sheet

With the rapid development of MEMS and micro electronic industry,there is a great demand of micro parts.Current micro forming technologies can not satisfy the mass fabrication of complex three-dimension micro parts.As the extension of traditional plastic forming,micro forming technology has a wide foreground for the fabrication of micro parts.However,as the part size decreases to several hundreds micro meters or less,size effects make the traditional forming theories not suitable in micro forming field.In this thesis,the constitutive behavior in micro forming considering the first order size effect was set up based on micro tensile tests.And then it was applied in the FE analysis to investigate the influence of process parameters on micro bending. Meanwhile,the simulation results can also supply references to the mould design.At last,the micro bending tests were also been done to compare with the simulation results and validate the constitutive model.The detail investigation is as follows:Firstly,the size effects in the micro forming were studied,and also divided into the first order size effect and the second order size effect.For the first order size effect, the theory model was established based on the surface layer model.The test results were given out and the size effects in the springback were also analyzed according to the free micro bending test.For the second order size effect,a measure method of characteristic length for H62(Y2) was put forward based on the former research,and the test results were also given out.Secondly,the influence of size effects on stress-strain relationship was investigated based on micro tensile tests of ultra-thin H62(Y2) sheet.The internal grain boundary model was also presented to explain the size effects in micro tensile tests.Based on that,the constitutive behavior in micro forming considering the first order size effect was set up,and then validated to be correct.Thirdly,the micro bending process of ultra-thin sheet was analyzed in ABAQUS using the modified surface layer model.The results were also compared with using the traditional material model.According to the simulation results,the parameters of mould structure were optimized in order to reduce the springback and supply references to the mould design.The influence of process parameters on bending force and springback was also studied.Finally,based on the simulation results and traditional mould design theory,the micro bending moulds were designed and fabricated.In order to investigate the influence of thickness,grain size,bending angle,friction condition and forming velocity on bending force and springback,several micro bending tests were also done on ultra-thin H62(Y2) sheet.The tests results agreed well with the simulation results.