Research On Liquid-filled Bending Process Of Metal Thin-walled Pipes

Thin-walled or extremely thin-walled elbow fittings have the characteristics of light weight,low consumption,and good structural performance.They are widely used in important fields such as gas-liquid transmission pipe lines,aerospace,and ships.However,due to the hollow structure of the pipe and the non-uniformity of metal flow during the bending process,the thin-walled elbow forming parts are prone to wrinkles,cracks and cross-sectional distortion.In this regard,this article proposes a new liquid-filled bending forming process suitable for metal thin-walled or extremely thin-walled pipes,which studies a new forming method for the bending of extremely thin pipes.This process uses a variable loading pressure liquid to support the inside of the pipe.Pull and thrust are applied to both ends of the pipe.The bending process of the pipe is completed inside the bending mold.The method of theoretical analysis,finite element simulation and process test is used to study the process.First of all,the principle of liquid-filled bending forming of thin-walled pipes is introduced in detail,and the deformation and defects of the pipes under pure bending are analyzed.The material performance parameters of 304 stainless steel pipes are obtained through uniaxial tensile tests at room temperature.Based on the ABAQUS finite element software,a finite element model of the thin-walled pipe liquid-filled bending forming process was established,and the stress and strain distribution under three different working conditions of separate push bending,separate stretch bending and push-pull bending and their effects in the bending process were compared and analyzed.The results show that the push-pull bending condition can effectively reduce the stress and strain concentration of the pipe and reduce the extreme value of the stress and strain.The three working conditions are compared and analyzed according to the forming defects and wall thickness changes.The results show that the push-pull bending conditions can better solve the problems of inner wrinkling and port distortion,and make the inner and outer wall thicknesses more uniform.Secondly,the results of bending forming of thin-walled pipes under different forming parameters are analyzed,and the influence of forming parameters is studied using single factor method.The results show that both the bending radius and the bending angle will have a certain degree of influence on the wall thickness change during the pipe bending process,and the inner and outer wall thickness changes of the push-pull bending condition are the most uniform,and the influence of the forming parameters is the least,and the thickness of the inner and outer walls increases.Both the thickness rate and the thinning rate are maintained at about 10%.The forming quality analysis under three different process parameters of internal pressure,die gap,and friction coefficient is carried out on push-pull bending conditions,and the significance of the three process parameters is analyzed by the orthogonal test method,which proves that the internal pressure is the most important factor affecting the forming quality under this working condition.The working conditions of different internal pressure loading paths and variable speed feed are simulated.Among them,the forming effect under gradient loading internal pressure and constant speed feed is the best,and the degree of wall thickness change is relatively small.Finally,according to the above research results,a thin-walled pipe liquid-filled bending forming process mold was designed and the process test was carried out.The test results show that the gradient-loaded internal pressure can successfully produce the elbow forming part.The comparison between the test results of the gradient loading internal pressure and the simulation results shows that the wall thickness of the inner and outer ridge lines are in good agreement with the circumferential wall thickness of the typical section.The ovality value of different sections remains the same,and the ovality value does not exceed 2%,which indicates this process has better control over the distortion of the pipe cross-section.