Optimization of the sheet metal stamping process: Closed-loop active drawbead control combined with in-die process sensing

The sheet metal forming process is subject to failure in several modes, the first of which is wrinkling in the flange region of the part, the second of which is fracture in the sidewall or bottom of the part. The difficulty of drawing complex part shapes is heightened when forming parts of aluminum or thinner high-strength steel alloys. This work focuses on developing a closed-loop method to optimize the sheet metal forming process using the drawbead as the active die element. Two independent highspeed hydraulic servo-actuators were built into the press bed of a hydraulic press (125 ton) to provide active drawbead capability. A non-axisymmetric die with force-resolving sensors built into the forming radius of the tooling was used in the press to investigate the sheet metal forming process with active drawbeads. A series of open-loop tests (no feedback from the in-die sensors) with various drawbead trajectories were run and the results showed that the choice of drawbead trajectory has a dramatic affect on maximum attainable draw depth at fracture. Increasing drawbead penetration from 0 to 5 mm in the early stages and then retracting back to 1 mm midway through the draw resulted in a 42% increase in draw depth compared to a constant 5 mm penetration (for 6111-T4 aluminum panels). Experimental part strains for various drawbead trajectories were obtained using gridded 6111-T4 panels. The results were in excellent agreement with finite element analysis of the process using LS-Dyna3D. The results show that the magnitude of the major strain in the failure zone (sidewall just above the onset of the punch nose radius) can be minimized by using early drawbead penetration. The sheet metal forming process was made more robust by closing the loop around the feedback provided by the in-die sensors. The closed-loop system was able to compensate for non-symmetric changes in sheet surface roughness and lubrication. Good parts (same draw depth as the base case) were produced in spite of the disturbances due to the ability of the system to sense changes in local sheet tensions and react accordingly by adjusting drawbead penetrations.