Numerical Simulation And Experimental Study Of Bellows Hydroforming

Bellows hydroforming is a metal plastic forming process that using liquid to apply high internal pressure to the tube, meanwhile applying axial thrust at both ends of the tube, so as to make the tube deformed along radial. Because the effects of the material, the die shape and the loading condition, the bellows may be limited by the failure modes of buckling, wrinkling and bursting during the hydroforming process. To avoid the forming failures and improve forming quality, studying the technology of bellows hydroforming have a very important practical significance.This paper used FEA software DYNAFORM to build the numerical model of bellows hydroforming and analyze the deformation of each part of bellows after hydroforming. Simultaneously, the hydroforming experiment of bellows is carried out to verify the numerical simulation results. Finally, the influence of axial feed, internal pressure and coefficient of thickness anisotropy to the thickness reduction ratio of bellows is analyzed.The main research work and achievements of this paper are as follows:(1) Build numerical model to simulate the process of bellows hydroforming. Hill yield with the consideration of coefficient of thickness anisotropy and J.H. Holloman strain hardening model is selected, the friction between the tube and the die is calculated by penalty function algorithm and modified coulomb friction law, the stress and strain field in the forming process of bellows is solved by using the dynamic explicit algorithm with strong contact analysis function.(2) Thickness reduction ratio is the main factor that reflects the forming quality of bellows,through numerical simulate to analyze the deformation of each part of bellows after hydroforming, finding the maximum thickness reduction ratio located at the peak. At the same time, hydroforming experiment of bellows and thickness measurement are carried out, the numerical simulation and experimental results of thickness reduction ratio of bellows are compared, finding both in good agreement.(3) The influence of axial feed parameters on thickness reduction ratio of bellows is studied. Four kinds of loading paths of axial feed displacement are compared: step path, bilinear path, single linear path and two curve path. The thickness reduction ratio of step path and two curve path are the largest, single linear path smaller, bilinear path is the smallest. Under the condition that the total axial feed displacement is unchanged, quick feed in the initial forming stage is favorable for reducing the thickness reduction ratio of bellows.(4) The influence of internal pressure on thickness reduction ratio of bellows is studied. The gradient pressure curve of maintaining after boosting is used, finding the thickness of bellows increases with the increase of the internal pressure. However, if the internal pressure is too low, due to the mismatch between internal pressure and axial feed, obvious fold phenomenon appeared at the peak.(5) In order to consider the anisotropy of the material in the process of bellows hydroforming, coefficient of thickness anisotropy is used to reflect the difference between the in-plane deformation and the thickness deformation. Through the numerical simulation of bellows forming with different coefficient of thickness anisotropy, the thickness of the material is increased with the increase of coefficient of thickness anisotropy.