Hydroforming of tubular materials at various temperatures
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. The analytical model based on an incremental strain theory was used to predict the wall thickness at the apex of the dome. 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 produced by roll forming process. The maximum bulge height at the bursting pressure was found to be the most sensitive variable. 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 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 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. 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 and strain rates. 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.
School:The Ohio State University
School Location:USA - Ohio
Source Type:Master's Thesis
Keywords:hydroforming tube warm tubular materials flow stress
Date of Publication:01/01/2007