The tube hydroforming process has gained increasing attention in recent years. Coordination of the internal pressurization and axial feeding curves is critical in the tube hydroforming process to generate successful parts without fracture or wrinkling failure. The stress state at a given time and location varies with the process history and the design and control of the load paths. A new process parameter, counter-pressure, is introduced to achieve a favorable tri-axial stress state during the deformation process. The new process is referred to as dual hydroforming. The benefits offered by dual hydroforming will be characterized based upon the amount of wall thinning, plastic instability limit and final bulged configuration. An analytical model is developed to analyze the stress and strain state in the part (tube) during the dual hydroforming process. The stress-strain condition analyzed will be used to evaluate and compare thinning for tube hydroforming and dual hydroforming. The effect of applying counter-pressure on the plastic instability of thin-walled tubes with only internal pressure and combination of internal pressure and independent axial loading is considered. Finite element analysis is used to quantify the merits of dual hydroforming in terms of final bulged configuration. A parametric study has been conducted to investigate the effectiveness of dual hydroforming based on the various material properties and process conditions. Dual hydroforming results in different stress and strain states compared to tube hydroforming. The counter-pressure enabled favorable tri-axial stress state during deformation that resulted in different thickness and percentage thinning. Finite element analysis showed that for a particular amount of wall thinning there is an increase of around 8% in bulge height for dual hydroforming. Dual hydroforming delays the onset of plastic instability. This increase in the value of effective strain to failure results in an increase of around 12% in bulge height for dual hydroforming as shown by finite element simulations. Results of this study indicate that dual hydroforming can increase expansion i.e. more difficult parts can be designed and manufactured. Also, for a given part geometry, higher strength and less formable materials can be used.
