Research On The Formability Evaluation Of Hydroformed Parts

The lightweight of auto-body is more and more important with the demand for savingcosts of fuel and raw material, and with the strict restrict of automobile exhausts from thecodes for environmental protection of many countries. To ensure safety of running, it shouldnot reduce the safety of auto bodies while reducing the weight of auto-body parts. Except forusing light materials, another important way is to apply hollow and multi-sectional parts inplace of traditional structural parts. Tube hydroforming is an advanced technology tomanufacture such hollow and light-weight parts. Compared with the parts manufacturedthrough stamping and welding, the structural parts of auto-body manufactured through tubehydroforming has many advantages, such as lighter weight, higher rigidity and strength,higher impact strength, and saving material and space, reducing manufacturing processes andso on. All these advantages cater to the demand of light weighting for auto bodies very well.Tube hydroforming has played an important role in the light weighting of auto bodies, and hasdrawn most auto manufactures’ attention. It can be foreseen that this technology will propelthe advancement and improvement of auto-body manufacturing. Some subjects on tubehydroforming are studied in the present dissertation. It is a very important work for promotingthe localization process of automobile industry of China.Researchers have made efforts on the process design of tube hydroforming, defectprediction and related subjects, and great achievements have been obtained. But there are stillmany aspects on tube hydroforming to study, for example, the product design, process design,the formability evaluation of hydroformed parts and so on. There are many difficulties in theapplication of tube hydroforming on automobiles. The first one is the modification of parts, i.e.modifying parts manufactured through stamping and welding processes to hydroformed parts.This is a key step for the design of hydroformed parts. There are still absent of necessarytheoretical bases for the design of key geometries of hydroformed parts. The second difficulty is in testing the properties of tube blanks. To test the properties of the materials correctly is thebasis for selecting material and for process design. There are still not effective ways to test thematerial properties applied to tube hydroforming. The third difficulty is designing reasonablehydroforming process according to the geometries of the part and the material properties,which can avoid any defects occurring on the hydroformed parts. The existing models fordefect prediction do not apply to complex hydoroforming conditions.In this dissertation, some subjects on the formability evaluation of hydroformed parts arestudied through experiments and theoretical analyses. At the beginning, the testing method forthe properties of tube blanks are studied, including the testing methods of the anisotropycoefficient, the strain-stress relationships of tube materials and the forming limit diagram(FLD). Then, the prediction model for bursting and wrinkling are established for circular andnon-circular parts, according to the main defects and the main geometric characteristics ofparts applied on automobiles. In the end of the dissertation, the design methods for tubehydroforming process are studied. It provides good theoretical bases for the formabilityevaluation and the design of key geometric dimensions to hydroformed parts.The main contents and the main innovations of the present dissertation are as follows.1. Formability evaluation for tube blanksThe main properties that affect the hydro-formability of tubes include the anisotropy andthe strain-hardening property. A method to test the anisotropy through hydroforming alongwith tensile tests of standard samples is put forward. And a method for testing the stress-strainproperty of tubes through hydroforming with the aid of several advanced tools is put forward,too. Compared with the method of uni-axial tension, these methods are more suitable for tubehydroforming, because the effects of the manufacturing processes are considered.2. Evaluation of the forming limit of tubesBased on the requirements for establishing forming limit diagrams (FLD), testingdevices of FLDs for tubes suitable for different strain states are invented. A method to designthe loading paths for FLD tests is put forward. Then a complete test scheme is put forwardbased on all these works.3. Study on the defect prediction model for hydroformed part The mechanics of all defects that may occur on the parts in hydroforming processes areanalyzed. Based on the theory of plastic stability of stiffened thin shells, the law of energyconservation, the theory of plastic forming and the theory of tube hydroforming, theprediction model for bursting and wrinkling are established. All these models provide atheoretical basis for the design of hydroformed products and hydroforming processes.4. Design methods for tube hydroforming processesThe factors that may affect the hydro-formability of tubes involve dies, properties of thematerial, the geometries of parts, et al. Some issues related to the tube hydroforming processare studied in this dissertation. The method for the design of hydroforming dies and for thedesign of key geometries of parts are put forward based on the prediction model that areestablished in this dissertation. At the end of the dissertation, the design of hydroformingprocesses for several typical parts are studied, and experiments are performed. Theexperimental results verify the methods provided in this dissertation very well.