Page 36 - MetalForming May 2015

Automotive Steels
replace the flywheel, motor and clutch/brake to provide full energy and constant torque even when speed is slowed to 1 stroke/min. Servo-drive presses also have been pro- grammed to make multiple hits on a stamping at or near bottom-dead center (BDC), using unique tooling designs, to help reduce springback.
When comparing flywheel-drive and servo-drive press technologies, remember that there are key differences between the two technologies in term of energy—the fun- damental principles of tonnage derating still apply to both. The tonnage rating of mechanical presses usually is deter- mined at a slide position between 1⁄32 and 1⁄2 in. above BDC. The available working force (tonnage) decreases in mechan- ical presses as the working distance above BDC increases. This derating occurs because the relationship between the crank- shaft angle and the pitman arm has reduced mechanical advantage higher up in the stroke. This is true for flywheel- drive mechanical presses and their servo-drive counterparts.
Tonnage derating and press-energy curves are specific to a given press design and its construction. They are particu- larly important to understand and apply to deep-drawing operations, where the drawing process can begin several inches above BDC.
Virtual Forming, and Failure Modes
The practice of engineering stamping dies generally
begins with the application of computer-aided-engineering (CAE) tools to model processes in a virtual world. One of the biggest concerns with forming simulation and AHSS grades is springback accuracy. The goal of springback simulation is to provide compensation direction and magnitude for die try- out. However, the selection of different yield criteria produces varying springback results, and these results are very sensi- tive to anisotropy values when anisotropic yield criteria are used.
Springback calculations in metalforming simulations typically are calculated in two-step processes—residual stresses for the deformed features are predicted first, followed by predicting the elastic displacements based on the calcu- lated relaxation of the stresses.
Two other failure modes also have been observed in AHSS stampings:
Splitting failure when bending over a sharp die radius, fol- lowed by simultaneous stretching as the material flows into the die cavity. The onset of necking may not precede this fail- ure; this failure mode appears to occur by rapid shear-band formation, a condition that may not be predictable by the conventional FLD approach.
Failure in hole expansions and in sheared-edge stretching. Again, failures may occur sooner than predicted and may not be accompanied by appreciable necking. Some AHSS grades pos- sess extraordinary sensitivity to sheared-edge quality. MF
34 MetalForming/May 2015
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