Axial hydro-forging sequence for variable-diameter tube of 6063 aluminum alloy

Abstract To overcome the problems in forming the variable-diameter tubes by tube hydroforming, such as excessive thinning and high internal pressure, a new approach named axial hydro-forging sequence was proposed. In this approach, the tube was expanded to fill the die cavity firstly and then the thinning ratio was reduced by axial compression deformation produced in axial hydro-forging sequence. To demonstrate the feasibility of this approach, two mechanical conditions were theoretically analyzed in this study include axial hydro-forging condition and deformation sequence. Based on the plastic energy method, a critical wrinkling model for axial hydro-forging sequence was established to calculate the critical wrinkling internal pressure. It was observed that there was a critical wrinkling internal pressure in the axial hydro-forging sequence, and the critical wrinkling internal pressure increased with increasing the tube axial shortening and material strength coefficient, but decreased with the increase of strain hardening coefficient. At the same time, an analytical model based on mechanical analysis was developed to discuss the deformation sequence for different deformation zones. The results showed that the deformation sequence of the three characteristic zones in the process was as follow: maximum diameter zone > transition zone > guide zone, which satisfied the requirement of reducing the thinning ratio. Finally, experiments along with the finite element simulation were performed to validate the analytical model, and effects of key process parameters on tube forming were revealed. As a result, variable-diameter tubes were successfully produced by this approach, which indicated that the axial hydro-forging sequence is a feasible forming approach for the fabrication of tubes with variable diameters.