Investigation On The Dynamic Frictional Characteristics Of Thin-walled Tubes Under Impact Loading

In the actual production,the quality of formed parts is affected by many complicated factors.Friction plays an important role in the tube hydroforming(THF),which has an adverse effect on formability of the metal thin-walled tube.Liquid impact forming(LIF)is a new type of forming technology based on the THF.It is an advanced manufacturing technology for forming thin-walled tubes or plates into parts of different shapes by combining ordinary stamping and hydroforming.In the process of LIF,the tube is bulged rapidly in a short period of time.The coefficient of friction(COF)may change dynamically with the high velocity plastic deformation of the tube.Moreover,impacting velocity and other parameters may have effects on the COF in the guiding zone and result in complicated frictional characteristics in LIF.As a result,it is of great significance to explore the frictional characteristics of tube under impact hydraulic circumstance and further reveal the effects of different impact loading parameters on COF.Based on the research methods of theoretical analysis,numerical simulation and experimental research,the dynamic frictional characteristics of thin-walled tube under impact loading are studied based on plastic mechanics,hydroforming technology,advanced sensing technology and on-line detection technology.The specific topics of this thesis are as follows:(1)Methods and principle of measuring the COF of metal plastic deformation under high velocity are compared,which lays the foundation for subsequent research.(2)The LIF experiments of SUS304 stainless steel tube and TP2 copper tube are carried out by using the COF measuring device developed by this group.The changes of the COF in the guiding zone of two kinds of tubes under impact loading are studied.(3)The curves of COF of two kinds of tubes are obtained,the effects of the impacting velocity on COF are revealed,and the difference and relation between the COF of TP2 copper tube under ordinary hydraulic circumstance and impact hydraulic circumstance are compared through the LIF experiments of TP2 copper tube.(4)Simultaneously,the finite element simulation with the same experiment conditions is carried out by the ABAQUS finite element analysis software.The accuracy of the finite element models are verified by comparing the simulation results with the experimental results.The results display that:(1)No matter which kinds of tube,the COF in the guiding zone has always changed dynamically during the forming process,and all of them show a downward trend.(2)For SUS304 stainless steel tube,with the increase of impacting velocity,the COF in the front part decreases faster in the latter part in the same length tube.At the same impacting velocity,as the length of tube increases,the COF in the front part of the forming time decreases faster,while in the later part of the forming time decreases slowly.(3)For the TP2 copper tube,as the impacting velocity increases,the decrease of the COF in the front part of the forming time becomes larger,and the descending velocity of the latter part becomes smaller and ultimately remains unchanged.With the increase of impacting velocity,the curves of the hydraulic pressure change more steeply.That is,the increasing velocity of the hydraulic pressure curve increases,and the maximum liquid pressure value required for forming is also greater,as the impacting velocity increases.(4)Under the ordinary hydraulic circumstance,the decrease of the COF is smaller,and the curve of the COF changes more smoothly.Under the impact hydraulic circumstance,the COF decreases more rapidly,that is,the decrease of the COF is faster.(5)By comparing the finite element simulation and experimental results,it is found that the precision of the constructed finite element model meets the requirements.