Study Of The Deformation Mechanical Properties On Pre-strain Ultrathin Stainless Steel Sheet

Micro channel structures with high aspect ratio were widely applied in high efficiency reactors and new heat transformation structures.Multistage micro stamping which was effective technology in manufacturing micro channels with high aspect ratio and high profile accuracy,could make full use of the deformation properties in ultra-thin sheet by asymptotical deformation.However,due to the pre-strain induced by former deforming,the mechanical properties of ultra-thin sheet in the following deforming stage were significantly influenced,which made the deformation design in pre-deformed ultra-thin sheet more complicate.It also influenced the profile accuracy and forming properties in manufacturing the micro channel structures whit ultra-thin sheet.It was of great importance to study the pre-strain influence on the micro/meso scale mechanical properties in ultra-thin sheet multi-stage deforming.The mechanical properties of pre-strained 316 L stainless steel sheet with thickness of 0.1mm were studied by experiments and modeling.In order to study the mechanical behavior of the pre-strained material in the same-oriented and reversal loading,uniaxial tensile,pre-strain tensile and pre-strain bending experiments were conducted.Based on experiment results,the stress-strain relationship of pre-strained material under varied loading path was studied and the constitute model of pre-strain material in reversal compression loading path was built.Multi-stage deforming experiments for micro channels by using ultra-thin stainless steel sheet were designed and the parameter influence on the micro channel forming quality was also analyzed,which provided the theoretical guidance in multi-stage forming design.The main work in this thesis:(1)Complex loading experiments of 316 L ultra-thin stainless steel sheetTwo types of 316 L ultra-thin stainless steel sheet with different grain sizes were prepared by heat treatment experiments,and their stress-strain curves in elastic,plastic deformation were obtained by uniaxial tensile experiments.The pre-strain loading experiments were designed and the varying of material mechanical properties in pre-strain tensile and prestrain bending was studied.It was found that the flow stress was lower in coarse grain material and the mechanical behaviors of pre-strained material in the same-oriented tensile loading was nearly identical with that in the uniaxial tensile experiments.But the mechanical behaviors of pre-strain material in reversal compression loading showed significant Bauschinger effect.The decrease of yield stress in reversal compression related to the amount of pre-strain.(2)Modeling and analysis of the mechanical behavior of the prestrained ultra-thin sheetBased on the experiment results,the elastic,plastic stress-strain relationship in pre-strain material were studied and modeled.The mathematical relationship between the average unloading Young's modulus and pre-strain was fitted and modeled based on the measurement in multi-stage tensile experiments.Based on the calculating of bending moment in the pre-strain bending experiments,the stress-strain relationship of the pre-strain material in reversal compression was modeled,which coincided well with experiment results.(3)Multi-stage forming of micro channels with ultra-thin stainless steel sheetAccording to the requirement of high aspect ratio micro channel structures,the FEM simulation model for multi-stage forming was built.Based on the mechanical properties of pre-strain material,parameter influence on the micro channel forming quality was analyzed.Experiments were designed for studying the influence of the parameters such as forming stroke,fillet corner radius,and number of stages on the micro channel forming quality in ultra-thin stainless sheet with different grain size.It was found that the micro channels' profile quality and aspect ratio were improved by multi-stage forming and the research provided guidance for deformation design.