Improving the formability limits of lightweight metal alloy sheet using advanced processes: Finite element modeling and experimental validation

Weight reduction is one of the major goals in the automotive, appliance and electronics industries. One way of achieving this goal is to use lightweight alloys such as aluminum and magnesium that have high strength to weight ratios. However, due to their limited formability at room temperature, advanced forming processes are needed. Room temperature and elevated temperature hydraulic bulge tests (using a submerged tool) were conducted for Al 5754-O and Mg AZ31-O to determine their mechanical properties. Experiments were conducted between room temperature and 225 C, at various approximate true strain rates. Strain values up to 0.7 were obtained under equi-biaxial state of stress at elevated temperatures. Flow stress curves were calculated using the membrane theory.; Deep drawability of aluminum and magnesium alloys is investigated through experiments and process simulation at room temperature (using solid dies), against liquid pressure (hydroforming) and at elevated temperatures (warm forming). Limiting Draw Ratio (LDR) of Al 5754-O is increased from 2.1 (room temperature) to 2.4 when hydroforming is used as the drawing process. This value is increased to 2.9 when warm forming is used. Formability of Mg AZ31-O is found to be limited at room temperature while LDR up to 3.2 is obtained at elevated temperatures. Warm forming experiments were conducted using a servo motor driven press and a heated tool set. The in-die dwelling concept is developed by using the flexibility of the servo press kinematics and blanks were heated in the tool set prior to forming. Temperature - time measurements were made at various blank holder interface pressures in order to determine the required dwell time to heat the blank to the forming temperature. Several lubricants for elevated temperature forming were evaluated using the deep draw test and a PTFE based film was selected as a lubricant at elevated temperatures. Deep drawing tests were conducted to determine the process window (max. punch velocity as functions of blank size and temperature) for Al 5754-O and Mg AZ31-O. Maximum punch velocities of 35 mm/s and 300 mm/s were obtained for the Al and Mg alloys, respectively. Comparisons for the Mg alloy sheets from two different suppliers were made and significant differences in formability were found. Additional experiments were conducted in order to understand the effect of constant and variable punch velocity and the temperature on the mechanics of deformation. Variable punch velocity is found to improve the thickness distribution of the formed part and provide 60% reduction in the drawing time. By calculating heat transfer coefficients using inverse optimization, computational models are developed and experimental results are used to validate the predictions from the computational model.