Modeling And Experiemtal Study Of The Friction Behaviors In Micro-forming Of Sheet Metal

Interfacial friction behavior is one of the most important boundary conditions in the forming process of sheet metal,which is the key factor of whether the part can be processed with high quality.With the decrease of sheet thickness,the proportion of contact interface increases,and the influence of interfacial friction on the forming process become more significant.When the sheet thickness decreases to micro-and meso-scale,the influence of intrinsic charactors such as grains,surface asperities of tool and workpiece on the interfacial friction can not be neglected.Traditional friction models such as Coulomb friction model,constant factor model and general friction model,etc.,is difficult to accurately describe the influence of these charactors,due to the lack of description of surface deformation and damage during the friction process.Therefore,it is of great value to study the interfacial friction behavior in the micro-sheet forming process.The aim of this paper is the precise prediction of the tool-workpiece interfacial friction behavior in metal forming.The mechanism and the grain size effect on the friction behavior of the metal polycrystalline material are analyzed and studied.The results are applied to the rolling process of the fine metal sheet.The quality of the surface scratch forming analysis and prediction,the main research work in this paper including: 1)Experimental study of grain size effect on friction behaviorAt present,there are controversies about the experimental research results of the scale effect of frictional grains of polycrystalline metals.Different experimental methods were used by different researchers to study the grain size effects on friction of ductile materials,but opposite results were obtained.In order to investigate the grain size effects,the hardness and roughness of the contact surface were designed based on the different characteristics of frictional adhesion and ploughing,and pin-disk friction experiments were conducted to analyze the difference in grain size effects.The results showed that the influence of grain size on the friction behavior of the adhesive and ploughing is opposite.With the increase of grain size,the friction coefficient of adhesion gradually decreases,and the friction coefficient of the ploughing increases gradually.The experimental conclusion explains the controversy of previous experiments.2)Modeling and analysis of grain size effect on adhesion frictionThe existing theoretical models for the behavior of adhesion friction do not fully characterize the yield and hardening of materials,and cannot accurately describe the effect of grain size on the adhesion friction behavior.Based on the traditional contact mechanics and adhesion friction theory,this paper establishes an adhesion friction model considering the nonlinear hardening of materials by means of finite element method,and introduces the law of flow and hardening with the change of grain size to the established adhesion friction.In the model,an accurate description of the grain size effect on the adhesion friction behavior is achieved.By comparing with the experiment,the accuracy of the model was verified.Based on the established friction model,the mechanism of the grain size on the adhesion friction behavior during the forming process was analyzed.The results showed that the increase of grain size on the one hand increases the proportion of the plastic deformation of the contact surface in the overall deformation,and on the other hand it inhibits the increase of the contact point.Both factors lead to a decrease in the coefficient of friction.3)Modeling and analysis of grain size effect on ploughing frictionIn this paper,surface scratching experiments were performed to simulate the ploughing behavior of the surface of the material in the process of forming,and the damage behavior of the material under different cone angles and grain sizes was analyzed.The results showed that changes in the grain size of the material can lead to a change in the failure mode of the material.The existing ploughing friction model is based on a single mechanism such as plastic deformation and crack propagation,and cannot describe the effect of grain size.In this paper,the material flow and fracture behavior during the scratch process were analysed to establish a surface scratch model that considers the failure mode of the material.The influence of grain size on material flow stress and fracture strain was introduced into the scratch model.The grain size effects on scratch resistance and the friction coefficient of ploughing were predicted and verified by experimental comparison.The model analysis and experimental observations showed that the increase of grain size reduces the content of grain boundaries in the material,and suppresses the generation of cracks in the material during the ploughing process,which results in an increase in the friction coefficient.4)FE analysis of microforming process considering the grain size effects on friction Based on the theoretical modeling of the scale effect of frictional grains on the adhesion and ploughing,a comprehensive friction model that considers the influence of grain size,workpiece and mold surface topography was established in the framework of the universal friction model,where the ploughing model was introduced to calculate the friction factor,and the adhesive area model was introduced to calculate the real contact area on the other hand.The friction model was further uses to analyse metal micro-rolling process and cylindrical upsetting process.The comparison of the simulation of the rolling process and experimental results showed that the FE model can accurately predict the depth,location of scratch morphorlogy on the surface of the part.As the grain size increases,the friction coefficient during rolling,the scratch depth on the surface of the part,and the thickness reduction of the sheet material will increase.The analysis of the process of cylinder upsetting at different scales and the experimental results show that,as the forming scale increases from microscopic to macroscopic,the bulge produced by upsetting decreases first and then increases.The reason for this size effect phenomenon is the main source of the constraints to grain rotation changes from surface contact and friction to the interaction between adjacent grains.In this paper,we focus on the interfacial friction behavior in sheet metal forming,and conduct experimental and theoretical research on the effect of friction grain size.The research results in this paper will provide the theoretical basis for the design of the forming process of the micro thin plate structure products under the fine conditions.