Study On The Surface Quality And Surface Material Flow Behavior During Ultrasonic-assistey Incremental Sheet Forming Of Aluminum Alloy Sheets

Incremental sheet forming process(ISF)has advantages in the field of plastic sheet forming due to its great flexibility and low-cost.However,the surface quality is an important limiting factor for the industrialization of the incremental sheet forming process.With the development of the ultrasonic assisted forming technology,positive effects of the ultrasonic vibration on the improvement of the surface quality and forming properties have been proved during other forming technologies.Therefore,this thesis dedicated investigates the surface quality and surface material flow behavior during ultrasonic-assisted incremental sheet forming of aluminum alloy sheets.Firstly,based on the method of three dimensional surface characterization,the height parameters and the function parameters of both internal and external surfaces were observed.The effect of forming parameters and the rolling mark orientation was explored by examining the Sa,Sku,Ssk,Sbi and Sci.And the optimal combination of process parameters of single index and the surface quality were obtained with both internal and external surfaces.Besides,the prediction model of the three dimensional surface characterization were established based on the influence of the step-down size,the forming tool radius,and the speed rotation with the internal surfaceSecondly,the ultrasonic vibration was added into the incremental sheet forming process.The effect of the ultrasonic vibration and the forming parameters(the step-down size and the roll mark orientation)on the surface topography were explored.The periodic dent feature on the surface was obtained by the white light interferometer.The effect of the periodic dent feature was explored by examining surface height parameters including the arithmetic average Sa,the kurtosis Sku,and the skewness Ssk of the roughness surface and the waviness surface,respectively.It can be found that the ultrasonic vibration can effectively reduce the arithmetic average Sa and the kurtosis Sku.Subsequently,the surface function parameters were analyzed to explore the relationship between surface topography and surface properties.It is obtained that the ultrasonic vibration has a positive effect on the improvement of the surface bearing capacity and lubrication performance.Thirdly,the material flow and deformation behavior of the ultrasonic assisted incremental sheet forming process(UISF)were investigated through three approaches including analytical modeling,finite element modeling,and experimental testing.It can be observed that the material flow area is significantly increased after applying ultrasonic vibration,leading to more materials participates in the plastic deformation.Meanwhile,the material flow area increases considerably with the increase of the ultrasonic amplitude,while it decreases slightly with the raise of ultrasonic frequency.The reduction of forming forces in UISF is attributed to the reduced flow stress due to the increased material flow area and the reduced contact time due to the separation effect caused by the dynamic effect of ultrasonic vibration.Besides,the ultrasonic vibration can effectively reduce the stress along the thickness direction and the Von Mises stress.This reduction plays a significant effect in the reduction of the forming force and the improvement of the formability.Finally,the surface performance and the forming performance of the forming parts during the ultrasonic assisted incremental sheet forming were investigated through reasonable experiments and testing methods.The effect of the ultrasonic vibration on the surface hardness,the surface residual stress,and the surface microstructure was explored.It can be found that the application of the ultrasonic vibration can reduce the surface hardness,uniform the residual stress distribution and increase the surface grain size.Subsequently,the influence of the ultrasonic vibration on the thickness of the forming parts and the forming limit was analyzed with different forming parameters.The results shown that the minimum thickness of the forming parts is improved with the help of the ultrasonic vibration.And the increase of the minimum thickness has significant effect on the forming limit during the ISF process.However,this improvement mainly exists in cases with small deformation.Finally,the tensile test was performed to explain the improvement of the plastic properties with the ultrasonic vibration.