Research On Bulging-upsetting Composite Hydroforming For Thin-walled Tube With Large Expansion Ratio

With the problem of energy shortage becoming more and more prominent,aerospace,automobile and other industries require more and more lightweight parts,and hydroforming parts have broad application prospects due to their lightweight characteristics.Thin-walled tube with large expansion ratio is used more and more widely in industrial,military,chemical and other fields.However,due to the characteristics of large expansion ratio and large diameter-tothickness ratio,it is easy to cause severe wall thickness reduction and even cracking,and low die-fitting ratio and other issues during hydroforming.Therefore,it is necessary to explore an appropriate hydroforming process in order to process thin-walled tube with large expansion ratio with better wall thickness uniformity and die-fitting effects,thereby improving the quality of Chinese manufacturing.Aiming at the problems of severe wall thickness reduction and low die-fitting ratio during conventional hydroforming of thin-walled tube with large expansion ratio,this paper firstly analyzed the material flow characteristics and feeding mechanism during hydroforming,adopted bulging and upsetting composite forming,a process of bulging-upsetting composite hydroforming was proposed.By comparing the forming process principles of the bulgingupsetting composite hydroforming and the conventional hydroforming,the difference in the feeding mechanism of the two processes was explained.The purpose is to improve the typical problem of poor uniformity of wall thickness and low die-fitting ratio in hydroforming,and realize good quality hydroforming of thin-walled tube with large expansion ratio.As a typical application of the bulging-upsetting composite hydroforming for thin-walled tube with large expansion ratio,taking the outer tube of a certain type of silencer,biconvex thinwalled tube,as an example.The bulging-upsetting composite hydroforming process was designed and the finite element analysis model was established based on ABAQUS.The wall thickness distribution and the stress-strain distribution of the forming parts obtained by numerical simulation were compared.The analysis shows that the maximum thinning ratio of the bulging-upsetting composite hydroforming is 30%,so the maximum thinning ratio has been significantly improved compared with the 67.1% of the conventional hydroforming.The uniformity of wall thickness distribution as well as the die-fitting effects of the formed part obtained by bulging-upsetting composite hydroforming is better,which proves that the bulgingupsetting composite hydroforming is an effective process for forming thin-walled tube with large expansion ratio.In view of the problem of multiple process parameters and strong interaction of factors in the bulging-upsetting composite hydroforming,a multi-parameter and multi-objective parameter optimization method in hydroforming based on the response surface method was presented.The response surface models of the maximum thinning ratio and the maximum fillet radius difference were established,the forming performance of the biconvex thin-walled tube under different loading paths was analyzed,and the loading path was optimized based on the quality evaluation parameters.The optimal loading path was determined to be linear loading,and the maximum axial feeding was 13 mm and the maximum internal pressure was 89 MPa.The error of the maximum thinning ratio,the maximum fillet radius difference between simulation and prediction were 1.70% and 0.82%,respectively.Therefore,the numerical simulation agrees well with the theoretical optimization.Finally,the forming experiments of the biconvex thin-walled tube in the bulging-upsetting composite hydroforming were carried out.Through the comparison between the experimental results and the numerical simulation results,the validity of the finite element model was verified.The theoretical optimization parameters obtained by response surface method were used to carry out forming experiments,and the theoretical optimization results,numerical simulation results,and experimental results were compared.The error of the maximum thinning ratio,the maximum fillet radius difference between experiment and prediction were 0.76% and 7.22%,respectively.The results indicated that the response surface method had good feasibility to optimize the loading path,and the biconvex tube with uniform wall thickness and shape size was obtained,which can provide a reference for the hydroforming of other pipe fittings.