Investigations for warm forming of lightweight sheet materials: Process optimization

The main objectives of this research are to broaden the fundamental understanding of warm forming mechanism, and to develop the predictive analytical and numerical models that can accurately characterize effect of process factors (i.e., material, temperature, lubrication, forming rate, binder force, etc.) on warm forming performance (i.e., formability and dimensional accuracy of a part), and that can assist in designing optimal process and tooling conditions effectively (i.e., reduced trial and error and short lead time). To achieve the stated objectives, analytical and FEA models considering the thermo-mechanically coupled characteristic of warm forming were developed for various model geometries. In addition, design guidelines to determine appropriate heating conditions were established using the developed FE models and design of experiments (DOE) techniques to examine the multiple process and tooling parameters. The two-phase sequential analysis combining the isothermal FEA/DOE and few additional non-isothermal validation FEA offered an efficient way to determine the optimal warm forming condition with the reduced number of simulations and much less CPU time usage. A multi-fidelity optimization module allowing to combine less accurate and less costly low fidelity analysis with expensive high fidelity analysis was also developed to systematically and accurately predict warm forming performance in a wide range of operating conditions. Moreover, the overall quality of a part in warm forming was evaluated by checking the dimensional accuracy of a formed part after unloading at elevated temperatures, and the effect of non-uniform heating and die distortion on part quality was analyzed by comparing the stress distribution, thickness variation, and springback amount between rigid and elastic tooling FEA models.