Theory And Trial Study On Optimization Design Of Process Parameters In Tube Hydroforming

Tube hydroforming is a plastic forming method in which tube in the die isdeformed by internal pressure and axial force then the desired component is formedfinally. Comparing to traditional process, THF has a number of advantages, such asreduction in the number of parts, reduction in the tooling and material costs, lighteningweight, better surface quality and mechanical property, etc. It is obtained widelyapplication in automotive and aviation aerospace industry in recent years. But as it is anew plastic forming method, so far its theoretical research is not contented enough,because the matching of internal pressure and axial force is so complex and the flowstate of materials is difficult to grasp. This is limiting the application and developmentof tube hydroforming greatly.In order to analyze the internal rule of tube hydroforming, this paper introducesthe history, recent development, focus and the main problems of the special process.The technology development of finite element method and the introduction of dynamicexplicit algorithm are also reported in the paper.Base on the broad comprehension of tube hydroforming, this paper study on thenormal strain increment dε t with incremental theory. Disclose the mechanicalessence of occurring wrinkle, burst and larger bulge. This paper focus on theshortcoming in the theoretical research, analyze macroscopic mechanics behavior inTHF with plastic mechanics. Give out the distribution of typical stress state on yieldellipse and the relation of internal pressure and axial force when tube occurs plasticforming. Base on Swift diffuse instability theory, this paper give out the equation ofinstable strain ε crand the influence of the parameter of stress-stateα , work-hardeningexponent n on the equivalent strain of instability is discussed.The finite element software DYNAFORM is issued to simulate the whole tubehydroforming process. The stress distribution, strain distribution and thicknessdistribution at different process conditions and different times are presented. The effectof different matching of internal pressure and axial force is analyzed. Reducing axialforce or increasing pressure alone would reduce the forming limits of tube. Thedifferent friction coefficient between tube and die is one of the important influences intube hydroforming. Reduce the friction as possible, it can not only benefit to tubeforming, but also reduce axial force effectively. In addition, the material characters andthickness of tube can affect forming evidently. When work-hardening exponent of tubematerial get bigger, the formability of the tube is better. When the thickness of tubeincrease, the formability of the tube is better too, but the internal pressure for tubehydroforming is higher.At last, the experiment of tube hydroforming was carried out in the guidance ofthe theory analysis. Experimental results of different load path are presented and therule of the effect of internal pressure and axial force are studied. Based on theexperiment, the basic rule of load control method is also discussed. The experimentalresults and the results of FEA are also compared. They coincide well.The research results in this thesis provide effective guidance and solid foundationfor the application of tube hydroforming process to tubular parts. They also offer abasic rule and a high-efficient way for parameters optimization. This can enhanceefficiency and competition of tube hydroforming.