The increase in using Advanced High Strength Steel (AHSS) and aluminum sheet materials is accompanied by many challenges in forming these alloys due to their unique mechanical properties and/or low formability. Therefore, developing a fundamental understanding of the mechanical properties of AHSS, as compared to conventional Draw Quality Steel (DQS), is critical to successful process/ tools design. Also, alternative forming operations, such as warm forming or sheet hydroforming, are potential solutions for the low formability problem of aluminum alloys.
In this study, room temperature uniaxial tensile and biaxial Viscous Pressure Bulge (VPB) tests were conducted for five AHSS sheet materials; DP 600, DP 780, DP 780-CR, DP 780-HY, and TRIP 780, and the resulting flow stress curves were compared. Strain ratios (R-values) were also determined in the tensile test and used to correct the biaxial flow stress curves for anisotropy. The pressure vs. dome height raw data in the VPB test was extrapolated to the burst pressure to obtain the flow stress curve up to fracture. Results of this work show that flow stress data can be obtained to higher strain values under biaxial state of stress. Moreover, it was observed that some materials behave differently if subjected to different state of stress. These two conclusions, and the fact that the state of stress in actual stamping processes is almost always biaxial, suggest that the bulge test is a more suitable test for obtaining the flow stress of AHSS sheet materials to be used as an input to FE models. An alternative methodology for obtaining the flow stress from the bulge test data, based on FE-optimization, was also applied and shown to work well for the AHSS sheet materials tested.
Elevated temperature bulge tests were made for three aluminum alloys; AA5754-O, AA5182-O, and AA3003-O, using a special machine where the tools and specimen are submerged in a fluid heated to the required temperature. Several challenges were faced in the experiments such as leakage of the bulging fluid and sample pre-bulging in the clamping stage prior to the test. Moreover, it was originally planned to measure the dome curvature by using three LVDTs; one at the dome apex, and the others at different off-center locations. However, the probes slightly penetrated the soft sheet. Consequently, the off-center probes deflected and gave incorrect data. As a result of these challenges, the pressure and dome height data was not considered reliable to be used in determining the flow stress curves. Only the experimental data is included in this report for documentation purposes, while the calculated flow stress curves are not included.
A Sheet Hydroforming with a Punch (SHF-P) process was successfully simulated using the FE software Pamstamp 2G 2007. The objective was to develop a fundamental understanding of the process to reduce the expensive experimental trial and error. A systematic methodology to design the process was suggested and applied using FE simulation. A considerable improvement in the thinning distribution in the part was achieved by properly selecting the blankholding and pot pressure curves.