Hydroforming of tubular materials at various temperatures

by 1973- Aue-u-lan, Yingyot

This dissertation research covered two main areas in tube hydroforming process. The first was to develop the methodology to determine the flow stress directly from the tube at room temperature. The hydraulic bulge test was selected for this purpose, because it emulates the real state of stress (biaxial state of stress) occurring during hydroforming. Dimensions of the hydroformed tube were used to calculate the flow stress. The analytical model based on an incremental strain theory (non-proportional strain path) was used to predict the wall thickness at the apex of the dome and curvature radius. The thickness predictions were compared with the measured data. The agreement was good. The application of the hydraulic bulge test was extended for use as a tool for a quality control of incoming tubular materials. The experiments were performed to investigate the variations in formability of the tubes due to the tube manufacturing processes (rolling process to produce a sheet and roll forming to bend the sheet to form the tube). Different criteria (maximum bulge height (h), strain hardening exponent (n) and maximum percentage thinning) were evaluated to determine the sensitivity of the material property variations to manufacturing processes. The maximum bulge height at the bursting pressure was found to be the most sensitive variable. ii The second portion of this research was to develop a prototype tube hydroforming system that could be used to form lightweight alloy tubes (aluminum and magnesium alloys) at elevated temperatures. The existing knowledge on process development, especially in equipment and process designs, for forming these materials at the elevated temperature was not sufficient. Therefore, a new design approach called “submerged concept”, was developed to reduce the heating and filling time and maintain uniform temperature in the tube during hydroforming. The prototype tube hydroforming system was used to investigate the effect of the tube extrusion processes (with mandrel –seamless and with porthole die –with seams) on the quality of tubes. Seamless extruded tubes were studied extensively regarding the effect of the process parameters (forming temperatures and forming rates) on the formability and loading behavior (internal pressure). The tubes with seams were found to have defects at the welding line that caused fracture during hydroforming. The results indicated that formability increases with increasing temperature. The forming pressure dropped before the tube touched the die surface, indicating of strain softening. Tensile test was used to obtain the flow stress of the tubes at different temperatures (100, 150, 200 and 250oC) and strain rates (0.001, 0.01 and 0.1 /s). These flow stress data were used in Finite Element simulations to predict process variables, i.e. pressure and axial feed versus time. The comparison between the simulation and experimental results showed reasonable agreement. iii